Vehicle location systems
United States Patent Haemmig et al.  
 Apr. 8, 1975 l l VEHICLE LOCATION SYSTEMS Primary Examiner-Malcolm A. Morrison [75] inventors: Adrian Haemmig, Silverado; James Asslsm&#39;lf jF $5 O] Gibson, Irvinew both of Calif. Attorney, Agent. or 1rm no 6, artens. son,  
  Hubbard &amp; Bear [73] Assignee: Products of Information Technology,  
  Inc., Costa Mesa. Calif. [57] ABSTRACT [22] Filed: 1973 A vehicle location system having a plurality of station- 211 App| 413 040 ary wayside stations positioned at predetermined geographical locations. The passage of a vehicle near a station references the location of the vehicle at that [52] US. Cl. 340/l46.l BA; 325/51; 325/53; point in time and automatically Supplies the remote 340/23 headquarters with information as to the whereabouts [51] Int. Cl 608g 1/12 of all vehicles Operating in the system Efficient uti|i [58] held of Search 340/l46-l l46-l zation of the communication link between the vehicle 340/22 24; 325/5L 64 and the remote headquarters is enabled by transmitting only updated location information to headquarl56l References C&#39;ted tersxThe system enables low power radio frequency UNITED STATES PATENTS signals to automatically communicate error free loca- 2.740106 3/l956 Phelps 340/l46.l BA information between the Wayside Station and the 3.252.138 5/1966 Young 340/1461 BA vehicle by repetitively transmitting identical digitally 3.644.883 2/1972 Borman et al. 340/23 coded messages and inhibiting utilization of the infor- 3.662.267 5/1972 Reed 335/51 mation contained in such message until receipt in suc- 3 69794l lO/l972 Chrlst a 340/23 cession of two location messages identical in code fun 3.732.541 5/1973 Neubaucr..... 340/14e1 BA mm 3.757.290 9/l973 Ross et al. 340/23 6 Claims, 11 Drawing Figures [454/626- lean/2 /75 MHz) W4 K6705 A92746L A 064 T/O/l/ VEAl/CZ 5 #54000475? 574770/V 7fl/V5M/77&#39;5 114555465 --1 2565/1 66 2565/1 67? TFfi/Vfi&#39;M/WEQ Vff/F/E? //50 MM!) 7&#39;I4/V6&#39;M/7&#39;7&#39;EE /75 me) /75M//z) /50 m/z/ l l k VE/l/CLE M55546! Tenn/541F758 57x5 050 ,w/z/  
  /6 4L /6/F/ l f 44 2 (6750420 rem/su/rriz V///(L p/spmy (04 7201. nan/2w mama/v a 10* AUG/C s&#39;rim-ra/s&#39;flm/ VEHICLE LOCATION SYSTEMS BACKGROUND OF THE INVENTION In the copending application of Adrian B. Haemmig. entitled Vehicle Location System,&#34; Ser. No. 413,039, filed Nov. 5, 1973, and assigned to Products of Information Technology, Inc., assignee of the present invention, is disclosed and claimed a practical system for referencing the location of vehicles that respect to a plurality of stationary wayside stations. This invention relates to certain improvements .in the basic system dis closed and claimed in this copending application.  
 SUMMARY OF THE INVENTION One improvement provided by the present invention is efficient utilization of the radio frequency channel connecting the vehicle to headquarters. Vehicle locating systems constructed in accordance with this invention are adapted to automatically transmit only updated vehicle location messages, i.e., a verified location message will be transmitted only if it is different than the message previously sent. Thus, although the patrol car may remain in the vicinity of a wayside station and receive numerous repeated messages identical in code format, only one data message containing this geographical location will be transmitted to headquarters.  
  Another advantage of vehicle locating systems constructed in accordance with this invention is an improved error prevention system. In its preferred embodiment. the present invention utilizes a pluality of stationary wayside radio transmissions each repetitively transmitting a lower power digitally coded message indicative of the geographic location thereof. Each vehicle carries a radio receiver for receiving this coded signal when the vehicle is in the proximity of the transmitter. Each such location message is compared with a temporarily stored, previously received location message. Only when the comparison is positive and the two successive location messages found to be identical is a coded message sent from the vehicle to the remote headquarters including this location information. In each instance that the comparison is negative, the stored location message is discarded, and the temporarily storage filled with the just received location message. The comparison is then repeated upon receipt of the next received location message. It has been found that when the system of this invention is used in combination with the message verifier system described and claimed in the copending application identified hereinabove, that the possibility of an error in the location in formation transmitted to the remote headquarters is so remote that the overall vehicle location system can be considered to be essentially error free.  
  Advantages of the error prevention systems constructed in accordance with this invention over prior art error detection systems employing a comparison technique are simplicity and reliability. For example, in the present invention, no variation in spacing between successive location messages is required. In addition, the absolute minimum number of identical. messages need be received in order for a positive comparison to be made.  
