U.S. patent application number 09/416477 was filed with the patent office on 2001-11-15 for system for processing directional signals.
Invention is credited to SMITH, MARK D..
Application Number | 20010040526 09/416477 |
Document ID | / |
Family ID | 23650138 |
Filed Date | 2001-11-15 |
United States Patent
Application |
20010040526 |
Kind Code |
A1 |
SMITH, MARK D. |
November 15, 2001 |
SYSTEM FOR PROCESSING DIRECTIONAL SIGNALS
Abstract
A system for calculating the bearing of a signal source, with a
directional antenna, provides corrections for distortion, such as
due to a small fuselage of the monitoring aircraft and the
elevation angle of an intruder aircraft with respect to the
monitoring aircraft. A correction is applied to the bearing
estimate that is based on relevant factors, such as the fuselage
size and the elevation angle of the intruder aircraft. The
correction can be calculated or applied through the use of a
look-up table, which may be either pre-selected or selected after
calculation of the elevation angle of the intruder aircraft.
Inventors: |
SMITH, MARK D.; (GLENDALE,
AZ) |
Correspondence
Address: |
John Leshinkski
L-3 Communications
Aviation, Communications & Surveillance Systems
21111 North 19th Ave., Mail Station H16A5
Phoenix
AZ
85027-2708
US
|
Family ID: |
23650138 |
Appl. No.: |
09/416477 |
Filed: |
October 12, 1999 |
Current U.S.
Class: |
342/418 |
Current CPC
Class: |
G01S 3/023 20130101;
G01S 13/933 20200101; G01S 3/28 20130101 |
Class at
Publication: |
342/418 |
International
Class: |
G01S 003/52 |
Claims
What is claimed is:
1. A method for calculating a bearing of a signal source using a
directional antenna having a plurality of receiving elements,
comprising the steps of: selecting a correction model from a
plurality of correction models; receiving a plurality of incoming
signals with the receiving elements; processing the incoming
signals to produce a plurality of electrical connection signals,
wherein each of the electrical connection signals corresponds to a
different quadrant of a polar coordinate system and each of the
electrical connection signals has an amplitude; selecting the
electrical connection signal with the strongest amplitude;
selecting the electrical connection signal with the second
strongest amplitude; identifying a first quadrant that contains the
electrical connection signal with the strongest amplitude;
identifying a second quadrant that contains the electrical
connection signal with the second strongest amplitude; calculating
an amplitude difference between the electrical connection signal
with the strongest amplitude and the electrical connection signal
with the second strongest amplitude; applying a correction to the
amplitude difference in order to obtain the bearing of the signal
source, wherein the correction is determined by the correction
model, the first quadrant, and the second quadrant.
2. The method according to claim 1, wherein the correction model
comprises a look-up table.
3. The method according to claim 1, wherein the signal source
comprises an intruder aircraft.
4. A method for calculating a bearing of a signal source using a
directional antenna having a plurality of receiving elements,
comprising the steps of: receiving a plurality of incoming signals
with the receiving elements; processing the incoming signals to
produce a plurality of electrical connection signals, wherein each
of the electrical connection signals corresponds to a different
quadrant of a polar coordinate system and each of the electrical
connection signals has an amplitude; selecting the electrical
connection signal with the strongest amplitude; selecting the
electrical connection signal with the second strongest amplitude;
identifying a first quadrant that contains the electrical
connection signal with the strongest amplitude; identifying a
second quadrant that contains the electrical connection signal with
the second strongest amplitude; calculating an amplitude difference
between the electrical connection signal with the strongest
amplitude and the electrical connection signal with the second
strongest amplitude; calculating an elevation angle of the signal
source; selecting a correction model from a plurality of correction
models based on the elevation angle; applying a correction to the
amplitude difference in order to obtain the bearing of the signal
source, wherein the correction is determined by the correction
model, the first quadrant, and the second quadrant.
5. The method according to claim 4, wherein the selecting step of
selecting a model further comprises selecting the correction model
based on a fuselage size of a monitoring aircraft.
Description
BACKGROUND
[0001] 1. Field of the Invention
[0002] The present invention generally relates to processing
signals received with a directional antenna, and more particularly,
to identifying the bearing of a signal source based on signals
received with a directional antenna.
[0003] 2. Description of the Related Art
[0004] As technology in air transportation has evolved, the demands
on the members of the flight deck have become increasingly severe.
To avoid flight path conflicts, the flight deck crew monitors
considerable aircraft status information for multiple surrounding
aircraft at a time when air traffic is dramatically increasing.
