Differential ECD detector for Loran C receiver
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to apparatus for processing Loran C signals and more particularly to apparatus for processing a received Loran C signal and compensating for envelope cycle discrepancy. 
     2. Description of the Prior Art 
     Loran C is a radio navigation system employed in the low frequency portion of the radio spectrum at a carrier frequency of 100 kilohertz. A typical Loran C chain of transmission stations includes a master station and at least two secondary stations each of which transmits a plurality of radio frequency pulses at pre-selected times relative to each other. The Loran C receiver operates by measuring the difference in time of arrival of the RF pulses transmitted by the transmitting stations. These time differences (TD) are the time measured between the arrival of the pulses from the master transmitter in each of the other secondary transmitters in the group. Each measured TD is a function of the receiver&#39;s distance from the master and the secondaries. To determine the location of the Loran C receiver, the TD between time of arrival of the first secondary station pulses and the master station pulses is determined and employed to plot a hyperbolic line of position on an appropriate map. The TD between the time of arrival of a second secondary station pulses and the master station pulses is determined and used to plot a second hyperbolic position intersecting with the above first mentioned line of position thus yielding the location of the Loran C receiver. 
     The signal transmitted from each station is a series of pulses of RF carrier at 100 kilohertz with a prescribed envelope, as shown in FIG. 1. The receiver locates and tracks pulses from each station by phase locking to one of the pulse&#39;s positive-going zero crossings, normally the third positive-going zero crossing designated as the pulse tracking reference (PTR). The TD&#39;s are typically obtained by gating on a counter at a time coincident with the arrival of the master station&#39;s tracked zero crossing and counting the number of time increments that occur until the counter is gated off at the arrival of the tracked zero crossing of the secondary. This method of measuring TD&#39;s requires that the same zero crossing is tracked on each station&#39;s pulses. If this were not true, the TD&#39;s measured would vary by increments of ten micro-seconds (one carrier cycle) each time it was necessary to acquire and track a station. The ambiguity would result in location errors of many miles, thus, the convention has been adapted to use the third positive-going zero crossing of the pulse as the PTR. 
     In normal operation, the Loran C receiver starts in a &#34;search&#34; mode, searching for and phase-locking to a zero crossing of the pulses transmitted by the master and the secondary stations. The Loran C receiver then enters the &#34;coarse settle&#34; mode, wherein a means is employed to jump the phase tracking point up toward the front of the pulse, such that it is known that the pulse tracking point is near the desired PTR. At this point, the receiver enters the &#34;fine settle&#34; mode, wherein the phase tracking point is jumped to the exact PTR. When fine settle has been completed on the master and at least two secondaries, TD measurement operations are started resulting in the output of location information. The critical operation is the &#34;fine settle&#34; mode. An error in tracking cycle selection in the fine settle mode can result in subsequent location errors of one or more miles. 
     Most Loran C receivers operate using pulses which have been hard limited to simplify the RF design and facilitate the interfacing of signal path through pulse sampling circuitry to digital circuitry. However, since the pulse envelope contains the information necessary to make a fine settle decision in the carrier cycle peak ratios, this information is lost in the hard limiting process. The first few cycles of a received Loran C pulse are shown in FIG. 2A, and the hard limited wave form as seen at the pulse sampling circuitry is shown in FIG. 2B. The limited waveform is binary in form and contains sufficient information for the receiver to carry out the search, coarse settle, and TD tracking functions. During the fine settle process, &#34;envelope deriver&#34; circuitry is switched into the RF signal path before the hard limiter for some of the received pulses. This circuitry adds a five microsecond delay and amplified version of the received pulse to the original pulse, resulting in a new pulse, (as shown in FIG. 3A) which contains a phase reversal at the PTR. This new waveform is hard limited producing a waveform such as that shown in FIG. 3B. The PTR can thus be determined by sampling the hard limited envelope derived pulses at 7.5 microseconds ahead and 2.5 microseconds behind the positive zero crossing times of the normal pulse near the one being tracked as a result of the coarse settle operation. When a change in state of the binary wave form is found along with a set of these leading (L) and trailing (T) samples, the PTR is the zero crossing point that lies between them. 
