U.S. patent number RE31,962 [Application Number 06/568,557] was granted by the patent office on 1985-07-30 for loran-c navigation apparatus.
This patent grant is currently assigned to Sanders Associates, Inc.. Invention is credited to Lester R. Brodeur.
United States Patent |
RE31,962 |
Brodeur |
July 30, 1985 |
**Please see images for:
( Certificate of Correction ) ** |
LORAN-C navigation apparatus
Abstract
LORAN-C navigation apparatus is disclosed wherein digital
circuitry and a microprocessor .[.is.]. .Iadd.are .Iaddend.used to
automatically identify LORAN transmitting stations and .[.makes.].
.Iadd.make .Iaddend.standard hyberbolic navigation measurements.
The equipment operator manually enters the group repetition rate
into the apparatus for a LORAN-C chain covering the area within
which the navigation apparatus is being operated. Initially, the
apparatus searches all incoming signals .Iadd.as they are received
.Iaddend.until signals from a master station are received regularly
at the stored group repetition rate. The apparatus then closely
determines the time of arrival of signals from the secondary
stations of the selected LORAN-C chain before changing to a fine
search mode in which the exact time of arrival of the secondary
station signals is determined; the phase code of the received
signals is checked to determine if the received signal is a ground
or sky wave, and a determination is made if there is a defective
secondary station blink code. The time difference of arrival
measurements are then output visually to be plotted in a well known
manner on a LORAN-C chart to locate the position of the craft upon
which the apparatus is located.
Inventors: |
Brodeur; Lester R. (Hudson,
NH) |
Assignee: |
Sanders Associates, Inc.
(Nashua, NH)
|
Family
ID: |
27361925 |
Appl.
No.: |
06/568,557 |
Filed: |
January 5, 1984 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
842706 |
Oct 17, 1977 |
|
|
|
Reissue of: |
022661 |
Mar 22, 1979 |
04318105 |
Mar 2, 1982 |
|
|
Current U.S.
Class: |
342/389;
701/493 |
Current CPC
Class: |
G01S
5/10 (20130101); G01S 1/245 (20130101) |
Current International
Class: |
G01S
1/24 (20060101); G01S 1/00 (20060101); G01S
5/10 (20060101); G01S 001/24 () |
Field of
Search: |
;343/388,389,390
;364/452 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Blum; Theodore M.
Attorney, Agent or Firm: Etlinger; Louis Porter, Jr.; Wm.
F.
Parent Case Text
This is a continuation of application Ser. No. 842,706, filed Oct.
17, 1977, now abandoned.
Claims
I claim:
1. A navigation receiver-indicator providing navigation information
by receiving and measuring differences in the time of arrival of
coded radio signals received from a plurality of navigation
transmitters, in groups of transmitters, comprising:
means for selecting a group of transmitters to be used for said
time difference of arrival measurements,
first logic means processing said radio signals as they are
received for determining when received signals are properly coded
indicating they are from said navigation transmitters,
and providing output indications of same,
processor means storing and analyzing said output indications from
said first logic means to determine when received radio signals are
from transmitters of a group of transmitters selected using said
selecting means, and
second logic means enabled by and functioning with said processor
means after said processor means has determined that received radio
signals are from said selected group of transmitters to calculate
the time of reception of subsequently received radio signals and
then to analyze said last-mentioned radio signals to thereby locate
a specific point of said last-mentioned radio signals used by said
processor means to accurately measure the difference in time
.Iadd.of .Iaddend.arrival of said radio signals from individual
transmitters of said selected group of transmitters and provide a
visual output of said measurements to provide navigation
information.
2. The invention in accordance with claim 1 wherein said processor
means receives feedback information from said second logic means
that enables said processor means to revise said calculated time of
radio signal reception to accurately make said time difference
measurements.
3. The invention in accordance with claim 2 wherein a plurality of
said output indications for said selected stations are stored
within said processor means which is programmed to take an average
of said output indications to calculate the time of reception of
said radio signals subsequently received from each of said selected
group of transmitting stations.
4. The invention in accordance with claim 3 wherein said feedback
signals received by said processor means from said second logic
means are stored in said processor means which takes an average of
said feedback signals to properly determine if said calculated time
of arrival of said radio signals is correct or needs revision
before making said radio signal time difference of arrival
measurements to obtain said navigation information.
5. The invention in accordance with claim 4 wherein said
transmitters are arranged in groups consisting of one master
transmitting station and a plurality of secondary transmitting
stations, wherein the radio signals transmitted by each of said
master and secondary stations comprises a series of pulses in a
pulse train with the pulse train transmitted by a master station
differing from the pulse train transmitted by a secondary station
to distinguish the two types of stations, wherein said time
difference of signal arrival measurements are always made between
the time of arrival of signals from a master station and selected
ones of said secondary stations, and wherein said first logic means
comprises,
a multistage shift register used for storing pulses of the pulse
chains as they are received from said master and secondary station
transmitters, and
third logic means connected to various of said stages of said shift
register to analyze a pulse chain stored therein to determine if it
is from a master or a secondary station and provide an appropriate
indication of said analysis to said processor means.
6. The invention in accordance with claim 5 wherein individual
pulses of said transmitted pulse trains are phase coded, phase
codes for different master and secondary transmitting stations
being stored in said processor means, and phase code correction
apparatus is provided wherein said phase codes for the sequentially
received signals from master and secondary stations of said
selected group of transmitters are input to said correction
apparatus and used to remove the phase coding from said
sequentially received signals before utilizing said received radio
signals to make said time difference of arrival measurements.
7. The invention in accordance with claim 6 wherein the
transmitters of each group of navigation transmitters all transmit
their pulse trains at a predetermined repetitive rate peculiar to
each group and wherein said selecting means is manually operable to
indicate the repetitive rate of a selected group of transmitters to
said processor means which utilizes said rate indication to first
identify signals received from said selected group of transmitters
and then to calculate the subsequent time of reception of said
radio signals from said selected group .Iadd.of
.Iaddend.transmitters.
8. The invention in accordance with claim 7 wherein said navigation
receiver-indicator further comprises,
a display functioning with said processor means to provide a visual
display of said time difference of signal arrival measurements used
for navigation, and
means for indicating to said processor means particular ones of
said secondary stations with respect to which said time difference
of signal arrival measurements should be made.
9. The invention in accordance with claim 6 wherein said processor
means generates an enable timing signal upon calculating the time
of reception of subsequently received radio signals from a selected
group of navigation transmitters and wherein said second logic
means comprises means enabled by said enable timing signal for
generating timing signals which cause samples of any received
signals to be input to said processor means which stores a
plurality of said samples and amplitude averages same for
analyzation to determine if said calculated time of signal
reception is correct to locate said specific point of said radio
signals from which said time difference of signal arrival
measurements are made to derive navigation information.
10. The invention in accordance with claim 9 wherein said processor
means generates an enable timing signal upon calculating the time
of reception of subsequently received radio signals, and wherein
said timing signal generating means comprises:
first timing means enabled by said enable timing signal to provide
a first timing signal causing a first sample of any signals
received by said navigation receiver to be input to said processor
means which stores a plurality of said first signal samples and
amplitude averages same, said average being zero when said signal
samples are taken outside of said pulses.
second timing means enabled by said enable timing signal to provide
a second timing signal causing a second sample of any received
signals to be input to said processor means which stores a
plurality of said second samples and amplitudes averages same, said
second sample average being other than zero when said second sample
is taken during receipt of any one of said pulses, and
third timing means enabled by said enable timing signal to provide
a third timing signal causing a third sample of any received
signals to be input to said processor means which stores a
plurality of said third samples and amplitude averages same, said
third sample average being other than zero when said third sample
is taken during any one of said pulses, and from said first, second
and third samples said processor means determines if said
calculated time of arrival of said radio signals is correct and
said processor means revises said calculated time until
predetermined pulse parameters are located from which said specific
point may be located to make said time difference of signal arrival
measurements.
11. The invention in accordance with claim 1 wherein said processor
means stores the results of the analyzation of said radio signals
made by said second logic means for a plurality of samples and said
processor means is programmed to derive signal to noise ratio
information from stored plurality of stored samples.
12. The invention in accordance with claim 11 wherein said
processor means stores said analyzation samples as binary zeros and
ones with pure noise input to said navigation receiver-indicator
resulting in an equal number of binary zeros and ones from said
plurality of samples and pure signals from said navigation
transmitters input to said receiver-indicator resulting in all
binary ones from said plurality of samples, and responsive to the
number of zeros and ones in said plurality of samples, said
microprocessor means causes a visual output to be given indicating
said signal to noise ratio.
13. A method for deriving position information for navigation
purposes by making measurements of the time period between receipt
of periodic signals in pulse trains from a selected first or master
transmitting station and selected ones of a plurality of secondary
transmitting stations associated with said master station in a
navigation system comprising the steps of:
analyzing the makeup of each pulse train as it is being received to
determine if it is from a master or secondary station,
storing an indication of the time of receipt of each pulse train
along with an indication whether it is from a master or secondary
station, said stored indications being used to identify said
selected master station and its associated secondary stations,
calculating the time of receipt of signals subsequently received
from said selected master station and said secondary stations after
determining which periodic signals are received therefrom,
analyzing each signal received at the indicated times of arrival
from said selected master station and said secondary stations to
determine if the indicated time of arrival is correct and to modify
said indicated times of arrival if necessary to locate a specific
point in each of said periodic signals.
measuring the difference in time of arrival between the specific
point of each of the periodic signals received from said master
station and each of the selected secondary stations, and
providing an output reflecting said time difference of signal
arrival measurement used for navigation purposes.