Higher aircraft speeds magnify the burden by reducing the time in
which the flight deck crew can respond to threatening
situations.
[0005] To assist the flight deck crew and enhance safety, several
systems have been and are being developed. Many aircraft carry
transponders (e.g., mode S, mode C, mode A) by which one aircraft
can communicate to a second aircraft both its identity and various
flight parameters. Typically, a monitoring aircraft transmits a
signal in a predetermined format which, upon receipt by an
intruding aircraft, causes the intruding aircraft to respond with a
transmission which includes information in a predetermined format.
Systems generally referred to as traffic alert and collision
avoidance systems (TCAS) process information received from intruder
aircraft along with the status parameters of the receiving aircraft
to identify potential collision situations. A TCAS also typically
provides the flight deck crew with advisory information suggesting
an action to avoid the collision situation.
[0006] A TCAS typically includes a directional antenna. The TCAS
uses the directional antenna to determine the bearing of an
intruder aircraft relative to the TCAS equipped monitoring
aircraft. When receiving signals from the intruder aircraft, TCAS
processes the signals to calculate an estimated bearing for the
intruder aircraft, and this information is displayed to the flight
deck crew to assist them in obtaining visual contact with the
intruder aircraft.
[0007] One approach used by TCAS systems is to estimate an intruder
aircraft's bearing by comparing magnitudes of signals received by
the components of the directional antenna. FIG. 1 illustrates the
radiation pattern of signals received by a typical directional
antenna having four antenna elements measured on a test four foot
diameter flat ground plane. This radiation pattern simulates the
performance of the antenna on a large transport aircraft, such as
an aircraft having a fuselage curvature radius greater than 80
inches. The performance of the antenna beams in each of the four
quadrants representing port, starboard, fore and aft, is virtually
identical. To estimate the bearing of an intruder aircraft,
conventional TCAS uses a model based on the radiation pattern
measured on the test ground plane, such as the radiation pattern
illustrated in FIG. 1. An exemplary model used by a conventional
TCAS signal processing scheme is illustrated in FIG. 2. When an
intruder aircraft is detected, the bearing of the intruder aircraft
is calculated by determining which beam (from the four beams
representing each of the four quadrants of the polar coordinate
system) has the largest amplitude and which beam has the second
largest amplitude and then taking the difference between the two.
Based on this difference, a bearing estimate can be generated using
a conventional TCAS model, such as the model illustrated in FIG.
2.
[0008] Various factors, however, may degrade the accuracy of the
bearing estimate. For example, monitoring aircraft having small
fuselages may detect transponder signals differently than larger
aircraft, which may degrade the bearing estimate accuracy.
Consequently, the model illustrated in FIG. 2 is not as accurate
with aircraft that have small fuselages, such as those aircraft
with a radius of fuselage curvature smaller than 64 inches. The
smaller fuselage causes a distortion such that the beam peak in the
port and starboard directions occurs at a lower elevation angle
than the beams of the model illustrated in FIG. 2. This
displacement may degrade the accuracy of the bearing estimate. The
degradation tends to be more pronounced in aircraft with smaller
fuselages than those with a larger fuselage. The amount of the
error is also dependent on the elevation angle of the intruder
aircraft.
SUMMARY OF THE INVENTION
[0009] In accordance with the present invention, a system for
calculating a bearing of a signal source, such as an intruder
aircraft, using an antenna, such as a directional antenna having a
plurality of receiving elements, may include selecting a correction
model from a plurality of correction models, wherein the correction
model selection may be based on the fuselage radius of the
monitoring aircraft or the estimated elevation angle of the
intruder aircraft. The monitoring aircraft receives a plurality of
incoming signals with the receiving elements and processes the
incoming signals to produce a plurality of electrical connection
signals, wherein each of the electrical connection signals
corresponds to a different quadrant of a polar coordinate system
and each of the electrical connection signals has an amplitude. The
system selects the electrical connection signals with the strongest
amplitude and the second strongest amplitude, calculates an
amplitude difference between the two selected signals and applies a
correction model to the amplitude difference in order to obtain the
bearing of the signal source. The correction model may be
pre-selected by the supplier or the operator, or alternatively, the
correction model may be automatically selected by the system.
[0010] In accordance with an embodiment of the present invention,
the correction model may comprise a look-up table.
