Integrated radio location and communication system

7~ ,s ~kgro~n~ ~f ~h~ Inventl_~ 1. Field of the I:nventionO This invention relates to radio location systems for determining the identity and location of a pluralit~ of locatortransmitter devices at arbitrary fixed or moving locations throughout a geographical location. My invention ~inds application in public safety systems, commercial trucklng and taxi dispatching operations, personal alarm systems, and the like. This inventivn is related in part to my invention entitled ~Ultra-Narrow Band Communication System'l, U. S.

Patent No.

~,117,405 2~ Prior Art Systems for locating vehicles and/or persons within a city or extended arèa, sometimes referred to as automatic vehicle monitoring (AVM) systems, have long held the interest of both public safety authorities and commercial enterprises for ~he control of vehicles, for dispatching operations~ and for citizens' alarm devices. Indeed, many different types of systems have been proposed in the past and some are currently undergoing testing. These systems utilize such radio methods as LORAN, OMEGA, radio pulse tri~lateration, and similar wireless techniques. Another radio location method employs what are referred to as "sign-posts" which are in fact either miniatur~ radio transmitters or receivers located at various street intersections and within city blocks. These latter devices are intended to communicate with passing vehicles and thereby determine their position subsequent to which this informat~on is 78 ~L r communicated to a central location within the city. The latter ~ys~ems suffer the problem that a considerable number o~ such sign-posts are necessary for accurat:e coverage within a ci~y7 these numbers ranging upwards of 10,000 or more sign-posts, and these pose difficult maintenance ancl installation problems a~ well as very hlgh cost. On the other hand the wireless or radio technique like LORAN are often inaccurate, have complex and costly communication devices and receivers are necessary on each vehicle. Furthermore, none of these prior art systems are readily adaptable to low cost miniature packaging such as might be necessary for on-person alarm ap lication (herein referred to as citi2ens' alarm~.

Another prior art radio location system which was designed by the present inventor utili~es three existing local radio broadcast stations phase-locked to each others that is~ one broadcast station serves as the "master" station and the other two derive their RF carrier by phase-locking to the master statlon and thus all three stations are "synchronized." A receiver within a vehicle is designed to tune to and "track" all three broadcast stations and make phase comparisons in a manner designed to disclose the position oE the vehicle, relative to some starting reference location.

r 78~Lil ~Ly n~ ~hæ Inv~n~Lnn The invention which I herein disclose represents a significant improvement over my prior three broadcast s~ation sy~tem because it utilizes only a single broadcast station and two ~ixed receivers and because the vehicle borne device (or the device carried on-person) is relatively small and simple. Each of sa d locator-transmitters is continuously phase-locked to the RF carrier of a nearby commercial broadcast station, which station is modified slightly to provide periodic transmission of a "range-tone" which is on the order of a few kilohertz in frequency and is phase-locked to the RF carrier. An identical phase-locked range-tone is likewise generated within each locator-transmitter. The two centrally located fixed receivers are also phase-locked to the broadcast station RF carrler and likewise generate a range-tone. Thus the broadcast station, the plurality of locator-transmitters and the two fixed receiver sites all generate identical range-tones and these are continuously kept in synchronism with each other by virtue of a phase-lock loop arrangement wherein all range-tones are derived from (and locked to) the RF carrier of the AM broadcast station signalr which RF carrier is capable of being detected throughout the area of coverage of this system. Each locator-transmitter incorporates a RF frequency synthesizer arrangement whereby its transmitter frequency (e.g. at 35 M~lz) is precisely generated and transmitted to the fixed receiver sites using very narrow-band techniques whenever the position of a locator-transmitter is to be determinedO The fixed receiver sites can compute the position of each locator-transmitter by comparing the phase of the rangetone sent to them directly by the locator-transmitter with the phase of the range-tone received directly from the broadcast station~ This phase difference is E)roportional to the di~er2nce in radio path len~th between signaLs received directly from the broadcast station relative to those received via the locakortransmitter. Such a range coTnparis0n is made simultaneously at both receiver fixed sites and based upon this information a computation can be made to determine the location of said locator-transmitter using well known mathematical relations. My system is capable of continuously determining the location of hundreds or thousands of locator-transmitters and communicating automatically said position information to two or more fixed receiver sites. A novel frame synchronization method is disclosed whereby the position (i.e., "time-slot") of each locator-transmitter emis~ion uniquely identifies the emittin~ ~ource and thus the location as well as identity of each of the many locator-transmitters can be automatically and continuously ascertained.