  Systems constructed in accordance with the present invention not only serve the needs in an urban environment, such as police, fire, cab service and delivery service, but also because of their reliability and relative low cost permit application ofa vehicle location system to new and different environments. One such environment, for example. is a large open pit mine in which the system of this invention automatically supplies updated information as to the whereabouts and status of the ore hauling. trucks to remote headquarters.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of the system of the in vention used as a police patrol car location system, particularly illustrating the relationship between the wayside stations, vehicles, portable transmitters and headquarters;  
  FIG. 2a illustrates graphically the pulse waveforms of a binary one and a binary zero;  
  FIG. 2b is a graphical representation of the digitally encoded waveform generated at a wayside station;  
  FIG. 3 is a simplified block diagram of the exemplary embodiment of the vehicle location system, particularly illustrating the communication paths connecting the wayside stations, vehicles, emergency transmitters and headquarters;  
  FIG. 4 is a detailed block diagram of the wayside station, emergency transmitter and message verifier portions of a vehicle location system including improvements constructed in accordance with this invention:  
  FIG. 5a illustrates graphically the pulse waveform of an emergency call from the emergency transmitter to the patrol car;  
  FIG. 5b, 5c and 5d illustrate waveforms within the portion of the system of FIG. 4 for decoding an emergency call;  
  FIG. 6 is a detailed block diagram of the vehicle message transmission portion of a vehicle location system including improvements constructed in accordance with this invention; and  
  FIG. 7 is a detailed block diagram of the vehicle message receiving portion of the vehicle location system.  
 OVERALL DESCRIPTION OF PATROL CAR LOCATION SYSTEM Referring to FIG. 1, a plurality of stationary wayside stations 10a, 10b, 10c, 10d and 10e strategically located at predetermined geographical locations. In the specific embodiment described herein of a police partrol car location system, these wayside stations are conveniently mounted to structures already existing in a municipality such as street lamps 11 and stop and go lights 12. As a patrol car 13a, 13b, or 13d is driven past a wayside station, the location of the vehicle is referenced at that point in time by virtue of receiving a low power digitally encoded message from the wayside station. The encoded message corresponds to the geographical location of the wayside station. Advantageously as shown, the encoded location message received by the vehicle is in turn transmitted from the vehicle to a remote headquarters 14 where the message is decoded and presented on a visual display 15. The positions of all patrol cars in use are conveniently displayed on a map 16 of the environment in which the system is installed. I  
  In addition, the system provides for a portable transmitter 20a, 20b small enough to be conveniently carried on an individual patrolman when he leaves the patrol car. By merely depressing an activator button on the unit, the patrolman is able to transmit to a patrol car such as car 13d and thence over a high power radio frequency link to the remote headquarters 14 a precoded emergency message and last known vehicle location and thus inform headquarters that an emergency condition exists.  
  The opertion of the system of FIG. 1 depends upon limiting the communication paths between the wayside stations and the vehicles 13 so that a vehicle 13 receives a particular encoded location signal only when it is in the proximity of the station sending the signal. A convenient manner for achieving this is to provide each of the wayside stations with a very low power radio transmitter so that the effective radius of trans mission is limited to a predetermined distance, for example, some 200 feet, represented by numerals 21, 22 and 23. Advantageously, this transmitted low power level at each wayside station may be adjusted to conform to the particular location of the wayside station. Thus, the transmitted power levels of stations 10a and 10b whose respective signals 21, 22 must be received by vehicles traveling along a pair of intersecting streets should be higher than station 10d whose signal 23 need only blanket the street area immediately adjacent this wayside station.  
  Such low power transmission as is provided by the wayside station 10 ordinarily produces serious communication problems since a driven vehicle 13 is constantly entering and leaving a fringe area of reception at the outer periphery of the radio transmitter zones, e.g.. 21, 22 or 23; during which time noise may produce significant error signals. A significant feature of this invention is that the message verifier system located within each vehicle 13 requires receipt of two identical location messages in succession so as to avoid transmitting erroneous location messages to the headquarter display 15.  
  As shown in FIG. 3, the wayside station 10 includes a low power 75 MHz radio transmitter coupled to an antenna 25. A vehicle within receiving range of this antenna receives a digitally encoded location message transmitted from the wayside station 10 on vehicle mounted antenna 26 coupled to a 75 MHz receiver 27. The validity of each received location message is determined in the location message verifier 28 and a valid location is retained in message store 29. Message store 29 also retains any status communication selected by the patrolman on the keyboard-display 30.  
  The portable transmitter advantageously operates on the same 75 MHz radio channel to provide on its antenna 35 a signal received on the vehicle antenna 26. The precoded message provided by the portable transmitter is verified by the emergency signal verify 36 which automatically actuates the keyboard-display 30.  
  Messages encoding the vehicle location, vehicle number and status are automatically transmitted from the vehicle 13 to headquarters 14 over another communications link which in the exemplary embodiment is a 150 MHz radio channel. These messages are automatically transmitted whenever: (i) an updated location message is contained in message store 29, (ii) the patrolman activates the portable transmitter 20, or (iii) the patrolman actuates his keyboard-display unit within the vehicle. When any one of these events occurs, a signal is supplied from either the location message verifier 28 or the keyboard-display 30 to the transmitter control logic 37. This logic is also responsive to the presence of another 150 MHZ signal on vehicle mounted antenna 38 and received by vehicle mounted receiver 39 so that the 150 MHz vehicle transmitter 40 is keyed-on only when the 150 MHZ channel is clear. The data stored in the message store 29 is then transmitted from antenna 41 to headquarters 14 on the 150 MHz channel. While transmitter 40 is keyed-on, a signal on lead 46 turns off vehicle receiver 39.  