[0011] The correction model is applied to improve the bearing
estimate accuracy of the intruder aircraft by, for example,
minimizing the distortion caused by the curvature of the monitoring
aircraft's fuselage.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] A more complete understanding of the present invention may
be derived by referring to the detailed description and claims when
considered in connection with the following illustrative Figures,
which may not be to scale. In the following Figures, like reference
numbers refer to similar elements throughout the Figures.
[0013] FIG. 1 illustrates the radiation pattern, in rectangular
coordinates, of a typical TCAS directional antenna mounted on a
standard ground plane;
[0014] FIG. 2 illustrates the bearing calculation model of a
typical TCAS directional antenna;
[0015] FIG. 3 is a block diagram of a typical TCAS system;
[0016] FIG. 4 illustrates the radiation pattern, in polar
coordinates, of a typical TCAS directional antenna having four
receiving elements;
[0017] FIG. 5 is a block diagram of the processing system, in the
context of the present invention, for calculating the bearing of
the source of signals received by a directional antenna;
[0018] FIG. 6 illustrates the radiation pattern, in rectangular
coordinates, of a TCAS directional antenna mounted on an aircraft
with a small fuselage;
[0019] FIG. 7 illustrates a representative correction model of the
present invention, for the monitoring aircraft of FIG. 6; and
[0020] FIG. 8 illustrates a look-up table of the representative
correction model illustrated in FIG. 7 in accordance with one
embodiment of the present invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0021] A signal processing system according to various aspects of
the present invention provides a system for calculating the bearing
of a signal source, such as an intruder aircraft, which is adjusted
according to various criteria, such as the size of the fuselage of
the monitoring aircraft or the relative elevation angle of an
intruder aircraft. Although various aspects of the invention may be
used in conjunction with a variety of systems that have a
directional antenna with a plurality of receiving elements, the
present invention is conveniently described below in connection
with a TCAS. This exemplary implementation, however, should in no
way be construed to limit the applicability of various aspects of
the invention in other environments or otherwise limit the
claims.
[0022] FIG. 3 is a block diagram of a conventional TCAS comprising
a TCAS directional antenna 300, a TCAS omnidirectional antenna 302,
and a TCAS computer unit 305 which includes a receiver 310, a
transmitter 320, and a processor 330. The TCAS also includes an
aural annuciator 340, a traffic advisory (TA) display 350, and
resolution advisory displays 360. A transponder is also shown
comprising a transponder unit 370, a control panel 380, and
transponder antennas 390 and 395. The TCAS and transponder operate
together to function as a collision avoidance system. The present
embodiment is merely illustrative of a typical TCAS and many other
configurations are possible, such as adding a second directional
antenna or utilizing a transceiver.
[0023] The operations of the TCAS and each component illustrated in
FIG. 3 are well known and therefore will not be described in
detail. A general description of TCAS technology, however, is
provided in Introduction to TCAS II published by the United States
Department of Transportation--Federal Aviation Administration.
[0024] Referring to FIG. 4, a typical radiation pattern of a TCAS
directional antenna having four receiving elements is shown in
polar coordinates. The TCAS directional antenna is mounted on a
monitoring aircraft 400. The directional antenna of monitoring
aircraft 400 receives a plurality of incoming signals on its four
antenna elements substantially simultaneously. The incoming signals
are processed inside the antenna to produce a plurality of
electrical connector signals 505 (shown in FIG. 5), such that each
electrical signal 505 represents a unique quadrant of the polar
coordinate system. The electrical connector signals 505 in the
present embodiment correspond to the four quadrants representing
fore 410, starboard 420, aft 430, and port 440. FIG. 4 illustrates
the signal amplitude received by each of the four elements from
various angles for receiving signals.
[0025] Referring to FIG. 5, the TCAS computer unit 305 calculates
an estimated intruder aircraft bearing based on the intruder's
transponder signal. As described above, directional antenna 300,
having a plurality of receiving elements, receives the incoming
transponder signals. Directional antenna 300 processes the incoming
signals and produces electrical connector signals 505 that are
routed to TCAS computer unit 305. TCAS computer unit 305 processes
signals 505 and produces the bearing estimate of intruder aircraft
450 for display on TA display 350.
[0026] More particularly, the signals received by antenna 300 are
routed to a selector 510. Selector 510 selects the signal with the
strongest amplitude and the signal with the next strongest
amplitude. The selected signals from selector 510 are applied to an
identifier 520 and to a comparator 530. Identifier 520 identifies
the corresponding quadrants (i.e., fore, starboard, aft, or port)
for the strongest signal and the second strongest signal selected
by selector 510. Comparator 530 compares the amplitudes for each of
the two selected signals and calculates an amplitude difference
that is the difference between the two amplitude values. The
quadrant identification and the amplitude difference of the two
selected signals are supplied to correction system 540, which then
suitably generates a corrected bearing estimate of intruder
aircraft 450. In accordance with various aspects of the present
invention, correction system 540 applies a correction to an initial
bearing estimate, which is suitably generated in any manner.