14. The method of deriving position information for navigation
purposes in accordance with claim 13 wherein said step of
calculating the time of receipt of said signals comprises the step
of analyzing said stored indications of time of receipt of pulse
trains by averaging the stored time indications for said selected
master station and its associated secondary stations to determine
the average of the time of arrival of said signal pulse trains.
15. The method of deriving position information for navigation
purposes in accordance with claim 13 further comprising the step
of
checking phase coding of each pulse of the signal pulse trains
received from each of the selected master stations and its
associated secondary stations with stored phase code information to
determine if the received pulse train from each of the master and
secondary stations is a sky wave reflected from the ionosphere
which pulse train is to be disregarded.
16. The method of deriving navigational position information in
accordance with claim 15 further comprising the steps of
removing the phase coding from the pulses of said pulse trains
received from said selected master station and its associated
secondary stations prior to storing the indication of time of
receipt of each pulse train in order to achieve accurate
measurements of the time period between receipt of signals from
said selected master station and each of said selected secondary
stations.
17. The method of deriving position information for navigation
purposes in accordance with claim 13 further comprising the steps
of
storing the polarity characteristic at a discrete point on each of
said periodic signals received from said selected master station
and from its associated secondary stations for a plurality of
samples, and
providing an indication of the number of times a particular
polarity occurs in said plurality of samples from each of said last
mentioned stations to provide an indication of the signal to noise
ratio of said signals received from said last-mentioned stations.
.Iadd.
18. A navigation receiver-indicator providing navigation
information by receiving and measuring differences in the time of
arrival of coded radio signals received from a plurality of
navigation transmitters, in groups of transmitters, each of said
coded radio signals comprising a train of discrete radio frequency
pulses having a known pulse repetition rate, the transmission of
said pulse trains from each of said navigation transmitters in a
given group being repeated at a known group repetition interval,
said receiver-indicator comprising:
means for selecting a group of transmitters to be used for said
time difference of arrival measurements,
first logic means processing said radio signals as they are
received for determining when received signals are properly coded
indicating they are from said navigation transmitters, said first
logic means comprising
shifter register means,
means for continuously shifting radio signals as they are received
through said shift register means,
said shift register means having a sufficient length to contain a
window time sample of duration at least as long as that of one of
said pulse trains from one of said navigation transmitters,
said shift register means further including a set of output
terminals spaced apart by the known pulse repetition rate of the
pulses in said pulse trains from said navigation transmitters,
and
decoding means connected to said output terminals and providing an
output indication whenever the signals contained in said shift
register means correspond to said coded radio signals from one of
said navigation transmitters,
processor means storing and analyzing said output indications from
said first logic means to determine when received radio signals are
from transmitters of a group of transmitters selected using said
selecting means, and
second logic means enabled by and functioning with said processor
means after said processor means has determined that received radio
signals are from said selected group of transmitters to calculate
the time of reception of subsequently received radio signals and
then to analyze said last-mentioned radio signals to thereby locate
a specific point of said last-mentioned radio signals used by said
processor means to accurately measure the difference in time of
arrival of said radio signals from individual transmitters of said
selected group of transmitters and provide a visual output of said
measurements to provide navigation information. .Iaddend. .Iadd.19.
The invention in accordance with claim 18 wherein said first logic
means further includes
limiter means for converting each pulse of said pulse trains from
said navigation transmitters as they are received to corresponding
binary digital signals, and wherein said shifting means
continuously shifts said binary digital signals through said shift
register means. .Iaddend. .Iadd.20. The invention in accordance
with claim 19 wherein each of said pulse trains comprises a
plurality of cycles of a radio frequency carrier of fixed
frequency, and wherein said shifting means shifts said binary
digital signals through said shift register means at a rate at
least equal to said fixed carrier frequency. .Iaddend. .Iadd.21.
The invention in accordance with claim 19 wherein said decoding
means comprises logic gate means responsive to the time coincidence
of binary digital signals on each of said output terminals for
providing said output indication. .Iaddend. .Iadd.22. The invention
in accordance with claim 19 wherein said shift register means
includes a plurality of stages connected in series, each of said
stages including a plurality of binary digit storage locations and
one of said output terminals. .Iaddend. .Iadd.23. The invention in
accordance with claim 18 wherein the individual pulses of said
pulse trains are phase coded with the pulse phase codes being
different for different ones of said navigation transmitters in a
group, and wherein said first logic means further includes means
for inverting the signals on selected ones of said output terminals
prior to the application thereof to said decoding means. .Iaddend.
.Iadd.24. A method for deriving position information for navigation
purposes by making measurements of the time period between receipt
of periodic signals in pulse trains from a selected first or master
transmitting station and selected ones of a plurality of secondary
transmitting stations associated with said master station in a
navigation system, each of said pulse trains comprising a plurality
of discrete radio frequency pulses having a known pulse repetition
rate, comprising the steps of:
analyzing the makeup of each pulse train as it is being received to
determine if it is from a master or secondary station, said
analyzing step including the steps of
continuously shifting the pulses of said pulse train as they are
received through shift register means of sufficient length to
contain a window time sample of duration at least as long as that
of one of said pulse trains from one of said secondary transmitting
stations,
extracting a plurality of outputs from said shift register means at
points spaced apart by the known pulse repetition rate of the
pulses in said pulse trains from said master and secondary
transmitting stations, and
decoding said outputs to provide an output indication whenever said
pulse train contained in said shift register means corresponds to
one from said master or secondary transmitting stations,
storing an indication of the time of receipt of each pulse train
along with an indication whether it is from a master or secondary
station, said stored indications being used to identify said
selected master station and its associated secondary stations,
calculating the time of receipt of signals subsequently received
from said selected master station and said secondary stations after
determining which periodic signals are received therefrom,
analyzing each signal received at the indicated times of arrival
from said selected master station and said secondary stations to
determine if the indicated time of arrival is correct and to modify
said indicated times of arrival if necessary to locate a specific
point in each of said periodic signals,
measuring the difference in time of arrival between the specific
point of each of the periodic signals received from said master
station and each of the selected secondary stations, and
providing an output reflecting said time difference of signal
arrival measurement used for navigation purposes. .Iaddend.
.Iadd.25. The method of deriving position information for
navigation purposes in accordance with claim 24 wherein said
analyzing step further includes the step of
converting each pulse of said pulse trains as they are received to
corresponding binary digital signals, and wherein said shifting
step comprises the step of continuously shifting said binary
digital signals through said shift register means. .Iaddend.
.Iadd.26. The method of deriving position information for
navigation purposes in accordance with claim 25 wherein each pulse
of said pulse trains comprises a plurality of cycles of a radio
frequency carrier of fixed frequency, and wherein said shifting
step comprises the step of shifting said binary digital signals
through shift register means at a rate at least equal to said fixed
carrier frequency. .Iaddend. .Iadd.27. The method of deriving
position information for navigation purposes in accordance with
claim 25 wherein said decoding step comprises the step of providing
said output indication in response to the time coincidence of
binary digital signals on each of said outputs from said shift
register means. .Iaddend. .Iadd.28. The method of deriving position
information for navigation purposes in accordance with claim 24
wherein the individual pulses of said pulse trains are phase coded
with the pulse phase codes being different for said master and
secondary stations in said system, and wherein said analyzing step
further includes the step of inverting the signals at selected ones
of said outputs from said shift register means prior to the
operation of said decoding step. .Iaddend. .Iadd.29. A navigation
receiver providing navigation information by receiving and
measuring differences in the time of arrival of radio frequency
pulse trains periodically transmitted by a plurality of navigation
transmitters in a group, the individual pulses in each of said
pulse trains having a known pulse repetition rate, said receiver
comprising:
means for analyzing pulse trains as they are received to determine
when a given pulse train being received is from one of said
transmitters, said analyzing means including
shift register means,
means for continuously shifting the pulses of said pulse trains as
they are received through said shift register means, said shift
register means having a sufficient length to contain a window time
sample of duration at least as long as that of one of said pulse
trains from one of said transmitters,
means for extracting a plurality of outputs from said shift
register means at points spaced apart by the known pulse repetition
rate, and
means for decoding said outputs to provide an output indication
whenever the pulse train contained in said shift register means
corresponds to one from said transmitters; and
processor means for processing said output indications from said
analyzing means to measure the difference in the time of arrival of
said pulse trains from individual ones of said transmitters in said
group. .Iaddend.
.Iadd.30. The invention in accordance with claim 29 wherein said
analyzing means further includes
means for converting each pulse of said pulse trains from said
transmitters as they are received to corresponding binary digital
signals, and wherein said shifting means continuously shifts said
binary digital signals through said shift register means. .Iaddend.