  Communications from the headquarters 14 to vehi cles 13 are provided via headquarters I50 MHZ receiver-transmitter 42 which transmits over antenna 43 a signal received on the antenna 38 of all patrol vehicles. Encoded communications are selected by the dispatcher at headquarters on keyboard 44. These mes sages are received on the vehicle receiver 39 and displayed on vehicle keyboard-display 30. The encoded location and status messages transmitted from the vehicle are displayed on the headquarters vehicle location and status display 45.  
  For convenience, the 150 MHZ receiver 39 and transmitter 40 and their respective antennas 38 and 41 are shown as separate elements herein. It will be under stood that combinations of these units are commercially available as transceivers. Also, a single antenna installed on the vehicle would ordinarily be used to both transmit and receive the 150 MHz radio signals.  
  In the exemplary embodiment, the wayside stations include individual radio transmitters which produce low power signals received on vehicle mounted antenna 26. Another embodiment of the invention not shown includes a low power transmitter mounted on each vehicle for transmitting the vehicle identification numbers and status to receivers located at each wayside station. Passage of a vehicle proximate to a wayside station automatically provides an updated location message back to headquarters 14. In such embodiment the location message verifier 28 is located at the wayside station 10 rather than in the vehicle 13 and operates in an identical manner as described below to protect the integrity of the signal, especially when the vehicle and wayside station are so situated as to result in fringe reception at the wayside station receiver.  
  It will further be apparent that this invention is not limited to a system in which radio links provide the entire communication path from the wayside station or vehicle to the headquarters. The invention may, for example, be used with particular systems advantageously wherein the communication links between the vehicle and wayside station are wireless paths subject to interference and substantially attenuated when any appreciable distance separates the vehicle from the wayside station. Thus, modulated light waves encoding a digital message may transmit information between the vehicle and wayside station. Telephone lines may be used to transmit messages between the wayside station and the remote headquarters. Although specific radio frequencies are given herein for exemplary purposes, it will be understood that these values are exemplary values and specific channels will be generally determined by the availability of frequencies in accordance with the FCC. regulations.  
 Detailed Description of Wayside Station As shown in FIG. 4, each wayside station 10 includes a timer 49 periodically activating repetitive code generator 50 and low power MHZ RF modulatortransmitter 51. When generator 50 and modulatortransmitter 51 are activated, a digitally coded message produced by generator 50 is transmitted over the 75 MHz channel on antenna 25.  
  Exemplary encoded waveforms produced by the repetitive code generator 50 are shown in FIGS. 2a and 2b. As shown in FIG. 2a, binary ones and zeros are distinguished by the time spacing between the leading edges of a pulse train. Thus, a five millisecond spacing between the leading edges 53, 54 of adjacent pulses defines a binary one whereas a two millisecond spacing between the leading edges 54, 55 defines a binary zero. The respective pulses are typically one half millisecond long.  
  Respective binary ones and zeros are combined to provide an encoded signal from each wayside station shown in FIGS/1, 3 and 4. As shown in FIG. 2b, the 8 bits defining a digitally encoded location message are accompanied by a predetermined digital code pattern which in the exemplary embodiment comprises an initial 4 bits defining a start code and a following 4 bits defining a stop code. In general, the start and stop codes will be common to all wayside stations, whereas a preprogrammed wayside station location is indicative of a unique geographical location. Thus, the complete encoded message shown in FIG. 2b communicates both the location of the station and a binary coded signal common to all stations. As described in the copending application of Adrian B. Haemmig, entitled Vehicle Location System&#34; identified above, the coding of the start and stop codes is keyed to a location message verifier 28 (FIGS. 3 and 4) within the vehicle so as to guard against reception of an erroneous location signal. This is particularly important in a vehicle locating system in which, as shown in FIG. 1, the vehicles are constantly moving within a fringe receiving area.  
  After transmission of the encoded signal of FIG. 2b, the modulator-transmitter 51 is caused to be turned off by the timer 49 for a predetermined time interval followed by a repetitive transmission of the identical digitally encoded message. This interval and the transmission radius of the wayside station are selected so that any vehicle operating at its maximum velocity past a station will receive at least two complete coded messages.  
 Detailed Description of Message Verifier System The encoded message of FIG. 3 is received on vehicle mounted antenna 26 (FIG. 4) coupled to the 75 MHz RF receiver and demodulator 27 mounted within the vehicle. The demodulator output signal is connected to a decoder 62 which distinguishes between an encoded binary one and a binary zero. A common form of decoder 62 produces a series of clock pulses on one output 63 with the presence or absence ofa pulse on a second output 64 in time coincidence with a clock pulse indicative of a binary one or zero. Such decoders are common in the art and therefore are not shown in further detail in the Figure.  
  The output of the decoder 62 is coupled to the location message verifier 28 which includes a multiple stage, serial load shift register 65 adapted to hold the entire message from the repetitive code generator 50. As shown, the shift register 65 includes a four stage storage capacity for the 4 start bits, an eightstage storage capacity for the 8 bits wayside station location message and a four stage storage capacity for the 4 bits of stop code.  
