U.S. patent application number 10/402518 was filed with the patent office on 2003-10-02 for terrain awareness system.
Invention is credited to Conner, Kevin J., Johnson, Steven C., Muller, Hans R..
Application Number | 20030184450 10/402518 |
Document ID | / |
Family ID | 24027588 |
Filed Date | 2003-10-02 |
United States Patent
Application |
20030184450 |
Kind Code |
A1 |
Muller, Hans R. ; et
al. |
October 2, 2003 |
Terrain awareness system
Abstract
A terrain awareness system (TAS) provides LOOK-AHEAD/LOOK-DOWN
as well as LOOK-UP terrain advisory and warning indications to the
pilot of an aircraft of a hazardous flight condition. The TAS
includes an airport data base as well as terrain data base that is
structured to provide various resolutions depending on the
topography of the particular geographic area of interest.
Navigational data from a satellite-based navigational system, such
as a global positioning system (GPS), is used to provide a
LOOK-AHEAD/LOOK-DOWN and LOOK-UP terrain advisory and terrain
warning indications based upon the current position and projected
flight path of the aircraft. Since the terrain advisory and the
warning signals are a function of the flight path of the aircraft,
nuisance warnings are minimized.
Inventors: |
Muller, Hans R.; (Redmond,
WA) ; Conner, Kevin J.; (Kent, WA) ; Johnson,
Steven C.; (Issaquah, WA) |
Correspondence
Address: |
Black Lowe & Graham, PLLC
816 Second Avenue
Seattle
WA
98104
US
|
Family ID: |
24027588 |
Appl. No.: |
10/402518 |
Filed: |
March 28, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10402518 |
Mar 28, 2003 |
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08509660 |
Jul 31, 1995 |
|
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6606034 |
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Current U.S.
Class: |
340/963 ;
340/970; 340/988 |
Current CPC
Class: |
G01S 19/41 20130101;
G01S 7/22 20130101; G08G 5/0086 20130101; G01S 13/953 20130101;
G01S 7/12 20130101; Y02A 90/18 20180101; G01S 19/47 20130101 |
Class at
Publication: |
340/963 ;
340/970; 340/988 |
International
Class: |
G08B 023/00 |
Claims
What is claimed and desired to be secured by Letters Patent of the
United States is:
1. A warning system for an aircraft comprising: first means for
receiving signals representative of the instantaneous flight path
angle; second means for receiving signals from a predetermined
satellite-based navigation system representative of the position of
the aircraft; third means for receiving signals representative of
the altitude of an aircraft; means for storing data representative
of terrain elevation information; and means responsive to said
first means, said second means, said third means and said storing
means for generating a first warning signal of a hazardous flight
condition, as a function of both the position of the aircraft
relative to said terrain elevation information and the flight path
angle of aircraft, for a predefined area relative to the aircraft
wherein said area includes areas both above and ahead of the
aircraft.
2. A warning system as recited in claim 1, wherein said
predetermined satellite-based navigation system is the global
positioning system GPS.
3. A warning system for an aircraft comprising: means for receiving
signals representative of the instantaneous flight path angle of
the aircraft; means for receiving signals representative of the
position of the aircraft; means for receiving signals
representative of the altitude of the aircraft; means for storing
data representative of actual terrain elevation information; and
means for generating a first warning signal of a hazardous flight
condition resulting from location of a subset of said predetermined
terrain information at a predetermined distance in front of the
aircraft, said first warning signal generated as a function of both
the position of the aircraft relative to said subset of said
predetermined terrain information and the flight path angle of the
aircraft.
4. A warning system as recited in claim 23, wherein said generating
means includes means for determining the relationship between the
flight path angle and a predetermined first datum and generating a
first warning signal when a predetermined relationship between the
flight path angle and said first predetermined datum exists.
5. A warning system as recited in claim 24, wherein said generating
means includes means for determining the relationship between the
flight path angle and a predetermined second datum and generating a
second warning signal when a predetermined relationship between
said flight path angle and said second predetermined datum
exists.
6. A warning system as recited in claim 25, wherein parameters for
determining said first predetermined datum and said second
predetermined datum are configurable.
7. A warning system as recited in claim 23, wherein said
predetermined distance is a function of the airspeed of the
aircraft.
8. A warning system as recited in claim 23, wherein said first
warning signal defines a plurality of boundaries relative to the
current position and altitude of the aircraft.
9. A warning system as recited in claim 28, further including means
for modifying said boundaries under predetermined conditions.
10. A warning system as recited in claim 29, wherein said modifying
means includes means for modifying said boundaries as a function of
the flight path angle of the aircraft.
11. The warning system of claim 1 wherein said area relative to the
aircraft further includes an area below the aircraft.
12. A method of providing terrain warnings for a vehicle, the
method comprising: receiving signals representative of an
instantaneous flight path angle of the vehicle; receiving signals
representative of the position of the vehicle; receiving signals
representative of terrain elevation information; and generating a
first warning signal as a function of both the position of the
vehicle relative to the terrain elevation information and the
flight path angle of the vehicle for a predefined area relative to
the vehicle wherein the area includes areas both above and ahead of
the vehicle.
13. A method of providing terrain warnings for a vehicle, the
method comprising: receiving signals representative of an
instantaneous flight path angle of the vehicle; receiving signals
representative of the position of the vehicle; receiving signals
representative of the altitude of the vehicle; receiving signals
representative of terrain elevation information; and generating a
first warning signal as a function of a subset of the terrain
elevation information at a predetermined distance in front of the
aircraft, said first warning signal generated as a function of both
the position of the vehicle relative to the subset of terrain
elevation information and the flight path angle of the vehicle.
Description
PRIORITY CLAIM
[0001] This application is a continuation of application Ser. No.
08/509,660, filed Jul. 31, 1995, attorney docket number
543-94-001.
BACKGROUND OF THE INVENTION
[0002] 1. Field of Invention
[0003] The present invention relates to a terrain awareness system
(TAS) and more particularly to a system for alerting a pilot of an
aircraft of a dangerous flight condition which monitors the
position as well as the trajectory of an aircraft based upon a
satellite-based navigation system, such as a global positioning
system (GPS), to provide a LOOK-AHEAD/LOOK-DOWN as well as LOOK-UP
terrain advisory and warning indications based upon stored terrain
data which provides relatively longer warning times than known
ground proximity warning systems while minimizing nuisance
warnings.
[0004] 2. Description of the Prior Art
[0005] Various systems are known in the art that provide warnings
and advisory indications of hazardous flight conditions. Among such
systems re systems generally known as ground proximity warning
systems (GPWS) which monitor the flight conditions of an aircraft
and provide a warning if flight conditions are such that
inadvertent contact with terrain is imminent. Among the flight
conditions normally monitored by such systems are radio altitude
and rate, barometric altitude and rate, air speed, flap and gear
positions. These parameters are monitored and an advisory signal
and/or warning signal is generated when the relationship between
the parameters is such that terrain impact is likely to occur.
Typical examples of such systems are disclosed in U.S. Pat. Nos.
3,715,718; 3,936,796; 3,958,218; 3,944,968; 3,947,808; 3,947,810;
3,934,221; 3,958,219; 3,925,751; 3,934,222; 4,060,793; 4,030,065;
4,215,334; and 4,319,218, all assigned to the same assignee as the
assignee of the present invention and hereby incorporated by
reference.
[0006] While the above-referenced systems do provide advisory and
warning signals in the event of proximity to terrain, the warnings
generated by such systems are based solely upon flight conditions
of the aircraft and do not utilize any navigational information.
Consequently, the sensitivity of such systems must be adjusted to
provide adequate warnings when a hazardous flight condition exists
without generating false or spurious warnings. However, such an
adjustment can result in a compromise that may still result in
nuisance warnings over terrain unique to particular geographic
areas and shorter than desired warning times in yet other
geographic areas.
[0007] Several attempts have been made to improve upon such ground
proximity warning systems utilizing ground-based navigational
information. For example, U.S. Pat. Nos. 4,567,483; 4,646,244;
4,675,823; and 4,914,436 all disclose ground proximity warning
systems which monitor the position of the aircraft relative to
stored terrain data in order to provide modified ground proximity
warnings. However, the utility of such systems is limited. For
example, the systems disclosed in U.S. Pat. Nos. 4,567,483 and
4,914,436 disclose ground proximity warning systems which utilize
navigational data to modify predetermined warning envelopes
surrounding certain particular airports.
[0008] U.S. Pat. Nos. 4,646,244 and 4,675,823 disclose terrain
advisory systems which utilize various ground-based navigational
inputs and stored terrain data to provide various ground proximity
warning systems based on the position of the aircraft. In order to
conserve memory, the terrain data is modeled in various geometric
shapes as a function of the elevation of the terrain within the
area defined by the geometric shape. The warning signals are
generated as a function of the position of the aircraft relative to
the model. While such systems do provide adequate terrain warnings,
such systems may cause nuisance warnings under certain conditions
since the predicted trajectory of the aircraft is not taken into
consideration.
[0009] Another problem with such systems is that they are based
upon ground-based navigational systems, such as VOR/DME and LORAN.
Such ground-based navigational systems are being replaced with
relatively more accurate satellite systems, such as the global
positioning system (GPS). Moreover, such ground-based systems are
only able to provide two-dimensional position data. Thus, with such
systems, the altitude of the aircraft must be supplied by an
auxiliary device, such as an altimeter, which increases the
complexity of the system as well as the cost.
SUMMARY OF THE INVENTION
[0010] It is an object of the present invention to solve various
problems associated with the prior art.
[0011] It is yet another object of the present invention to provide
a terrain awareness system which provides increased warning times
to the pilot of an aircraft of a hazardous flight condition while
minimizing nuisance warnings.
[0012] It is yet another object of the present invention to provide
a system which provides a LOOK-AHEAD/LOOK-DOWN as well as a LOOK-UP
terrain advisory and warning indications to the pilot of an
aircraft of a hazardous flight condition based upon the predicted
trajectory of the aircraft.
[0013] It is yet another object of the present invention to provide
a terrain awareness system which utilizes inputs from a
satellite-based navigation system, such as the global positioning
system (GPS).
[0014] Briefly, the present invention relates to a terrain
awareness system (TAS) which provides LOOK-AHEAD/LOOK-DOWN as well
as LOOK-UP terrain advisory and warning indications to the pilot of
an aircraft of a hazardous flight condition. The TAS includes an
airport data base as well as a terrain data base that is structured
to provide various resolutions depending on the topography of the
particular geographic area of interest. Navigational data from a
satellite-based navigation system, such as a global positioning
system (GPS), is used to provide a LOOK-AHEAD/LOOK-DOWN and LOOK-UP
terrain advisory and terrain warning indications based upon the
current position and projected flight path of the aircraft. Since
the terrain advisory and the warning signals are a function of the
flight path of the aircraft, nuisance warnings are minimized.
