Vehicle location system

Abstract

Signals from a number of NAVSTAR global positioning system (GPS) satellites (11,12,13,14) are received by a receiver (16) in a vehicle (15) and a segment of the signals is stored in a memory (18) prior to retransmission by a transmitter (19). A base station (35) receives these transmissions from the mobile unit using a first receiver (36). The base station also receives signals directly from the NAVSTAR GPS satellites using a second receiver (38). A control and calculating apparatus (37) within the base station can determine the ephemeris (course) information for the satellites and can measure the transmission times or propagation delays of signals between the satellites and the vehicle and with this information the control and calculating apparatus can calculate the position of the vehicle unit.

Description

This invention relates to a by call cell of a cellular radio system is being used to retransmit the signals to the base station. If the vehicle position is not known to this degree of accuracy (100 km may be less than one hour's motoring) the data bit edges on the satellite signals can act as timing marks with a spacing of 20 ms. Since the modulation of the satellite signal by the data is synchronised to an atomic clock, the position of the data bit edges in the received, despread signals gives a coarse measure of transit time which is nonetheless accurate to within one millisecond. To use this measurement technique, at least 20 ms of satellite signals will need to be recorded to ensure that the recording contains a data bit edge from each satellite. A third alternative is to use the Doppler shift on the received GPS signals to calculate an approximate user position. However, this method still requires at least 20 ms of satellite data and is mathematically more complex, especially if the user's vehicle is in motion.

FIG. 4 is a block schematic diagram of a mobile receiver and transmitter suitable for use in a vehicle locating system in accordance with the present invention. Satellite signals are received at an antenna 20 which feeds an rf amplifier 22. The input stage of the rf amplifier 22 will usually include a bandpass filter. The output of the amplifier 22 is mixed with the output of local oscillator 24 in a mixer 23 and the output of the mixer is filtered by a bandpass filter 26. Although only one down-conversion stage is shown, the front end of the receiver could include two or more such stages. The nominal intermediate frequency to which the satellite signals are mixed down could be anything from zero to several MHz. In the case of a zero IF receiver, the filter 26 would be a low pass type. The output of filter 26 is digitised in an analogue to digital converter 27 whose sampling rate is determined by the Nyquist sampling criterion.

The output of the analogue to digital converter 27 is stored in a random access memory (RAM) 28 which is addressed by a counter 31, the counter itself being under the control of a receiver controller 30. The size of this RAM will be determined by the rate of sampling and the length of time that the incoming satellite signals are to be recorded for. For example, sampling at 2.046 MHz (to satisfy the Nyquist criterion) for 8 ms will require just under 16 kbits of memory. The contents of the RAM 28 are transmitted serially by transmitter 32 via antenna 33. In a practical system the transmitter 32 may be part of an existing transceiver within the mobile unit.

These signals are received and processed by the base station, an embodiment of which is shown in block schematic form in FIG. 5. The retransmitted signals from the mobile unit are received by antenna 43 and fed to a transceiver 44. Again, the transceiver 44 could be part of an existing communications link. A base station controller 42 is connected to the transceiver and in addition to receiving the signals from the mobiles and calculating their positions it maintains an up to date copy of the ephemeris data for all the satellites currently in view. The GPS signals are received by a GPS receiver 38 via an antenna 40. The purpose of this receiver is to decode satellite ephemeris and clock correction data and it will probably also decode almanac data to facilitate satellite signal acquisition. Since positional information is not required for the base station it does not need to determine the propagation delays of the GPS signals. It is thus possible to use a signal despreading technique based on non-coherent demodulation which does not use any logically generated C/A codes. In all other respects the satellite data is received as described previously for a conventional system and stored in a RAM 41 for use by the base station controller 42 in calculating the satellite pseudoranges in respect of the or each vehicle. One advantage of using a complete GPS receiver at the base station rather than one employing a non-coherent demodulation technique is that it permits location fixes to be made by a differential technique. The base station uses the GPS to determine its own position and, since this is already known accurately, can calculate an up to date error term for the GPS. When the mobile unit(s) position is calculated, this error can be removed from the mobile unit's pseudoranges which gives an improvement in the accuracy of the positional fix. The transceiver 44 enables request signals to be passed from the base station to the mobile units for commencement of data logging and/or data transmission. It can also, if required, relay vehicle position or directions back to the driver of the vehicle.