In addition, wherein my aforesaid prior three broadcast station system was capable of determining the position of a vehicle, no simple means were available for transmitting this information to a centxally located d.ispatching point. In my invention disclosed herein, this problem of communicating the position information to a central location is inherently and simply solved in a manner hereinafter described~ Therefore my current invention simultaneously so:Lves the location problem and the communications problem and I therefore refer to it as the "integrated radio location and communication" (IRLC) system.

The communication aspects of the system which I hereln disclose is related intimately to the system disclosed in "Narrow 78~ 01 Band Communication System", U.S.

Patent No. 4,117,405, 02 issued September 26, 1978.

Stated simply, this latter 03 communication system is a very narrow band system which 04 has inherently high signal-to-noise ratio, natural 05 immunity to both intentional and unin-tentional radio 06 signal in-terference, and ver~ high traffic capacity.

The 07 reader is referred to this latter patent application Eor 08 further detail.

09 In general, according to the present invention there is provided a radio location and 11 communication s~stem, comprising a broadcast circuit for 12 transmitting an RF carrier signal and a periodic carrier 13 range tone signal.

One or more locator transmission 14 circuits are phase-locked to the RF carrier signal from the broadcast circuit, for generating and relaying a 16 transmi-tter range tone signal. A plurality of receiving 17 circuits are provided for detecting the carrier range 18 tone signal from the broadcast circuit and the 19 transmitter range tone signal from the locator transmission circuits and phase comparing these signals 21 to compute the location of the locator transmission 22 circuits.

23 24 ,...

01 ~rief Description of the Drawings 02 Figure 1 illustrates symbolically my 03 integrated radio location and communication system 0~ includ.ing a broadcast s-tation, plurality of 05 locator-transmitters, and two centrally located fixe~ 06 receiver si-tes, 07 Figure 2 illustrates the broadcast station 08 modifications which are necessary to make -the 09 broadcast station compatible with my system, Figure 3 shows schematic arrangement of a 11 typical radio location and communication transmitter 12 (herein called "locator-transmitter"), 13 Figure 4 shows a schematic arrangement of 14 a centrally located fixed receiver site, and Figure S shows a schematic arrangement of 16 a narrow band receiver, according to the present 17 invention.

18 19 21 22 23 24 26 27 - 6 .78~ ~g~L~L~iQn Q~ ~h~ El~s~u~s~ Embodim~n~ Flgure 1 illustrates a preferred embodiment showing the overall arrangement of the integrated radio location and communlcation system wherein broadcast station 2, designed to transmit conventional broadcast material, is modified to transmit a "range-tone" fr~ for example at 3 KHz, periodically for my system, which tone fr in no way interferes with the normal operation of said broadcast station. Receiving antenna 3 detects these broadcast signals and sends them to typical locatortransmitters 7, which transmitters may be located on vehicles 5 or hand carried. Transmitters 7 are phase-locked to the RF carrier of station 2 and generate therefrom a 3 KHz range-tone fr (for example by fre~uency divider means), which range-tone circuit is further designed in a manner to be synchronized to the occa~ional periodic transmission of range-tone fr from broadcast station 2. For example, broadcast station 2 may transmit a range-tone burst once every 30 minutes for a period of perhaps five seconds so as to enable range-tone synchroni~ation of transmitters 7 and receivers 8 and 12, these range-tones being synthesized from the broadcast station RF carrier by frequency divider means, thus phase-locked to that RF carrier.

Transmitter 7 includes a receiver which is phase-locked to the carrier of broadcast station 2 and a range-tone generating circuit adap~ed to frequency divide down the broadcast station signal frequency fc to 3 KHz, thus ~o enable this la~ter rangetone to be synchronized to the range-tone burst which is periodically transmitted from broadcast station 2. Therefore transmitters 7, being always phase-locked to the carrier broadcast signal fc of station 2, continuously generates a range ~2~ tone of 3 R~z which i5 in synchronism with the ranye-tone generated by station 2, however, it is displaced in phase from the latter range-tone by an amount proportional to the propagation time of the siynal from broadcast station 2 to transmitters 7~ This phase lag, being on the order of 100 microseconds when broadcast station 2 is approximately 20 miles away from transmitters 7, provides the necessary information for computing range, thus location.