BRIEF DESCRIPTION OF THE DRAWING
[0015] These and other objects of the present invention will be
readily apparent with reference to the following specification and
attached drawings, wherein:
[0016] FIGS. 1A and 1B represent a block diagram of a terrain
awareness system (TAS) in accordance with the present
invention;
[0017] FIG. 2 is a diagram of the world broken down into various
latitude segments and longitude segments in accordance with the
present invention;
[0018] FIG. 3 is a graphical representation of the various memory
map and resolution sequencing in accordance with the present
invention;
[0019] FIG. 4A is an exemplary representation of the digital header
and subsquare mask word for identifying geographical areas in
accordance with the present invention;
[0020] FIG. 4B is a terrain map alternative to FIG. 3;
[0021] FIG. 5 is a graphical illustration of the LOOK-AHEAD
distance assuming the aircraft turns at a 30.degree. angle in
accordance with the present invention;
[0022] FIG. 6 is a graphical representation of a .DELTA.H terrain
floor boundary which forms the bases for a terrain advisory signal
and a terrain warning signal in accordance with the present
invention;
[0023] FIG. 7 is a graphical illustration of a terrain advisory
signal for an aircraft relative to the terrain and an airport for a
condition when the aircraft flight path angle is less than a first
predetermined reference plane or datum in accordance with the
present invention;
[0024] FIG. 8 is similar to FIG. 7 but for a condition when the
aircraft flight path angle is greater than the first predetermined
reference plane;
[0025] FIG. 9 is a graphical illustration of a terrain warning
signal for an aircraft relative to the terrain and an airport for a
condition when the aircraft flight path angle is greater than a
second reference plane or datum in accordance with the present
invention;
[0026] FIG. 10 is similar to FIG. 9 but for when the flight path
angle of the aircraft is less than the second reference plane or
datum;
[0027] FIG. 11 is a graphical illustration of a cut-off angle
correction boundary for a level flight condition in accordance with
the present invention;
[0028] FIG. 12 is similar to FIG. 11 but for a condition when the
flight path angle of the aircraft is greater than a predetermined
reference plane or datum;
[0029] FIG. 13 is similar to FIG. 11 but for a condition when the
flight path angle of the aircraft is less than a predetermined
reference plane or datum and also illustrates a BETA sink rate
enhancement boundary in accordance with the present invention;
[0030] FIG. 14 is a graphical illustration of the terrain advisory
and superimposed terrain warning signals for an aircraft relative
to the terrain for a condition when the aircraft flight path angle
is less than a first reference plane or datum;
[0031] FIG. 15 is similar to FIG. 14 but for a condition when the
flight path angle is between a first datum and a second datum;
[0032] FIG. 16 is similar to FIG. 14 but for a condition when the
flight path angle is less than the second datum;
[0033] FIG. 17 is a graphical illustration of LOOK-AHEAD/LOOK-DOWN
terrain advisory and warning boundaries in accordance with the
present invention for a condition when the aircraft is
descending;
[0034] FIG. 18 is similar to FIG. 17 but for a condition when an
aircraft is climbing;
[0035] FIG. 19 is a graphical illustration of LOOK-UP terrain
advisory and warning boundaries in accordance with the present
invention;
[0036] FIG. 20 is a graphical illustration of an aircraft during a
pull-up maneuver;
[0037] FIG. 21 is an exemplary block diagram for a system for
generating a signal representative of the altitude loss due to
pilot reaction time ALPT, as well as altitude loss due to a pull-up
maneuver ALPU in accordance with the present invention;
[0038] FIG. 22 is a graphical illustration of an alternative
methodology for generating a cut-off altitude boundary in
accordance with the present invention;
[0039] FIG. 23 is a block diagram of the display system in
accordance with the present invention;
[0040] FIG. 24 is a functional diagram of the display data format
in accordance with the ARINC 708/453 standard for a weather radar
display;
[0041] FIG. 25 is a plan view of a background terrain display
illustrating the variable density dot pattern in accordance with
the present invention;
[0042] FIG. 26 is similar to FIG. 25 but additionally illustrates
the terrain threat indications;
[0043] FIG. 27 is a block diagram illustrating the configuration of
a flash ROM for a terrain data base and a RAM used with the present
invention;
[0044] FIG. 28 illustrates a wedding-cake configuration for a RAM
utilized in the present invention;
[0045] FIG. 29 is a diagram of the file format of the files in the
terrain data base in accordance with the present invention;
[0046] FIG. 30 illustrates a terrain map of the terrain data base
in accordance with the present invention illustrating the highest
elevation in each of the cells within the map;
[0047] FIG. 31 is similar to FIG. 30 and illustrates a method for
correcting various cells in the vicinity of a runway;
[0048] FIG. 32 is a simplified elevational view of certain data
illustrated in FIG. 31;
[0049] FIG. 33 is a diagram of a display illustrating the weather
radar sweep spokes and the determination of the range
increments;
[0050] FIG. 34 is a block diagram of the display system in
accordance with the present invention illustrating the memory
configuration and the data flow;
[0051] FIG. 34A is a block diagram of a configuration alternative
to that shown in FIG. 34;
[0052] FIG. 35 is a diagram of a terrain map illustrating the
determination of the look-ahead vector array in accordance with the
present invention;
[0053] FIG. 36 is a diagram of a display illustrating a radar sweep
spoke and a method for determining the range increments of the
radar sweep spoke;
[0054] FIG. 37 is a terrain map illustrating the computation of the
hazardous terrain in accordance with the present invention;
[0055] FIG. 38 is a diagram of a terrain map superimposed on a
display screen illustrating the sweep range and the determination
of the incremental range increments;
[0056] FIG. 39 is a diagram of a pixel map of the display in
accordance with the present invention;
[0057] FIG. 40 is a diagram of the different fractal patterns used
to provide the variable density display of non-hazardous display
indications in accordance with the present invention;
[0058] FIG. 41 is a diagram illustrating the determination of the X
and Y increments of the display screen; and
[0059] FIG. 42 is a diagram of the method for superimposing the
fractal patterns on the displayed pixel map in accordance with the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0060] Referring to FIG. 1, a terrain awareness system (TAS),
generally identified with the reference number 20, is illustrated.
As will be discussed in more detail below, the TAS 20 utilizes
inputs from a satellite-based navigational system, such as global
positioning system (GPS) 22 (longitude, latitude, altitude,
groundtrack, ground speed), and/or an FMS/IRS navigational system
which may be updated by the GPS and/or DME/DME, a terrain data base
24, an airport data base 26 and corrected barometric altitude, for
example from a barometric altimeter 28 to provide a LOOK-AHEAD
warning system which provides relatively longer warning times while
minimizing nuisance warnings. Since the TAS 20 does not require a
radio altimeter input, the system can be used with certain
aircraft, such as commuter aircraft, which are not normally
equipped with a radio altimeter.
[0061] The current longitude and latitude of the aircraft from the
GPS 22 are applied to an Airport and Terrain Search Algorithm,
indicated by a block 29, which includes location search logic for
determining the terrain data, as well as the airport data
surrounding the aircraft. Such search logic is described in detail
in U.S. Pat. Nos. 4,675,823 and 4,914,436 assigned to the same
assignee as the present invention and hereby incorporated by
reference. The GPS inputs, along with terrain and airport data
surrounding the aircraft from the search algorithm, indicated by
the block 29, are applied to the LOOK-AHEAD warning generator 30,
which provides both terrain advisory and terrain warning signals
based upon the position and projected flight path of the aircraft.
The LOOK-AHEAD warning generator 30 may provide both an aural
warning by way of a voice warning generator 32 and speaker 34,
and/or a visual warning by way of a map or a display 36, as
discussed below.
Global Positioning System
[0062] The primary positional information for the aircraft is
provided by the GPS 22, such as disclosed in U.S. Pat. Nos.
4,894,655; 4,903,212; 4,912,645; 4,954,959; 5,155,688; 5,257,195;
5,265,025; 5,293,163; 5,293,318; and 5,337,242, all hereby
incorporated by reference. The GPS 22 includes a GPS receiver 37
and a combination GPS receiver and monitor 38. The positional
inputs from the GPS 22 are processed by a dead reckoning algorithm,
indicated by the block 40, for example as discussed in detail in
U.S. Pat. No. 5,257,195, to compensate for temporary outages of the
GPS 22, such as loss or masking of satellites. The dead reckoning
algorithm can be replaced by an additional navigational system,
such as an FMS or an IRS, which can be used during GPS outages.
[0063] A differential GPS receiver 42 can also be used to provide
an indication of a GPS altitude. As long as the differential GPS
information is received and four more satellites are visible, the
differential GPS information from the differential GPS receiver 42
is sufficiently accurate for the TAS 20. If the differential GPS
receiver 42 is not available, a compound altitude signal may be
generated by the block 44 and connected to the LOOK-AHEAD warning
generator 30 by way of a single-pole, double-throw switch 46, under
the control of a signal "DIFF GPS AVAILABLE", which indicates when
the differential GPS receiver 42 is available. In a first mode when
differential GPS information is available, the switch 46 connects
the GPS altitude signal from the GPS 22 to the LOOK-AHEAD warning
generator 30. In this position, the GPS altitude signal is
generated either from the GPS 22 or from the differential GPS
receiver 42, as discussed above. When a differential GPS receiver
42 is not available, a compound altitude signal is provided by the
block 44 to the LOOK-AHEAD warning generator 30. During this
condition, the switch 46, under the control of a "DIFF GPS
AVAILABLE" signal, connects the block 44 to the LOOK-AHEAD warning
generator 30 to enable the compound altitude signal to be provided
to the LOOK-AHEAD warning generator 30. Within the Differential GPC
Receiver 42, the barometric altitude is compared with the GPS
altitude to generate a compound altitude signal which could be used
to limit the maximum difference between the GPS output altitude
signal and the barometric altitude to the maximum expected GPS
altitude error. Such a configuration would reduce the effect of
erroneous pressure correction settings on the barometric altimeter
28.
Terrain and Airport Data Bases
[0064] As mentioned above, the airport data and terrain data are
separated into two different data bases 26 and 24, respectively.
Such a configuration allows either of the data bases 24 or 26 to be
updated without the need to update the other.
[0065] The airport data base 26 contains various types of
information relating to airports, such as runway midpoint
coordinates, runway length, runway heading, runway elevation and
airport/runway designation data. Additional data relating to
obstacles along the approach path (i.e. high-rise hotels adjacent
Heathrow Airport in England) and a nominal final approach slope
could also be included in the airport data base 26. A separate
obstacle data base may also be provided, structured similar to the
airport data base.
[0066] In order for the TAS 20 to be useful, all airports, for
example, which commuter planes without radio altimeters can land,
would have to be included in the airport data base 26. In areas
where the airport data base 26 is not complete, the
search-algorithm 29 would indicate NO DATA. Since the airport data
base 26 is organized in a similar fashion as the terrain data base
24 as described below, additional airport data could be added in a
piecewise fashion when available. The estimated size of the airport
data base 26 is contemplated to be relatively small, compared to
the terrain data base 24, for example less than 200 kilobytes
depending on the amount of data stored per airport.
[0067] The terrain data base 24 may require up to 40 megabytes of
storage space, which makes it compatible with available flash
erasable programmable read only memory (ROM) devices, such as a
flash EEPROM, or mini hard disk drives, which could take advantage
of known data compression techniques to reduce the amount of
storage space required and also promote easy data retrieval by the
search algorithm 29.
[0068] An important aspect of the invention is that the terrain
data base 24 is structured to provide varying resolutions of
terrain data as a function of the topography of the terrain, as
well as distance to airports. For example, a relatively high
resolution can be provided close to the airport on the order of 1/4
to 1/8 nautical miles and a medium resolution, for example 1/2 to 1
nautical miles within a 30-mile radius of the airport. Outside of
the 30-mile radius from the airport, a coarser resolution is
sufficient as will be described below.
[0069] FIGS. 2 through 4 illustrate the organization of the terrain
data base. Referring first to FIG. 2, the world is divided into a
plurality of latitude bands 50, for example each about 4.degree.
wide. Each latitude band 50 is then divided into a plurality of
longitudinal segments 52, which are about 4.degree. wide around the
equator, such that each longitudinal segment 52 is about
256.times.256 nautical miles. In order to maintain a relatively
constant segment size, the number of longitudinal segments 52 per
latitude band 50 are reduced closer to the poles.
[0070] Since the number of latitude bands 50 and the number of
longitudinal segments 52 per latitude band 50 is fixed,
determination of the particular segment corresponding to the
current aircraft position is readily determined. For example, the
aircraft current latitude X is used to determine the latitude band
number. Next, the number of longitudinal segments 52 associated
with that band 50 is determined either by way of a LOOK-UP table or
by calculation. Once the number of longitudinal segments 52 in the
particular latitude band 50 of interest is determined, the current
longitudinal position Y of the aircraft can be used to readily
determine the longitudinal segment number.
[0071] The minimum data associated with each longitudinal segment
52 corresponds to the highest altitude within that segment. As
mentioned above, each of the segments 52 is approximately
256.times.256 nautical miles. As shown in FIG. 3, these
longitudinal segments 52 can be broken down into various subsquares
to provide varying levels of resolution. For example, each segment
52 may be broken down into a plurality of subsquares 54, each
subsquare 54 being 64.times.64 nautical miles to provide a very
coarse resolution. The subsquares 54, can, in turn, be further
subdivided into a number of subsquares 56, for example, each
16.times.16 nautical miles, to provide a coarse resolution. These
subsquares 56, in turn, can be subdivided into a plurality of
subsquares 58, for example, each 4.times.4 nautical miles, to
provide a medium resolution. The subsquares 58 can also be broken
down into a plurality of subsquares 60, for example 1.times.1
nautical miles, to provide a fine resolution. In the vicinity
around airports, it may even be desirable to break down the
subsquares 60 down into smaller subsquares 62 to provide even finer
resolution, for example 1/4.times.1/4 nautical miles.
[0072] As shown in FIG. 4A, the minimum data associated with each
longitudinal segment 52 consists of a header 65 which includes a
multiple data byte 66 which includes the reference altitude which
corresponds to the highest altitude for all the subsquares within
the segment, which assumes that the highest elevation in a square
is representative of the entire square elevation. The multiple data
byte 66 may also include a flag to indicate when no further
subdividing is required for certain geographical areas. For
example, for segments representing the ocean, all subsquares would
have the same maximum altitude and thus no further subdividing
would be required. In order to enable the data base to be created
and updated on a piecewise basis, the multiple data byte 66 may
also contain a flag bit or code indicating that data corresponding
to further subdivisions does not exist for a particular segment 52
containing a code indicating that no map data exists in the
segment.
