CA1141845A - Method and apparatus for removing noise in a loran-c navigation receiver - Google Patents

Abstract

D-? ?B
JEF:eo A METHOD AND APPARATUS FOR REMOVING NOISE
IN A LORAN-C NAVIGATION RECEIVER

Abstract Noise generated in the front end of a LORAN-C receiver that causes noise bias interfering with carrier cycle detection is effectively removed by periodically shifting the phase of all received signals by 180° in the front end of the receiver and then removing any phase shift elsewhere in the receiver where noise is no longer a problem but before the phase shift can affect time difference of signal arrival measurements normally made in LORAN-C receivers. In this manner, noise generated in the front end of the receiver is averaged out and doesn't introduce bias levels to the received signal.

Description

ll~}i845 D-3t JEF:eo 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 D-36~B 1141~45 JEF:eo 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 D-36~B
JEF:~o 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 ~4~845 D-36~5B
JEF:eo 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 D-36' , 1141845 JEF:eo 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 D-36 i 11 4 1 8 4 5 JEF:eo 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 JEF.eo 11418~

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.

JEF36e~ 1141845 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.

D-36 `~ il 418~

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 D-3~ ~ il41845 JEF:eo 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 D-3~ ,B 1~41~
JEF:eo 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 ~ 3655B ~1~1845 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 .~

~1845 JEF:eo 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 D-3~ 3 JEF:eo 1~41845 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.

Claims (4)

JEF:eo The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for removing the effects of noise generated internal to a navigation receiver-indicator that provides navigation information comprising:
means for periodically generating a first signal, first means responsive to said first signal for periodically inverting the phase of said received navigation signals in the front end of said receiver to cause noise generated in said front end of said receiver to average out and thereby be effectively removed, and second means responsive to said first signal for reinverting the phase of said navigation signals to remove the phase inversion introduced by said first means, said phase reinversion of said navigation signals being accomplished at a point within said receiver whereat internally generated noise is not a significant problem and before phase reinversion introduces problems deriving said navigation information.

2. The apparatus in accordance with claim 1 wherein said first signal generating means comprises counter means incremented for each of said period navigation signals and divider means responsive to the count in said counter means to periodically generate said first signal.

3. The apparatus in accordance with claim 2 wherein said first means comprises an amplifier that normally passes navigation signals input thereto through without phase inversion but in response to said first signal inverts the phase of said navigation signals passing therethrough.

4. The apparatus in accordance with claim 3 wherein said second means comprises an exclusive OR logic gate to a first input of which is applied said navigation signals and to a second input of which is applied said first signal, said logic gate inverting the phase of said navigation signals applied thereto upon said first signal being applied to said second inputs.