Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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 United States 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 trans-
mitting chains in operation. This new navigational system will
result in an eventual phase-out of the earlier LORAN-A naviga-
tional system.
LORAN-C is a pulsed low-frequency (100 kilohertz), hyper-
bolic radio navigation system. LORAN-C radio navigation systems
employ three or more synchronized ground stations that each
t~ansmit radio pulse chains having, at their respective start of
tranmissions, a fixed time relation to each other. The first
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station to transmit is referred to as the master station while
the other stations are referred to as the secondary stations.
The pulse chains are radiated to receiving equip~ent that is
generally located on aircraft or ships whose positions is to be
accurately determined. The pulse chains transmitted by each of
the master and secondary stations is a series of pulses wherein
each pulse has an exact envelope shape, each pulse chain is
transmitted at a constant precise repetition rate, and each
pulse is separated in time from a subsequent pulse by a precise
fixed time interval. In addition, the secondary station pulse
chain 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 from the ~.aster 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
e~uipment.
The focus of all points on a LO~AN-C chart representing a
constant difference in distant from a master and a secondary
station, and indicated by a fixed time difference of arrival of
their 100 kilohertz carrier pulse chains, describes a hyperbola.
The LORAN-C navigation system makes it possible Lor a navigator
to exploit this hyperbolic relationship and precisely determine
his position using a LORAN-C chart. By using a moderately low
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frequency such as 100 kilohertz, which is characterized by low
attentuation, and by measuring the time difference between the
reception of the signals from master and secondary stations, the
modern day LORAN-C system provides equipment position location
accurate 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 Radio 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, United States Coast Guard, Number
CG-462, dated August, 1974, and entitled "LORAN-C User Hand-
book."
The LORAN-C system of the type described in the aforemen-
tioned 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 previously mentioned.
All secondary stations radiate pulse chains 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 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
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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
corrresponding point on the envelope of a preceding or
subsequent pulse within the eight pulse chains. 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. 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.
This requires that oscillators internal to the LORAN-C receiver
be calibrated frequently in order to avoid measurement errors
caused by oscillator inaccuracy.
The signals presently received by LO~AN-C navigation
receivers have very low signals to noise ratios and it is
difficult to locate the third cycle positive zero crossing
conventionally used in making the time difference measurements
between signals received from the master and secondary stations.
This problem is exacerbated by noise generated within the
circuitry of LORAN-C navigation receivers and particularly in
the receiver front end circuitry in the signal path immediately
following the receiver antenna.
Thus, there is a need in the art for improved circuitry and
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techniques to minimize the noise generated internal to LORAN-C
receivers. It is a feature of this invention to minimize the
effect of noise generated internally to a receiver by averaging
out the noise.
There is also a need in the art for inexpensive oscillators
within LORAN-C receivers that never require calibration yet the
operation of the receivers is as if the oscillators are as
accurate as a laboratory standard oscillator. Such oscillators
increase the accuracy and reliability of navigation information
output from the receiver.
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,
minimizing the effec~s of noise generated to the receiver and
requiring no calibration of the receiver oscillator/clock.
Four thumbwheel switches on my LORAN-C equipment are used by
the operator to enter the group repetition interval information
for a selected LORAN-C chain covering the area within which the
LORAN-C equipment is being operated. This information entered
via the thumbwheel switches is used in the process of locating
the signals from the master and secondary stations of the chosen
LORAN-C chain and providing an output.
The receiver of my equipment receives all signals that
appear within a small bandwidth centered upon the 100 KHz
operating frequency of the LO~AN-C network. A shift register
clocked at 100 KHz is coupled with logic circuitry continuously
check all received signals to search for the unique pulse trains
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transmitted by LORAN-C master and secondary stations. The
microprocessor and other circuits internal to my novel LORAN-C
equipment analyze outputs from the register and associated logic
circuitry indicating that signals from master or secondary
stations have been received to first determine which received
signals match the group repetition interval rate for the
selected LORAN-C chain. Once the receiver has identified the
pulse trains from the selected master station and can predict
future receipt of same, the microprocessor causes other
circuitry to go into a fine search mode.
In the fine search mode the microprocessor enables a phase-
lock-loop made up of a computer program and other circuitry,
including a cycle detector, to analyze and locate the third
cycle positive zero crossing point of each received master
station pulse. In the event the third cycle positive zero
crossing of each master station pulse is not located at the time
calculated by the microprocessor, the cycle detector provides
outputs used by the microprocessor to determine whether
multiples of 10 microseconds should be added to or subtracted
from the calculated time. The microprocessor then repeats the
fine search mode analyzation process. This analyzation process
and revision of the calculated time is repeated using feedback
from the cycle detector until the third cycle positive zero
crossing of each pulse of the master station pulse train is
located.
