State entry beacon system
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     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND OF THE INVENTION 
     This invention relates to automatic sensing and recording of an indication of the crossing of a boundary, and more specifically, it relates to a system for recording the passage of a commercial load-carrying vehicle across a state line with an accurate indication of the point of crossing for revenue generating purposes. 
     Interstate commercial vehicles are subject to tax liability on the basis of mileage within a state. Heretofore, the drivers of interstate commercial vehicles have been required to gather the information manually and to report to state tax authorities in a document known as a driver trip envelope such information as highway used, odometer reading at border crossings, starting locations and destinations. Inaccurate information, particularly inaccurate odometer readings due to human error or due to the inconvenience of manually recording an odometer reading in a moving vehicle, is a frequent source of concern. 
     Interstate trucks are now frequently equipped with an on-board computer (OBC). The OBC is used for various mobile data gathering and processing tasks. One of the data gathering tasks of an OBC is the recording of odometer readings upon receipt of an indication signal. 
     There are significant problems with each possible technology for issuing an index of a boundary crossing. Inductive signals generated by a signal source embedded in a roadway is extremely limited in range and requires that the roadway itself be modified, which is an extremely expensive proposition. Optical or other signals generated from a true line of sight source at a roadside to a passing vehicle are subject to interference and blockage from fog or intervening vehicles and are thus easily defeated. Radio frequency or microwave signals generated from a roadside source have an indeterminate range and are difficult to distinguish in a moving vehicle on a crowded multiple-lane roadway, and they are also difficult to locate precisely and isolate from other similar sources in an extended proximity, particularly due to reflections off of moving and fixed sources. 
     The following patents and publications have come to the attention of the inventors in relationship to this invention: 
     EP 289,657A (Sumitomo) describes a two-way communication system between a single directional antenna serving a beacon function on a fixed structure and a close moving vehicle. 
     EP 254,262A (Sumitomo) describes a two-way communication system with a beacon function on a fixed structure and a close moving vehicle as part of a vehicle navigation system. 
     U.S. Pat. No. 4,962,457 discloses an interactive vehicle highway system for providing site or specific information to a vehicle operator for navigation purposes. Communication is two-way and employs an antenna system embedded in the ground. 
     U.S. Pat. No. 3,996,554 discloses a vehicle data gathering system wherein an inductive receiver in the vehicle picks up information from an external transmitter driving an inductive loop antenna buried in the pavement as the vehicle passes over. 
     U.S. Pat. No. 4,858,133 discloses use of an on-board computer similar to the type which might be used as an information-gathering apparatus in connection with the invention herein. However, nothing is disclosed regarding the capturing of information from a remote source via radio or the automatic logging of landmark information. 
     U.S. Pat. No. 4,926,331 discloses another on-board processor with automatic data recording capabilities for information about the vehicle. 
     U.S. Pat. No. 4,939,652 discloses still another on-board processor with data recording capabilities about the vehicle plus provision for manual input of state location. 
     U.S. Pat. No. 4,677,429 discloses an on-board processor with data recording capabilities in connection with provision for manual input of state location information from the vehicle, together with a provision for a radio frequency link to a fixed receiver to download information from the on-board computer to the fixed station. 
     U.S. Pat. No. 4,067,061 discloses simple on-board data capturing device which has provision for manual entry of state location information. 
     It is desirable to provide automated recording of odometer readings of interstate vehicles which takes advantage of the availability of an on-board computer system to minimize inaccuracies in data recording and to provide a convenient and accurate method for issuing, capturing and accurately logging an indication of a boundary crossing such as at a state border. 
     SUMMARY OF THE INVENTION 
     According to the invention, a beacon system is provided wherein a radio beacon station is placed at a landmark such as a state line adjacent a major highway to broadcast information-containing alternating directional signals. In addition, each vehicle may be equipped with a radio receiver, a receiving decoder and a data analysis processor for receiving data signals from the beacon station. The beacon station includes an information encoder for generating first and second alternating but distinct location signals, first and second directional antennas disposed to radiate in opposite directions along the path of travel, the directional antennas having a significant back attenuation, a radio frequency transmitter for broadcasting the two location signals, preferably as data packets, to vehicle-mounted receivers along the path of travel, and an r.f. switcher for switching signal output of the transmitter between the two antennas in synchronization with the two location signals. The data analysis processor associated with each vehicle-carried receiver includes a half-circular buffer operative to capture a succession of packets received from the receiving decoder, to preserve initially-received complete packets of the first location signals and to preserve last-received complete packets of the second location signals. The data analysis processor further includes means for processing the preserved packets in the half-circular buffer to determine the exact crossover point of the landmark marked by location of the transmitter. 