.Iadd.31. The invention in accordance with claim 30 wherein each
pulse of said pulse trains comprises a plurality of cycles of a
radio frequency carrier of fixed frequency, and wherein said
shifting means shifts said binary digital signals through said
shift register at a rate at least equal to said fixed carrier
frequency. .Iaddend. .Iadd.32. The invention in accordance with
claim 30 wherein said decoding means comprises logic gate means
responsive to the time coincidence of binary digital signals at
each of said outputs from shift register means for providing said
output indication. .Iaddend. .Iadd.33. The invention in accordance
with claim 30 wherein said shift register means includes a
plurality of stages connected in series, each of said stages
including a plurality of binary digit storage locations, and
wherein said extracting means extracts an output from each of said
stages. .Iaddend. .Iadd.34. The invention in accordance with claim
29 wherein the individual pulses of said pulse trains are phase
coded with the pulse phase codes being different for different ones
of said transmitters in said group, and wherein said analyzing
means further includes means for inverting selected ones of said
outputs from said shift register means prior to the application
thereof to said decoding means. .Iaddend. .Iadd.35. A method of
providing navigation information by receiving and measuring
differences in the time of arrival of radio frequency pulse trains
periodically transmitted by a plurality of navigation transmitters
in a group, the individual pulses in each of said pulse trains
having a known pulse repetition rate, said method comprising the
steps of:
analyzing pulse trains as they are received to determine when a
given pulse train being received is from one of said transmitters,
said analyzing step including the steps of
continuously shifting the pulses of said pulse trains as they are
received through shift register means of sufficient length to
contain a window time sample of duration at least as long as that
of one of said pulse trains from one of said transmitters,
extracting a plurality of outputs from said shift register means at
points spaced apart by the known pulse repetition rate, and
decoding said outputs from said shift register means to provide an
output indication whenever said pulse train contained in said shift
register means corresponds to one from said transmitters; and
processing said output indications to measure the difference in the
time of arrival of said pulse trains from individual ones of said
transmitters in
said group. .Iaddend. .Iadd.36. The method in accordance with claim
35 wherein said analyzing step further includes the step of
converting each pulse of said pulse trains as they are received to
corresponding binary digital signals, and wherein said shifting
step comprises the step of continuously shifting said binary
digital signals through said shift register means. .Iaddend.
.Iadd.37. The method in accordance with claim 36 wherein each pulse
of said pulse trains comprises a plurality of cycles of a radio
frequency carrier of fixed frequency, and wherein said shifting
step comprises the step of shifting said binary digital signals
through said shift register means at a rate at least equal to said
fixed carrier frequency. .Iaddend. .Iadd.38. The method in
accordance with claim 36 wherein said decoding step comprises the
step of providing said output indication in response to the time
coincidence of binary digital signals at each of said outputs from
said shift register means. .Iaddend. .Iadd.39. The method in
accordance with claim 35 wherein the individual pulses of said
pulse are phase coded with the pulse phase codes being different
for different ones of said transmitters in said group, and wherein
said analyzing step further includes the step of inverting selected
ones of said outputs from said shift register means prior to the
operation of said decoding step..Iaddend.
Description
FIELD OF THE INVENTION
This invention relates to navigational equipment and more
particularly to hyperbolic navigational equipment utilizing the
time difference in the propagation of radio frequency pulses from
synchronized ground transmitting stations.
BACKGROUND OF THE INVENTION
Throughout maritime history navigators have sought an accurate
reliable method of determining their position on the surface of the
earth and many instruments such as the sextant were devised. During
the second world war, a long range radio-navigation system,
LORAN-A, was developed and was implemented under the auspices of
the U.S. Coast Guard to fulfill wartime operational needs. At the
end of the war there were seventy LORAN-A transmitting stations in
existence and all commercial ships, having been equipped with
LORAN-A receivers for wartime service, continued to use this
navigational system. This navigational system served its purpose
but shortcomings therein were overcome by a new navigational system
called LORAN-C.
Presently, there are eight LORAN-C multi-station transmitting
chains in operation .[.by 1980.].. This new navigational system
will result in an eventual phase-out of the earlier LORAN-A
navigational system. LORAN-C is a pulsed low frequency (100
kilohertz), hyperbolic radio navigation system LORAN-C radio
navigation systems employ three or more synchronized ground
stations that each transmit radio pulse .[.chains.]. .Iadd.trains
.Iaddend.having, at their respective start of transmissions, a
fixed time relation to each other. The first station to transmit is
referred to as the master station while the other stations are
referred to as the secondary stations. The pulse .[.chains.].
.Iadd.trains .Iaddend.are radiated to receiving equipment that is
generally located on aircraft or ships whose position is to be
accurately determined. The pulse .[.chains.]. .Iadd.trains
.Iaddend.transmitted by each of the master and secondary stations
.[.is.]. .Iadd.comprise .Iaddend.a series of pulses, each pulse
having an exact envelope shape, each pulse .[.chain.]. .Iadd.train
.Iaddend.transmitted at a constant precise repetition rate, and
each pulse separated in time from a subsequent pulse by a precise
fixed time interval. In addition, the secondary station pulse
.[.chain.]. .Iadd.train .Iaddend.transmissions are delayed a
sufficient amount of time after the master station pulse train
transmissions to assure that their time of arrival at receiving
equipment anywhere within the operational area of the particular
LORAN-C system will follow receipt of the pulse .[.chain.].
.Iadd.train .Iaddend.from the master station.
Since the series of pulses transmitted by the master and secondary
stations is in the form of pulses of electromagnetic energy which
are propagated at a constant velocity, the difference in time of
arrival of pulses from a master and a secondary station represents
the difference in the length of the transmission paths from these
stations to the LORAN-C receiving equipment.
.[.The focus.]. .Iadd.The locus .Iaddend.of all points of a LORAN-C
chart representing a constant difference in distance from a master
and a secondary station, and indicated by a fixed time difference
of arrival of their 100 kilohertz carrier pulse .[.chains,
described.]. .Iadd.trains, describes .Iaddend.a hyperbola. The
LORAN-C navigation system makes it possible for a navigator to
exploit this hyperbolic relationship and precisely determine his
position using a LORAN-C chart. By using a moderately low frequency
such as 100 kilohertz, which is characterized by low attenuation,
and by measuring the time difference between the reception of the
signals from master and secondary stations, the modern-day LORAN-C
system provides .Iadd.an .Iaddend.equipment position location
accuracy within two hundred feet and with a repeatability of within
fifty feet.
The theory and operation of the LORAN-C radio navigation system is
described in greater detail in an article by W. P. Frantz, W. Dean,
and R. L. Frank entitled "A precision Multi-Purpose Radion
Navigation System," 1957 I.R.E. Convention Record, Part 8, page 79.
The theory and operation of the LORAN-C radio navigation system is
also described in a pamphlet put out by the Department of
Transportation, U.S. Coast Guard, No. CG-462, dated August, 1974,
and entitled "LORAN-C User Handbook".
The LORAN-C system of the type described in the aforementioned
article and pamphlet and employed at the present time, is a pulse
type system, the energy of which is radiated by the master station
and by each secondary station in the form of pulse trains which
include a number of precisely shaped and timed bursts of radio
frequency energy as priorly mentioned. All secondary stations each
radiate pulse .[.chains.]. .Iadd.trains .Iaddend.of eight discrete
time-spaced pulses, and all master stations transmit the same eight
discrete time-spaced pulses but also transmit an identifying ninth
pulse which is accurately spaced from the first eight pulses. Each
pulse of the pulse .[.chains.]. .Iadd.trains .Iaddend.transmitted
by the master and secondary stations has a 100 kilohertz carrier
frequency, so that it may be distinguished from the much higher
frequency carrier used in the predecessor LORAN-A system.
The discrete pulses radiated by each master and each secondary
LORAN-C transmitter are characterized by an extremely precise
spacing of 1,000 microseconds between adjacent pulses. Any given
point on the precisely shaped envelope of each pulse is also
separated by exactly 1,000 microseconds from the corresponding
point of the envelope of a preceding or subsequent pulse within the
eight pulse .[.chains pulses.]. .Iadd.train.Iaddend.. To insure
such precise time accuracy, each master and secondary station
transmitter is controlled by a cesium frequency standard clock and
the clocks of master and secondary stations are synchronized with
each other.
As mentioned previously, LORAN-C receiving equipment is utilized to
measure the time difference of arrival of the series of pulses from
a master station and the series of pulses from a selected secondary
station, both stations being within a given LORAN-C chain. This
time difference of arrival measurement is utilized with special
maps having time difference of arrival hyperbola information
printed thereon. These maps are standard LORAN-C hydrographic
charts prepared by the U.S. Coast Guard and the hyperbola curves
printed thereon for each secondary station are marked with time
difference of arrival information. Thus, the difference in time
arrival between series of pulses received from a master station and
selected ones of the associated secondary stations must be
accurately measured to enable the navigator to locate the hyperbola
on the chart representing the time difference measured. By using
the time difference of arrival information between a master station
and two or more secondary stations, two or more corresponding
hyperbolae can be located on the chart and their common point of
intersection accurately identifies the position of the LORAN-C
receiver. It is clear that any inaccuracies in measuring time
difference of arrival of signals from master and secondary
transmitting stations results in position determination errors.
There are other hyperbolic navigation systems in operation around
the world similar to LORAN-C, and with which my novel receiver can
readily be adapted to operate by one skilled in the art. There is a
LORAN-D system utilized by the military forces of the United
States, as well as the aforementioned LORAN-A system. Others are
DECCA, DELRAC, OMEGA, CYTAC, GEE and the French radio WEB, all of
which operate in various portions of the radio frequency spectrum
and provide varying degrees of positional accuracy.
LORAN-C receiving equipment presently in use is relatively large in
size, heavy and requires relatively large amounts of power. In
addition, present LORAN-C receivers are relatively expensive and,
accordingly, are found only on larger ships and aircraft. Due to
the cost, size, weight, and power requirements of present LORAN-C
receiving equipment, such equipment is not in general use on small
aircraft, fishing boats and pleasure boats. In addition, LORAN-C
receiving equipment presently in use .[.required.]. .Iadd.requires
.Iaddend.anywhere from five to ten minutes to warm up and provide
time difference measurement information. Further, present LORAN-C
equipment is rather complex, having many controls, and the operator
thereof usually must have some training in the use of the
equipment.