Transmitters 7 also contrain a frequency synthesizer which i5 phase-locked to broadcast carrier fc of station 2 and which generates therefrom two transmitter RF carrier frequencies, for instance at a frequency of 35 MHz, and th~ "Beat" b~tween these ~wo 35 MHz frequencies is equal to and in phase with the rangetone fr generated by transmitters 7. Transmitters 7 will then transmit via a~tenna 4 a range~tone fr (i.eO, location information) to centrally located receivers 8 and 12 on occurance of a specified condition, for example every 20 seconds based on clock trigger ~ulses.

The centrally located receivers 8 and 12 are designed to detect radio signals directly from broadcast station 2 using antennas 9 and 11, and also detect the signals radiated from transmitter 7 via antennas 6 and 10. These centrally located receivers 8 and 12 each develop a first range-tone based on the signal received directly from broadcast station 2 and a second range-tone based on the signals received from tran~mitter~ 7;

these two range-tones are phase compared and the location oE transmitters 7 is therefrom computed based on well known mathematical relationships based on the findings of receiver 8 in conjunction with findings of receiver 12.

~7~ Figure 2 shows the modificatiorls whlch are necessary to a typical AM broadcast station to oper3te in conJunction with my sy~em. A master oscillator 16, typically used in all broadcas~ ~tations and which i8 usually an accurate crystal controlled oscillatorr generates the basic radio frequency of the broadcast station at, in l:his example, 640 RHæ. Frequency divider 18 is locked to this master oscillator and develops in this example a tone fr~ the latter frequency being the 3 KHz range-tone mentioned before. Frequenc~ divider 20 further divides the output of frequency divider 18 and generates a low clock frequency of, for example, one impulse every 30 minutes. This is sent to master clock 22 which develops a gating signal to permit range-tone f~ to be periodically gated through time gate 24 to phase modulator 26L For example, the 3 KHz range-~one could be gated on for five seconds every 30 minutes. This range-tone "burst" phase modulates the broadcast carrier fc in a manner designed to minimize deleterious effects on the AM broadcast station conventional signal so that it may perform its normal function of transmitting AM program material without annoying its listeners. This could be typical "time trackn. For example, phase modulator 26 could be designed to provide a + 30 phase angle modulation in correspondence with the range-tone frequency fr and this would not be detectable by conventional AM receivers.

Such a modulation process would give rise to only two significant side bands and all harmonic side bands would be negligible~ as i3 well known in the phase modulation theory relating to "~mall angle" modulation. Thus the broadcast station ~ can be modified in a manner which in no way interferes with its normal operation ~2~ yet is capable of transmitting a range~tone simultaneously to many locator-transmitters 7 for synchronization purposes~ Figure 3 shows an arrangement o~ a typical radio locatortransmitter 7. The signal broadcast by station 2 is detected by antenna 3 and sent to phase-lock receiver 34. Receiver 34 employs a phase-lock loop arrangement wherein a local reference oscillator is synchronized with the incoming broadcast station RF carrier and wherein the amplitude modulated audio signals are stripped away from the broadcast si(3nal. Such a receiver is described in detai.l in V. S.

Patent No. 4,117t405.