     The invention will be better understood upon reference to the following detailed description in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a beacon system in accordance with the invention. 
     FIG. 2 is a block diagram of a beacon system in accordance with the invention. 
     FIG. 3 is an illustration of a typical message format. 
     FIG. 4 is a timing diagram illustrating a typical data pattern by packet type. 
     FIG. 5 is a block diagram of a data acquisition subsystem showing in a receiver dual half-circular buffers. 
     FIG. 6 is a block diagram of an illustrative half-circular buffer. 
     FIG. 7 is a flow chart for illustrating operation of a data analysis subsystem. 
     FIG. 8 is a flow chart for illustrating a boundary crossing discrimination method. 
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENT 
     Referring to FIG. 1, there is shown in side view an environment for a beacon system 10 according to the invention which includes a transmitter beacon station 12 and at least one receiving subsystem 14 mounted in a mobile road vehicle such as a truck 16 moving on a highway 18 which passes the transmitter beacon station 12. The transmitter beacon station 12 is typically mounted on a pole 20 which is in plain view in both directions along a path of travel represented by the highway 18. Referring to FIG. 2 in connection with FIG. 1, the transmitter beacon station 12 comprises a first antenna 22 and a second antenna 24 each of which produces a highly directional radiation pattern. A typical antenna is a vertically-polarized, multiple-element yagi. Antennas 22 and 24 may be in close proximity to one another oriented to radiate in substantially opposite directions and having a reflective shield 26, such as a flat metal plate, separating the two antennas. The first antenna 22 produces a forward radiating pattern or first forward lobe 28 directed up the highway 18 (toward an approaching vehicle with a receiver 14) and a lower intensity generally short range first back lobe 30 directed down the highway 18. The second antenna 24 produces a second forward lobe 32 oppositely directed down the highway with a second back lobe 34 directed up the highway 18. Under ideal propagation conditions, that is, in the absence of multi-path reflections or unusual atmospheric conditions, reception of the signal produced by a forward lobe can be expected to occur ideally without detection of signals produced by the back lobe of the other antenna. In close proximity to the beacon station 12, the back lobe radiation power level is above a squelch level for detection and thus alternating signals can be expected to be received from both antennas without difficulty. 
     The transmitter beacon station 12 comprises in addition a switcher 36 coupled to each of the antennas 22 and 24 for alternately switching an input RF signal between the two antennas. The switcher is driven by a transmitter 38 generating signals in preferably the 1.4 GHz frequency band (L-band) at power levels of less than a few watts and ideally at power levels of only a few milliwatts. The transmitter 38 is driven by a modulator/controller 49. The modulator/ controller 49 has as its primary function the generation of alternating first and second location signals to alternately switch the switcher between the respective first and second antennas 22 and 24 via a control line 31. The modulator/controller 49 provides switching synchronization signals to the switcher 36 (via line 42). Preferably the modulator/controller 49 produces digitally-generated modulation signals representing packets of information as explained hereinafter, and the control signal on line 31 merely causes the switcher 36 to alternate the transmitter 38 signal output between the first antenna 22 and the second antenna 24 with each alternating packet signal transmission. 
     The modulator/controller 49 may comprise for example a terminal nod controller which is programmed to send alternating beacon signals through an internal processor. The controller portion could also be a simple external exciter which generates alternating beacon signals and controls the switcher 36. 
     Further according to the invention, the receiver subsystem 14, which is installed in the truck 16, comprises an omnidirectional antenna 44 coupled to a receiver 46 which in turn drives a demodulator 48. The demodulator 48 provides digital signals to a digital analyzer 50, the operation of which is hereinafter explained. The output of the digital analyzer is provided to a conventional on-board computer (OBC) 51, which is carried in or mounted to the vehicle as part of the normal control and monitoring function of an interstate truck. The receiver 46 is a receiver matched to the frequency and modulation of the transmitter 38. The demodulator 48 is for demodulating the signals produced by the modulator/controller 49. The demodulator 48 may for example, be a packet terminal node controller or equivalent device for demodulating packet signals sent in a frequency-shift keyed (FSK) format. 
     Because the beacon system 10 produces RF signals which are fixed in source and only approximately directionally sensitive, it is helpful for the location signals produced by the transmitter beacon station 12 to contain information useful for identifying location of the beacon and direction of travel of a receiving subsystem 14 having only an omnidirectional receiving antenna. Unlike expensive and relatively-complex radial direction finding systems of the prior art, the present invention employs information embedded in a pair of alternating location signals to determine direction and further analyzes the reception pattern of those location signals to determine location. Moreover, other information may also be conveyed in packetized format from the beacon station 12 to the receiving subsystem 14, such as station identification. 