Thus, there is a need in the art for a new LORAN-C receiver that is
small, light in weight, has few controls and is therefore easy to
operate by inexperienced people, requires a small amount of
electrical power, and is relatively low in cost. Such equipment
would fill the needs of those who do not now have LORAN-C receiving
equipment.
SUMMARY OF THE INVENTION
The foregoing needs of the prior art are satisfied by my novel
LORAN-C receiver. I eliminate much of the complex and costly
automatic acquisition and tracking circuitry in prior art LORAN-C
navigation receivers and provide a small, light weight, inexpensive
receiver using relatively little electrical power.
Four thumbwheel switches on my LORAN-C equipment are used by the
operator to enter the group repetition rate information for a
LORAN-C chain covering the area within which the LORAN-C equipment
is being operated. Thus information entered via the thumbwheel
switches is used by an internal microprocessor to locate the
signals from the master and secondary stations of the chosen
LORAN-C chain.
The receiver of my equipment receives all signals that appear
within a small bandwith centered upon the 100 Khz. operating
frequency of the LORAN-C network. A digital register coupled with
logic circuitry .Iadd.including a group of serially connected shift
registers .Iaddend.is then used to continuously check all received
signals .Iadd.as they are received .Iaddend.to search for the
unique pulse trains transmitted by the master and secondary
stations. The microprocessor internal to my novel LORAN-C equipment
analyzes all signals output from the .[.register.]. .Iadd.registers
.Iaddend.and logic circuitry indicating that signals from master or
secondary stations have been received to first determine if they
match the group repetition rate for the selected LORAN-C chain and
then to develop a histogram of the time of arrival of the signals
from the secondary stations. Once the equipment has approximately
located and is receiving the pulse trains from the selected master
and secondary stations, the microprocessor causes other circuitry
to go into a fine search mode.
In the fine search mode the microprocessor disables the equipment
from analyzing any signals other than those received within 35
microseconds of the approximate time of arrival of the signals from
the secondary stations as determined using the histogram. The
microprocessor also enables other equipment to analyze the phase of
each pulse and to locate the third cycle zero crossing point of
each received pulse. In the event the third cycle zero crossing of
a pulse is not located at the approximate time indicated by the
microprocessor, the analyzation circuitry indicates to the
microprocessor whether to add or subcontract 10 microseconds to the
approximate time of arrival and then repeats the analyzation
process. This analyzation process and shifting of the approximate
search point is repeated until the third cycle zero crossing of the
desired pulse of the selected master and secondary station pulse
trains is located. Using an accurate crystal controlled clock
internal to my novel equipment, the microprocessor then makes
accurate time difference of arrival measurements between the time
of arrival of signals from the master station of the selected chain
and the arrival of the pulse trains from the secondary stations.
The equipment operator utilizes other thumbwheel switches to
indicate two secondary stations, the time difference of arrival
information to be visually displayed. The operator of the LORAN-C
equipment utilizes these two read-outs using a LORAN-C hydrographic
chart to locate the physical position of the navigation equipment
on the surface of the earth.
In an alternative embodiment of my invention a front panel keyboard
may be utilized rather than thumbwheel switches and the
microprocessor can be programmed to perform other functions
including, but not limited to, use as a calculator. Other possible
uses are limited only by the amount of storage provided within the
microprocessor or auxiliary memory adjunct to the processor in a
well known manner, and by the imagination of the equipment
designer.
The operator of my novel LORAN-C navigation receiver can quickly
and easily calibrate the receiver master oscillator, unlike prior
art receivers. To accomplish this, the operator places the
equipment in a calibration mode wherein the output of the
oscillator is compared against the group repetition interval [GRI]
information which has been entered via the thumbwheel switches. The
display is used to indicate to the operator if the equipment is in
calibration or requires a simple adjustment by the operator.
The Applicant's novel LORAN-C navigation receiver will be better
understood upon a review of the detailed description given
hereinafter in conjunction with the drawing in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a general block diagram of the Applicant's LORAN-C
navigation receiver;
FIG. 2 shows the shape of each pulse of the pulse trains
transmitted by all LORAN-C master and secondary stations;
FIG. 3 is a graphical representation of the pulse trains
transmitted by the master and secondary stations within a LORAN-C
chain.
FIG. 4 is a representation of a portion of a LORAN-C navigation
chart;
FIGS. 5, 6, and 7 are detailed block diagrams of the Applicant's
navigation receiver;
FIG. 8 is a detailed block diagram of the smart shift register
shown in FIG. 5; and
FIG. 9 shows the manner in which FIGS. 4, 5, and 6 should be
arranged with respect to each other when reading the detailed
description.
GENERAL DESCRIPTION
To understand the general or detailed operation of my novel LORAN-C
receiver, it is best to first understand the makeup of the signals
transmitted by LORAN-C stations and being received by my novel
receiver. Representation of these signals are shown in FIGS. 2 and
3 which will now be discussed.
All master and secondary stations transmit groups of pulses as
briefly mentioned above, at a specified group repetition interval
which is defined as shown in FIG. 3. Each pulse has a 100 Khz.
carrier and is of a carefully selected shape shown in FIG. 2. For
each LORAN-C chain a group repetition interval (GRI) is selected of
sufficient length so that it contains time for transmission of the
pulse .[.chains.]. .Iadd.trains .Iaddend.from the master station
and each associated secondary station, plus time between the
transmission of each pulse train from the master station so that
the signals received from two or more stations within the chain
will never overlap each other when received anywhere in the LORAN-C
chain coverage area. Each station transmits one pulse .[.chain.].
.Iadd.train .Iaddend.ofbeight or nine pulses per GRI as shown in
FIG. 3. The master station pulse .[.chain.]. .Iadd.train
.Iaddend.consists of eight pulses, each shaped like the pulse shown
in FIG. 2, with each of the eight pulses spaced exactly 1,000
microseconds apart, and with a ninth pulse spaced exactly 2,000
microseconds after the eighth pulse. The pulse .[.chain.].
.Iadd.train .Iaddend.for each of the secondary stations X, Y and Z
contains eight pulses shaped as shown in FIG. 2, and each of the
eight pulses is also spaced exactly 1,000 microseconds apart. The
pictorial representation of the pulses transmitted by the master
station and the three secondary stations X, Y and Z associated
therewith shown in FIG. 3 shows that the pulse trains never overlap
each other and all are received within the group repetition
interval. FIG. 3 also shows a representative time difference of
arrival of the pulse train from each of the secondary stations with
respect to the master station. These time difference of arrival
figures are designated Tx, Ty and Tz and are the time differences
measured using my receiver.
It is to be recognized that the time difference of arrival between
reception of the pulse train from the master station and the pulse
trains from each of the X, Y and Z secondary stations will vary
depending upon the location of the LORAN-C receiving equipment with
the coverage area of a LORAN-C chain. In addition, the signal
strength of the received signals from the master and secondary
stations will also vary depending upon the location of the
receiving equipment, as represented by the different heights of the
representative pulse lines shown in FIG. 3.
The delayed or spaced ninth pulse of each master station not only
identifies the pulse train as being from a master station, but the
ninth pulse is also turned on and off by the Coast Guard in a
"blink" code, well known in the art, to indicate particular faulty
secondary stations in a LORAN-C chain. These "blink" codes are
published by the Coast Guard on the LORAN-C charts.
In World War II when the LORAN-C systems were installed, carrier
phase coding was used as a military security method, but after the
war when the need for military security ceased, the phase coding
was called a skywave unscrambling aid. In skywave unscrambling the
100 Khz. carrier pulses from the master station and the secondary
stations in a LORAN-C chain are changed in phase to correct for
skywave interference in a manner well known in the art. Skywaves
are echoes of the transmitted pulses which are reflected back to
earth from the ionosphere. Such skywaves may arrive at the LORAN-C
receiver anywhere between 35 microseconds to 1,000 microseconds
after the ground wave for the same pulse is received. In the 35
microsecond case, the skywave will overlap its own groundwave while
in the 1,000 microsecond case the skywave will overlap the
groundwave of the succeeding pulse. In either case the received
skywave signal has distortion in the form of fading and pulse shape
changes, both of which can cause positional errors. In addition, a
skywave may be received at higher levels than a ground wave. To
prevent the long delay skywaves from affecting time difference
measurements, the phase of the 100 Khz. carrier is changed for
selected pulses of a pulse train in accordance with a predetermined
pattern. These phase code patterns are published by the Coast Guard
on the LORAN-C charts.
The exact pulse envelope shape of each of the pulses transmitted by
all master and secondary stations is also very carefully selected
to aid in measuring the exact time difference in arrival between a
pulse train from a master station and a pulse train from a
secondary station as is known to those skilled in the art. To make
exact time difference measurement, one method the prior art teaches
is superpositions matching pulse envelopes of pulses from a master
station and a selected secondary station. Another method which I
also utilize, is detection of a specific zero-crossing of the 100
Khz. carrier of the master and secondary station pulses.
Now that the reader has an understanding of the nature of the
signals transmitted by the LORAN-C master and secondary stations
and how they are used for navigation purposes, the reader can
better understand the operation of my novel LORAN-C receiver which
will now be described.
In FIG. 1 is seen a general block diagram of my novel LORAN-C
navigation equipment. Filter and preamplifier 1 and antenna 2 are
of a conventional design of the type used in all LORAN-C receivers
and is permanently tuned to a center frequency of 100 Khz., which
is the operating frequency of all LORAN-C transmitting stations.