  After an entire message has been entered into the shift register 65 in serial fashion, the entire message is analyzed in parallel fashion to detect whether or not the received signal is a true or erroneous signal. The  
 four stages storing the 4 bits of the start code are respectively connected to a first binary-to-decimal converter 70. Similarly, each of the four stages storing the 4 bits of the stop code are connected to a secondary binary-to-decimal converter 71. Each of the converters 70, 71 convert the code pattern registered in the start and stop stages of shift register-65 into another code format. Thus, these converters have respective unique outputs 72 and 73 coupled to an AND gate 74. These unique outputs correspond to a pair of numbers programmed at and common to all of the repetitive code generators 50. Thus, in the exemplary message shown in FIG. 3, the start code bits in the binary 0100 pattern digitally encode the decimal number 4 and the stop code bits in the binary 0011 pattern digitally encode the decimal number 3. Each time these start and stop codes are received and stored in the first four and last four stages of the shift register 65, binary-to-decimal converter supplies a signal on its output 72 corresponding to the decimal digit 4 and binary-to-decimal converter 73 supplies a signal on its output 73 corresponding to the decimal digit 3. Simultaneous energization of output 72 and 73 provide an enable signal at the output 75 of AND gate 74. As described below, the location message in the shift register is discarded if no signal appears at this enable output.  
  A feature of vehicle locating systems constructed in accordance with this invention is that the integrity of the signal is further protected by store register and digital comparator 81. The function of these elements is to inhibit utilization of a location message unless and until two identical location messages are received in succession. The store register 80 is connected in parallel with the eight stages of the shift register 65 which contain the station location information. This data is retained in the store register 80 and supplied over plural leads 82 to one set of inputs of comparator 81 until receipt of the succeeding message from the wayside station 10. At such time, the output of the previous station location message, now retained in the store register 80, is compared with the current station location message located in the shift register 65 and supplied over plural leads 83 to a second set of inputs of comparator 81. If the comparison is negative, i.e., the combination of 8 bits in the store register 80 and shift register 65 do not compare, there is a signal applied to the strobe (no compare) output 84 of the comparator 81 for entering the shift register information into the store register 80. This signal also resets transmission enable flip flop 85 and the comparison repeated upon receipt of the next received location message in the shift register 65. Thus, until a positive compare is achieved, each location message is used twice in the comparison system, once when located in the shift register 65 and once when temporarily stored in the store register 80. This provides a simple and economical system having a high degree of reliability. If, however, the comparison is positive, i.e., presently received location message is identical to the previously received message, a signal is applied on the compare output 86 to AND gate 87. AND gate 87 is also responsively connected to the enable output 75 so that simultaneous energization of the compare lead 86 and enable output 75 result in a signal on the output 88 of AND gate 87 which triggers the flip flop 85. Flip flop 85 when triggered supplies a signal over lead 89 to initiate automatic transmittal of an updated vehicle location message from the vehicle to headquarters as described below.  
  It will be understood that the operation time of the logic elements described above is very short, e.g., a few hundred nanoseconds at most, in comparison with the milliseconds of time taken to shift each binary bit of in formation into the shift register 65. Thus, once the binary converters 70, 71 detect a proper start and stop codes in the shift register 65, they are able to apply a signal to the output of enable gate 74 for automatically initiating transmittal of a vehicle location message to headquarters as described below well within the time interval that the digital data is retained in the store register 80.  
 Portable Transmitter The portable transmitter 20 when activated transmits a chain of uniformly spaced binary one (FIG. pulses 95 for a predetermined length of time. In the specific embodiment shown in FIG. 4, these pulses are generated by clock 98 when the activator switch 96 is depressed, causing a I50 millisecond interval timer 97 to run and supply the burst of binary one pulses 95 shown in FIG. 5 to the 75 MHZ modulator-transmitter 99 for the 150 millisecond period. These pulses modulate a 75 MHz carrier signal supplied to antenna 35. At the end of this 150 millisecond time interval, there is a time interval of I50 milliseconds followed by a second burst of pulses as shown in FIG. 5. After a plurality of such pulse bursts. the time 97 shuts off and terminates the flow of pulses from the clock 98 and also shuts off the transmitter 99 so that no additional signals are transmitted from the modulator-transmitter 99 until the switch 96 is again actuated. At such time, the unit is recycled and an identical series of pulse bursts are generated and transmitted on a 75 MHz carrier.  
 Reception of Signal From Portable Transmitter The modulated pulses generated by the portable transmitter are received on antenna 26 of any vehicle mounted receiver 27 located within the receiving range of the emergency transmitter. The signal is demodulated in receiver 27 and supplied to decoder 62 which, as described above, produces a pulse on output 64 for each binary one bit.  
  These pulses are supplied to the emergency signal verify logic 36 so that the first such pulse triggers a retriggerable 5.5 millisecond one shot multivibrator 110, thereby providing a voltage rise at its output 111 as indicated by waveform 112 of FIG. 5b. So long as the train of binary one pulses is supplied to the input of the retriggerable 5.5 millisecond one shot multivibrator 110, its output remains at its high level as shown in FIG. 5b.  
  The voltage pulse on output 111 in turn triggersthe 120 millisecond one shot multivibrator 113. Unless previously cleared, this latter multivibrator will automatically reset at the end of 120 milliseconds resulting in a voltage rise at its output 114 as shown by Waveform 115 in FIG. 50. During the interval that the voltage levels on both of the outputs 111 and 114 are high, the voltage level on the output 116 of AND gate 117 is also high as shown as waveform 118 in FIG. 5d. Since the pulse burst from the portable transmitter continues for some milliseconds after the one shot 113 has reset, the pulse 118 has a duration of some 35.5 milliseconds equal to the sum of the 30 milliseconds interval and the 5.5 millisecond period of one shot multivibrator 110. As described below, this pulse may, for example, function as an emergency transmit control signal and is supplied at the operator keyboard 30 to activate the same circuitry that is activated when the operator in the patrol car depresses a key for transmitting a pre coded message from the vehicle to headquarters. Typically, this message is reserved for the 10-999 or emer gency message.  