     Referring to FIG. 3, there is shown the format for a broadcast message packet in a specific embodiment of the invention. The packet format 152 comprises for example twelve characters 153-164, each of which is an ASCII 10-bit or an 8-bit character. Characters 153 and 154 are each binary packet framing characters. Frames 155 and 156 are a two-character mnemonic for a state, province, country or the like indicating the jurisdiction which is being approached by a receiving station and which is broadcast from the directional antenna with preference toward the approaching vehicle. Characters 157-160 are for the route number or more specifically a transmitter location identifier. Characters 161-163 are a representation of the distance between the beacon site and the landmark or boundary to be marked, for those systems where the beacon must be displaced from the exact boundary or a state border. Character 164 is a checksum of the previous characters for use by the receiver in verifying the integrity of the received data packet. Other information could be included in an extended packet, such as date and time. However, a shorter beacon signal packet is preferred over a longer beacon signal packet due to the susceptibility of packets to corruption in actual operating environments. 
     FIG. 4 illustrates a typical data pattern received at a mobile receiver from a fixed beacon station 12 broadcasting alternating packets in nominally opposite directions as a mobile receiver 14 approaches and passes the fixed beacon station 12 (FIG. 1). In this example, the data pattern 66 is represented by the two mnemonics: &#34;OR&#34; (for Oregon) and &#34;CA&#34; (for California). This data pattern is received by a vehicle 16 approaching the California border from Oregon along a highway which is subject to multi-path reflections. In a preferred embodiment, the timing diagram for this data pattern would represent approximately three packets per second, or 180 packets over a period of one minute where, if the packet is twelve characters long, about 21,000 bits are transmitted during a minute. The typical data pattern is that a certain number of packets are detected and received at the beginning of signal acquisition from both antennas (Period A). Thereafter, consecutive packets are received from the antenna whose main lobe points toward the approaching receiver (Period B). Thereafter, as the receiver nears and passes the station, both signals are detectable by the receiver and hence detects an alternating pattern of signals (Period C). Thereafter, as the moving receiver moves further from the site of the beacon transmitter, there will be a sequence of packets from the second antenna which does not alternate with the packets from the first antenna (Period D). Finally, there may be a period when the packets again alternate, due to multi-path effects (Period E). The first period A and the last period E are uncommon but generally brief when they occur. However, such occurrences must be taken into account. Periods B and C generally extend for a substantial amount of time, as does Period D. A typical sequence involves receipt of about eighty complete and error-free packets. 
     The fact that there is a sequence of like packets can be used to identify direction in accordance with the invention. The onset and termination of both periods B and D of the sequences of like packets can be used in connection with a simple clock to determine with relative precision the location of the beacon signal relative to the path of travel. This analysis is carried out in the digital analyzer 50. 
     Referring to FIG. 5, there is shown one possible embodiment of a digital analyzer 50 in accordance with the invention. The digital analyzer 50 comprises a packet input 52 coupled to a first half-circular buffer 54 and through a data filter 56 to a second half-circular buffer 58. The first half-circular buffer 54 provides output to a detector 60 which in turn feeds a selector 62. The first half-circular buffer 54 is for capturing and analyzing only the state mnemonics (fields 55 and 56 of FIG. 3). The selector 62 is for designating to the second half-circular buffer which packets are to be used for further analysis. The output of the second half-circular buffer 58 is provided to a crossing time calculator 61 whose function is to calculate the crossing time from the relative timing of the packets received in the second half-circular buffer 58. The crossing time calculator 61 provides its output in the form of a time mark or an interrupt to an output interface 63, which in turn is coupled to the on-board computer 51 (FIG. 2). 
     The data filter 56 is used to reject packets incompletely received or received in error. The data filter 56 is operative to compare the checksum field 64 (FIG. 3) with the sum of bits in the associated packet. Only those packets which are received correctly are stored in the second half-circular buffer 58. 
     In contrast, the first half-circular buffer 54 captures the state mnemonics, whether or not the associated packet is received free of error. 