Filter 1 has a bandpass of 20 Kilohertz. Received signals are
applied to smart shift register 3 and zero crossing detector 6.
The signal input to zero crossing detector 6 is first amplitude
limited so that each .Iadd.positive half .Iaddend.cycle of each
pulse is represented by a binary one and each negative half cycle
is represented by a binary zero. The leading or positive edge of
each binary one exactly corresponds to the positive .[.slope.].
.Iadd.going zero crossing .Iaddend.of each sine wave comprising
each pulse. Thus, detector 6 is a positive zero-crossing detector.
As will be described in detail further in this specification logic
circuit 16 also provides an input to zero crossing detector 6, not
shown in FIG. 1, which sets a 10 microsecond window only within
which the leading edge of each binary 1 may be detected. The end
result is that only the positive zero-crossing of the third cycle
of each pulse of the train pulse trains transmitted by each LORAN-C
station is detected and an output provided by detector 6.
It can be seen that latch 5 has inputs from zero crossing detector
6 and logic circuit 4. Clock/counter 7 is a crystal controlled
clock which is running continuously while my novel LORAN-C receiver
is in operation. The count present in counter 7 at the moment that
zero crossing detector 6 indicates a third cycle positive zero
crossing is stored in latch 5, the contents of which are then
applied to multiplexer 8. Multiplexer 8 is a time division
multiplexer used to multiplex the many leads from logic circuit 16,
latch 5, clock/counter 7, thumbwheel switches 11, 61 and 62 through
to microprocessor 9. The count in latch 5 indicates to
microprocessor 9 the time at which each positive zero crossing is
detected.
Smart shift register 3 has a filter at its input causing it to
receive the output from receiver 1 within a narrower bandpass of
five kilohertz centered on the carrier frequency of 100 Khz. The
signal input to register 3 is also amplitude limited so that a
pulse train of 1's and 0's is produced that is input to a shift
register therein which is shifted at a 100 Khz. rate. Because of
the 100 Khz. shifting frequency only the pulse trains from LORAN-C
master and secondary stations will result in outputs from each of
the individual stages of the shift .[.register.]. .Iadd.registers
.Iaddend.internal to smart shift register 3. Logic circuitry within
register 3 is used to analyze the contents of the shift
.[.register.]. .Iadd.registers .Iaddend.internal to register 3 to
first determine if the signals represent a pulse train from a
LORAN-C station, secondly to determine if the pulse train is from a
master or a secondary station, and finally to indicate the
particular phase coding of the signals being received from a
LORAN-C station. Logic circuit 4 includes a latch and a circuit to
store information from register 3 indicating whether a pulse train
is from a master or a secondary station and further indicating the
phase code transmitted. This information stored within the latch of
logic circuit 4 is applied to microprocessor 9 via multiplexer 8
for use in processing received LORAN-C signals. At the same time
the information is stored on the latch within logic circuit 4 there
is an output from circuit 4 enabling latch 5 to store the count in
clock/counter 7 which will indicate the time of occurrence. It
should be noted that clock/counter 7 also has an input to
multiplexer 8 so that microprocessor 9 can keep track of a
continuous running time as indicated by recycles of counter 7.
The output of thumbwheel switches 11 are also input to multiplexer
8 allowing the operator of my novel LORAN-C equipment to input the
group repetition rate of a selected LORAN-C chain to microprocessor
9. The group repetition rate is also called the Group Repetition
Interval (GRI). In alternative embodiments of my invention
thumbwheel switches 11 may be replaced by a keyboard which can be
used by the operator to access microprocessor 9 to do many things
including perform navigation calculations.
With the various types of information being input to microprocessor
9 via multiplexer 8 from the circuits previously described,
microprocessor 9 determines if and when signals being received via
filter 1 are from the master and secondary stations of the selected
LORAN-C chain. Once the microprocessor 9 locates the signals from
the selected master station, as determined by a match of the GRI
number input thereto via the four thumbwheel switches 11 with the
difference in time arrival between each pulse train transmitted by
the selected master station, microprocessor 9 similarly locates the
corresponding secondary station signals. To locate the secondary
stations microprocessor 9 creates a histogram from time of arrival
information of any and all secondary station signals which are
stored in twenty bins or slots created by the microprocessor in its
own memory between the arrival of any two consecutive master
station pulse trains. When signals from the secondary stations of
the selected LORAN-C chain are located by secondary station signal
counts appearing in the histogram bins at the same rate as the GRI
of the selected LORAN-C chain, the microprocessor 9 performs a
finer search by creating histogram bins of a shorter time duration.
Each of the histogram bins in which are stored the time of arrival
counts of the signals of the appropriate secondary stations is then
subdivided by microprocessor 9 into one hundred smaller time slot
histogram bins to more closely determine the time of arrival of the
pulse trains from the secondary stations of the selected LORAN-C
chain. Each of these smaller histogram bins or slots stores counts
corresponding to the time of receipt of signals received in
consecutive twelve microsecond periods. In this manner,
microprocessor 9 closely determines the time of arrival of pulse
trains from the master and secondary stations of the selected
LORAN-C chain within twelve microsecond periods.
Once microprocessor 9 determines the particular twelve microsecond
histogram time slots in which the secondary station signals are
being received, the microprocessor causes an enable timing signal
which causes the equipment to go into a fine search mode utilizing
logic circuit 16 to accurately find the third cycle positive zero
crossing of each pulse of the selected master and secondary station
pulse trains. To accomplish this function, the approximate time of
arrival of sequentially received pulses of the master and secondary
station pulse trains are sequentially entered into latch 15 and the
contents thereof are applied to comparator 14. Comparator 14
compares the contents of latch 15 with the contents of
clock/counter 7 and upon there being a match, comparator 14
provides an output signal to logic circuit 16. The time entered
into latch 15 is actually a time calculated to be 35 microseconds
before the time of arrival of each pulse of the pulse train from a
selected secondary station. The output from comparator 14 to logic
circuit 16 is used to store three timing signals therein which are
received from microprocessor 9. These three timing signals
represent lines which occur 2.5 microseconds, 12.5 microseconds,
and 30.0 microseconds after the output signal from comparator 14.
At the end of each of these three timed sequences, the phase coding
of a received pulse is checked against phase coding permanently
stored in microprocessor 9. With the phase coding information,
microprocessor 9 is able to accurately locate the third cycle zero
crossing of each pulse of the pulse trains from the master and
secondary stations. In the event that the previously described
signal characteristics immediately prior to and at fixed points
during a pulse are not received, microprocessor 9 knows that there
is an error in its calculated time placed in latch 15 and
microprocessor 9 either increases or decreases the calculated time
of subsequent pulse trains by 10 microseconds and the new
calculated time figure is placed in latch 15. Logic circuit 16
again analyzes incoming signals at the aforementioned points. This
process of adding or subtracting 10 microseconds to the calculated
time is repeated until microprocessor 9 accurately locates the
third positive zero crossing of each pulse of the pulse trains
transmitted by each of the master and secondary stations of the
selected LORAN-C chain .[.;.]. .Iadd.. The microprocessor 9
.Iaddend.then determines if the received pulse trains are from a
master or a secondary station, and further determines the
particular skywave phase code being transmitted by each of the
stations.
Once microprocessor 9 functioning with the other circuits in my
LORAN-C receiver has located and locked onto the pulse trains being
transmitted by the master and secondary stations of the selected
LORAN-C chain and has made the desired time difference of arrival
measurement that is required in LORAN-C operation, microprocessor 9
causes a visual indication to be given to the equipment operator
via display 12. The output information is plotted on a LORAN-C
hydrographic chart in a well known manner to locate the physical
position of the LORAN-C receiver.
DETAILED DESCRIPTION
Turning now to describe in detail the operation of my novel LORAN-C
equipment.
In FIG. 2 is seen the shape or waveform of every pulse transmitted
by both master and secondary LORAN-C stations. The waveform of this
pulse is very carefully chosen to aid in the detection of the third
carrier cycle zero crossing in a manner well known in the art. One
method known in the art is to take the first derivative of the
curve represented by the envelope of the pulse shown in FIG. 2, and
this first derivative clearly indicates a point at 25 microseconds
from the beginning of the pulse. The next zero crossing following
this indication is the desired zero crossing of the third cycle of
the carrier frequency. Similar to the prior art method just
described, my novel LORAN-C receiver detects the third zero
crossing for each pulse of the master station and each secondary
station. The precise time difference of arrival measurements to be
made utilizing a LORAN-C receiver are made by measuring from the
third cycle zero crossing of the fifth pulse of the master station
pulse train and the third carrier cycle zero crossing of the fifth
pulse of the manually selected secondary station.
In FIG. 3 is shown a representation of the nine pulse and eight
pulse signals transmitted by a master station and the secondary
stations of a LORAN-C chain. The small vertical lines each
represent a pulse waveform such as shown in FIG. 2. The height of
the vertical lines represents the relative signal strength of the
pulses as received at a LORAN-C receiver. It can be seen that the
signal strength of the pulses from the master station and each of
the secondary stations are not identical.
It can be seen in FIG. 3 that the group repetition interval (GRI)
is defined as the period between the first pulses of two
consecutive master station pulse trains for a given LORAN-C chain.
This information is found on standard LORAN-C hydrographic charts
and is used to calibrate the oscillator in my novel LORAN-C
receiver as will be described to greater detail further in this
specification.
In a manner well known in the art, LORAN-C receiving equipment is
used to measure the time difference of arrival between the pulse
train from a master station pulse train and the pulse trains from
two or more secondary stations associated with the master station.