Once the third cycle positive zero crossing of each pulse
from the master transmitting station of the selected LORAN-C
chain is located, the receiver operates to locate the associated
secondary stations. The microprocessor creates a small number
of time bins between the arrival of each pulse train from the
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master station and creates a coarse histogram by putting a count
in an appropriate bin when a secondary station signal is
detected. Once particular bins are found to contain counts
representing receipt of signals from secondary stations, the
microprocessor breaks those particular bins down into a large
number of time bins creating a fine histogram to more closel~
determine the time of signal arrival of secondary station
signals. The cycle detector is then utilized in conjunction
with the microprocessor in a phase-lock-loop mode to identify
the third cycle positive zero crossing of each received pulse
from a secondary station.
The microprocessor then makes accurate time difference of
arrival measurements between the time of arrival of signals from
the master station and the secondary stations. The equipment
operator utilizes other thumbwheel switches to indicate
secondary stations, the time difference of signal arrival
information which is to be visually displayed. The operator of
the LORAN-C equipment plots these visual read-outs on a LORAN-C
hydrographic chart to locate the physical position of the
LORAN~C receiver on the surface of the earth.
My novel LORAN-C navigation receiver need never have its
internal oscillator calibrated unlike prior art receivers. The
microprocessor, having the GRI (Group Repetition Interval) input
thereto by the receiver operator, knows how many cycles of the
internal oscillator must occur within the cesium clock standard
GRI between two consecutive received master station pulse
trains. Any error is noted and interpolated over the GRI period
and correction factors are added or subtracted t~ internal
circuit clock counts of interest to thereby achieve highly
accurate time difference of signal arrival measurements.
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The LO~AN-C receiver circuitry keeps track of the receipt of
master and secondary transmitter station signals each periodi-
cally transmitted at the Group Repetition Interval (GRI) and
periodically causes the phase of the received signals to be
inverted or shifted 180. This is done at the front end of the
receiver immediately after the antenna with preamplifier and 100
KHz input filter. It is the front end of receivers where
troublesome internal noise is generated. These periodic phase
reversals cause the internally generated noise to average out
and thus it does not introduce a bias level to the received
signals, which bias level causes erroneous time difference of
signal arrival measurements to be made by the receiver. The
periodic phase reversals are removed or compensated for further
in the signal path through the receiver before the time period
of the 180~ phase shift interferes with the time difference of
signal arrival measurements standardly made in LORAN-C
receivers.
The Applicant's novel LORAN-C navigation receiver will be
better understood upon a review of the description given
hereinafter in conjunction with the drawing in which:
Brief Description of the Drawing
Figure 1 is a general block diagram of the Applicant's
LORAN-C navigation receiver.
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Descr~_ion
In Figure 1 is seen a block diagram of my LORAN-C navigation
equipment utilizing my novel noise removal circuit. 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 via inverting
amplifier 81 to cycle detector 82 and to zero crossing detector
10 6.
The signal input to zero crossing detector 6 is first
amplitude limited so that each 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 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 Figure 1, which
20 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 is provided by detector 6.
It can be seen that latch 5 has its input from zero crossing
detector 6. Clock/counter 7 is a crystal controlled clock which
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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, logic circuit 4, cycle detector 82, latch 5, clock/counter
7, and thumbwheel switches 11 and 12, through to microprocessor
9. The count in latch 5 indicates to microprocessor 9 the time
at which each positive zero crossing is detected.
The signal input to smart shift register 3 from detector 6
is a pulse train of l's and 0's which is shifted through the
shift register digital delay line which is tapped at 1 milli-
second intervals. Because of the logic circuits connected to
each tap thereof, only the pulse trains from LORAN-C master and
secondary stations will result in outputs from the logic
circuits of register 3. The logic circuits within register 3
are used to analyze the contents of the shift register delay
line to first determine if the signals represent a pulse train
from a LORAN-C master or secondary station, and secondly, to
indicate the particular phase coding of the signals being
received. Logic circuit 4 stores information from register 3
indicating whether a pulse train is from a master or secondary
station and further indicating the particular phase code
transmitted. This information stored within logic circuit 4 is
applied to microprocessor 9 via multiplexer 8 for use in
processing received LORAN-C signals. At the same time that
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information is s~ored within logic circuit 4, detector 6 causes
latch 5 to store the present count in clock/counter 7 which
indicates 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 continuous running time as
indicated by recycles of counter 7.