     In actual operation in a noise filled environment, statistical averaging of samples from many pulses is used for the fine settle operation. For example, five sampling strobes (see FS1 to FS5 in FIG. 3B) may be used on each pulse. They take samples of the derived pulse 17.5 and 7.5 microseconds ahead of and 2.5, 12.5, and 22.5 microseconds after the phase tracked zero crossing which was reached at the end of the coarse settle mode. The samples from each pulse are processed to increment or decrement the designated accumulator in a group of five corresponding to the samples FS1 through FS5. After samples from many pulses have been accumulated, a fine settle operation is started to examine each adjacent pair of sample accumulators as a potential leading or trailing phase reversal pair, starting with FS1 and FS2. When an adjacent pair of accumulators is found such that the leading 1 (designated L) is positive and greater than some predetermined positive fine settle threshold, and the trailing one (designated T) is negative and less than some predetermined negative fine settle threshold, the PTR is considered found and the phase tracking point is locked to the positive zero crossing that lies between the leading and trailing sample point. Once this has occurred, the envelope deriver circuitry is switched out of the RF signal path and the receiver now tracks the PTR of the hard limited pulse. If no adjacent pair of accumulators pass the fine settle threshold test, another set of fine settle samples is added to the existing accumulations and the detection operation is run again. The process is repeated until the PTR is found. 
     The fine settle thresholds are chosen to keep the probability of making an incorrect decision on the PTR to an acceptable level while maximizing the probability of making a correct decision in as short a time period as possible. A major factor which affects the PTR determination is a type of pulse distortion called Envelope Cycle Discrepancy (ECD). ECD is a differential phase delay to group delay distortion which manifests itself as a difference in arrival time between the RF pulse carrier and its envelope. Thus, although the pulse generated at a transmitter is the pulse shown in FIG. 1, the received pulse may have its carrier phase-shifted under the envelope. This alters the leading/trailing cycle peak ratio on either side of the PTR, distorting the information required for fine settle. The effect this has on a hard limited receiver is to decrease the signal level at the leading sample point for a negative ECD and to decrease the signal level at the trailing point for a positive ECD. Thus, a non-zero ECD degrades the fine settle performance by increasing both the PTR selection time and the percentage error. 
     It is well known that the average value of ECD of the pulses received from a particular transmitter is a function of the distance from that transmitter. It varies from a maximum of approximately a positive 2.8 microseconds within a hundred miles of a transmitter to about a negative 0.5 microseconds at a distance of 800 miles. In prior art systems using average values or emperically derived values, a fine settle threshold is chosen to meet the required error criteria (typical 1 in 10,000). This procedure is used in the design of marine Loran C receivers resulting in a receiver which meets required performance for the marine environment. While this fine settle method works well for a Loran C receiver operated in the marine environment, it does not perform well when a receiver is operated in the land mobile environment. This is due to the fact that in a marine environment, signal level and ECD variations are dominated by a dependence on distance from the receiver to transmitter. Over land, this is true only in an average sense. The proximity of the receiver to bridges, tunnels, buildings, and power lines, etc. can significantly affect received signal distortion. The total ECD observed at the receiver therefore contains a randomly varying component in addition to the distance dependent non-dynamic component. Thus, prior art systems, although adequate for marine environments, are unacceptable in a Loran C receiver which is to be used in a land mobile vehicle location system. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide a novel means of compensating for the effect of ECD cycle selection error. 
     It is another object of the invention to provide a novel Loran C receiver suitable for use in a land mobile environment. 
     It is yet another object of the invention to provide fine settle apparatus for a Loran C receiver which compares the average ECD for a station to a short term ECD to maximize accuracy and speed in the cycle selection operation. 
     Briefly, according to the invention, Loran C signal processing apparatus is provided for compensating for enveloping cycle discrepancy in a Loran C receiver including an accumulator for each of a plurality of predetermined sample points wherein a sample is taken and accumulated on envelope derived Loran C pulses at each of the sample points. In addition, the accumulators are scanned to make a fine settle determination to identify a leading accumulator (L) and a trailing accumulator (T). The improvement of the invention comprises circuitry for calculating a measure (M) of envelope cycle discrepancy determined by M=(L+T)/(L-T) in response to a fine settle determination, and circuitry for generating an ECD reference, E. In addition, circuitry is provided to compare the reference E to the M calculated for the most recent fine settle determination to produce a control signal if the difference is greater than a predetermined threshold, and circuitry to reject the fine settle determination and initiate a new fine settle determination in response to the control signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention itself, together with further objects, features and advantages thereof, may best be understood by reference to the following description when taken in conjunction with the accompanying drawings. 