[0073] For geographical areas, such as mountainous areas and areas
in the vicinity of an airport, the longitudinal segments 52 are
subdivided as discussed above. In such a situation, the stored data
would include a 2-byte mask word 68, which points to the subsquares
having different altitudes. For example, as shown in FIG. 4, the
2-byte mask word 68 is illustrated with a pointer in the box 15
within the subsquare 54 within the longitudinal segment 52, which
represents a different altitude which points to the next finer
layer of resolution as discussed above. The subsquares each include
a header 70 including a mask as described above to enable a pointer
for each subsquare having a different altitude to point to the next
finer layer until the finest resolution level desire is reached as
shown.
[0074] The data base structure provides for flexibility as to
resolution required and can be constructed on a piecewise basis at
different times. As such, the structure contains the entire
framework necessary to add new data or modify existing data without
changing any of the software.
[0075] The size of the data base is proportional to the resolution
and the size of the area to be covered. In order to cover the
entire earth's surface, about 149 million square nautical miles,
only about 2,500 to 3,500 longitudinal segments 52 are required.
Thus, the overhead for the minimum header sizes is relatively
small.
[0076] Alternatively, the world may be divided into a plurality of,
for example, 1.degree..times.1.degree. map files or cells 51 as
shown in FIG. 4B. The various map files 51 may be formed with
variable resolution. More particularly, each of the map files may
be subdivided into a plurality of subcells 53 with the highest
terrain elevation in each subcell indicated, with the resolution of
all of the subcells within a particular map file being of the same
resolution. As indicated above, the maps are generally
1.degree..times.1.degree., which may be combined for coarser
resolution (i.e. 2.degree..times.2.degree. or
10.degree..times.10.degree.)
[0077] The size of the subcells 53 is varied to provide varying
degrees of resolution. For example, for relatively high resolution,
the size of the subcells 53 may be selected as either 15.times.15
arc seconds or 30.times.30 arc seconds. The size of the subcells 53
may also be varied as a function of the terrain in particular
geographic areas. For example, the size of the subcells 53 may be
selected as 30.times.60 arc seconds at relatively far north
longitudes and as 30.times.120 arc seconds at longitudes even
further north. The size of the subcells 53 may also be varied as a
function of the distance to the nearest airport. For example, for
distances greater than 64 miles from an airport, the size of the
subcells may be selected as 60.times.60 arc seconds while at
distances greater than, for example, 128 miles from the airport,
size of the subcell 53 may be selected as 120.times.120 arc
seconds. For a course resolution, for example for use during a
cruise mode as discussed below, the size of subcells may be
selected as 5.times.5 arc minutes.
[0078] As discussed below and as illustrated in FIG. 27, the data
base may be compressed by a compression algorithm and stored in a
flash read only memory (ROM), configured as a wedding cake as
illustrated in FIG. 28. Various compression algorithms are known
and suitable for this application. The compression algorithm does
not form a part of the present invention.
[0079] In order to simplify computation, each of the map files
includes a header 55, which includes, inter alia, the resolution of
the particular map file 51 as well as the location of one or more
corners, for example, the northwest and southeast corners. The
headers 55 as well as the map files 51 are discussed in more detail
below and illustrated in more detail in FIG. 29.
[0080] For certain terrain, for example, terrain adjacent a runway,
the resolution of the-map files 51 (i.e. relatively large size of
the subcells 53) may result in errors in runway elevation. More
particularly, since each subcell 53 contains the highest elevation
within the subcell 53, certain errors relating to runway elevations
can occur, depending on the resolution of the map file 51.
Correction of the elevations adjacent the runways is discussed in
more detail below in conjunction with FIGS. 30-33. Systematic
errors may also be introduced by the maps, which are digitized, for
example, by the Defense Mapping Agency, using known averaging
algorithms, which are known to fill in valleys and cut-off the tops
of sharp peaks of the terrain.
LOOK-AHEAD Warning Generator
[0081] As mentioned above, the LOOK-AHEAD warning generator 30
generates both a terrain advisory signal and a terrain warning
signal based upon the position and trajectory of the aircraft
relative to stored terrain data. There are two aspects of the
terrain advisory and terrain warning signals: LOOK-AHEAD
distance/direction; and terrain threat boundaries.
LOOK-AHEAD Distance/Direction
[0082] The LOOK-AHEAD direction for detecting threatening terrain
is along the groundtrack of the aircraft. In order to prevent
nuisance warnings, the LOOK-AHEAD distance is limited, as will be
discussed below. Otherwise, potentially threatening terrain along
the current flight path of the aircraft relatively far from its
current position could produce nuisance warnings.
[0083] Two different LOOK-AHEAD distances (LAD) are utilized. The
first LAD is used for a terrain advisory signal (also referred to
as a yellow alert LAD). A second LAD is used for terrain warning
signals which require immediate evasive action (also referred to as
a red-alert LAD).
[0084] The LAD for a terrain advisory condition is considered first
in determining the LAD because it is assumed that the pilot could
make a 30.degree. bank turn at any time at a turning radius R. The
LAD is equal to the product of the speed of aircraft and the total
LOOK-AHEAD time. As shown in FIG. 5, the total LOOK-AHEAD time is
equal to the sum of the LOOK-AHEAD time T.sub.1 for a single
turning radius R; the LOOK-AHEAD time T.sub.2 for terrain clearance
at the top of the turn (i.e. point 73) plus a predetermined
reaction T.sub.3.
[0085] The terrain clearance at the top 73 of the turn is provided
to prevert inadvertent terrain contact as a result of the turn.
This terrain clearance may be selected as a fixed distance or may
be made equal to the turning radius R.
[0086] As shown in equation (1), the turning radius R is
proportional to the square of the speed of the aircraft and
inversely proportional to the bank angle TG (ROLL).
R=V.sup.2/G.times.TG(Roll) (1)
[0087] For a 30.degree. bank angle, the turning radius R in
nautical miles (nM) as a function of the speed in knots is given by
equation (2).
R=0.000025284*V.sup.2 (2)
[0088] The LOOK-AHEAD time T.sub.1 for a single turning radius is
given by equation (3).
T.sub.1=R/V (3)
[0089] Substituting R from equation (1) into equation (3) yields
the LOOK-AHEAD time T.sub.1 for a single turn radius as shown in
equation (4) as a function of the speed of the aircraft and the
bank angle.
T=V/G.times.TG(Roll) (4)
[0090] TABLE 1 provides various LOOK-AHEAD times T.sub.1 at various
turning radii and ground speeds.
1TABLE 1 Radius Speed T.sub.1 (nM) (knots) (sec) 1/2 140 13 1 200
18 2 280 26 3 345 31 4 400 38 5 445 40 6 490 44
[0091] By assuming that the fixed terrain clearance at the top 73
of the turn is equal to one turning radius R, the total LOOK-AHEAD
time for two turn radii (i.e. T.sub.1+T.sub.2) is simply twice the
time for a single turning radius. Thus, the total LOOK-AHEAD time
is 2*T.sub.1 plus the predetermined reaction time T.sub.3, for
example 10 seconds, as given by equation (5).
T.sub.TOTAL=2*T.sub.1+T.sub.3 (5)
[0092] By substituting R from equation (2) into equation (4), the
LOOK-AHEAD time for a single turn radius is given by equation
(6).
T.sub.1=0.000025284*V (6)
[0093] Thus, if it is assumed that the fixed clearance X at the top
of the turn 73 is also equal to the turning radius R, the total
LOOK-AHEAD time for two turning radii is given by equation (7)
below, simply twice the value determined in equation (6)
T.sub.1+T.sub.2=2*0.000025284*V (7)
[0094] For a predetermined reaction time T.sub.3, for example 10
seconds, the LOOK-AHEAD distance (LAD) in nautical miles for a
terrain advisory signal can be determined simply by multiplying the
speed of the aircraft (V) by the total time T.sub.TOTAL as shown in
equation (8).
LAD=V*(0.0000278+2*0.000252854*V)+fixed distance*K, (8)
[0095] where K is a constant, for example 0.
[0096] However, in order to provide optimal conditions during a
terrain advisory condition, the LAD is limited with an upper limit
and a lower limit. The lower limit may be a configurable amount,
for example, either 0.75, 1 or 1-1/2 nautical miles at relatively
low speeds (i.e. speeds less than 150 knots) and limited to, for
example, 4 nautical miles at speeds, for example, greater than 250
knots. The LAD may also be limited to a fixed amount regardless of
the speed when the distance to the runway is less than a
predetermined amount, for example 2 nautical miles, except when the
aircraft altitude is greater than 3,000 feet, relative to the
runway.
[0097] The terrain warning LAD is given by equation (9) below.
LAD=k.sub.1*LAD(terrain for LOOK-DOWN/LOOK-AHEAD advisory
indication),
k.sub.2*LAD(terrain LOOK-DOWN/LOOK-AHEAD warning),
k.sub.3*LAD(terrain LOOK-UP advisory), (9)
[0098] where k.sub.1=1.5, except when the LAD is limited at its
lower limit, in which case k.sub.1=1, k.sub.3=1 and where
k.sub.2=0.5, k.sub.3=2.
Terrain Threat Boundaries
[0099] The terrain threat boundaries include a terrain floor
boundary, terrain advisory boundaries (yellow alert), as well as
terrain warning boundaries (red alert) and are provided along the
groundtrack of the aircraft.
[0100] Terrain Floor Boundary
[0101] The terrain floor boundary is the basis for the terrain
threat boundaries and is similar to the terrain floor developed for
the GPWS. The terrain floor relates to a distance .DELTA.H below
the aircraft and is proportional to the distance to the closest
runway to prevent nuisance warnings when the aircraft is taking off
and landing, while providing adequate protection in other modes of
operation. As illustrated in equation (10), the terrain floor
boundary below the aircraft is essentially based upon providing 100
feet clearance per nautical mile from the runway, limited to 800
feet.
.DELTA.H=100(ft/nM)*(distance to runway threshold-an
offset)+100(ft/nM)*(distance to runway center-12 nautical miles)
(10)
[0102] Equation (10) is shown graphically in FIG. 6. Referring to
FIG. 6, the horizontal axis represents the distance from the
runway, while the vertical axis represents the .DELTA.H terrain
floor boundary beneath the aircraft. The first segment 71
represents the runway length, while the point 72 represents the
runway center. After a small offset 74, the next segment 78, which
starts at 0 feet, slopes at 100 feet per nautical mile distance
from the runway threshold, identified by the point 80, up to a
maximum of 500 feet. The 500 foot maximum continues along a segment
82 until the distance to the runway center 72 is a predetermined
distance D, for example 12 nautical miles. The next segment 84
slopes up at a 100 feet per nautical mile distance until the
distance rises 300 feet from the segment 82 for a total of 800
feet.
[0103] The terrain floor .DELTA.H boundary beneath the aircraft is
limited such that the segment 78 begins at 0 and the segment 82
never goes above a predetermined maximum, for example 500 feet. In
addition, the terrain floor .DELTA.H boundary is also limited so
that the segment 84 never goes below the vertical height of the
segment 82 and rises no more than 300 feet relative to the segment
82. During conditions when the aircraft is 3,000 feet above the
terrain, the terrain floor .DELTA.H boundary is maintained at the
upper limit of 800 feet.
[0104] Terrain Advisory Boundaries
[0105] Two terrain advisory boundaries are shown graphically in
FIGS. 7 and 8. FIG. 7 represents a condition when the aircraft is
descending while FIG. 8 represents a condition when the aircraft is
ascending. As will be discussed in more detail below, the terrain
advisory boundaries are based upon the relationship between the
flight path angle .gamma. and a first configurable datum,
THETA1.
[0106] Referring first to FIG. 7, the terrain advisory (yellow
alert) boundaries are shown. The first segment of the terrain
advisory boundary, identified with the reference numeral 92,
corresponds to the .DELTA.H terrain floor boundary. As mentioned
above, the .DELTA.H terrain floor boundary is a function of the
distance from the runway 94. In order to determine the bottom
segment 100 of the terrain advisory boundary, it is necessary to
determine whether the flight path angle .gamma. is more or less
than a configurable datum, identified as THETA1. As shown and
described herein, THETA1 is set for 0.degree.. However, it should
be clear that other angles for THETA1 are within the broad scope of
the present invention. For the condition illustrated in FIG. 7, the
flight path angle .gamma. is less than THETA1. Thus, the segment
100 extends from the .DELTA.H terrain floor boundary at the angle
THETA1 to the LOOK-AHEAD distance for a terrain advisory. The final
segment 102 is generally parallel to the segment 92 and extends in
a vertical direction along the LAD for the terrain advisory
boundary.
[0107] If the flight path angle .gamma. is greater than THETA1 as
shown in FIG. 8., then different terrain advisory boundaries are
provided. The segment 92 will be the same as shown in FIG. 7 and
described above. However, when the flight path angle .gamma. is
greater than THETA1 as shown in FIG. 8, a bottom segment 106 is
formed by extending a line segment from the bottom of the segment
92, which represents the .DELTA.H terrain floor, along a direction
parallel with the flight path angle .gamma. up to the LAD. A
vertical segment 108 extends from the bottom segment 106 along the
LAD to form the terrain advisory boundaries.