Thumbwheel switches 11 are used to input the GRI of a
selected LORAN-C chain to the receiver. The output of
thumbwheel switches 11 are also input to multiplexer 8 to apply
the GRI of the selected LORAN-C chain to microprocessor 9.
With the various types of information being input to
microprocessor 9 via multiplexer 8 from the circuits previously
described, microprocessor 9 determines when received signals are
from the master and secondary stations of the selected LORAN-C
chain. Once microprocessor 9 closely locates the signals from
the selected master station, as determined by a match of the GRI
number input thereto via thumbwheel switches 11 with the
difference in time of receiving each pulse train transmitted by
the master station of the selected chain, the receiver goes into
a fine search mode utilizing a phase-lock-loop implemented with
a computer program in microprocessor 9 and the loop being closed
by an input from cycle detector 82 to locate the desired radio
frequency carrier third cycle positive zero crossing in
conjunction with zero crossing detector 6. The receiver then
switches to locate the secondary station signals of the selected
chain. To locate the secondary stations, microprocessor 9 first
creates a coarse histogram and then a fine histogram by storing
the time of receiving all secondary station signals in time slot
bins created by the microprocessor in its own memory between the
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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 coarse histogram time slot bins at the same rate as the
GRI of the selected LORAN-C chain, the microprocessor 9 creates
a fine histogram having time slot bins of shorter time duration.
In this manner, microprocessor 9 closely determines the time of
arrival of pulse trains from the secondary stations of the
selected LOXAN-C chain.
Once microprocessor 9 closely determines the time of
receiving secondary station signals and can calculate the time
of receipt of subsequently received secondary station pulse
trains, the microprocessor causes the receiver to go into a fine
search mode utilizing the same phase-locked-loop arrangement
generally described above to accurately locate the third cycle
positive zero crossing of each pulse of the secondary station
pulse trains.
Once microprocessor 9 functioning with the other circuits in
my LO~N-C receiver has located and locked onto the pulse trains
being trallsmitted by the master and secondary stations of the
selected LORAN-C chain, it makes the desired time difference of
arrival measurements that are required in LORAN-C operation.
Microprocessor 9 then causes a visual indication to be given via
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display 12. The output information is plotted on a LOR~-C
hydrographic chart in a well-known manner to locate the physical
position of the LORAN-C receiver.
There are lamps on the front panel display 12 of the
receiver which initially all flash on and off when the receiver
is first turned on. As the signals of the master and each
secondary station of the selected LORAN-C chain are located and
it is determined by microprocessor 9 that each station's signals
can be utilized to make accurate time difference of signal
arrival measurements, the lamp associated with that station is
changed to be lit steady. This gives an indication to the
receiver operator of the confidence he may have in selecting
stations with switches 11 to make time difference of signal
arrival measurements.
The oscillator internal to the LORAN-C receiver never needs
calibration, unlike prior art receivers. Microprocessor 9 knows
exactly the time difference of signal arrival of the pulse
trains from the master station of the selected chain because of
the GRI input thereto via switches 11. This information is
compared with the output of a master oscillator within the
receiver to determine the frequency error of the oscillator.
Microprocessor 9 then interpolates the error over the time
period between receipt of signals from the master station and a
correction factor is added or subtracted to internal clock
indications of time of rec~ipt of all pulses from the master and
secondary stations to thereafter make accurate time difference
of signal arrival measurements.
In accordance with the teaching of my invention a phase
shifting function is accomplished within the receiver to average
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out internally generated noise within the front end circuitry of
the receiver, which noise normally creates a bias level which
seriously affects the ability to locate the third cycle positive
zero crossing of each pulse. After the receipt of two master
station pulse trains the phase of all signals is periodically
inverted within the receiver to average out the noise.
Smart shift register 3 provides an output signal each time
signals from a master or secondary station are detected, which
output signal is applied via logic circuit 4 and multiplexer 8
to microprocessor 9. Microprocessor 9 determines which master
and secondary station signals are from the selected LORAN-C
station chain and provides a signal to logic circuit 16 each
time the selected chain master station signal is detected.
Logic circuit 16 counts these signals from microprocessor 9 and
applies a signal to the control input of inverting input of
amplifier 81. This causes all ~eceived signals to undergo a 180
degree phase shift every other time the signal from the selected
master station is detected. The effect of this periodically
alternating phase shift is removed at zero crossing detector ~
where internally generated noise is no longer a problem. Each
time the inverting signal applied to inverting amplifier 81
changes state another inverting signal applied to zero crossing
detector 6 also changes state to remove the effects of the phase
inversion introduced at amplifier 81.