From reading the present disclosure other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of GPS systems and component parts thereof and which may be used instead of or in addition to features already described herein. Athough claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

1. A vehicle location system for use in a global positioning system (GPS), comprising at least one vehicle mounted equipment including means for receiving signals directly from the GFS, a fixedly sited base station including first means for receiving signals directly from the GPS, characterised in that the vehicle mounted equipment includes means for recording the received GPS signals and means for retransmitting the recorded GPS signals to the base station, and in that the fixedly sited base station includes second means for receiving the recorded GPS signals retransmitted by the vehicle mounted equipment, and position determining means coupled to the first and second receiving means for determining the position of the vehicle at the time when the vehicle mounted equipment received the GPS signals.

2. A vehicle location system as claimed in claim 1, wherein the vehicle mounted equipment transmits a time of arrival (TOA) signal in addition to retransmitting the recorded GPS signals.

2. A mobile equipment location system as claimed in claim 17, wherein the base station includes a means for obtaining the GPS ephemeris for satellites of the GPS. 33. A mobile equipment location system as claimed in claim 18, wherein the mobile equipment transmits a time of arrival (TOA) signal in addition to retransmitting the recorded GPS signals. 34. A mobile equipment location system as claimed in claim 18, wherein the rate at which the mobile equipment retransmits the GPS signals is lower than that at which the signals were recorded. 35. A mobile equipment location system as claimed in claim 34, wherein the mobile equipment further comprises control means coupled to the recording means such that signals from the control means cause the recording means to record the GPS signals at preset intervals. 36. A mobile equipment location system as claimed in claim 33, wherein the GPS is the satellite-based NAVSTAR GPS, and in that the base station has means for obtaining the GPS ephemeris for the satellites in use. 37. A mobile equipment location system as claimed in claim 36, wherein the means for obtaining the GPS ephemeris in the base station has means for despreading the NAVSTAR GPS signals without using any locally generated pseudo random noise codes. 38. A mobile equipment for use in a global positioning system (GPS) with a mobile equipment location system and including means for receiving GPS signals, the mobile equipment location system comprising a base station at a known location including first means for receiving signals directly from the GPS, second means for receiving recorded GPS signals transmitted by the mobile equipment, and position determining means coupled to the first and second receiving means for determining the position of the mobile equipment at the time when the mobile equipment received the GPS signals, said mobile equipment including means for recording the received GPS signals and means for retransmitting the recorded GPS signals. 39. A mobile equipment for use in a global positioning system (GPS) with a mobile equipment location system and including means for receiving GPS signals, the mobile equipment location system comprising a base station at a known location including first means for receiving signals directly from the GPS, second means for receiving recorded GPS signals transmitted by the mobile equipment, and position determining means coupled to the first and second receiving means for determining the position of the mobile equipment at the time when the mobile equipment received the GPS signals, said mobile equipment including means for recording the received GPS signals and means for retransmitting the recorded GPS signals and a time of arrival (TOA) signal. 40. A base station at a known location for use in a qlobal positioning system (GPS) with a mobile equipment location system comprising at least one mobile equipment including means for receiving signals directly from the GPS, means for recording the received GPS signals, and means for retransmitting the recorded GPS signals to the base station, said base station including first means for receiving GPS signals directly from the GPS, second means for receiving a retransmission of GPS signals from the mobile equipment and means coupled to said first and second receiving means for determining the position of the mobile equipment at the time the GPS signals were received. 41. A base station as claimed in claim 40, wherein the means for determining the position of the mobile equipment calculates the position of the base station using the GPS and then calculates the position of the mobile equipment using a differential technique. 42. A mobile equipment location system as claimed in claim 33, wherein the rate at which the mobile equipment retransmits the GPS signals is lower than that at which the signals are recorded. 43. A mobile equipment location system as claimed in claim 42, wherein the mobile equipment further comprises control means coupled to the recording means such that signals from the control means cause the recording means to record GPS signals at preset intervals. 44. A mobile equipment location system as claimed in claim 33, wherein the mobile equipment further comprises control means coupled to the recording means such that signals from the control means cause the recording means to record GPS signals at preset intervals. 45. A mobile equipment location system as claimed in claim 18, wherein the mobile equipment further comprises control means coupled to the recording means such that signals from the control means cause the recording means to record GPS signals at preset intervals. 46. A mobile equipment location system as claimed in claim 18, wherein the base station includes a means for obtaining the GPS ephemeris for satellites of the GPS. 47. A mobile equipment location system as claimed in claim 17, wherein the means for determining the position of the mobile equipment calculates the position of the base station using the GPS and then calculates the position of the mobile equipment using a differential technique. 48. A mobile equipment location system as claimed in claim 18, wherein the means for determining the position of the mobile equipment calculates the position of the base station using the GPS and then calculates the position of the mobile equipment using a differential technique.