  The portable signaling system is such that the pulse burst received from the transmitter 99 will uniquely produce the 35.5 millisecond pulse 118 at the output of AND gate 117. Thus, for example, any train of pulses received from another source, e.g., such as a wayside station, containing any binary Zeros will cause a time interval greater than 5.5 milliseconds between the successive pulse inputs to the 5.5 millisecond one shot 110. If a pulse is not received by one shot within 5.5 milliseconds, this one shot resets causing the voltage at its output to change and this voltage change is supplied via output 111 to the clear input of the 120 millisecond one shot causing it also to be reset. As a result, only a steady train of binary one pulses for a period of time longer than 120 milliseconds will provide the requisite pair of high inputs to the AND gate 117 necessary to produce a signal on output 116.  
  It will also be seen that the message verify system of FIG. 4 will inherently screen out the portable transmitter signal. Thus, although the binary one pulses on decoder output 62 will continue to fill up the shift register 65, binary ones in its start and stop stages will each encode the decimal number 15,21 number in excess ofthe output of either of the binary-to-decimal converters 70, 71. Accordingly, the portable transmitter signal (as with any other series of bits not having a proper start and stop code) will not cause a false location message to be sent from the vehicle.  
 Vehicle Message Transmitter System The system for transmitting the location message and other information from the vehicle 13 to the headquarters 14 is shown in FIG. 6. The message store 29 comprises a multiple stage, parallel-load shift register 125. Eight stages of this register are parallel loaded with the encoded data in store register 80 over plural leads 82. As described above, this data comprises 8 bits of digitally encoded information corresponding to the vehicle location as received from the wayside station. The remaining portion of the shift register is advantageously divided into a predetermined number of stages for a status message and the patrol car number. In the exemplary embodiment shown, each of these portions also store 8 bits of coded information. The data com municating a status message and patrol car number are supplied from the keyboard 30 located in the vehicle over respective multiple leads 127 and 128. The integrity of the message transmitted from the vehicle to the headquarters 14 is protected by a start code and a stop code in the same manner as described hereinabove. Thus, these twenty-four message stages are bounded by four stages of the shift register 125 loaded from a stop generator 129 and four stages loaded from a start code generator 130.  
  The status message derived from the keyboard 30 is either selected by depressing the appropriate button or buttons on the keyboard or in the case of an emergency message received from the portable transmitter, is automatically provided each time the emergency signal transmit control pulse 118 is supplied at the output 116 of AND gate 117 (FIG. 4). The keyboard 30 advantageously includes a plurality of keys 131 each selecting a precoded message. In the exemplary embodiment shown, representative precoded status messages from the patrol field officer in the vehicle dispatcher at headquarters include:  
 10-4 O.K.; Acknowledgement 10-9 Repeat Last Message 10-82 Transmit on Channel 2 10-84 Field Check 10-86 Send Back-Up Unit 10-97 Arrived At Scene; Officer Leaving Mobile Unit 10-98 Leaving Scene. In Service.  
 Available for Assignment l0-99 Dispatcher Alert; Contact Officer \&#39;ia Radio. Or Emergency Conditions The patrol car number is also supplied from the manual keyboard 30. Advantageously this number may be manually dialed by the officer on a pair of thumbwheel knobs 132 to distinguish his vehicle from the other vehicles currently in use. As described below, this number also serves to discriminate between communications received from headquarters so that the only messages displayed on keyboard readout 207 are those directed by the headquarters dispatcher to a particular vehicle or vehicles.  
  Transmittal of the information stored in shift register 125 from the vehicle 13 to the headquarters 14 is initiated in the following manner: A signal rise occurs at the output 89 of the transmit enable flip flop 85 (FIG. 4) when this flip flop is triggered to its set&#34; state or a signal rise also occurs on lead 140 from the keyboard 30 when the operator selects one of the keys 131 or when an emergency signal produces a pulse 118 on lead 116. Leads 89 and 140 are connected to respective inputs of OR gate 141. Thus, a signal rise from flip flop 85 or keyboard 30 at the input of OR gate 141 will produce a corresponding signal rise on the output 142 of OR gate 141 which is supplied as one of the inputs of AND gate 143. The other input 90 of this AND gate is responsive to the presence of another radio signal being transmitted on the same 150 MHz radio channel. In the exemplary embodiment described herein, this information is derived from the squelch circuitry within the vehicle radio receiver 39 (FIGS. 3 and 7) tuned to the 150 MHZ channel. If no other 150 MHZ signal is present, a signal rise from either the enable flip flop 85 or officer keyboard 30 on lead 140 causes a signal to appear at the output 144 of AND gate 143 and set the transmitter control flip flop 145.  
  The output 146 of transmitter control flip flop keys on the 150 MHz radio frequency transmitter 40 and is also applied through a signal delay network 147 to the input of shift register control AND gate 148 and to the enable input 149 of count down counter 150. A signal applied to counter input 149 enables this counter and results in application ofa signal to the parallel load inhibit portion 152 of the shift register 125. Accordingly, as long as the count down counter 150 is enabled, parallel loading of the register 125 is inhibited.  