This time difference of arrival information is shown on FIG. 3 as
T.sub.x, T.sub.y, and T.sub.z.
In FIG. 4 is shown a representative figure of a LORAN-C
hydrographic chart. On this chart are shown three sets of arcuate
curves, each set of curves having a five digit number thereon and
suffixed by one of the letters, x, y or z. The numbers directly
correspond to the time difference of arrival information T.sub.x,
T.sub.y and T.sub.z shown in FIG. 3 and measured by a LORAN-C
receiver. In FIG. 3 the particular secondary station with which a
set of the arcuate curves is associated is indicated by the suffix
x, y, or z after the numbers on the curves.
LORAN-C charts show land masses such as island 80 on FIG. 4. For an
example, the operator of my LORAN-C receiver located on boat 81
near island 80 would measure the time difference of arrival
information between the master station and at least two of the
three secondary stations in the LORAN-C chain. The operator, in
making a measurement with respect to the X secondary station would
measure 379000 on my LORAN-C receiver. As can be seen in FIG. 4,
the line of position (LOP) 379000 is shown passing through boat 81.
In a similar manner, the operator would measure the time difference
arrival information with respect to the Y secondary station and
would come up with the number 699800 on the receiver. Again, the
LOP for this receiver reading passes through boat 81. If the
operator of the LORAN-C receiver measures the time difference of
arrival information with respect to the Z secondary station the
reading would show 493500 and the LOP for this reading also passes
through boat 81. Thus, the operator can accurately fix the position
of boat 81 on the LORAN-C chart. From this position information on
the map of FIG. 4, boat 81 may, for example, be accurately
navigated toward harbor 82 of island 80.
It will be noted that the sample LORAN-C chart shown in FIG. 4 has
only five digits on each LOP, but my LORAN-C receiver, has six
digits. The lowest order or sixth digit is used to interpolate
between two LOPs on the LORAN-C chart in a manner well known in the
art. In the simple example given above, boat 81 is located exactly
on three LOPs on so no interpolation need be done to locate a LOP
between those shown on the chart of FIG. 4. Thus, it should be
noted that the six digit numbers obtained utilizing my equipment
each included an extra zero suffixed to the end of the five digit
LOP numbers shown on the LORAN-C chart. A sixth digit other than
zero on the receiver would require interpolation between the LOP
lines on the chart.
In FIGS. 5, 6, and 7 is shown a detailed block diagram schematic of
my novel LORAN-C receiver which I will now describe in detail.
FIGS. 5, 6, and 7 should be arranged as shown in FIG. 9 to best
understand the description found hereinafter.
LORAN-C signals are received by a conventionally designed antenna 2
and conventionally designed filter and preamplifier 1, in a manner
well known in the art. Interference caused by miscellaneous radio
frequency signals and signals from other navigational systems are
essentially eliminated by filter 1 which utilizes filters having a
20 Khz, bandwidth centered on 100 Khz, with a sharp drop off at
either side of this band. Filter 1, being of a conventional design
utilized in many LORAN-C receivers, is not described in further
detail herein. Similarly, the choice of antenna 2 and/or the design
thereof is also well known in the art and is not disclosed herein
in detail for the purpose of not cluttering up the specification
with details that are well known in the art and would detract from
an understanding of the invention. The output filter 1 is
undemodulated and is applied to limiter 17 in zero crossing
detector 6 and to 5 Khz, bandwidth filter 19.
When my novel LORAN-C equipment is initially placed in operation,
it is in a coarse search mode wherein it is only trying to
generally locate the pulse trains from the master and secondary
stations of the selected chain. This function is accomplished by
smart shift register 3 as now described. Filter 19 has a five Khz.
bandwidth centered on the 100 Khz. carrier frequency of the LORAN-C
signals and causes rejection of most spurious signals. LORAN-C
signals and a few spurious signals are passed through filter 10 to
limiter 20. Limiter 20 demodulates and hard limits the signals
input thereto so that only a chain of binary 1's is output from the
limiter. Each .Iadd.chain .Iaddend.of the binary 1's output from
limiter 20 corresponds to a spurious signal pulse or to .[.each.].
.Iadd.one .Iaddend.of the pulses in the pulse trains from master
and secondary stations. These .[.pulses.]. .Iadd.binary signals
.Iaddend.are applied to smart shift register 3 which is shown in
block diagram form in FIG. 5, but is shown in detail in FIG. 8 and
will be described in detail further in this specification.
Smart shift register 3 is made up of ten serially connected shift
registers, all of which are clocked or shifted at the same period
as the pulses from master and secondary LORAN-C stations are
received and logic gates. This is a one-thousand microsecond period
as shown in FIG. 3. These ten shift registers store a window time
sample of received signals which are analyzed to determine if the
signal stored in the shift registers represents a pulse train from
a LORAN-C master or secondary station. Due to the clocking the
sample moves in time. The logic gates connected to various stages
of shift registers provide outputs that are used to analyze the
signals temporarily stored in the register to determine if received
signals are from a master or secondary station and to determine if
the received signals have what the U.S. Coast Guard refers to as
group repetition interval A or B phase coding. These phase codes
are well known to those skilled in the art. Upon smart shift
register 3 determining that a pulse train has been received from a
master or secondary station the internal logic gates, which are
described in greater detail further in the specification, apply an
output signal on one of leads MA, MB, SA, or SB, indicating if the
signals are from a master or secondary station and the particular
phase coding thereof. A signal indication that the received signals
are from either a master of a secondary station is stored in latch
21. In addition, the last named signal output from register 3 is
applied via OR gate 22 to the SET input of R/S flip-flop 23 to
place this flip-flop in its set state with its 1 output high
indicating that a pulse train from either a master or secondary
station has been received. The 1 output of R/S flip-flop 23 is
applied via OR gate 24 to latch 5. More particularly, this output
signal from flip-flop 23 is applied to the clock input CK of latch
5 and causes the latch to store the contents of BCD counter 26 in
clock/counter 7 at the moment in time that it is determined that
signals have been received from the master or secondary station as
indicated by the signal at input CK. The .[.sored.]. .Iadd.stored
.Iaddend.count is indicative of the real time at which the pulse
train was received. As previously briefly described, the contents
stored in latch 5 are applied to multiplexer 8 in FIG. 6 to
thereafter be input to microprocessor 9.
Multiplexer 8 in FIG. 6 is required to input signals to
microprocessor 9 in FIG. 7 due to the limited number of input
terminals to microprocessor 9 and the large number of leads over
which signals must be applied to the microprocessor. Multiplexer 8
accomplishes this task utilizing time division multiplexing
techniques. Integrated circuit multiplexers are available on the
market, but may also be made up of a plurality of two input logic
AND gates, one input of each of which is connected to the leads on
which are the signals to be multiplexed, and the other input of
each of which is connected to a clock and counter arrangement which
causes ones or groups of the logic gates to have their other inputs
sequentially energized in a cyclic manner. In this embodiment of my
invention multiplexer 8 comprises Texas Instrument TI74151
multiplexers.
It can be seen in FIG. 6 that there are inputs to multiplexer 8
from logic circuit 4, latch 5, clock/counter 7, thumbwheel switches
11, 61 and 62, logic circuit 16 and microprocessor 9. The signals
input to multiplexer 8 from microprocessor 9 on leads 40 are used
to control the operation of multiplexer 8.
The contents of BCD counter 26 which are stored in latch 5 in
response to the receipt of a pulse train from a master or secondary
station are applied via multiplexer 8 to microprocessor 9 and
indicate to the microprocessor the time of receipt of a valid pulse
train from a master or secondary station.
Following microprocessor 9 receiving the contents of latch 5 via
multiplexer 8, indicating the time of receipt of a pulse train from
a master or a secondary station, the microprocessor outputs a
signal on LATCH RESET which is applied to reset latch 21 and clear
the information stored therein in preparation of storing a
subsequent master or secondary station indication. In addition, the
CATCH RESET is applied via OR gate 60 to place flip-flop 23 in its
reset state.
As signals being input to microprocessor 9 from latch 5 will
represent the receipt of master and secondary station signals from
more than one LORAN-C station chain, microprocessor 9 requires an
input from the equipment operator using thumbwheel switches 11 to
indicate a particular LORAN-C chain of interest. The operator first
consults a LORAN-C hydrographic chart published by the U.S. Coast
Guard and finds the group repetition interval (GRI) for the LORAN-C
station chain of interest. Using the four switches 11 the operator
enters the repetition rate or GRI.
As previously described, latch 5 is used to store the count present
in BCD counter 25 each time a pulse train from a master or
secondary station is detected by smart shift register 3. At the
same time, the information stored in latch 21 is also applied to
microprocessor 9 via multiplexer 8 to indicate the signal is from a
master or secondary station and the phase coding thereof. In the
previously mentioned initial coarse search mode microprocessor 9
analyzes master and secondary station information being input
thereto via latch 5 to determine which indication represent signals
from the stations of the selected LORAN-C chain. Microprocessor 9
stores the time signal reception of the pulse trains from all
master and secondary stations as indicated by the counts stored in
latch 5 until it has definitely located and locked onto the
selected stations and can therefore calculate the time of arrival
of subsequent pulse chains therefrom.
The microprocessor is programmed to create twenty bins or slots
each corresponding to one of twenty sequential time periods of
approximately twelve hundred microseconds duration each. The count
stored in latch 5 when logic circuit 4 indicates a pulse train has
been received from a master or secondary station causes a count to
be stored internal to microprocessor 9 in the corresponding one of
the twenty slots or bins. The microprocessor 9 is programmed to
store the counts stored in these twenty bins, which make up a
histogram to determine which bins contain counts indicating receipt
of master and secondary station pulse trains at the correct
GRI.