3. A vehicle location system as claimed in claim 1 wherein the rate at which the vehicle mounted equipment retransmits the GPS signals is lower than that at which the signals were recorded.

4. A vehicle location system as claimed in claim 3, wherein the vehicle mounted equipment further comprises control means coupled to the recording means such that signals from the control means cause the recording means to record the GPS signals at preset intervals.

5. A vehicle location system as claimed in claim 2, wherein the GPS is the satellite-based NAVSTAR GPS, and in that the base station has means for obtaining the GPS ephemeris for the satellites in use.

6. A vehicle location system as claimed in claim 5, wherein the means for obtaining the GPS ephemeris in the base station has means for despreading the NAVSTAR GPS signals without using any locally generated pseudo random noise codes.

7. A vehicle mounted equipment for use in a global positioning system (GPS) with the system as claimed in claim 1 a vehicle location system and including means for receiving the GPS signals, wherein the equipment also includes the vehicle location system comprising a fixedly sited base station including first means for receiving signals directly from the GPS, second means for receiving recorded GPS signals transmitted by the vehicle mounted equipment, and position determining means coupled to the first and second receiving means for determining the position of the vehicle at the time when the vehicle mounted equipment received the GPS signals, said vehicle mounted equipment including means for recording the received GPS signals and means for retransmitting the recorded GPS signals.

8. A vehicle mounted equipment for use in a global positioning system (GPS) with the system as claimed in claim 2 a vehicle location system and including means for receiving the GPS signals, wherein the equipment includes the vehicle location system comprising a fixedly sited base station including first means for receiving signals directly from the GPS, second means for receiving recorded GPS signals transmitted by the vehicle mounted equipment, and position determining means coupled to the first and second receiving means for determining the position of the vehicle at the time when the vehicle mounted equipment received the GPS signals, said vehicle mounted equipment including means for recording the received GPS signals and means for retransmitting the recorded GPS signals and a time of arrival (TOA) signal.

9. A fixedly sited base station for use in a global positioning system (GPS) with the system as claimed in claim 1 a vehicle location system comprising at least one vehicle mounted equipment including means for receiving signals directly from the GPS, means for recording the received GPS signals, and means for retransmitting the recorded GPS signals to the base station, said fixedly sited base station including first means for receiving GPS signals directly from the GPS, wherein the base station also includes second means for receiving a retransmission of GPS signals from a the vehicle mounted equipment and means coupled to said first and second receiving means for determining the position of the vehicle mounted equipment at the time the GPS signals were received.

10. A fixedly sited base station as claimed in claim 9, wherein the means for determining the position of the vehicle mounted equipment calculates the position of the base station using the GPS and then calculates the position of the vehicle mounted equipment using a differential technique.

11. A vehicle location system as claimed in claim 2, wherein the rate at which the vehicle mounted equipment retransmits the GPS signals is lower than that at which the signals were recorded.

12. A vehicle location system as claimed in claim 11, wherein the vehicle mounted equipment further comprises control means coupled to the recording means such that signals from the control means cause the recording means to record GPS signals at preset intervals.

13. A vehicle location system as claimed in claim 2, wherein the vehicle mounted equipment further comprises control means coupled to the recording means such that signals from the control means cause the recording means to record GPS signals at preset intervals.

14. A vehicle location system as claimed in claim 1, wherein the vehicle mounted equipment further comprises control means coupled to the recording means such that signals from the control means cause the recording means to record GPS signals at preset intervals.