  The encoded data stored in the shift register 125 is shifted out in serial format to the transmitter 40 in the following manner: A control clock 155 has one output 156 connected to another input of shift register control AND gate 148. As noted above, the other input of this AND gate is connected to the output of transmitter control flip flop 145. Thus, when this flip flop is set, pulses from the clock. 155 are applied to the clock input of shift register 125, resulting in a serial shifting out of digital bits on shift register output 157. Each clock pulse causes one data bit to be applied to the input of a frequency shift encoder 158. Thus, the first data removed are the four start code bits and the last data removed are the four stop code bits.  
  Encoder 158 is supplied with two different frequency signals from the control clock 155 over leads 159 and 160 to provide a signal on output 161 which shifts from one frequency to the other corresponding to whether the bit is a binary one or zero. This frequency shift signal on output 161 is applied to the 150 MHZ radio frequency modulator-transmitter 40 where it modulates the 150 MHz carrier for transmittal on antenna 41.  
  During the entry of clock pulses into the shift register 125, the count down counter 150 counts down to zero. In the example shown, this counter is preset to the number thirty-two, the data bit capacity of the register 125. Thus, when thirty-two clock pulses have been ap plied to the input of this counter, all of the bits in the shift register 125 will have been serially shifted out on output 157. Receipt of thirty-two clock pulses cause the counter 149 to remove the inhibit signal on lead 151 to the parallel load inhibit portion 152 of the shift register 125 and also reset transmitter control flip flop 145. The shift register 125 is then free to accept new data from the store register (FIG. 4) and the officer keyboard 30.  
  The signal delay network 147 delays transmittal of the enabling signal from flip flop 145 to the AND gate 148. This delay, typically 375 milliseconds, is selected longer than the warm up time of the transmitter so as to insure that the transmitter is on at full power for transmission ofa message from the vehicle to the headquarters. During the interval after the transmitter has been turned on but before a signal appears at the output of delay network 147, the 150 MHZ transmitter 40 transmits a radio frequency signal at whichever of the clock frequencies on leads 159, 160 is tuned the high Q resonant circuit 171 (FIG. 7) in the 150 MHz radio receiving stages of the other vehicles and headquarters. As described below this high Q circuit is used to automatically disconnect the vehicle audio speaker during transmission of a data signal. Delay network 147 thus insures that the high Q circuit in each vehicle and at headquarters is operative before the AND gate 148 is enabled.  
  A significant feature of the present invention is that the comparator 81 and enable flip flop shown in FIG. 4 and transmitter control flip flop shown in FIG. 6 provide an efficient utilization of the MHz channel by limiting automatic transmission of location messages to updated information. Flip flop 145 is edge triggered, i.e., it is triggered to its set state only when a signal rise occurs on input lead 44. Such a signal rise occurs when the transmit enable flip flop 85 is set and does not reoccur on lead 89 until flip flop 85 has first been reset by a signal on the comparator output strobe lead 84 (FIG. 4) and subsequently triggered to its set state by a compare signal on comparator output 86. It will be recalled that the transmit-enable fiip flop 85 is set when two successive station location messages are identical so that no additional rise signals are supplied on lead 89 until the flip flop is first reset and subsequently set. So long as the location information in store 80 remains unchanged, the flip flop 85 is retained in its set state and no signal rise occurs on lead 89 to trigger flip flop 145. As a result. although the vehicle 13 may remain in the immediate vicinity of a wayside station for a period of time during which time it receives numerous location messages identical in code format, only one such message is used for automatically initiating a location communication to headquarters over the 150 MHz channel from the vehicle to headquarters.  
 Reception of Audio and Digital Encoded Messages From Headquarters Referring to FIG. 7,-each patrol car carries an RF receiver-demodulator 39 tuned to the headquarters transmitter frequency of 150 MHz. This receiverdemodulator advantageously includes what is well known in the art as a squelch system which distinguishes between a carrier signal on the 150 MHz channel and high frequency noise. The operation of this squelch circuitry is such as to provide a signal on lead 90 when the squelch is operative indicative that the 150 MHz channel is clear.  
  As described and claimed in the copending application of Adrian B. Haemmig, entitled Vehicle Location System, identified above, the audio speaker is automatically disconnected during receipt of a digital data signal as follows: The output of the RF receiverdemodulator 39 is coupled to a first input of AND gate 170, the input of a tuned high Q resonant circuit 171 and one side (terminal 172) of a single pole, single throw switch relay 173. The tuned high Q resonant circuit 171 in each vehicle is tuned to the clock frequency transmitted from the vehicle transmitter 40 during the interval between when the transmitter is keyed on and an enable signal appears at the output of delay network 147 (FIG. 6). Each data transmission over the 150 MHz channel from any vehicle as well as headquarters is preceded by this tone which results in energization of the tuned high Q resonant circuit and triggering of one shot multivibrator 174 to supply a signal to a second input of AND gate 170 and also apply a signal to relay coil 175 to disconnect movable relay contact 176 from terminal 172. The period of one shot multivibrator 174 is sufficiently long so to energize the relay 173 for the length of time that a digital signal is applied to antenna 38. The movable contact 176 of relay 173 is adapted to open or close the path from the RF receiverdemodulator 39 to the audio speaker 169. Energizing relay 173 thus disconnects a digitally encoded message on the 150 MHz channel from the loudspeaker in the patrol car so as to avoid compelling the occupants of the vehicle to listen to the unpleasant and distracting noise which would otherwise result from the digital information transmitted over the 150 MHz band. Accordingly, a common radio channel may be used-for both digital and audio information without inconveniencing the vehicle occupants.  