Typically, the corrected bearing estimate of the intruder aircraft
(i.e., intruder aircraft icon 360 on TA display 350) is then shown
relative to the monitoring aircraft (i.e., monitoring aircraft icon
370) on TA display 350.
[0027] As illustrated in FIG. 6, the radiation pattern of a
directional antenna mounted on an aircraft with a small fuselage,
such as a fuselage having a radius of curvature of approximately 64
inches or less, has beam peaks of different amplitudes for each of
the four quadrants for a specified elevation angle. The radiation
pattern illustrated in FIG. 6 is for an elevation angle of 90
degrees (i.e., horizon). As stated previously, the rounding of the
port and starboard sides of the aircraft fuselage has little effect
on the fore and aft beams of the directional antenna. However, the
beam peaks of the port and starboard beams occur at a lower
elevation angle for an antenna mounted on the fuselage than for an
antenna mounted on the flat ground plane. Thus, for directional
antenna azimuth radiation patterns at lower elevation angles (i.e.,
near or below the horizon), the port and starboard beams are
stronger than the fore and aft beams, creating a radiation pattern
similar to that shown in FIG. 6. The reciprocal effect is that at
higher elevation angles, the fore and aft beams are stronger than
the port and starboard beams. There is a particular elevation angle
at which the fore and aft beam peak amplitudes are equal to the
port and starboard beam peak amplitudes. At this unique elevation
angle, the antenna radiation approximates FIG. 1 very well, but for
all other elevation angles, the system based on FIG. 1 inherently
contains errors. Since the amplitudes of the beam peaks affect the
calculation of the bearing of the intruding aircraft, the
correction system 540, according to various aspects of the present
invention performs a correction that accounts for the different
beam peaks.
[0028] In accordance with the present invention, a correction
model, such as the correction model 700 illustrated in FIG. 7, is
applied by the correction system 540 to improve the bearing
estimate accuracy of the intruder aircraft based on, for example,
the size of the monitoring aircraft's fuselage and/or the elevation
angle of the intruder aircraft. The specific values of the model
suitably vary depending on fuselage size and elevation angle, but
the application of the model may remain the same.
[0029] In the present embodiment, correction model 700
approximates, in a piece-wise linear pattern, the radiation pattern
illustrated in FIG. 6. The model is divided into four
quadrants--fore 410, port 440, aft 430, and starboard 420. Each
quadrant has a cardinal angle that corresponds to that quadrant's
position in the polar coordinate system. Fore 410 has a cardinal
angle 415 of 0 degrees. Port 440 has a cardinal angle 445 of 270
degrees. Aft 430 has a cardinal angle 435 of 180 degrees and
starboard 420 has a cardinal angle 425 of 90 degrees. In addition,
each quadrant has a primary crossover spacing 702A, B that varies
depending on any relevant factors, such as the fuselage size and
the elevation angle of the intruder aircraft in the present
embodiment. The primary crossover spacing for a quadrant is the
spacing between the crossovers of the beam for the current quadrant
and the beams for the adjacent quadrants. For example, primary
crossover spacing 702A for port quadrant 440 illustrated in FIG. 7
is 100 degrees, and primary crossover spacing 702B for aft quadrant
430 is 80 degrees. Similarly, the primary crossover spacing for
starboard quadrant 420 is 100 degrees and the primary crossover
spacing for fore quadrant 410 is 80 degrees.
[0030] Each quadrant of the correction model also has a depth of a
secondary crossover. The depth of the secondary crossover for a
quadrant is the depth or difference, such as in decibels (dB),
between the beam peak of the quadrant and the point at which the
beams for the two adjacent quadrants intersect. For example, the
depth of the secondary crossover for port quadrant 440 illustrated
in FIG. 7 is 13.0 dB, and the depth of aft quadrant 430 is 11.0 dB.
Similarly, the depth of starboard quadrant 420 is 13.0 dB, and the
depth of fore quadrant 410 is 11.0 dB.
[0031] The correction model 700 applied to the bearing estimate of
an intruder aircraft, in accordance with various aspects of the
present invention, uses the cardinal angle, the primary crossover
spacing, and the depth of the secondary crossover, as well as any
other suitable criteria, to correct for distortion of the signal.