Once microprocessor 9 is consistently receiving signals from the
master station of the selected LORAN-C chain, it causes a front
panel light designated "M" to be lit indicating that the receiver
has locked onto the correct master station signals. As
microprocessor 9 locates each secondary station associated with the
selected LORAN-C chain, it causes a corresponding front panel light
"51", "52", "53" and "54" to be lit as each secondary station is
locked onto. This indicates to the operator which secondary
stations are acceptable to use to make LORAN-C measurements.
Microprocessor 9 then takes only the ones of the twenty histogram
bins in which the selected chain master and secondary station
signal counts are stored and subdivides each of these bins into
one-hundred bins corresponding to sequential time slots of twelve
microseconds duration each. The process just described is repeated
for the shorter duration histogram bins created in memory internal
to microprocessor 9 to more closely determine the time of arrival
or receipt of the pulse trains from the secondary stations of the
selected LORAN-C chain. When the above histogram processing has
been accomplished to determine the time of receipt of master and
secondary station pulse trains within twelve microseconds accuracy,
microprocessor 9 generates an enable timing signal which causes the
equipment to switch from the coarse search mode to a fine search
mode to accurately make the LORAN-C time difference measurements as
is described further in this specification.
To place the equipment in the fine search mode, microprocessor 9
outputs a signal on its output COARSE DISABLE. The last named
signal is applied via OR gate 60 to the reset input R of flip-flop
23 which prevents signals from register 3 being applied to the set
input S and placing flip-flop 23 in its set or one state.
Microprocessor 9 also applies a signal to its FINE ENABLE output
causing the equipment to go into the fine search mode wherein the
time of arrival of subsequently received signals is accurately made
and a readout is provided on display 12.
More particularly, the FINE ENABLE signal is applied to comparator
14 in FIG. 7 to enable same. One of the two inputs to comparator 14
is the output from BCD counter 25 in clock 7 on lead REAL TIME. The
other input to comparator 14 is a number stored in latch 15 and
this number is calculated by microprocessor 9 as is now described.
Once microprocessor 9 determines the time of arrival of the signal
trains from the master and secondary stations of the selected chain
in the coarse search mode, and then switches to the fine search
mode, it calculates the time of arrival of the subsequent pulse
trains of the master and secondary stations from the secondary or
fine (12 microsecond) histogram. Using the fine histogram,
microprocessor 9 actually calculates a time 35 microseconds prior
to the expected time of arrival of a subsequent master or secondary
pulse train and loads this information into latch 15 over lead
PRE-TIME under the control of another microprocessor generated
signal on the CONTROL input. Comparator 14 compares the signal from
clock 7 with the number stored in latch 15 and upon there being a
match between these two digit numbers, there is an output from
comparator 14 which places flip-flop 30 in logic circuit 16 into
its set or one state. The one output of the flip-flop 30 is
connected to the reset input R of counter 31 and to one of the two
inputs or OR gate 32. Being in its one state the output of
flip-flop 30 is high and this is applied via OR gate 32 to the set
input S of flip-flop 33 which is thereby placed in its set state
with its one output high.
The high one output of flip-flop 30 being supplied to reset input R
of counter 31 causes this counter to reset to zero. Once reset to
zero, counter 31 counts to a count of 8, stops counting and causes
its TC output to go high. The TC output of counter 31 is applied to
the reset input R of counter 34 which is disabled from counting
once counter 31 reaches a count of eight and is thereby disabled
from counting. This occurs because flip-flop 30 being placed in its
set state with its one output high enables counter 31 to count by
resetting it to zero whereby its TC output goes to zero, thereby
removing the signal to the reset input R of counter 34. Counter 34,
which is reset to zero count, is thereby enabled to count in
response to the 1 MHz signal being input to its clock input CK.
Counter 34 is different than counter 31 in that it counts up to its
maximum count of 10,000 and then resets itself to zero to recount
to 10,000 again and again. Because of counter 34 counting and
recounting to 10,000, its output TC has a signal thereat which
occurs at a 1,000 microsecond rate due to the dividing action by
counter 34 of the 1 MHz signal at its CK input. Thus, counter 34 is
providing output signals at the same rate that each of the pulses
are being received in the pulse trains from the master and
secondary stations. The TC output of counter 34 is applied to the
second input of OR gate 32 and is also applied to the clocking
input CK of counter 31. This causes the count in counter 31 to be
.[.increment.]. .Iadd.incremented .Iaddend.by one each time counter
34 counts to 10,000. Thus, at the end of 8,000 microseconds counter
31 will have reached its full count and its output TC is high
which, being applied to the reset input R of counter 34, causes
counter 34 to be reset to zero and to cease counting. Counter 31
will not be reset to zero until flip-flop 30 is returned to its
reset state with its one output low. This happens when output TC
goes high, which being connected to reset input R to flip-flop 30,
causes it to be reset to its zero state. This removes the high
input to reset input R of counter 31, leaving the counter at its
full count with its output TC high.
One of the purposes for the timing function accomplished by
counters 31 and 34 is to check the phase coding of the pulse trains
being received from the selected master and secondary stations.
Upon microprocessor 9 changing over the receiver to the fine search
mode, the microprocessor parallel loads the phase coding for the
first eight pulses of the master and secondary station pulse trains
of the selected LORAN-C chain into parallel/serial converter 35 of
logic circuit 16. Converter 35 is a conventional shift register
well-known in the art which may be loaded in parallel and then
shifted out in serial to perform parallel to serial conversion. As
is well known in the art, each of the pulses of the pulse trains
received from master station and secondary stations has a
particular phase coding. The phase coding is stored in
microprocessor 9 and is selected by information input to the
equipment by the operator using thumbwheel switches 11. It can be
seen that the clocking input CK to converter 35 is the same 1,000
microsecond signal output from counter 34. Thus, the contents of
converter 35 are serially shifted out at its output Q at a 1,000
microsecond rate. It should be noted that the output Q of converter
35 is connected to one of the two inputs of exclusive OR gate 36 in
zero crossing detector 6. Exclusive OR gate 36 functions as an
inverter in this case in a manner known to circuit designers. When
a particular one of the pulses of the pulse trains received from a
master or secondary station is of a positive phase there is no
signal or a zero on output Q from converter 35 if the phase codes
match. The result is that each radio frequency cycle of the
particular pulse which is hard limited by limiter 17 will pass
directly through exclusive OR gate 36 to flip-flop 37 unchanged.
Upon the expected receipt of each particular pulse of the pulse
trains from the master and secondary stations which are to be of a
negative phase, converter 35 will have shifted its contents such
its output Q will be high or a one. This high input applied to the
second input of exclusive OR gate 36 causes OR gate 36 to invert
the phase of the pulse output from limiter 17. That is, the signal
being input to detector 6 is effectively shifted 180.degree.
thereby eliminating the negative phase coding applied to the
particular pulse. This is done in order that there will be an
output from exclusive OR gate 36 to place flip-flop 37 in its set
state at exactly the beginning of each pulse of the pulse trains
from the master and secondary stations.
Fiip-flop 37 in detector 6 being placed in its set state with its
one output high as described heretofore, causes latch 5 to store
the contents of counter 26 at that particular moment in time.
Microprocessor 9 thereby receives a time indication of the
beginning of each radio frequency cycle of each of the pulses and
this information is used to make the required time difference of
arrival measurements which are the basis .[.or.]. .Iadd.of
.Iaddend.the LORAN-C system. Flip-flop 37 is returned to its reset
state before the beginning of the first cycle of a subsequent pulse
received from a master or secondary station by the LATCH RESET
signal as described heretofore.
Microprocessor 9 determines the estimated time of arrival of the
third cycle positive zero crossing of each of the pulses of the
next to be received pulse train from the selected master and
secondary stations. Microprocessor 9 then substracts 35
microseconds from this time which results in a time that should
occur five microseconds before the beginning of the first radio
frequency cycle of each pulse of the master and secondary station
pulse trains. This point in time occuring 5 microseconds before the
beginning of each pulse of the pulse trains is output from
microprocessor 9 on its output leads PRE-TIME and is input to latch
15 under control of signals from the microprocessor on the input
CONTROL. The contents of latch 15 are applied to comparator 14
which is enabled by the microprocessor energizing input E upon the
equipment being placed in the fine search mode. It should be noted
that comparator 14 also has an input thereto designated REAL TIME,
which is the lock output from BCD counter 26 of clock/counter 7 in
FIG. 5. Upon there being a match of the two inputs to the
comparator 14, there is an output therefrom which places flip-flop
30 in logic circuit 16 into its set state and its one output goes
high. As mentioned heretofore, this enables counters 31 and 34 to
commence counting as previously described. The one output of
flip-flop 30 is also coupled by an OR gate 32 to the set input S of
flip-flop 33 to place this flip-flop in its set state with its one
output high. As seen in FIG. 6, the one output of flip-flop 33 is
connected to the reset inputs of counters 38, 39 and 41, and to the
clocking input CK of flip-flop 42, all in logic circuit 16. The
purpose of these last listed circuit elements is to help
microprocessor 9 analyze each received pulse of the pulse trains
from the master and secondary stations to accurately determine the
time of arrival of the third cycle positive zero crossing of each
pulse.