Briefly then, phase-lock receiver 34 is continuously and tightly locked to the RF carrier fc from station 2 and derives therefrom 2 reference fre~uency fO which is sent to frequency synthesizer 40, and a range-tone fr derived from the phase modulation which is periodically put on broadcast station signal fO by range-tone circuits which I have described in relation to figure 2. Range-tone detector 38 detects the presence o the range-tone from station 2, which range-tone is only momentarily transmitted by station 2, for example, for five seconds every 30 minutes (this periodic rate is called fs in figure 2 herein) and this is used to synchronize range-tone oscillator 36 and also to provide nframe synchronizationn. Oscillator 36 is on continuously and phase-locked to the reference oscillator which is part of a phase-lock receiver 34. It is expected that once locked in, range tone oscillator 36 w.ill maintain its phase relationsh{p to the range-tone oscillator contained within broadcast station 2, however, should oscillator 36 become unsynchronized ~or some unexpected reason, it will come back into synchronization by virtue of ~he operation of range-tone detector 1() ~l2~78~a~ 38, at the next occurance oE a range~tone burst transmis~ion rom broadcast station 2. Thus, in this example, range~tone o~cillator 36 will never be out of synchronization with the correspondlng range-tone oscillator in station 2 for a length of time in excess of 30 minutes because station 2 transmits such a range-tone at least once every 30 minutes, or more frequently if desired.

Frequency synthesizer 40 accepts the reference frequency fO from receiver 34 and the range-tone from oscillator 36 and synthesizes therefrom two RF frequencies fl and f2 whose frequency difference is equal to the frequency of range-tone o~cillator 36 and in such a manner so that the effective "beat frequency~ signal between fl and f2 is accurately in phase with the signa' from range-tone oscillator 36; for example, a well known single side band modulation process could be used~ RF Amplifier 42, when keyed ON from master clock 44, transmits signals 1 and f2 to the centrally located receivers 8 and 12 via antenna 4.

The crucial functions of transmitters 7 are thus the accurate phase-lock to broadcast station signal fc and generation therefrom of a range-tone and RF transmitter frequencies fl and f2 in such a manner wherein the beat signal between these two frequencies fl and f2 comprises the range-tone frequency fr and consequently effectively relaying range information to central receivers 8 and 12 so they can determine the position and identity of transmitters 7.

The identity of any one vf a plurality of transmitter~ 7 is determined by virtue of a time and frequency division coding which will now be described. Whenever broadcast station 2 7~ transmits a range-tone burst, in this example a five seconds burst every 30 minutes (i.e., at a rate of fs)~ range-tone detector 38 transmits a frame synchronization^pul~e to frequency divider and master ciock 44, which clock 44 also continuou~ly receives the synchronized ran~e-tone signal r from oscillator 36, which uses these two inputs to accuratel~ synthesize a transmltter gating pulse fi at an accurately prescribed time relative to the frame synchronization burst which is transmitted from the broadcast station 2, and this "time-slotn, together with the assigned RF transmitter frequency comprise a multiplex concept wherein the specific RF frequencies fl and f2 and the "time-slot" of transmis~ion uniquely identifies any particular transmitter 7 and furthermore, the location of transmitter 7 within a city is determined on the basis of the range-tone phase information communicated from transmitter 7, which information is conveyed in the beat frequency between signals fl and f2.

The range-tone periodically transmitted Erom station 2 at rate f$ thus serves to not only provide phase synchronizat~on for determining the location of transmitters, but also serves to synchronize the time at which transmitters 7 are to report thelr position. The actual rate at which transmitter 7 reports its position and identification is shown in figure 3 as fl which~ for exampler might be every 20 seconds, within a unique "time-~lot.~ For example, if we define 64 unique frequency pairs fl and f2 for ~frequency-division" identification purposes, and 128 "timeslotsn, then 8,192 di~ferent transmitters 7 (e.g. ~,192 vehicles) could be separately "tracked~ and identified, each reporting ~for example3 for .156 seconds every 20 seconds.

L784~ Figure 4 shows a schematic arrangement of a r~celver at central locations. At least two of these recelvers are necessary to make a location determination and these receivers should be placed, for example; at opposite side.s of a city and opposite the broadcast station so ~hat location errors can be minimized~ For example, the two central receivers and broadcast station might be located on the three points of an equilateral triangle.

Reerring to figure 4, antenna 6 detects the R~ signal from broadcast station 2 and sends it to phase-lock receiver 34.

Range-tone detector 38, range-tone oscillator 36, and frequency divider and master clock 44 function in a manner equivalent to a typical radio locator-transmi~ter shown in figure 3, h~reinbefore described. Thus the circuits shown in the lower half of figure 4 are essentially equivalent to those shown in figure 3 whose purpose is to phase-lock a receiver to the broadcast station signal fc and to derive therefrom a range-tone fr and a frame synchronizing signal fs which will enable the remaining circuits of the central receiver to determine the identity and location o each of the plurality of transmitters 7.