[0108] Thus, as shown, if the airplane is climbing, only terrain
which penetrates the upward-sloping terrain advisory boundaries
will cause an advisory signal, such that if the aircraft clears the
terrain by a safe margin, no advisory signal is given, even if the
terrain ahead is at or above the present aircraft altitude.
However, if terrain penetrates the terrain advisory boundaries, an
aural warning, such as "CAUTION, TERRAIN" aural advisory may be
given. In addition, visual warnings may be provided.
[0109] Terrain Warning Boundaries
[0110] The terrain warning boundaries are shown in FIGS. 9 and 10
and, in general, indicate to the pilot of an aircraft conditions
when evasive action is required to avoid terrain contact. The
terrain warning boundaries are based on the relationship of the
flight path angle of the aircraft relative to a second configurable
datum, THETA2. The datum THETA2 may be selected with an upslope of,
for example, 6.degree., which is equal to the average climb
capability of airliners. The datum THETA2 could be modified, taking
into consideration aircraft type, configuration, altitude and time
for takeoff. However, because of the longer LOOK-AHEAD distance as
discussed above for the terrain warning boundaries, a terrain
warning could occur before a terrain advisory indication for
extreme terrain conditions.
[0111] Referring to FIGS. 9 and 10, two different terrain warning
boundaries are illustrated. In particular, FIG. 9 illustrates a
condition when the flight path angle .gamma. is greater than the
second datum THETA2. FIG. 10 illustrates a condition when the
flight path angle .gamma. is less than the second datum THETA2. In
both conditions, the first segment 114 of the terrain warning
boundary is extended below the aircraft for a distance 1/2 of the
.DELTA.H terrain floor, which, as mentioned above, is a function of
the distance of the aircraft from a runway. As mentioned above, the
terrain warning boundaries are dependent on the relationship
between the flight path angle .gamma. of the aircraft and the datum
THETA2.
[0112] Referring first to FIG. 9, since the flight path .gamma. is
less than the second datum THETA2, the bottom segment 118 of the
terrain warning boundary is formed by extending a line from the
segment 114 at an angle equal to the flight path angle .gamma. up
to the LOOK-AHEAD distance for a terrain warning as discussed
above. A vertical segment 120 extends from the lower segment 118
along the LAD for a terrain warning, forming the terrain warning
boundaries illustrated in FIG. 9.
[0113] If the flight path angle of the aircraft is less than the
slope of the second datum THETA2 as shown in FIG. 10, a bottom
segment 122 extends from the segment 114 up to the LOOK-AHEAD
distance at an angle equal to the flight path angle .gamma.. A
segment 124 extends upwardly along the LAD from the segment 122. If
any terrain along the groundtrack of the aircraft out to the LAD
for the terrain warning penetrates the envelope, aural and/or
visual terrain ahead warnings may be given.
[0114] Cut-Off Boundaries
[0115] In order to avoid spurious warnings when the aircraft
overflies a ridge at relatively low altitudes, the warning
boundaries may include cut-off boundaries, for example, as
illustrated in FIGS. 11, 12 and 13. Without the cut-off boundaries,
warnings would be given, although the terrain is virtually below
the aircraft and no terrain is visible ahead. Referring first to
FIG. 11, the cut-off boundary 126 begins at a predetermined cut-off
offset 128 below the aircraft and extends in a direction in front
of the aircraft at a predetermined envelope cut-off angle 130. The
envelope cut-off angle 130 is equal to the flight path angle
.gamma. plus a configurable predetermined cut-off angle, described
and illustrated as -6.degree.. For level flight as shown in FIG.
11, the cut-off boundary 126 extends from the cut-off offset 128 in
the direction of the envelope cut-off angle 130 toward the front of
the aircraft to a point 132 where it intersects a terrain advisory
boundary or terrain warning boundary, identified with the reference
numeral 134. For level flight, as shown in FIG. 11, the flight path
angle .gamma. is zero. Thus, the cut-off boundary 126 illustrated
in FIG. 11 will extend from the cut-off offset 128 along an angle
equal to the cut-off angle, which, as mentioned above, is selected
as -6.degree. for illustration. As mentioned above, the cut-off
boundary 126 extends from the cut-off offset 128 to the point 132
where it intersects the terrain advisory boundary 134 discussed
above. The warning boundary is then selected to be the highest of
the terrain advisory boundary 134 and the envelope cut-off boundary
126. Thus, for the example illustrated in FIG. 11, the terrain
advisory boundary would consist of the cut-off boundary 126 up to
the point 132, where the envelope cut-off boundary 124 intersects
the warning envelope 126. From the point 132 forward, the normal
terrain advisory boundary 134, corresponding, for example, to a
THETA1 slope, is utilized. Thus, if either a terrain advisory
boundary or terrain warning boundary is below the cut-off boundary
126, the cut-off boundary 126 becomes the new boundary for the
advisory or warning signal.
[0116] The cut-off angle 130 is limited such that if the flight
path angle .gamma. is a relatively high climb angle, the cut-off
angle 130 does not exceed a predetermined limit, for example a
configurable limit between 0 to 6.degree. as will be illustrated in
FIG. 12. In particular, referring to FIG. 12, the flight path angle
.gamma. is shown at a relatively high climb rate (i.e.
.gamma.=9.degree.). An unlimited cut-off envelope angle 130 is
illustrated by the partial segment 136. As shown, this segment 132
is 6.degree. below the flight path angle .gamma. However, in order
to decrease the sensitivity of the system 20 during conditions when
the flight path angle is relatively large, for example 9.degree.,
indicative of a fairly steep climb, the cut-off envelope angle 130
is limited. In the example illustrated in FIG. 12, the cut-off
angle limit is selected, for example, to be 1.4.degree.. Thus, at
flight path angles .gamma. greater than 7.4.degree., the cut-off
angle will result in an envelope cut-off boundary 140, which is
more than 6.degree. below the flight path angle .gamma.. As shown,
since the cut-off angle limit was selected as 1.4.degree., the
cut-off angle will be limited to 1.4.degree. anytime the flight
path angle is greater than 7.4.degree., even though the cut-off
angle will be equal to or more than 6.degree. less than the flight
path angle .gamma.. Thus, for the conditions illustrated in FIG.
12, the segment 140 begins at the cut-off offset 128 and continues
along a line that is 1.4.degree. from the horizon until it
intersects the terrain advisory or terrain warning boundary 142 at
the point 144. The actual advisory or warning boundary will be
formed by the limited cut-off boundary 140 up to the point 144 and
will continue with the normal warning boundary 142 beyond the point
144.
[0117] FIG. 13 illustrates a situation when the aircraft is
descending and thus the flight path angle .gamma. is less than 0.
During conditions when the flight path angle .gamma. is less than
the normal descent angle, the warning, boundary is lowered past the
cut-off boundary for high sink-rate situations in order to prevent
nuisance warnings during step-down approaches. In particular, a
BETA sink rate enhancement is provided and is effective anytime the
flight path .gamma. is below a predetermined configurable descent
bias GBBIAS, for example, -4.degree.. During such conditions, an
angle BETA is added to the terrain advisory datum (THETA1) as
indicated below. In particular, when the flight path angle .gamma.
is greater than the descent bias angle GBBIAS or the flight path
angle .gamma. is less than zero, BETA is selected as 0. When the
flight path angle .gamma. is greater than zero, BETA is given by
equation (11).
BETA=.gamma.-THETA1; for y>0 (11)
[0118] When the flight path angle .gamma. is less than GBBIAS, the
value for BETA is selected in accordance with equation (12)
below.
BETA=K*(.gamma.-GBBIAS), (12)
[0119] where k=0.5, .gamma.=flight path angle and GBBIAS is
selected as -4.degree..
[0120] FIG. 13 illustrates a condition when the flight path angle
.gamma. is -8.degree.. During this condition, the cut-off angle 130
is selected to be 6.degree. below the flight path angle .gamma. and
thus defines an envelope cut-off boundary 150 that extends from the
cut-off offset 128 along a line which is -14.degree. from the
horizontal axis. Since the flight path angle .gamma. is less than
GBBIAS, the angle BETA is added to the normal warning boundary
according to equation (12) above. Thus, for a flight path angle of
-8.degree., the BETA angle will be 2.degree.. Thus, the terrain
advisory or warning boundary, indicated by the boundary 151,
instead of extending along the THETA1 slope, will extend along an
angle which is 2.degree. below the THETA1 slope to define the
enhanced boundary 152. The warning cut-off boundary 152 thus will
extend to the point 156 where it intersects the normal terrain
advisory or terrain warning boundary 152. Since the cut-off angle
boundary 150 is higher than boundary 152 up to the point 156, the
cut-off boundary 150 will be utilized as the warning boundary up to
the point 156. Beyond point 156, the BETA-enhanced warning boundary
152 will define the warning envelope.
[0121] FIGS. 14, 15 and 16 illustrate the composite terrain
advisory and terrain warning boundaries for different conditions.
The cross-hatched area illustrates terrain in the vicinity of the
aircraft. Three conditions are illustrated. In particular, FIG. 14
represents a condition when the flight path angle .gamma. of the
aircraft is less than the first datum THETA1. FIG. 15 represents a
condition when the flight path angle .gamma. is between the first
datum THETA1 and the second datum THETA2. FIG. 16 represents a
condition when the flight path angle .gamma. is greater than the
second datum THETA2. The terrain advisory boundaries are indicated
in solid line while the terrain warning boundaries are illustrated
by dashed lines.
[0122] Referring first to FIG. 14, the terrain advisory boundaries
include a boundary 155, which extends from the aircraft to the
.DELTA.H terrain floor. Normally, the lower terrain advisory
boundary 157 would extend from the segment 155 along an angle equal
to the THETA1 slope. However, in this situation, the flight path
angle .gamma. is illustrated to be greater than the GBBIAS angle;
therefore, a BETA-enhanced boundary 159, as discussed above, is
utilized which extends along an angle that is equal to the THETA1
angle plus the BETA angle, up to the LOOK-AHEAD distance for a
terrain advisory. A vertical boundary 158 extends from the first
datum THETA1 vertically upwardly along the LOOK-AHEAD distance for
a terrain advisory. Since the cut-off boundary 157 is higher than
the terrain advisory boundary 157 up to the point 169 where the two
boundaries intersect, the cut-off boundary 157 becomes the terrain
advisory boundary up to the point 169. Beyond the point 169, the
boundary 159 is utilized as the warning boundary.
[0123] The terrain warning boundary includes a boundary 162
extending to a point equal to 1/2 of the terrain floor as discussed
above. A boundary 165 extends from the boundary 162 at the THETA2
slope. A vertical boundary 167 extends from the boundary 165 along
the LOOK-AHEAD distance for a terrain warning as discussed
above.
[0124] FIG. 15 illustrates a situation when the flight path angle
.gamma. is greater than the THETA1 angle and less than or equal to
the THETA2 angle. During this condition, the boundary for the
terrain warning extends along a segment 180, which is at 1/2 of the
.DELTA.H terrain floor. A sloping boundary 184 extends from the
segment 180 to the LOOK-AHEAD distance for a terrain warning (point
186). From there, a vertical boundary 188 extends vertically
upwardly to define the terrain warning boundaries.
[0125] The terrain advisory boundary includes a boundary 185 which
extends downwardly to the terrain floor, i.e. point 187. From point
187, the terrain advisory boundary extends along a segment 189 at
the flight path angle .gamma. up to a point 191, the LOOK-AHEAD
distance for a terrain advisory. A vertical segment 194 extends
along the LAD from the boundary 189 at the point 191.
[0126] In this illustration, an envelope cut-off boundary 193 cuts
off a portion of the terrain advisory indication boundary since the
flight path angle .gamma. of the aircraft is greater than a
predetermined limit. Thus, the portions of the warning boundary 189
below the cut-off angle boundary 193 are ignored; and the segment
193 becomes the effective terrain advisory indication up to the
point 195, where the cut-off boundary 193 intersects the terrain
advisory indication boundary 189. After the point 155, the boundary
189 will be the effective warning boundary.
[0127] FIG. 16 illustrates a situation when the flight path angle
.gamma. is greater than THETA2. In this situation, a terrain
advisory boundary 200 extends from the aircraft to the boundary
terrain floor. A lower boundary 202 extends at the flight path
angle .gamma. from the boundary 200 up to the LOOK-AHEAD distance
for a terrain advisory. A vertical terrain advisory boundary 204
extends upwardly along the LOOK-AHEAD distance for the terrain
advisory. Since the flight path angle .gamma. is illustrated to the
greater than GBBIAS, a segment 211, the cut-off boundary, defines
the effective warning boundary up to a point 213 where the terrain
advisory boundary intersects the cut-off warning boundary. After
that point, segment 202, which is higher, becomes the warning
boundary.