It can be seen that the clocking input CK to each of counters, 38,
39 and 41 is driven by a clock signal on lead CLK. The source of
this clocking signal is the 10 megahertz clock 45 in clock/counter
7 in FIG. 5. Flip-flop 33 being placed in its one state energizes
the reset input R of each of counters 38, 39 and 41, thereby
resetting these counters to zero and enabling these counters to
commence counting. As can be seen in FIG. 6, counter 38 is
designated a 30 microsecond counter. This means it counts and
provides a signal at its output TC 30 microseconds after this
counter is enabled to count. Similarly, counter 39 has an output
signal on output TC 2.5 microseconds after this counter is enabled
to count. Also, counter 41 has an output signal at output TC 12.5
microseconds after this counter is enabled to count. Thus, 2.5
microseconds after comparator 14 caused flip-flop 30 to be placed
in its set state, which thereby causes flip-flop 33 to be placed in
its set state, there is an output from counter 39 to the clocking
input CK of flip-flop 43 of logic circuit 16. The output TC of
counter 39 remains high until its reset input R is deenergized.
Similarly, 12.5 microseconds after counter 41 is enabled by
resetting there is an output therefrom to the clocking input CK of
flip-flop 44. Flip-flop 43 is a D type flip-flop which will store
whatever signal is present at its D input upon its clocking input
CK being energized. It should be noted that the D input of
flip-flop 43, as well as the D input of flip-flops 42 and 44 is
obtained from the output of exclusive OR gate 36 in zero-crossing
detector 6 in FIG. 5. The output of OR gate 36 is a square wave
pulse corresponding to each radio frequency cycle of each pulse of
the pulse trains received from the master and secondary LORAN-C
stations and also inverted to account for phase coding as
previously described.
Counter 39 will time out and cause the clocking input CK of
flip-flop 43 to go high at a point in time 32.5 microseconds before
the expected arrival of the third cycle positive zero crossing of
each pulse. It should be noted that this 32.5 microsecond point
occurs 2.5 microseconds before the first cycle of each pulse. At
that point in time only noise should be received by the LORAN-C
equipment and, more particularly, only noise of a frequency that
falls within the 10 kilohertz bandwidth of filter 1. Statistically
noise pulses applied to the D input of flip-flop 43 will occur as
often as they do not occur. Thus, counter 39 energizing clocking
input CK of flip-flop 43 will cause this flip-flop to store either
zero's or one's on a proportionally equal basis if the
microprocessor 9 has accurately determined the third cycle positive
zero crossing and the output signal from counter 39 does occur
prior to the beginning of each pulse. The Q output of flip-flop 43,
as well as the Q outputs of flip-flops 42 and 44, are coupled via
multiplexer 8 to microprocessor 9 as can be seen in FIGS. 6 and 7.
Microprocessor 9 receives and stores the output of flip-flop 43 for
a total of 2,000 samples and is programmed to average these samples
received from flip-flop 43. There will be approximately an equal
number of zero's and one's received therefrom if the input to the D
input of flip-flop 43 is received prior to any pulse of the pulse
trains from the master and secondary stations.
Counter 41 completes its count 12.5 microseconds after it is
enabled by the output signal from comparator 14 as previously
described. The output from counter 41 occurs 7.5 microseconds after
the beginning of the first cycle of each pulse of the pulse trains
if microprocessor 9 has accurately determined the position of the
third cycle positive zero crossing of each pulse. This point in
time will occur during the mid-point of the negative cycle of the
first radio frequency cycle of each pulse. Thus, the moment counter
41 energizes clocking input CK of flip-flop 44, the D input of this
flip-flop from exclusive OR gate 36 will be a zero. The result is
that the Q output of flip-flop 44 will also be a zero which will be
forwarded to microprocessor 9 via multiplexer 8 as previously
described. Microprocessor 9 also stores each output from flip-flop
44 for 10,000 samples, one per pulse, and is programmed to average
these samples to determine if they are predominantly zero
representing a negative half cycle.
In the event microprocessor 9 does not initially accurately
determine the location of the third cycle positive zero crossing of
each pulse of the pulse trains from the master and secondary
stations, and this will usually happen upon microprocessor 9
initially switching the LORAN-C equipment into its fine search
mode, the outputs from flip-flops 43 and 44 will not be as
described immediately hereinabove. When the estimated time is too
long, the sample points clocked into flip-flops 43 and 44 by
counters 39 and 41 respectively will both occur during each pulse
of the pulse trains. As a result, the averages made by
microprocessor 9 for flip-flops 43 and 44 will yield positive or
negative averages and will not yield a zero average. In response to
this condition, microprocessor 9 substracts 10 microseconds from
the estimated time of arrival and the sequence described above is
repeated. When the estimated time is too short the average of the
stored samples at the 2.5 microsecond and 12.5 microsecond points
will both be zero and microprocessor 9 will add ten microseconds to
the estimated time of arrival. This recalculation and repeat of the
circuit operation just described is repeated until the output from
flip-flop 43 yields a zero average to microprocessor 9 and the
output from flip-flop 44 yields a negative average. As
microprocessor 9 gets closer to the exact time of arrival, the
microprocessor can add or substract less than 10 microseconds to
the calculated time to determine the exact estimated time of
arrival figure.
Counter 38, which is also enabled to count upon receipt of the
output signal from comparator 14 via flip-flop 33, counts to time a
period of 30 microseconds at the end of which it provides an output
at its output TC. Output TC from counter 38 is connected to the
reset input R of flip-flop 37 in zero-crossing detector 6 and to
the reset input R of flip-flop 33. Flip-flop 37 is thereby placed
in its reset state with its one output low immediately prior to the
receipt of the third cycle positive zero crossing of each received
pulse of the pulse trains from the master and secondary stations of
the selected LORAN-C chain. The hard limited output from limiter 17
occurring immediately after flip-flop 37 is placed in its reset
state is responsive to the third cycle positive zero crossing of
each pulse. As a result, the one output of flip-flop 37 goes high
in direct correspondence with the leading edge of the hard limited
square wave pulse output from limiter 17 and corresponding to the
third cycle position zero crossing. As previously described, this
causes the count contents of BCD counter 25 to be clocked into
latch 5 and indicates the exact time of receipt of the third cycle
positive zero crossing of each pulse of the pulse trains. This
information is applied via multiplexer 8 to microprocessor 9 as
previously described for processing. In response to this
information, microprocessor 9 can make the desired time difference
of arrival measurements required in LORAN-C equipment. Upon the
time difference of arrival measurements being made, microprocessor
9 provides appropriate outputs on its DISPLAY outputs leads which
are input to display 12.
The signals output from microprocessor 9 to display 12 are applied
to the appropriate digital display units therein. Digital display
unit 51 is used to visually display the time difference of arrival
information for one selected secondary station, and digital display
52 is used to visually display the time difference of arrival
information for a second selected secondary station. The inputs of
these digital displays is encoded and is appropriately decoded by
anode drivers 46 and 47, anode driver 48 and decoder/drivers 40 and
50 to drive digital displays 51 and 52 respectively. These displays
along with their associated decoding and driving circuitry are well
known in the art and are commercially available. In this embodiment
of my invention, displays 51 and 52 are Itron FG612A1 flourescent
displays, but they may also be light emitting diode displays or
liquid crystal displays, or any other form of visual display.
To select the secondary stations, the time difference of arrival
measurements for which are to be displayed on displays 51 and 52,
thumbwheel switches 61 and 62 are provided. Switch 61 is physically
adjacent to display 51 and one of the numbers "1" to "4" are
selected with this switch to indicate to processor 9 the
information to be displayed. Similarly, thumbwheel switch 62 is
associated with display 52 and is used by the equipment operator to
indicate the particular secondary station arrival measurement to be
displayed on display 52. Switch 11 shows no details but is made up
of right individual switch such as represented by switch 61 in FIG.
7. The operator of a detented thumbwheel brings numbers into a
window and output terminals of the switch indicates the chosen
number.
A signal to noise button 62 is also located on the front panel of
the equipment which while depressed by the operator causes the
existing display on displays 51 and 52 to be replaced by a signal
to noise figure for the same secondary stations indicated by the
position of the corresponding ones of switches 61 and 62.
Microprocessor 9 is programmed to calculate the signal to noise
figures to be displayed and responds to the operation of button 62
to change the display on displays 51 and 52. To make this signal to
noise ratio check, microprocessor 9 stores fourteen-thousand
samples of the first negative half cycle of each pulse as indicated
by counter 41 described in detail hereinabove. As is easily
understood, pure noise would yield seven-thousand detected negative
half cycles and seven thousand positive half cycles, and a perfect
signal would yield fourteen thousand detected negative half cycles.
Accordingly, numbers between seven thousand and fourteen thousand
indicate the signal to noise ratio with this ratio getting higher
as the count of detected negative half cycles increases toward the
sample number of fourteen thousand. It is numbers between seven
thousand and fourteen thousand that will be displayed on displays
51 and 52 when signal/noise button 62 on the front panel is
operated.
It can readily be seen that microprocessor 9 can be programmed to
display numbers from 0 to 100 corresponding to the range of seven
thousand to fourteen thousand by using a simple interpolation
algorithm. Any other number scheme may also be used to indicate
signal to noise.
While that which has been described hereinabove is at present
considered to be the preferred embodiment of the invention, it is
illustrative only, and the rapid changes in technology will make
various changes and modifications obvious to those skilled in the
art without departing from the scope of the invention as claimed
below.
Thus, for example, programming may be added to the microprocessor
and the keyboard may be used or input and the display as output to
perform calculations of all kinds, or the display may, in addition,
be used to provide a digital clock with day, date and other
information. In another variation the microprocessor may provide
navigation instructions via the display.
* * * * *