  Received encoded messages from headquarters are temporarily stored and verified as follows: The output signal from the one shot multivibrator 174 enables the AND gate 170 so as to supply the demodulated signal from the receiver-demodulator 39 to the input of the decoder 180. Decoder 180 supplies a train of clock pulses on one of its outputs 181 and a series of pulses on its other output 182 corresponding to binary ones. Accordingly, the presence or absence of a pulse on output 182 in time coincidence with a clock pulse determines whether a binary one or zero is serially shifted into the multiple stage, serial load shift register 183.  
 This register includes 4 bits of storage capacity for start code, 8 bits of storage capacity for the patrol car number and 8 bits of storage capacity for the dis patcher status message selected on keyboard 44 (FIG. 3), and 4 bits of storage capacity for the stop code.  
  Binary-to-decimal converters 185 and 186 are respectively coupled to those stages of shift register which encode the start and stop codes. These converters function in the manner described hereinabove to protect the integrity of the signal received from the headquarters transmitter 42 (FIG. 3). Thus, only when the appropriate start and stop codes are contained in the headquarters message are signals applied to both of the converter outputs 187 and 188 to provide a signal at the output 190 of AND gate 189.  
  Those stages of shift register 183 which hold the encoded patrol car number are supplied as plural inputs 194 to a digital comparator 195. Another set of inputs 196 to the comparator connect the patrol number selected by the thumbwheel 132 on the vehicle keyboard 30. If the patrol car number stored in the shift register 183 corresponds to the patrol car number selected on the vehicle keyboard, a signal is applied at the output 197 of comparator 195 to one input of AND gate 198. A second input of this AND gate is connected to output 190 of AND gate 189. Simultaneous inputs on 190 and 197 to AND gate 198 produce a signal on its output 199 which cause a horn 200 to honk in the patrol vehicle advising the patrolman that a message encoding his patrol car number has been received from headquarters. In addition, the signal on output 199 is applied to latch gate 205. The encoded status message in the shift register 183 is then coupled through plural conductors 206 to the officer keyboard 30 to energize its readout display 207 (FIG. 6).  
  The signal at the output 199 of AND gate 198 is also applied to the clear input of the one shot multivibrator 174. As a result, this multivibrator is reset and AND gate 170 is disabled. Gate 170 then inhibits entry of any noise or other transient signals from the receiver 39 to the shift register and thus avoids premature shifting out of the information stored therein. AND gate 170 remains disabled until receipt of a subsequent headquarters transmitted signal on antenna 38 at the input of the RF receiver-demodulator 39. Such a signal, as described above, is preceded by a tone of predetermined frequency for energizing the high Q resonant circuit 171 and resultant enabling of AND gate 170 after triggering of one shot multivibrator 174.  
 What is claimed is:  
  1. In a vehicle locating system wherein stationary wayside stations are used to reference the location of said vehicle at a given point in time and wireless paths are used for transmitting digitally encoded messages between said stationary wayside stations and said vehicle, the improvement for efficiently utilizing the communication path between said vehicle and another site comprising:  
 means for temporarily storing said digitally encoded messages,  
 comparator means for comparing a transmitted digitally encoded message with said temporarily stored message,  
 means responsive to said comparator means for automatically initiating re-transmission of said digitally encoded message to said other site, said automatic re-transmission being initiated in response to an updated location message being received by said vehicle and independently of any control stimulus from said other site, and  
 means for inhibiting additional re-transmissions of said digitally encoded message to said other site so long as the received digitally encoded messages correspond to said temporarily stored message so that only updated messages are re-transmitted thereto.  
  2. In a vehicle locating system wherein stationary wayside stations are used to reference the location of said vehicle at a given point in time and wireless paths subject to interference and substantial attenuation when any appreciable distance separates the vehicle from a wayside station are used for transmitting digitally encoded messages between said stationary wayside stations and said vehicle, the improvement for maintaining the integrity of said messages by detecting errors resulting from the interference and attenuation of said wireless path comprising:  
 repetitively transmitting identical digitally encoded messages between said stationary wayside station and said vehicle,  
 means for temporarily storing one of said digitally encoded messages,  
 means for comparing the next received digitally encoded message with the temporarily stored message and providing an output signal indicative of a compare or a non-compare therebetween,  
 means responsive to a non-compare output signal for entering said next received digitally encoded message in place of the message in said temporary storage, said comparison being repeated as said digitally encoded messages are received until the received message and the temporarily stored messages are identical at which time a compare output signal is generated, each such message thereby being used twice in said comparison, once when it is compared with a message in said temporary store means and once when it is stored in said temporary store means, and  
 means responsive to said non-compare output signal for inhibiting utilization of said digitally encoded message.  
  3. In a vehicle locating system wherein stationary wayside stations are used to reference the location of said vehicle at a given point in time and wireless paths are used for transmitting digitally encoded messages between said stationary wayside stations and said vehicle, the improvement for efficiently utilizing the communication path between said vehicle and another site comprising: 1  
 register means for registering each received encoded message, temporary store means for temporarily retaining an encoded message,  
 comparator means responsive to said register means for comparing said registered message with a previously received message retained in said temporary store means, and  
 means responsive to said comparator means for automatically initiating only a single transmission of said message over said communication path between said vehicle and another site when said registered message corresponds to said temporarily stored message, said automatic message transmission between said vehicle and another site being initiated in response to an updated location message being received by said vehicle and independently of any control stimulus from said other site.  