     FIG. 1 is an amplitude vs. time representation of a typical Loran C pulse. 
     FIG. 2a is an amplitude vs. time representation of the first few cycles of a typical Loran C pulse. 
     FIG. 2b is an amplitude vs. time representation of a hard limited Loran C pulse. 
     FIG. 3a is an amplitude vs. time representation of the phase reversal portion of an envelope derived Loran C pulse. 
     FIG. 3b is an amplitude vs. time representation of a hard limited envelope derived Loran C pulse. 
     FIG. 4 is a block diagram of Loran C receiver apparatus according to the invention. 
     FIG. 5 is a program flow diagram for the ECD detector routine of the computer program for the invention. 
     FIG. 6 is a program flow diagram for the reference routine of the computer program for the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 4 shows Loran C signal processing apparatus 30 in accordance with the present invention. The processing apparatus 30 includes RF selectivity in gain stage 40 coupled to an antenna 42 for receiving and amplifying Loran C signals. The received signals a are coupled, as shown, to a summing circuit 48 and an envelope deriver circuit 44 which generates envelope derived Loran C signals. Envelope deriver circuits are well known to those skilled in the art. As already discussed, such envelope deriver circuits as circuit 44 inject a phase reversal at the location in the RF envelope of the Loran C pulses corresponding to the pulse tracking reference (PTR) of such pulses. The envelope deriver 44 is coupled, as shown, to a switch 46 for selectively switching the envelope deriver 44 into the signal path, thereby coupling the envelope deriver 44 to the summing circuit 48. When the envelope deriver 44 is coupled to the summing circuit 48, the output thereof is a waveform such as that shown in FIG. 3a. The switch 46 is controlled by a multiplex control signal provided via the conductor 53 from the microprocessor logic 54, as shown. 
     The output of the summing circuit 48 is coupled, as shown, to a hard limiter 50 which hard limits the Loran C pulses, thereby generating a signal such as that shown in FIG. 2b, and hard limiting envelope derived pulses when the envelope deriver 44 is switched into the circuit, thus generating signals such as shown in FIG. 3b. The output of the hard limiter 50 is coupled to the input of the sampling circuit 52, as shown. Sampling circuit 52 samples the hard limited Loran C pulses and hard limited envelope derived Loran C pulses at times determined by sampling strobes coupled to the sampling circuit 52 via the conductor 55 from the microprocessor logic 54, as shown. (A more detailed description is disclosed in an application filed by Robert V. Janc on Jan. 5, 1981, bearing Ser. No. 222,458 and assigned to Motorola, Inc.) The samples are coupled to microprocessor logic 54 which processes the samples to perform coarse settle and fine settle operations as previously described. A random access memory 56, is coupled, as shown, to the microprocessor logic 54 via an address/data bus 58 to provide storage for the microprocessor logic 54. The microprocessor logic 54 utilizes a microprocessor such as a Motorola MC6808. 
     The invention is an additional process amended to previously described fine settle operations relying on the fact that the average ECD doesn&#39;t change quickly with time for a moving vehicle. Thus, the invention involves determining a measure of ECD during the fine settle operation and comparing it to an average ECD for the Loran C station involved. If the difference is greater than a predetermined threshold, the decision made by the fine settle operation is rejected and the fine settle process is re-started. If the difference is less than the predetermined threshold, the fine settle decision is accepted as being the true PTR. The threshold can not be made arbitrarily small because many correct decisions would then be thrown out due to noise and phase tracking errors, thereby unduly increasing the average amount of time to fine settle. The preferred measure of ECD is (L+T)/(L-T) which has been found to have maximum insensitivity to signal to noise ratio variations and cross rate interference. However, it should be noted that other measures of ECD may be used (e.g., L/T, L+T, etc.). 