[0128] The terrain warning boundary extends along a segment 208
down to a point of 1/2 the terrain floor. A lower boundary 210
extends along an angle equal to flight path angle .gamma. since it
is greater than the THETA2 slope. A vertical terrain warning
boundary 212 extends upwardly from the boundary 210 along the
LOOK-AHEAD distance for a terrain warning.
[0129] In order to provide additional sensitivity of the warning
system, terrain advisory signals within a predetermined portion of
the LAD for a terrain advisory, for example 1/2 LAD, may be treated
as terrain warnings. Thus, as shown in FIGS. 14, 15 and 16,
vertical boundaries 161, 181 and 201, respectively, are shown, for
example, at 1/2 LAD. Based upon the flight path angles shown, these
segments define effective terrain advisory envelopes 166, 182 and
203.
Alternative Terrain Threat Boundaries
[0130] Alternative terrain threat boundaries are illustrated in
FIGS. 17-22. Similar to the terrain threat boundary discussed above
and illustrated in FIGS. 5-16, the alternative terrain threat
boundaries include a terrain floor boundary, terrain advisory
boundaries (yellow alert) and terrain warning boundaries (red
alert). In the alternative embodiment, the advisory and warning
boundaries are further divided into two parts, a
LOOK-AHEAD/LOOK-DOWN boundary for detecting terrain ahead or below
the aircraft (FIGS. 17 and 18) and a LOOK-UP boundary for detecting
precipitous high terrain ahead of the aircraft which may be
difficult to clear (FIG. 19).
LOOK-AHEAD/LOOK-DOWN Terrain Advisory and Warning Boundaries
[0131] As mentioned above, the LOOK-AHEAD/LOOK-DOWN boundaries are
illustrated in FIGS. 17 and 18. Referring first to FIG. 17,
LOOK-AHEAD/LOOK-DOWN advisory and warning boundaries are
illustrated for a condition when the aircraft is descending (i.e.
.gamma.<0) During such a configuration, the first segment of the
LOOK-AHEAD/LOOK-DOWN terrain advisory boundary, identified with the
reference numeral 300, corresponds to the .DELTA.H terrain floor
boundary. As discussed above, the .DELTA.H terrain floor boundary
is a function of the aircraft from the runway. In order to
determine the bottom segment 302 of the LOOK-AHEAD/LOOK-DOWN
terrain advisory boundary, the flight path angle .gamma. is
compared with a configurable datum, THETA1, for example 0.degree..
During descent conditions, the flight path angle .gamma. will thus
be less than THETA1. Thus, the LOOK-AHEAD/LOOK-DOWN terrain
advisory boundary segment will extend from the .DELTA.H terrain
floor boundary segment 300 along the angle THETA1 to the look-ahead
distance for a terrain advisory (LAD). The final segment 304
extends vertically upwardly from the segment 302 along the LAD.
[0132] The LOOK-AHEAD/LOOK-DOWN terrain advisory boundary may also
be modified by the BETA sink rate enhancement as discussed above.
In this embodiment, the BETA sink rate enhancement ensures that an
advisory indication always precedes a warning indication when the
aircraft descends into or on top of terrain. The BETA sink rate
enhancement is determined as a function of the flight path angle
.gamma. and two (2) configurable constants KBETA and GBIAS. The
BETA sink rate enhancement BETA1 for a LOOK-AHEAD/LOOK-DOWN terrain
advisory is provided in equation (13) below for a condition when
the aircraft is descending.
BETA1=KBETA*(.gamma.-GBIAS), (13)
[0133] where GBIAS is a configurable constant, selected for
example, to be zero (0) and KBETA is also a configurable constant
selected, for example, to be 0.5.
[0134] Referring to FIG. 17, the BETA sink rate enhancement BETA1
for the LOOK-AHEAD/LOOK-DOWN terrain advisory boundary provides an
advisory warning at a distance less than 1/2 LAD, unlike the
terrain advisory boundary described above). More particularly, for
the values defined above for the configurable constants KBETA and
GBIAS, the BETA sink rate enhancement BETA1 results in a segment
306 which extends from the .DELTA.H terrain floor segments 300 at
an angle equal to .gamma./2 up to 1/2 of the LAD. Beyond 1/2 LAD, a
segment 308 extends at the angle THETA1 to a distance equal to the
LAD. A vertical segment 310 extends along the LAD to connect the
segments 308 to the segment 304.
[0135] As discussed above, a cut-off boundary may be provided to
reduce nuisance warnings. Referring to FIG. 17, the cut-off
boundary is identified with-the reference numeral 312. The cut-off
boundary 312 extends from a vertical distance 314 below the
aircraft along a cut-off angle up to the point of intersection 316
with the terrain advisory boundary. For distances less than the
intersection 316, the cut-off boundary 312 forms the terrain
advisory boundary. For distances beyond the point of intersection
316, the boundaries 306 and 308 form the terrain advisory
boundaries.
[0136] The terrain warning boundary includes the segment 300
extending from the aircraft along the .DELTA.H terrain floor. A
bottom segment 318 connects to the segment 300 and extends along a
BETA sink rate enhancement angle BETA2. The BETA sink rate
enhancement angle is determined as a function of the flight path
angle .gamma. and a configurable constant KBETA2 as well as the
constant GBIAS discussed above and provided in equation (14)
below.
BETA2=KBETA2*(GAMMA-GBIAS), (14)
[0137] where GBIAS is a configurable constant selected, for
example, to be 0 and KBETA2 is also a configurable constant
selected, for example, to be 0.25.
[0138] For such values of the constants KBETA2 and GBIAS, the BETA
enhancement angle KBETA2 will be 1/4*.gamma.. Thus, the segment 318
extends from the segment 300 at an angle equal to 1/4*.gamma. up to
1/2 the LAD. A vertical segment 320 extends along a distance equal
to 1/2*LAD from the segment 318 to define the terrain warning
boundary.
[0139] Similar to the above, the terrain warning boundaries are
also limited by the cut-off boundary 312. Thus, the cut-off
boundary 312 forms the terrain warning boundary up to a point 322,
where the cut-off boundary 312 intersects the lower terrain warning
boundary 318. At distances beyond the point of intersection 322,
the segment 318 forms the lower terrain warning boundary up to a
distance equal to 1/2 of the LAD.
[0140] The terrain advisory and terrain warning boundaries for a
condition when the aircraft is climbing (i.e. .gamma.>0) is
illustrated in FIG. 18. During such a condition, the BETA sink rate
enhancement angles BETA1 and BETA2 are set to a configurable
constant, for example, zero (0).
[0141] The terrain advisory boundary during a climbing condition is
formed by extending a vertical segment 324 from the aircraft for a
distance below the aircraft equal to the .DELTA.H terrain floor.
During a climbing condition, a segment 326 is extended from the
segment 324 to the LAD at an angle equal to the flight path angle
.gamma.. At a point 328 where the segment 326 intersects a position
equal to 1/2 of the LAD, a vertical segment 330 is extended up from
the segment 326, forming a first vertical boundary for the terrain
advisory condition. The line segment 326 from the point 328 to the
LAD forms the lower terrain advisory boundary while a line segment
332 extending vertically upward from the line segment 326 along the
LAD forms a second vertical boundary.
[0142] For the exemplary condition illustrated, a cut-off boundary
334 does not intersect the terrain advisory boundaries. Thus, the
terrain advisory boundaries for the exemplary condition illustrated
is formed by the segments 330 and 332 and that portion of the line
segment 326 between the line segments 330 and 332.
[0143] The terrain warning boundaries for a condition when the
aircraft is climbing includes the vertical segment 324 (FIG. 18)
which extends from the aircraft to vertical distance equal to the
.DELTA.H terrain floor below the aircraft forming a first vertical
boundary. For a condition when the aircraft is climbing, the line
segment 326 extends from the segment 324 at the flight path angle
.gamma. to form the lower terrain warning boundary. The vertical
segment 330 at a distance equal to 1/2 of the LAD forms the second
vertical terrain warning boundary.
[0144] The cut-off boundary 334 limits a portion of the terrain
warning boundary. In particular, the cut-off boundary 334 forms the
lower terrain warning boundary up to a point 340, where the cut-off
boundary 334 intersects the line segment 326. Beyond the point 340,
a portion 342 of the line segment 340 forms the balance of the
lower terrain warning boundary up to a distance equal to 1/2 of the
LAD.
LOOK-UP Terrain Advisory and Warning Boundaries
[0145] The LOOK-UP terrain advisory and terrain warning boundaries
are illustrated in FIG. 19. As will be discussed in more detail
below, the LOOK-UP terrain advisory and warning boundaries start at
altitudes DHYEL2 and DHRED2, respectively, below the aircraft.
These altitudes DHYEL2 and DHRED2 are modulated by the
instantaneous sink rate (i.e. vertical speed, HDOT of the
aircraft). The amount of modulation is equal to the estimated
altitude loss for a pull-up maneuver, for example, at 1/4G (i.e. 8
ft/sec.sup.2) at the present sink rate. The altitudes DHRED2 and
DHYEL2 are dependent upon the altitude loss during a pull-up
maneuver (AlPU) and the altitude loss due to reaction time (ALRT)
discussed below.
[0146] The estimated altitude loss due to a pull-up maneuver ALPU
is best understood with reference to FIG. 20, wherein the vertical
axis relates to altitude in feet and the horizontal axis relates to
time in seconds. The trajectory of the aircraft from a time shortly
before pull-up is initiated to a time when the aircraft has
recovered is shown and identified with the reference numeral
344.
[0147] Assuming an advisory or warning indication is generated at
time T.sub.1 at a point 346 along the trajectory 344, the altitude
loss due to the reaction time of the pilot (ALRT) is given by
equation (15) below.
ALRT=HDOT*T.sub.R, (15)
[0148] where HDOT equals the vertical acceleration of the aircraft
in feet/sec and T.sub.R equals the total reaction time of the pilot
in seconds.
[0149] Assuming a pull-up maneuver is initiated at time T.sub.2,
the altitude loss due to the pull-up maneuver ALPU may be
determined by integrating the vertical velocity HDOT with respect
to time as set forth in equation (16) below.
HDOT(t)=a*t+HDOT.sub.0, (16)
[0150] where "a" equals the pull-up acceleration and HDOT.sub.0 is
a constant.
[0151] Integrating both sides of equation (16) yields the altitude
loss as a function of time H(t) as provided in equation (17)
below.
H(t)=1/2*a*t.sup.2+HDOT.sub.0t (17)
[0152] Assuming a constant acceleration during the pull-up
maneuver, the time t until vertical speed reaches zero (point 348)
is given by equation (18).
t=-HDOT.sub.0/a (18)
[0153] Substituting equation (18) into equation (17) yields
equation (19).
ALPU=-(HDOT.sub.0).sup.2/(2*a) (19)
[0154] Equation (19) thus represents the altitude loss during the
pull-up maneuver.
[0155] An exemplary block diagram for generating the signals ALTR
and ALPU is illustrated in FIG. 21. In particular, a signal
representative of the vertical velocity of the aircraft HDOT,
available, for example, from a barometric altimeter rate circuit
(not shown), is applied to a filter 350 in order to reduce nuisance
warnings due to turbulence. The filter 350 may be selected with a
transfer function of 1/(TAHDOT*S+1); where TAHDOT is equal to one
second. The output of the filter 350 is a signal HDOTf, which
represents the filtered instantaneous vertical speed; positive
during climbing and negative during descent.
[0156] In order to generate the altitude loss due to reaction time
signal ALTR, the signal HDOTf is applied to a multiplier 352.
Assuming a pilot reaction time Tr, for example, of 5 seconds, a
constant 354 equal to 5 seconds is applied to another input of the
multiplier 352. The output of the multiplier 352 represents the
signal ALTR, which is positive when HDOTf is negative and set to
zero if the signal HDOTf is positive, which represents a climbing
condition. More particularly, the signal HDOTf is applied to a
comparator 356 and compared with a reference value, for example,
zero. If the comparator 356 indicates that the signal HDOTf is
negative, the signal HDOTf is applied to the multiplier 352. During
climbing conditions, the signal HDOTf will be positive. During such
conditions, the comparator 356 will apply a zero to the multiplier
352.
[0157] The altitude loss due to the pull-up maneuver signal ALPU is
developed by a square device 358, a divider 360 and a multiplier
362. The filtered instantaneous vertical speed signal HDOTf is
applied to the square device 358. Assuming a constant acceleration
during the pull-up maneuver, for example, equal to 8 feet/sec.sup.2
(0.25 G), a constant is applied to the multiplier 362 to generate
the signal 2a. This signal, 2a, is applied to the divider 360 along
with the output of the square device 350. The output of the divider
360 is a signal (HDOT f).sup.2/2a, which represents the altitude
loss during a pull-up maneuver signal ALPU.