More specifically, in the present embodiment, the correction may be
applied in accordance with the following formula:
(cardinal angle)+((sign)*(primary crossover spacing/2))-((amplitude
delta)*(sign)*(ratio))=bearing estimate of intruder aircraft in
degrees
[0032] where,
[0033] cardinal angle=cardinal angle, in degrees, of the quadrant
containing the strongest beam (i.e., highest amplitude), received
from the intruder aircraft;
[0034] sign=multiplier that determines whether the offset due to
the amplitude delta is added or subtracted from the cardinal angle
as discussed below;
[0035] primary crossover spacing=primary crossover spacing, in
degrees, of the quadrant containing the strongest beam;
[0036] amplitude delta=difference, in dB, between the amplitudes of
the strongest beam, and the next strongest beam of the signals
received from the intruder aircraft; and
[0037] ratio=(primary crossover spacing/2)/(depth of the secondary
crossover), where the primary crossover spacing and the depth of
the secondary crossover are for the quadrant containing the
strongest beam.
[0038] In accordance with one embodiment of the present invention,
the correction model can be implemented by a look-up table, such as
the look-up table shown in FIG. 8 for the correction model
illustrated in FIG. 7. The look-up table is suitably organized by
the quadrant of the strongest beam and the quadrant of the next
strongest beam. The cardinal angle, primary crossover spacing,
sign, and ratio may be stored in the look-up table.
[0039] The value for sign is determined in the present embodiment
based on an initial bearing estimate using the quadrants of the
strongest and the next strongest beam. The only positions on the
antenna pattern where the difference between the amplitudes of the
strongest and the next strongest beam is known occurs at the
primary crossover points where the differences are zero (e.g.,
primary crossover points 610, 620, 630 and 640 in FIG. 6). The
formula uses these points as anchor points. Depending on which half
of the quadrant in which the intruder is located according to the
initially estimate bearing, the formula either subtracts a fixed
offset (i.e., primary crossover spacing divided by 2) while adding
the variable offset (i.e., amplitude delta*ratio) or the formula
adds the fixed offset while subtracting the variable offset.
[0040] The look-up table is suitably pre-calculated to provide
cardinal angle, sign, primary spacing, and ratio, for various
elevation angles and various fuselage sizes. The operator of the
monitoring aircraft may preselect the look-up table, from a
plurality of look-up tables, based on the fuselage size of the
monitoring aircraft. Alternatively, the TCAS may automatically
select an appropriate look-up table, for example upon entry of the
aircraft model number. In addition, since air traffic is heaviest
at the horizon, the supplier or operator may pre-select a look-up
table for the horizon (i.e., elevation angle of 90 degrees), so
that the bearing estimate would be more accurate for the greatest
number of intruder aircraft. Alternatively, the look-up table may
be selected for any suitable elevation angle. In this embodiment,
the correction applied would be dependent only on the fuselage
size.
[0041] The radiation pattern illustrated in FIG. 6 is for an
elevation angle of 90 degrees (i.e., horizon). The values of the
beam peaks, and therefore the characteristics of an appropriate
correction model 700, tend to differ depending on the elevation
angle of the intruder aircraft as well as the radius of curvature
of the monitoring aircraft's fuselage.
[0042] In accordance with another embodiment of the present
invention, the correction model may be applied by automatically
selecting a look-up table based on the elevation angle of the
intruder aircraft. A plurality of look-up tables for different
fuselage sizes and for different elevation angles may be
pre-calculated. The correct look-up table is suitably selected
after calculating the elevation angle of the intruder aircraft.
This selection may also be based on the fuselage size of the
monitoring aircraft. For example, the elevation angle of the
intruder aircraft may be calculated using altitude information
available in the incoming signals from the intruder aircraft and
the distance to the intruder aircraft. This distance may be
determined in any manner, such as the duration of the delay between
a transmitted signal and response from the intruder aircraft.
[0043] The present invention has been described above with
reference to a preferred embodiment. However, those skilled in the
art having read this disclosure will recognize that changes and
modifications may be made to the preferred embodiment without
departing from the scope of the present invention. For example,
instead of applying the correction model by a look-up table, the
correction model could be applied by a mathematical formula that
varies the correction depending on stored correction values for
different fuselage sizes and elevation angles. These and other
changes or modifications are intended to be included within the
scope of the present invention, as expressed in the following
claims.
* * * * *