Phase-lock receiver 34 generates a reference frequency fO which it sends to frequency synthesizer 48 which in turn synthesizes a plurality of frequenc~ pairs fl and f2 identical to those which are sent from each of the plurality of loc~tortransmitters~ Each pair of signals fl and f2 define a frequency channel, which channel compri~es a part of the identificatlon of the emitting locator-transmitter 7.

Narrow band receiver channel ~6 is one of a plurality of 3uch chanrlels corresponding to the many dlferent frequency pairs fl and f2 employed in a par~icular system. These RF channels 46 ~L2~7~ function in a manner which will now be described. Signals transmitted by locator-transmitters 7 are detected by antenna 9 (e.g., 35 MHz) and these signals are sent to all the parallel channels ln narrow band receiver 46. Narrow band receiYer 46 contains (see figure 5), within each channel, RF preamplifiers 52 and 60 followed by a pair of phase-lock receiver circuit~ wherein oscillators 58 and 68 within each phase-lock loop are brought into "soft" synchronism with anticipated incoming signals fl and f2 through use of "priming" signals f'l and f'2 from frequency synthesizer 48. In other words, each channel of receiver 46 is effectively a phase-lock loop arrangement wherein the "freerunning" frequency of oscillators 58 and 68 are kept accurately in tune, on the average, by signals derived from frequency synthesizer 48. However, when the incoming signals fl and f2 from a transmi~ter 7 arrive at the input to the phase-lock loop receiver, the oscillators 58 and 68 of the phase-lock loops are captured by the incoming signal and brought into "hardN synchronism with the incoming signals f.l and f2. By this process the ~wo independent oscillators oE each o~ the two phaselock loops within each receiver channel is brought into synchronism with the two incoming signals fl and f2 from each locator~transmitter 7.

When the two oscillators 58 ancl 68 of the phase-lock loops within each receiver channel are brought into "hard" ~ynchronism with the incoming signal, then the beat signal developed when output of oscillators 58 and 68 are mixed in mixer 70 compri~es the range-tone signal which we desire.

There are mar.y parallel channels such as shown in figure 5 in a central receiver ~ystem and each of these channels corresponds with a pair of frequencies 1~ ~2~ fl and f2 used in the many locator-transmitters of any one ~ystem.

Re~erring again to figure 4, the range-tone fr from receiver 46 together with timing in~ormation ~i and the second referenced ranging tone developed in range-tone oscillator 36 are taken a~ input signal~ to delta range detector and coder 50 which derives therefrom identification information and differential range information for each particular locator-transmitter 7 that may be currently transmitting a siynal, and sends this information in digital form to its companion central receiver which in turn computes the location of said locator-transmitter 7 and identifies it. Delta range detector and coder 50 circuits are relatively straight forward; i.e., conventional well known digital techniques provide means which can readily compare the phase of range~tones from oscillator 36 with the phase of rangetone from receiver 46 and derive therefrom a digital code proportional to the phase difference between these two rangetonesO ~his is the information necessary for location computationO In addition, they can derive a binary digital code that identifies the particular locator-transmitter presently emitting based on its time position relative to the subframe synchroniæation signal fio It will be obvious to tho~e skilled in the art of radio and digital ~echniques that many variations of my basic approach are possible and that these recitals are primarily an illustrative example of my basic invention. Those skilled in the art will also recognize the numerous additional applications which thi system can be put to, for example~ in aircraft collision avoidance systems, in the control of ships within a harbor, in mas~ fire fighting efforts wherein the position of indi~iclual piece~ of fire fighting equipment and fire Eighting per~onnel can be determined from a central locaticn for fire control purposes7 and the like.

It i~ also obvious that a specially contructed RF transmltter could be employed in place of broadcast station 2 for certain ~pecialized applications such as fire fighting or some military applications.

I also anticipate the use of doppler frequency shift information from RF carriers fl and f2 as detected in the central receiver stations to compute velocity and direction information on each o the locator-transmitters 7 to thereby further refine and update location information~