[0158] These signals ALRT and ALPU are used to modulate the
distance below the aircraft where terrain advisory and terrain
warning boundaries begin during a LOOK-UP mode of operation. More
particularly, during such a mode of operation, .DELTA.H terrain
floor segment of the terrain advisory boundary DHYEL2 is modulated
by the signals ALRT and ALPU while the .DELTA.H terrain floor
segment of the terrain warning boundary DHRED2 is modulated by the
signal ALPU as indicated in equations (20) and (21),
respectively.
DHYEL2={fraction (3/4)}*.DELTA.H+ALRT+ALPU (20)
DHRED2={fraction (1/2)}*.DELTA.+ALPU, (21)
[0159] where .DELTA.H represents the terrain floor as discussed
above.
[0160] Thus, referring to FIG. 19, the LOOK-UP terrain advisory
warning begins at a point 364 below the aircraft; equal to DHYEL2.
If the flight path angle .gamma. is less than a configurable datum
THETA2, a terrain advisory boundary 366 extends from the point 364
to the advisory LAD at an angle equal to THETA2. Should the flight
path angle .gamma. be greater than THETA2, the lower advisory
boundary, identified with the reference numeral 368, will extend
from the point 364 at an angle equal to the flight path angle
.gamma..
[0161] Similarly, the LOOK-UP terrain warning boundary begins at
point 370 below the aircraft; equal to DHRED2. If the flight path
angle .gamma. is less than the angle THETA2, a warning boundary 372
extends from the point 370 at angle THETA2 to the warning LAD.
Should the flight path angle .gamma. be greater than THETA2, a
warning boundary 374 will extend at an angle equal to the flight
path angle .gamma. between the point 370 and the warning LAD.
Cut-Off Altitude
[0162] The cut-off altitude is an altitude relative to the nearest
runway elevation; set at, for example, 500 feet. Altitudes below
the cut-of altitude are not displayed and are ignored by the
terrain advisory and terrain warning computations.
[0163] The use of a cut-off altitude declutters the display
discussed below around airports, especially during a final approach
when the aircraft approaches the ground.
[0164] A second advantage is that nuisance warnings on a final
approach, due to altitude errors, terrain data base resolution and
accuracy errors are minimized.
[0165] However, the use of a cut-off altitude during certain
conditions, such as an approach to an airport on a bluff (i.e.
Paine Field) at a relatively low altitude or even at an altitude
below the airport altitude, may compromise system performance. More
particularly, during such conditions, the use of a cut-off altitude
may prevent a terrain warning from being generated because the
threatening terrain may be below the cut-off altitude. In order to
account of such situations, the system selects the lower of two
cut-off altitudes; a nearest runway cut-off altitude (NRCA) and a
cut-off altitude relative to aircraft (CARA). The NRCA is a fixed
cut-off altitude, relative to the nearest runway. The CARA is an
altitude below the instantaneous aircraft altitude (ACA) by an
amount essentially equivalent to .DELTA.H terrain floor boundary
for an advisory condition, which, as discussed above, is a function
of the aircraft distance to the nearest runway.
[0166] Equations (22) and (23) below set forth the NRCA and CARA.
As mentioned above, the absolute cut-off altitude (ACOA) is the
lower of the NRCA and CARA as set forth in equation (24).
NRCA=COH+RE, (22)
[0167] where COH relates to the cut-off height and is a fixed
configurable value, initially set between 400 feet and 500 feet;
and RE equals the runway elevation.
CARA=ACA-.DELTA.H-DHO, (23)
[0168] where ACA is the instantaneous aircraft altitude; .DELTA.H
is the terrain floor which is proportional to the distance to the
runway as discussed above; and DHO is a configurable bias, set to,
for example, 50 feet.
ACOA=lower of CARA, NRCA, (24)
[0169] except where .DELTA.H DH1, in which case ACOA is always
equal to NRCA.
[0170] DH1 is a point at which the ACOA is forced to be equal to
NRCA independent of the aircraft altitude. The point DH1 is related
to COH, .DELTA.H and DHO such that on a nominal three (3) degree
approach slope, CARA is equal to NRCA when the aircraft is at a
distance equal to a distance DH1 from the airport, as illustrated
in TABLE 2 below.
2TABLE 2 COH DH1 DISTANCE TO (feet) (feet) RUNWAY (n mile) 300 50 1
400 100 1.5 500 150 2
[0171] The point DH1 forces the cut-off altitude (COH) above the
runway whenever the aircraft is close to the runway to ensure
robustness against nuisance warnings caused by altitude errors and
terrain data base resolution margins to disable the terrain
advisory and warning indications when the aircraft is within the
aircraft perimeter. There are trade-offs between nuisance warnings
and legitimate warnings. In particular, the lower COH, the closer
the advisory and warning indications are given, making the system
more nuisance prone. As indicated above, for a COH of 400, terrain
advisory and warning indications are effectively disabled when the
aircraft is closer than 1.5 n miles from the airport runway.
[0172] FIG. 22 illustrates the operation of the alternative cut-off
altitude boundaries. In particular, FIG. 22 illustrates a condition
when the COH is set to 300 feet with DH1 equal to 50 feet. The
cut-off altitude substantially for area from the runway, for
example, greater than 4 nautical miles, is 300 feet, as indicated
by the segment 378 when the glide slope angle is less than a
predetermined angle, for example, 3.degree.. As the aircraft gets
closer to the runway, the COH is lowered, as illustrated by the
segment 388, until the aircraft is within one (1) nautical mile of
the runway, at which point the COH is forced to be 300 feet, which
effectively disables any terrain advisory and warning indications
within the aircraft is closer than one (1) nautical mile from the
runway, as represented by the segment 382.
[0173] During a condition when the aircraft is on a, for example,
3.degree. glide slope angle, the ACOA is forced to be the NRCA. As
shown, the NRCA is illustrated by the segment 382.
Display System
[0174] A display system, generally identified with the reference
numeral 400, is illustrated in FIGS. 23-42. The display system 400
is used to provide a visual indication of the terrain advisory and
terrain warning indications discussed above as a function of the
current position of the aircraft. Background terrain information is
also provided which provides an indication of significant terrain
relative to the current position of the aircraft.
[0175] In order to declutter the display, background information
may be displayed in terms of predetermined dot patterns whose
density varies as a function of the elevation of the terrain
relative to the altitude of the aircraft. Terrain advisory and
terrain warning indications may be displayed in solid colors, such
as yellow and red, relative to the groundtrack of the aircraft.
[0176] An important aspect of the invention is that the terrain
background information as well as the terrain advisory and warning
indications may be displayed on a navigational or weather display,
normally existing within an aircraft, which reduces the cost of the
system and obviates the need for extensive modifications to
existing displays, such as a navigational and weather radar type
display. In particular, the terrain data is converted to a weather
radar format in accordance with the standard ARINC 708/453 for
digital buses on civil aircraft. The terrain data, masked as
weather data, can then be easily displayed on an existing
navigational or a dedicated weather radar display that conforms
with the ARINC 708/453 serial interface standard.
[0177] Referring to FIG. 23, the display system 400 includes a
weather radar display or navigational display 36 connected to a
control bus 404 conforming to an ARINC 429 standard and a terrain
data bus switch 406. The terrain data bus switch 406 enables
selection between terrain data and weather data for display. More
particularly, the data bus switch 406 includes a common pole 408
connected to the display 36 by way of a serial display bus 410. One
contact 412 on the data bus switch 406 is connected to a serial
weather data bus 414 and, in turn, to a weather radar R/T unit 416,
which transmits weather radar data over the weather data bus 414 in
conformance with the ARINC 708/453 standard.
[0178] An antenna 418 provides weather radar data to the weather
radar R/T unit 416. The weather radar R/T unit 416 is also
connected to the control bus 404, which is used for range and mode
data.
[0179] In accordance with an important aspect of the invention, the
terrain advisory and terrain warning indications, as well as the
background information discussed above, are converted to
"RHO/THETA" format. The converted terrain data is then applied to a
terrain data serial bus 420, which conforms to the ARINC 453
standard, and connected to a contact 422 on the data bus switch 406
to enable selective display of either weather radar or terrain data
on the display 36.
[0180] The system for converting the terrain advisory and warning
indications as well as background terrain data to an ARINC 708/453
standard serial format is indicated as a functional block 424 in
FIG. 23 and is described in more detail below. By converting such
data into an ARINC 708/453 standard serial format, the data can be
displayed on an existing display 36, which conforms to the ARINC
453 standard without the need for relatively complex and expensive
modifications to the display's symbol generator.
[0181] Referring to FIG. 24, each word 425, in accordance with the
ARINC 453/708 standard, is 1600 bits long and represents one spoke
or radial 426 of the display 36. Normally, 512 spokes or radials
426 are transmitted for each complete sweep of the display 36. At a
transmission rate of, for example, 1 megahertz (MHz), each word 425
takes about 1.6 milliseconds (msec) to transmit. For 512 spokes,
one complete sweep will thus take about 4 seconds.
[0182] The 1600-bit words 425 include a header 428 and 512 range
bins 430. The header 428 includes the control data on the control
bus 404 and also includes a 12-bit code representing the antenna
scan angle; the angle of the spoke relative to the aircraft heading
equivalent to an indicator UP direction. The range bins 430 cover
the distance from the antenna 418 to the maximum selected. Each of
the 512 range bins includes three intensity bits, encoded into
colors as indicated in TABLE 3 below.
3TABLE 3 3-BIT RANGE CODE COLOR 000 BLACK 100 GREEN 010 YELLOW 110
RED 001 -- 101 CYAN 011 MAGENTA 111 --
[0183] As will be discussed in more detail below, the display
system 400 is capable of displaying background terrain information
as well as terrain threat indications as a function of the current
position of the aircraft. The threat detection algorithm is run
along the groundtrack of the aircraft. Thus, terrain threat
indications are typically displayed along the groundtrack while
background terrain is displayed relative to the heading of the
aircraft.
[0184] Referring to FIG. 25, the terrain background information is
shown on the display 36. As will be discussed in more detail below,
the elevation of the highest terrain relative to the altitude of
the aircraft is shown as a series of dot patterns whose density
varies as a function of the distance between the aircraft and the
terrain. For example, a relatively dense dot pattern 432 may be
used to indicate terrain that is, for example, 500 feet or less
below the aircraft. A medium dense dot pattern 434 may be used to
represent terrain that is 1000 feet or less below the aircraft,
while a lightly dotted pattern 436 may be used to indicate terrain
2000 feet or less below the aircraft. In order to declutter the
display 402, terrain more than, for example, 2000 feet below the
aircraft, is not shown. The dots may be displayed in one of the
colors indicated in TABLE 3 above, for example, yellow (amber). In
addition, in the vicinity of an airport, a runway graph 438, for
example, in green, may also be provided. The apparent display
resolution may be increased by using more than one color along with
the variable density dot patterns, for example, as shown in TABLE 4
below.
4 TABLE 4 DOT DENSITY PATTERN HIGH 3000 0 MEDIUM 4000 1000 LOW 5000
2000 COLOR GREEN YELLOW
[0185] The display of a terrain threat indication is illustrated in
FIG. 26. As shown, the terrain threat indication may be displayed
contemporaneously with the background terrain information. Terrain
advisory and terrain warning indications are displayed in solid
shapes 440 and 442, respectively, for example, "squares"; the
displayed terrain map cells which represent a threat painted solid
yellow or red. More particularly, at an aspect ratio for the
display of, for example, 3.times.4 (vertical to horizontal), the
terrain cells will appear "square", particularly at relatively
low-range settings. Colors are used to distinguish between terrain
advisory and terrain warning indications. For example, red may be
used to represent a terrain warning indication 442 while yellow or
amber is used to represent a terrain advisory indication 440. By
using colored shapes for terrain threat indications and dot
patterns of variable density for the terrain background
information, clutter of the display 402 is minimized.
[0186] The terrain data from the terrain data base 24, for example,
as illustrated in FIG. 4B, is used to compute the terrain threat
indications as well as background information for the display 36.
As discussed above, the terrain data base 24 may be stored in a
flash ROM 444 (FIG. 27) for convenience of updates. In order to
facilitate update of the display 36 as a function of the position
of the aircraft, terrain data from the flash ROM 444 is transferred
to a random access memory (RAM) A, identified with the reference
numeral 446. As will be discussed in more detail below, the RAM A
is mapped with terrain data from the flash ROM 444, centered about
the current position of the aircraft, as illustrated in FIG.
27.
[0187] In order to provide a relatively large display range, for
example, 160 nautical miles, while minimizing processing time as
well as the size of the RAM A, the RAM A is configured as a
"wedding cake" as generally illustrated in FIG. 28 and configured
such that the highest terrain resolution is near the position of
the aircraft and the terrain farthest from the aircraft position
has the lowest resolution. For example, the RAM A may be provided
with four (4) layers. As shown, the instantaneous aircraft position
is shown generally centered in layer 1, having the highest
resolution. The resolution of layer 2 lies between the resolution
of layer 3 and layer 1. Layer 4 (not shown) is provided with a
relatively coarse resolution for use in a cruise mode.