     Referring to FIG. 5, there is shown a flow diagram of the ECD detector routine. The ECD detector routine is entered at block 60 from the main program after a fine settle operation has been completed wherein a tentative PTR has been located such that the leading accumulator L and the trailing accumulator T has been identified as described hereinbefore. Program flow proceeds directly to block 62 where an ECD value M is calculated using the formula (L+T)/(L-T). The program flow then continues to block 64 to test for the existence of an ECD reference. If none exists, indicating that this is the first fine settle operation, program flow proceeds to block 78 where the fine settle decision is accepted. The fail count is then set to zero, as shown, and program control is returned to the main program where the newly determined PTR is tracked. However, if at block 64 the result was positive, program flow continues to block 66 where the reference E (to be discussed in detail hereinafter) is compared to the ECD measure M to determine if the difference is less than the predetermined threshold (e.g., in the preferred embodiment approximately 1.5 microseconds). If the difference is less than the threshold, program flow proceeds to block 78 where the fine settle decision is accepted, and as before program control is returned to the tracking operation. If the difference at block 66 is greater than the predetermined threshold, program flow proceeds to block 68 to test if the fail count is equal to zero to determine whether this is the first failure of the reference test. If this is not the first reference test failure (i.e., if the fail count is not equal to zero) program flow proceeds directly to block 74 where the fail count is incremented. If the fail count is equal to zero, the program flow proceeds to block 70 to test whether the vehicle is moving. If the vehicle is not moving, the program flow proceeds to block 72 rejecting the fine settle decision, clearing the accumulators to restart the fine settle operation, and then returning program control to the fine settle operation as indicated at block 73. If the vehicle is moving, the program flow proceeds from block 70 to block 74, as shown, where the fail count is incremented and then to block 76. At block 76 a test is performed to determine if the fail count equals 8, and if so, program flow proceeds to block 78, where the fine settle decision is accepted, the fail count is set to zero, and the program control is returned to the main program as indicated at block 79. If at block 76 the fail count does not equal 8, the program flow proceeds to block 72 to reject the fine settle decision and restart the fine settle operation by clearing the accumulators and returning the program control to the main program as indicated at block 73. In this way, the fine settle decision can be accepted without passing the reference test if there have been eight consecutive reference test failures, after a reference test failure occurs when the vehicle is moving. 
     FIG. 6 is a program flow diagram of the reference routine of the computer program for the invention. The reference routine is entered, as shown, at block 80 from the main program after having completed a successful fine settle operation and after a predetermined number of Loran C pulses has been tracked. The L and T accumulations for this routine are accumulated in the tracking mode and thus cannot exist before the first successful fine settle decision. During tracking, the first seven of the eight Loran C pulses in a group are sampled normally in a PLL tracking loop. However, the eighth pulse is envelope derived and sampled as a pulse would be during fine settle selection. In the preferred embodiment, when 6300 of these samples have been accumulated to form an L accumulation and a T accumulation, then the reference routine is entered. These L and T accumulations, made over an extended period of time are used in the reference routine to establish the ECD reference, E. The program flow starting from block 80 proceeds directly to block 82 to calculate the ECD measure M utilizing the formula (L+T)/(L-T). Program flow proceeds from block 82 to block 84 to test if an ECD reference has previously been determined. If no reference existed, the reference E is set equal to the ECD measure M as shown at block 86, and then program flow proceeds to block 100, as shown. If the ECD reference did exist at block 84, program flow proceeds to test block 88 to determine if the old reference E is within a predetermined threshold of the new measure M. (The threshold in the preferred embodiment is approximately 1.5 microseconds). If the difference at block 88 is less than the threshold, the program flow proceeds to block 90 to calculate a new reference E utilizing the formula (E+M)/2, and then to clear the temporary value M1. Program flow then proceeds to block 100, as shown. However, if the result at block 88 was negative, program flow proceeds to block 92 to test to determine if there is a value M1, and if not, M1 is set equal to M as indicated at block 96. If the result at block 92 is positive, program flow proceeds to block 94 to test to determine if the difference between M1 and M is less than the predetermined threshold, and if so, the reference E is set equal to M as indicated at block 98. If not, the program flow proceeds to block 96 where M1 is set equal to M and then to block 100 where the accumulations of L and T are cleared to start a new accumulation. Program flow is then returned to the main program to continue tracking the PTR and accumulate a new set of L and T accumulations. 
     In summary, a novel differential ECD detector for a Loran C receiver has been described which is well adapted for a land mobile automatic vehicle location system. The invention minimizes false fine settle tracking decisions, decreasing PTR selection time and error rates. 
     While a preferred embodiment of the invention has been described in detail it should be apparent that many modifications and variations thereto are possible, all of which fall within the true spirit and scope of the invention. 
     APPENDIX I 
     The following is an Hex memory dump of the computer program for the invention utilizing a MC6808 microcomputer. ##SPC1##