     An important feature of the invention is the half-circular buffer. FIG. 6 illustrates one possible embodiment of a half-circular buffer 54. A half-circular buffer is a digital storage device which in this embodiment loads initially-received data in a first half of the buffer and last-received data in a second half of the buffer, the last-received data being determined by successively overwriting the storage locations or cells in the last half of the buffer. The half circular buffer 54 may be a digital memory device of sufficient word width such that each cell can contain an entire packet or that portion of the packet of interest in accordance with the invention. The cell address to which a packet is written is determined by the following process: 
     For cells numbered 1 through N, the address is X, where X is a variable; 
     For cells numbered N+1 through 2N, the address is: 
     
         N+X modulo M. 
    
     This addressing scheme can be depicted as in FIG. 6 wherein a first or monotonic switch 68 demultiplexes packet data input at an input port 70 to storage locations 1-15, where N=15, and then directs data to a second or recirculating switch 72 so that the address cycles between for example, 16 and 30. Thus, in half-circular buffer 54, the first half of the buffer 74 always contains the first fifteen packets and the last half of the buffer 76 always contains the last fifteen packets. Output of the packets is provided over a first bus 78 and a second bus 80 to crossing time calculator 60 (FIG. 5). 
     FIG. 7 is a flow chart illustrating the operation of processing in a digital analyzer 50 in accordance with the invention. In a software-controlled operation, this process is based on instructions carried out in a microprocessor. However, a microprocessor is not necessary for the operation of a system according to the invention. 
     Each cycle starts with a determination that an event is in process (step F). If the test indicates that an event is in progress, the operation proceeds. The processor tests to determine whether there are characters at an input port to the receive modem which are ready to be read (step J). If yes, the processor shifts in a character (step K) and tests to determine whether a sync frame is present (step L). If no sync frame is present, the processor tests to determine whether the data is a valid and recognized state mnemonic (step M). If there is a recognized state mnemonic, the state mnemonic is inserted in the next available insertion point in the first half-circular buffer 54 (step N). If not, the system tests whether there is a message of proper length (step 0) and if so, it determines whether it is a completely valid message (step P). If it is a valid message, the message buffer is updated along with its category (step Q) and then resets counters (step R). If it is not a valid message, it simply resets the counters (step R). If the message is not of the proper length, the system also resets the counters (Step R). If the sync frame has been received (at step L), the input buffer is reset (step S) and the processor shifts in a new signal strength reading (if the receiver is equipped to provide such a reading), signal strength being one of the parameters which is monitored through the receiver (step T). Thereupon the counters are also reset (step R). 
     If there are characters at the port ready to be read in (at step J), then the processor tests to determine whether the event has timed out, that is, whether the approach and passing of the beacon has occurred (step U). If the event has not timed out, interrupts are enabled (step V) and the counters are reset (step R). If the event has timed out, then the crossing point is determined by analyzing the crossing (step W), as hereinafter explained. 
     FIG. 8 is a flow chart for a specific embodiment of a crossing analysis (step W, FIG. 7). Initially, the first ten state mnemonics from the first half-circular buffer 54 are examined to determine if in those first ten mnemonics there is a series of three identical state mnemonics in a row (step AA). If not, it is immediately concluded that no crossing has occurred (step AB), the analysis is terminated, returned and the counters reset (FIG. 7 step R). If three identical state mnemonics in a row are detected, then the first three of such series are selected and designated state A (step AC). Next, the last ten state mnemonics received are examined from the first half-circular buffer 54 and tested to see if there is a series of three identical state mnemonics in a row (step AD). If not, it is immediately concluded that no crossing has occurred (step AB). If three identical state mnemonics in a row are identified, then the processor chooses the last of such series and categorizes them as a state B (step AE). Thereafter, the processor determines whether state A and state B are the same (step AF). If so, it is immediately indicated that no crossing has occurred (step AB). If not, then the processor tests to determine whether valid site information exists in the second half-circular buffer 58 (FIG. 5) which corresponds to the mnemonics for states A and B (step AG). If such information does exist, the processor tests to determine whether the site information for said states A and B agree, indicating that the packets producing that site information came from the same site (step AH). If not, no crossing could have occurred (step AB). However, if the site information agrees, the processor calculates the estimated border crossing time (step AI). The calculation is made by comparing the time of last occurrence of the first state A with the time of first occurrence of the second state B and dividing the time by two. Thereafter, the processor reports that a border crossing has occurred, along with the relative time of crossing and offset from the boundary of interest, to the on-board computer 51 (step AJ) and returns to reset the counters (step R FIG. 7). A source code listing in the c language of the relevant processes herein described is attached in an appendix. 
     The invention has now been explained with reference to a specific embodiment. Other embodiments will be apparent to those of ordinary skill in the art. It is therefore not intended that the invention be limited, except as indicated by the appended claims. ##SPC1##