[0188] Such a variable resolution display, configured as a layered
wedding cake, minimizes the memory size of RAM A as well as the
processing time. In order to improve the performance of the system,
the flash ROM 444 may also be configured similar to a "wedding
cake", as generally illustrated in FIG. 27.
[0189] As discussed above, the terrain data base 24 may be provided
with variable resolution with a relatively fine resolution of, for
example, 30.times.20 arc sec (i.e. 1/2.times.1/2 arc min). The
relatively coarse resolution of layers 2 and 3 can be formed by
combining finer resolution maps. With such a resolution, the
configuration of RAM A may be selected in accordance with TABLE
5.
5TABLE 5 RESOLUTION LAYER SIZE LAYER (MINUTES) (MINUTES) 1 1/2
.times. 1/2 32 .times. 32 2 1 .times. 1 64 .times. 64 3 2 .times. 2
128 .times. 128 4 5 .times. 5 REMAINING AREA
[0190] Each layer is generally centered relative to the aircraft
position as shown in FIG. 28 at the time of the update of RAM A.
Each layer of the "wedding cake" has its own update boundary. For
example, layer 1 of the RAM A includes an update boundary 448. As
the aircraft passes through the update boundary 448, the RAM A is
updated with new terrain data from the flash ROM 444 (FIG. 27) as
will be discussed in more detail below.
[0191] Because of the refresh rates required for the display 36,
the terrain data may be stored in files alternative to the format
illustrated in FIG. 4A. For example, the file format of the terrain
data may include an index file or header 55, stored in one or more
128 K byte storage blocks and a plurality of data files 51, as
generally shown in FIG. 29.
[0192] As shown, each index file 55 may include pointers to
specific data files 51, the length of the map file, as well as the
position of one or more of the corner boundaries of the file 51,
for example, northwest and southeast corners of each map file 51,
which facilitates updates of the display 36 as a function of the
current aircraft position. The index file 55 may contain
information regarding the length of the map file 51, a start of
file or altitude offset, the resolution of the map file 51, and
whether and how the file 51 is compressed to enable use of various
compression algorithms.
[0193] The data files 51 may include a file header 454 as well as
data blocks 456 which, as shown, can be grouped according to
resolution size. The file header 454 indicates various information
about the file. The format of the header 454 may be structured as
follows:
[0194] <MAJOR VERSION BYTE><MINOR VERSION BYTE><FILE
STATUS BYTE><ATTRIB BYTE>
[0195] <FILE NAME (8 CHARACTERS)><EXTENSION (4
CHARACTERS)>
[0196] <FILE LENGTH LONGWORD>
[0197] <TIME STAMP LONGWORD>
[0198] <CRC LONGWORD>
[0199] <SPME LONGWORD>
[0200] The major and minor version bytes relate to the revision
status of the data in the data files 51. The minor version is set
as the inverse of the byte value of the major version such that a
default erase will indicate a zero version. The file status byte
457 provides various information regarding the status of the file
as discussed below, while the attribute byte is reserved. The file
name, extension file length, and spare data are self-explanatory.
The time stamp relates to the operational time of the system (i.e.
length of time in use) while the CRC data relates to the data files
51.
[0201] The status byte may be used to provide the following
information regarding the data blocks 456, for example, as set
forth in TABLE 6 below.
6TABLE 6 FLAG BYTE VALUE INFORMATION FF EMPTY FE DOWNLOADING FO
VALID EO READY TO ERASE . RESERVED . RESERVED 00 RESERVED
[0202] The EMPTY status (FF) is set by erasing the data blocks 456,
while the DOWNLOADING status (FE) is set as part of the image. The
VALID status (FO) is set when the file is loaded. Lastly, the READY
TO ERASE (EO) is set by the file system for obsolete blocks. The
file structure may also be as described in Appendix 1.
[0203] As indicated below, various corrections of the data files 51
may be required in areas near runways. FIG. 30 illustrates a
portion of the terrain data base 24 around a particular airport,
Boeing Field, with a resolution of, for example, 1/2.times.1/2
minute subcells 455, shown with a runway 456 drawn in. The
elevations, as discussed above, are selected to be the highest
elevations within the cells 455, rounded up to the next highest
resolution increment, for example 100 feet. While such a system
provides useful and conservative terrain data for use with the
terrain data base 24, such a system in the vicinity of an airport
can cause the elevations of the runway 456 to be too high or too
low. For example, the elevation of the runway 456 at Boeing Field
is known to be 17/15 feet; rounded off to zero (0) feet. FIG. 30
shows the elevation of runway 456 to be 200/300 feet, for example,
by the averaging algorithm discussed above. Such errors in the
runway elevation can degrade system performance, and, in
particular, cause improper performance of the terrain advisory and
terrain warning cut-off boundaries below 500 feet, for example, as
illustrated in FIG. 22.
[0204] In order to solve this problem, the actual elevation of the
runway 456, rounded to the nearest resolution, is used for cells
455 around a perimeter of the runway 456, as generally shown in
FIG. 31. In particular, a first perimeter, illustrated by the
dashed box 458, is selected to be about .+-.1 cell (i.e. 1/2
minute) from the centerline of the runway 456. The elevations in
all of the cells 455 intersected by the dashed box 458 are set to
the actual runway elevation and rounded to the nearest resolution.
For Boeing Field, the elevation for such cells is 0 feet.
[0205] Consideration is also given for a narrow approach and a wide
approach. The dashed box 460 represents a wide approach while the
dashed box 462 represents a narrow approach. The cells 464 and 466,
intersected by the wide approach box 460, are set on the actual
runway elevation, rounded to 0 feet. Inside the approach perimeter,
as indicated by the arc 467, the elevation for the cells is
selected to be either the runway elevation or the .DELTA.H terrain
floor (as discussed above). For example, if the corner of the cell
468, closest to the runway 456, is 1.386 nM from the runway 456,
the .DELTA.H terrain floor will be 86 feet; rounded to nearest 100
feet=100 feet. Thus, the elevation of cell 468 is selected as 100
feet.
[0206] FIG. 32 illustrates the differences between the elevations
in selected cells 468, 470 and 472 before and after correction.
Before correction, the elevation of cells 468, 470 and 472 was 300
feet. After correction, the elevation of cells 470 and 472 are
corrected to the runway elevation and rounded to the nearest
resolution (i.e. 0 feet). The cell 468 is corrected to the .DELTA.H
terrain floor elevation of 100 feet.
[0207] FIG. 33 illustrates another situation where the elevation of
cells, due to the rounding-off method discussed above, can result
in a situation where the cell elevations adjacent a runway,
represented as a staircase 474, are below the runway elevation. The
correction method, discussed above, can also be used to correct the
various cells around the runway 456 up to the runway elevation in a
manner as generally discussed above.
[0208] FIG. 34 represents a simplified block diagram for
implementation for the display system 400 in accordance with the
present invention. The display system 400 may include a
microprocessor, for example, an Intel type 80486, 25 MHz type
microprocessor ("486") and an Analog Devices type digital signal
processor (DSP). The DSP is primarily used for calculating the
RHO/THETA conversions to off load the 486.
[0209] The display system 400 may have a normal mode and a cruise
mode. In a normal mode, the display system 400 displays terrain out
to, for example, 100 nautical miles, while in cruise mode out to
about 320 nautical miles. In the cruise mode, it is contemplated
that the background terrain display levels can be lowered by the
pilot such that terrain, for example 10,000 feet or 20,000 feet
below the aircraft, can be displayed. Alternatively, the display
level could automatically be adjusted to the highest terrain within
the selected display range; the value of this level set forth in
amber, for example.
[0210] The 486 runs a "file picker" routine which searches the
terrain data base 24 for terrain data covering the area relative to
the current position of the aircraft. For example, in a normal
mode, the file picker routine searches the terrain data base 24 for
terrain data covering an area approximately 256.times.256 minutes
relative to the current position of the aircraft. As indicated
above, the terrain data base 24 includes index files 55 (FIGS. 4B
and 29) which may include the northwest and southeast corners of
each map file 51 in order to facilitate the search. A list of files
covering the area is assembled and maintained.
[0211] As indicated above, the index file 55 also includes
information as to whether and how the data files 51 are compressed.
The files may be compressed/decompressed by a modified Hoffman
code, such as PKZIP as manufactured by PKWARE, INC.
[0212] If the data files 51 are compressed, the files are
decompressed in the background because of the relatively long
decompression time and stored in a decompressed file RAM (DFR) 476,
for example 256 K bytes. The decompressed files are then
transferred to RAM A, identified with the reference number 446. On
power-up, one or two files around the aircraft are decompressed and
displayed to allow immediate display of a limited range, relative
to the position of the aircraft. As more terrain data files are
decompressed, the additional range is displayed.
[0213] The RAM A may be configured in a plurality of cells 479
(FIG. 34), representative of a terrain area 1/2.times.1/2 minutes,
with true north appearing at the top of RAM A as shown. Each cell
479 is two bytes wide and includes data regarding the highest
altitude in the cell as well as the identity of each cell 479. As
such, the RAM A is addressable by X and Y coordinates and thus
represents a map with an altitude in each cell 479. The identity of
the cell may be used to distinguish between terrain data, for
example, from the terrain data base 24 and obstacle data. As
discussed above, in some applications, an obstacle data base may be
utilized.
[0214] As indicated above, the RAM A may be configured as a wedding
cake with various layers, as generally shown in FIG. 28 with the
top layer (i.e. layer 1) having the highest resolution (i.e.
1/2.times.1/2 minute cells). Terrain data is loaded into the RAM A
such that the aircraft is roughly centered in the top layer, where
the terrain advisory and warning detection algorithms are run. Once
the aircraft moves outside the update boundary 448 (FIG. 28), the
top layer of the RAM A is reloaded with terrain data with the
aircraft centered. The lower layers may also be provided with
update boundaries, which enable the lower layers to be updated
independently and less frequently than the top layer.
[0215] As discussed above and illustrated in FIG. 27, the flash ROM
444 for the terrain data base 24 may also be configured as a
wedding cake as illustrated in FIG. 28. In such an implementation,
the highest resolution is provided near the airports while the
coarser resolution is provided away from the airports as generally
shown in FIG. 27. For example, as illustrated in FIG. 34A, the
terrain data base 24 may be configured with
1.degree..times.1.degree., 2.degree..times.2.degree. or
10.degree..times.10.degree. map files. Some of the map files may
overlap, similar to a "wedding cake". As shown, the terrain maps
closest to airports are provided with the highest resolution, for
example 15.times.15 arc seconds. Maps further away from the airport
may be provided with resolutions of, for example, 1.times.1 minute
or 2.times.2 minutes while a coarse resolution (i.e.
2.degree..times.2.degree. or 10.degree..times.10.degree.) may be
provided even further from the airport.
[0216] An upload routine may be used to upsample or downsample
terrain or obstacle data into the RAM A from the DFR RAM 476 as
shown in FIG. 34. Such a routine is well within the ordinary skill
in the art. The top three layers (i.e. layers 1, 2 and 3) of the
RAM A (FIG. 28) are updated from the map file source from the flash
ROM 444 for the terrain data base 24 having the highest resolution,
except for the 5.times.5 minute layer, which is updated only from a
5.times.5 minute file source to reduce processing time.
[0217] Alternatively, as shown in FIG. 34A, an additional RAM A*,
identified with the reference numeral 477, may be provided and
structured similar to the RAM A 446, forming a "ping-pong" memory
arrangement. In this embodiment, one terrain data file 51 is
decompressed one file at a time and loaded into the DFR RAM 476 and
then upsampled to either the RAM A or RAM A*. For example, while
the RAM A is being loaded with additional decompressed files, the
terrain threat and display algorithms are executed from the RAM A*.
Once the RAM A is updated, the RAM A is then used for the
computations while the other RAM A* is updated with new data from
the DFR RAM 476 to account for changes in the aircraft
position.
[0218] The 5.times.5 minute resolution layer is primarily intended
for use in a cruise mode to provide a look-ahead range of about 320
nM. As discussed above and illustrated in FIG. 29, each file
includes an index file 55 which contains data relative to the
resolution of the terrain data in a particular map file 51. Such
information can be used to vary the resolution of the map files as
a function of longitude and latitude position. For example, the
latitude resolutions of the map files can be decreased at higher
latitudes in a northward direction. Moreover, longitude resolution
in the RAM A can be varied and provided with, for example, a one
degree latitude hysteresis to prevent flipping back and forth as an
aircraft cruises northwardly. Once the terrain data from the
terrain data base 24 is loaded into the RAM A, the terrain advisory
and warning algorithms ("detection algorithms") are run once per
second as discussed below.