 4. Apparatus as in claim 3 further including:  
 transmit enable means triggered to a first state when said registered message corresponds to said temporarily stored message and to a second state when said registered message is different than said stored message, said means being retained in said first state during receipt of successive encoded messages so long as the registered message corresponds to said temporarily stored message,  
 transmitter means for transmitting the message in said temporary storage means over said communication path, and  
 means responsively connecting said transmitter means to said transmit enable means for automatically initiating a transmission of the message in said temporary storage only when said transmit enable means is triggered to said first state.  
  5. In a vehicle locating system wherein stationary wayside stations are used to reference the location of said vehicle at a given point in time and wireless paths subject to interference and substantial attenuation when any appreciable distance separates the vehicle from a wayside station are used for transmitting digitally encoded messages between said stationary wayside stations and said vehicle, the improvement comprising:  
 repetitively transmitting identical digitally encoded messages between said stationary wayside station and said vehicle,  
 means for temporarily storing one of said digitally encoded messages, comparator means for comparing the next received digitally encoded message with the temporarily stored message, and providing an output signal indicative of a compare or a non-compare therebetween,  
 means responsive to a non-compare output signal for entering said next received digitally encoded message in place of the message in said temporary storage, said comparison being repeated as said identical digitally encoded messages are received until the received message and the temporarily stored message are identical at which time a compare output signal is generated,  
 means responsive to said non-compare output signal for inhibiting re-transmission of said digitally encoded message,  
 means responsive to said compare output signal for automatically initiating re-transmission of said digitally encoded message, said automatic retransmission being initiated in response to an updated location message being received by said vehicle and independently of any control stimulus from said other site, said means inhibiting retransmission of said digitally encoded message so long as the received digitally encoded messages correspond to said temporarily stored message so that only updated messages are re-transmitted.  
  6. In a vehicle locating system wherein stationary wayside stations are used to reference the location of said vehicle at a given point in time and wireless radio frequency channel are used for transmitting digitally encoded location messages between said stationary wayside stations and said vehicle and between said vehicle and a remote headquarters, the improvement for efficiently utilizing the radio frequency channel between said vehicle and said remote headquarters comprising:  
 radio receiver means carried by said vehicle for receiving said digitally encoded location message when said vehicle is near a stationary wayside stanon, means coupled to said receiver means for temporarily storing said digitally encoded message, comparator means for comparing a subsequently received message with said temporarily stored message, radio transmitter means carried by said vehicle for transmitting digitally encoded messages from said vehicle to said remote headquarters means coupling said temporary store means to said radio transmitter,  
 means for transmitting a digital signal from said vehi cle to said headquarters which encodes both the vehicle identification and said location message and means responsive to said comparator means and 0p eratively coupled to said radio transmitter means for automatically initiating transmission of an updated location message from said vehicle to said remote headquarters in response to an updated location message being received by said radio receiver means and independent of any control stimulus from said remote headquarters. said means also inhibiting additional transmissions of the same location signal so that only updated location messages are automatically transmitted from said vehicle to said remote headquarters.  
  TJNTTTD STATES PATENT OFFICE CERETMQATE @F (JORREQTION PATENT NO. &#39;1 3,875i930 DATED April 8 1.975  
 INVENTOMS) Adrian Ba E-iaemmig a James A. Gibson It is certified that error appears in the aboveidentif&#39;red patent and that said Letters Patent are hereby corrected as shown below;  
 Cole 1., line 30, lualitw should be -plurality-; Col. 3  
 line 4,, &#34;o ertion&#34; SuOul i --operation--; Col. 6, line 1, &#34;secondary&#34; should be --secc-nd-; Col. &#39;5, line 29, &#34;time should be -tiI-1cr&#34;---; Colo 10, line 56, &#34;44&#34; should be --l44--;  
 Col 14, .Llllii. 66 &#34;channel&#34; should be --=channels---.  
 Signed and Scaled this nineteen h D 3y Of A ugust I 9 75 [$EAL] Arrest:  
 RUTH Q. MSON C. MARSHALL DANN Ariestrng Officer (mnmis r&#39;mzer nj&#39;lalents and Trademurkx UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,876,930 DATED April 8, 1375 INVENTOR(S) Adrian B. Eiaemig &amp; James A. Gibson It is certified that error appears in the above-identified patent and that said Letters Patent I are hereby corrected as shown below:  
 Col. 1, line 30, lualit should be -plurality; co1. line.4, &#34;opertion&#34; snould De -operation-*-;-Col. &#39;0, 4.11m 4, &#34;secondary&#34; should be --second--; Col.- 7, line 29, &#34;time shpuld be Col. 14, 4.1413 66 channel&#34; should he -c&#39;namnels--.  
 Signal and Scaled this nineteenth D y of August 1975 [SEAL] AtteSt.  
 C. MARSHALL DANN (mnmissiuner oj&#39;latents and Trademarks RUTH C. MASON Auesting Officer -timur--; Col. 10, line 56, &#34;44&#34; should be -l44--;