[0219] The terrain threat algorithms discussed above are based on a
single vector along the groundtrack. The single vector provides
acceptable results because the relatively crude resolution of the
terrain data base 24 is based upon the highest elevation per cell
and the natural noisiness of the groundtrack angle and position
information. However, the single vector approach does not take into
account various errors, due to, for example, the terrain data base
24, GPS longitude and latitude errors as well as the published
track angle accuracy. As such, the single vector along the
groundtrack may be substituted with an array of vectors, as
generally shown in FIG. 35, which takes such errors into account.
In particular, the groundtrack vector is identified with the
reference numeral 482 originating at the instantaneous aircraft
position X.sub.0, Y.sub.0. A pair of vectors 484 and 486 are
disposed at a configurable distance D.sub.OFF, for example 0.2 nM
along a segment 488, perpendicular to the vector 482 at points
X.sub.1, Y.sub.1, and X.sub.r, Y.sub.r. A pair of outside vectors
490 and 492, originating at points X.sub.1, Y.sub.1 and X.sub.r,
Y.sub.r, respectively, are directed at a configurable angle
D.sub.ALPHA, relative to the vectors 484 and 486. The configurable
angle D.sub.ALPHA may be selected as 3 degrees, equivalent to the
groundtrack accuracy of the GPS.
[0220] The coordinates X.sub.1, Y.sub.1 and X.sub.r, Y.sub.r, for
the vectors 490 and 492, respectively, are determined. More
particularly, the incremental X and Y vectors, DX.sub.OFF and
DY.sub.OFF, are determined as set forth in equations (25) and (26)
as set forth below.
DX.sub.OFF={D.sub.OFF/COS(LAT)}*COS (GROUNDTRACK), (25)
[0221] where D.sub.OFF is a configurable distance between X.sub.0,
Y.sub.0 and X.sub.1, Y.sub.1, for example 0.2 nm and the factor
1/COS(LAT) is used to convert DX.sub.OFF to nautical miles when
DX.sub.OFF, DY.sub.OFF is in minutes and D.sub.OFF is in nautical
miles.
DY.sub.OFF=D.sub.OFF*SIN(GROUNDTRACK) (26)
[0222] The coordinates X.sub.r, Y.sub.r can be determined in a
similar manner.
[0223] The threat detection algorithms are initiated by calculating
the range increments DELTA X.sub.NS and DELTA Y.sub.NS along a
north-south coordinate system along the groundtrack. Referring to
FIG. 36, each spoke 426 has 512 size range bins as discussed above,
or 256 double range bins. In order to convert from an X-Y
coordinate system to the north-south coordinate system, the range
increments DELTA X.sub.NS, DELTA Y.sub.NS are determined in
accordance with equations (27) and (28). (The term 1/COS (LATITUDE)
is used to convert from arc minutes to nautical miles.)
DELTA X.sub.NS=DETECTION STEP SIZE*SIN
(GROUNDTRACK)*1/COS(LATITUDE) (27)
DELTA Y.sub.NS=DETECTION STEP SIZE*COS (GROUNDTRACK) (28)
[0224] After the range increments DELTA X.sub.NS and DELTA Y.sub.NS
are determined, the threat detection algorithm computation is
initiated at the instantaneous position X.sub.0, Y.sub.0 of the
aircraft (FIG. 37). The position is then incremented by the range
increments DELTA X.sub.NS and DELTA Y.sub.NS and the range is
incremented by a configurable step size, for example 1/8 unit, as
discussed above. After the new coordinates are determined, the
current terrain altitude at the new coordinate is determined. The
terrain detection algorithms, as discussed above, are then computed
for the current advisory LAD, as indicated by the arc 494, defining
a yellow threat detection vector 496. In order to provide
additional warning time for a terrain warning, the terrain warning
indication can be computed out to a distance equal to 1.5*LAD, as
indicated by the arc 498 to define a red threat detection vector
500. The above steps are repeated for the look-ahead vector arrays
490 and 492 (FIG. 35). The entire computation is done at a rate of
once per second.
[0225] Whenever the threat detection algorithm detects terrain
along the look-ahead vector arrays 490 and 492 (FIG. 35), the
offending terrain is painted in an expanded cone around the
groundtrack as shown in FIG. 37. In particular, the expanded cone
includes a plurality of vectors, spaced, for example, 4.degree.
apart (configurable value), for .+-.90.degree. (configurable value)
on each side of the groundtrack. For an expanded terrain warning,
the cone may be expanded out distances, for example as set forth in
TABLE 7 below.
7TABLE 7 (For an expansion factor of 2) NORMAL EXPANDED LOOK- LOOK-
TYPE OF AHEAD/ AHEAD/ THREAT LOOK-DOWN LOOK-UP LOOK-DOWN LOOK-UP
ADVISORY 1.0 * 2.0 * 2.0 * 4.0 * LAD LAD LAD LAD WARNING 0.5 * 1.5
* 1.0 * 3.0 * LAD LAD LAD LAD
[0226] The terrain threat is painted on the display 36 by
calculating the starting vector angle DISPA as set forth in
equation (29) when a terrain threat along the groundtrack has been
detected.
DISPA=GROUNDTRACK+30 degrees (29)
[0227] The range increments are then determined as set forth in
equations (30) and (31).
DELTA X.sub.NS=DISPLAY STEP SIZE*SIN(DISPA)*1/COS(LATITUDE)
(30)
DELTA Y.sub.NS=DISPLAY STEP SIZE*COS(DISPA), (31)
[0228] where the display step size is a configurable constant, for
example 1/2 unit.
[0229] After the range increments DELTA X.sub.NS and DELTA Y.sub.NS
are determined, the instantaneous position of the aircraft X.sub.0,
Y.sub.0 is incremented by the same and the range is incremented by
the step size. The altitude is then determined for the current
position on the terrain map. Subsequently, the threat detection
algorithms are run for the distances as discussed above. Should a
threat be detected, a color is loaded into a RAM B, identified with
the reference numeral 504 (FIG. 34). At the end of the yellow and
red threat vectors 496 and 500, the vector angle DISPA is
incremented by 4 degrees with the above steps being repeated until
all vectors are done.
[0230] In addition to the terrain detection and display algorithms,
a background terrain algorithm is also executed relative to the
terrain data in RAM A (FIG. 34) once every screen update, about 4
seconds. The background terrain algorithm simply compares the
current altitude of the aircraft with the various terrain altitude
data in the RAM A. Any of the dot patterns 432, 434 and 436 (FIG.
25) resulting from the computation are stored in the RAM B (FIG.
34) organized in a wedding-cake configuration (FIG. 28), similar to
the RAM A (FIG. 34). In addition, the RAM B is organized in single
byte cells containing the color/dot pattern as discussed above. The
closest runway may also be stored in the RAM B as either a circular
or square pattern in, for example, a green color.
[0231] The color/dot pattern in the RAM B (FIG. 34) is transferred
to the DSP memory RAM B*, identified with the reference numeral
506, once per screen update (i.e. 4 sec) and synchronized to the
DSP screen painting period. If a threat is detected as discussed
above, a new RAM B image is transferred to the DSP RAM B* 206
immediately. The image transfer time typically fits between two
spokes. If not, the DSP will send an old spoke stored in its spoke
memory 508, 510 twice.
[0232] The DSP then performs the RHO/THETA conversion on the data
in RAM B* 506 using the updated aircraft position, selected range
and heading data from the 486, as discussed below. Changes in the
selected range are implemented on the fly (i.e. the next spoke)
without restarting the sweep.
[0233] As discussed above, the ARINC 708/453 weather radar format
consists of 512 spokes covering a configurable scan range of
typically .+-.90 degrees and a range selected, for example, on a
weather data control panel. For ARINC systems, each spoke is
calculated twice, once for the left selected range and once for the
right selected range. The spokes are then transmitted by way of a
serial port 512, interleaved left spoke and right spoke to the bus
switch 406 (FIG. 23) by way of a pair of drivers 514 and 516 (FIG.
34).
[0234] The resolution of the scan conversion is 256 range bins;
each range bin actually being a double range bin per the ARINC
708/453 standard format. Such a resolution is sufficient for the
dot patterns 518 illustrated in FIGS. 25 and 26.
[0235] The RHO/THETA conversion is best understood with reference
to FIG. 38. In particular, once per sweep (i.e. every 4 seconds)
the range increment R1 is determined. The range increment is the
selected range divided by 256. After the range increment is
determined, the starting spoke angle is determined (i.e., the
aircraft true heading plus 90 degrees). Lastly, the incremental
spoke angle DELTA ALPHA is determined. The incremental spoke angle
DELTA ALPHA is the sweep range divided by 512; the total number of
spokes per sweep.
[0236] Once per spoke (i.e. 512 times per 4 second sweep), the
incremental range coordinates DELTA X.sub.ns, DELTA Y.sub.ns are
computed and the spoke angle is incremented by DELTA ALPHA, as
discussed above. The incremental range increment coordinates
X.sub.ns and Y.sub.ns are computed in accordance with equations
(32) and (33), respectively.
DELTA X.sub.ns=RI*SIN(SPOKE ANGLE)*1/COS(LATITUDE) (32)
DELTA Y.sub.ns=RI*COS(SPOKE ANGLE) (33)
[0237] Once per range increment (i.e. 512.times.256 times per 4
second sweep), the instantaneous aircraft position X.sub.0, Y.sub.0
is incremented by DELTA X.sub.ns and DELTA Y.sub.ns. The
color/pattern for the new position is looked up and output to the
display 36. These steps are repeated for the rest of the spoke.
[0238] With the use of the DSP, two computations can be done in
parallel. In particular, the color/pattern of each range bin
including the conversion of the longitude to nautical miles (i.e.
multiplication by 1/COS (LAT)). With reference to FIG. 38, this
computation is performed relative to the true north map coordinate
system with the aircraft true heading pointing up and the spoke
angles referenced to it. The second computation addresses each
display indicator pixel in order to paint the required dot pattern,
relative to the display screen coordinate system.
[0239] The generation of dot pattern is illustrated in FIGS. 39-42.
In general, the scan conversion is simulated by overlaying each
RHO/THETA range bin over a calculated indicator pixel map which
includes corrections for display aspect ratio using fractal
8.times.8 dot patterns.
[0240] The fractal patterns corresponding to the variable density
dot patterns 432, 434 and 436 (FIG. 25) are illustrated in FIG. 40.
The fractal pattern size is selected to be 8.times.8 pixels. The
densest pattern 432 includes 1/2, or 32, of the 64 pixels filled.
In the medium density pattern 434, 16, or 1/4, of the pixels are
filled. In the least dense pattern 4 of 64, or {fraction (1/16)},
of the pixels are filled.
[0241] Referring to FIG. 39, the display screen is divided into a
pixel map, consisting of, for example, 256.times.256 pixels 520. As
mentioned above, the 8.times.8 fractal patterns are overlaid onto
the pixel map to provide the variable density dot patterns 432, 434
and 436 illustrated in FIG. 25.
[0242] The computation of the dot patterns is best understood with
reference to FIGS. 39, 41 and 42. Using an X-Y coordinate system,
each spoke is determined at the bottom center of the display 36.
For a 256.times.256 pixel display, the starting point corresponds
to 120, 0. Next, the incremental X and Y coordinates relative to
the scan angle ALPHA are determined according to equations (34) and
(35).
DELTA X=RI*SIN(ALPHA) (34)
DELTA Y=RI*COS(ALPHA), (35)
[0243] where RI=a double range increment and ALPHA=the scan angle.
These increments are added to the starting point 128, 0. After each
increment, the selected fractal pattern is looked up in the RAM B *
to determine whether to send the last range increment as either
block or colored.
[0244] Referring to FIG. 42, the increments DELTA X and DELTA Y are
applied to a pair of adders 522 and 524, which, in turn, are
applied to an X-counter 526 and a Y-counter 528. As discussed
above, the X counter 526 starts at 128 and counts up if DELTA
X>0. If DELTA X<0, the X-counter counts down, indicating a
pixel on the left (FIG. 39) of the starting position. Since the
starting pixel is at the bottom of the display, the Y-counter 528
starts at zero and always counts up. A range counter 528 counts up
to 256; the maximum number of double range bins. The range counter
528 is driven by an increment clock such that the pattern/color is
painted once per range increment. The output indicates whether the
pixel is to be painted left black.
[0245] As discussed above, the background terrain algorithm
determines the particular fractal pattern to be used. This data is
applied to the DSP to enable the pixels on the pixel map (FIG. 29)
to be painted or left blank. In particular, the three least
significant bits from the X counter 526 and Y counter 528 are used
to select the fractal pattern as shown.
[0246] The scan conversion simulation is performed in parallel with
the RHO/THETA conversion which determines the color/dot pattern of
the range bin to be transmitted. The color/dot pattern provided by
the RHO/THETA conversion is then overlaid with the fractal dot
pattern to determine whether the range bin color should be black or
painted to provide the proper dot pattern on the display.
[0247] Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. Thus, it is
to be understood that, within the scope of the appended claims, the
invention may be practiced otherwise than as specifically described
above.
* * * * *