15. A vehicle location system as claimed in claim 1, wherein the base station includes a means for obtaining the GPS ephemeris for satellites of the GPS.

16. A vehicle location system as claimed in claim 1, wherein the means for determining the position of the vehicle mounted equipment calculates the position of the base station using the GPS and then calculates the position of the vehicle mounted equipment using a differential technique.

17. A mobile equipment location system for use in a global positioning system (GPS), comprising at least one mobile equipment including means for receiving signals directly from the GPS, a base station including first means for receiving signals directly from the GPS, characterized in that the mobile equipment includes means for recording the received GPS signals and means for retransmitting the recorded GPS signals to the base station, and in that the base station includes second means for receiving the recorded GPS signals retransmitted by the mobile equipment, and position determining means coupled to the first and second receiving means for determining the position of the mobile equipment at the time when the mobile equipment received the GPS signals. 18. A mobile equipment location system for use in a global positioning system (GPS), comprising at least one mobile equipment including means for receiving signals directly from the GPS, a base station at a known, location, including first means for receiving signals directly from the GPS, characterised in that the mobile equipment includes means for recording the received GPS signals and means for retransmitting the recorded GPS signals to the base station, and in that the base station includes second means for receiving the recorded GPS signals retransmitted by the mobile equipment, and position determining means coupled to the first and second receiving means for determining the position of the mobile equipment at the time when the mobile equipment received the GPS signals.

19. A mobile equipment location system as claimed in claim 17, wherein the mobile equipment transmits a time of arrival (TOA) signal in addition to retransmitting the recorded GPS signals. 20. A mobile equipment location system as claimed in claim 17, wherein the rate at which the mobile equipment retransmits the GPS signals is lower than that at which the signals were recorded. 21. A mobile equipment location system as claimed in claim 20, wherein the mobile equipment further comprises control means coupled to the recording means such that signals from the control means cause the recording means to record the GPS signals at preset intervals. 22. A mobile equipment location system as claimed in claim 19, wherein the GPS is the satellite-based NAVSTAR GPS, and in that the base station has means for obtaining the GPS ephemeris for the satellites in use. 23. A mobile equipment location system as claimed in claim 22, wherein the means for obtaining the GPS ephemeris in the base station has means for despreading the NAVSTAR GPS signals without using any locally generated pseudo random noise codes. 24. A mobile equipment for use in a global positioning system (GPS) with a mobile equipment location system and including means for receiving GPS signals, the mobile equipment location system comprising a base station including first means for receiving signals directly from the GPS, second means for receiving recorded GPS signals transmitted by the mobile equipment, and position determining means coupled to the first and second receiving means for determining the position of the mobile equipment at the time when the mobile equipment received the GPS signals, said mobile equipment including means for recording the received GPS signals and means for retransmitting the recorded GPS signals. 25. A mobile equipment for use in a global positioning system (GPS) with a mobile equipment location system and including means for receiving GPS signals, the mobile equipment location system comprising a base station including first means for receiving signals directly from the GPS, second means for receiving recorded GPS signals transmitted by the mobile equipment, and position determining means coupled to the first and second receiving means for determining the position of the mobile equipment at the time when the mobile equipment received the GPS signals, said mobile equipment including means for recording the received GPS signals and means for retransmitting the recorded GPS signals and a time of arrival (TOA) signal. 26. A base station for use in a global positioning system (GPS) with a mobile equipment location system comprising at least one mobile equipment including means for receiving signals directly from the GPS, means for recording the received GPS signals, and means for retransmitting the recorded GPS signals to the base station, said base station including first means for receiving GPS signals directly from the GPS, second means for receiving a retransmission of GPS signals from the mobile equipment and means coupled to said first and second receiving means for determining the position of the mobile equipment at the time the GPS signals were received. 27. A base station as claimed in claim 26, wherein the means for determining the position of the mobile equipment calculates the position of the base station using the GPS and then calculates the position of the mobile equipment using a differential technique. 28. A mobile equipment location system as claimed in claim 19, wherein the rate at which the mobile equipment retransmits the GPS signals is lower than that at which the signals are recorded. 29. A mobile equipment location system as claimed in claim 28, wherein the mobile equipment further comprises control means coupled to the recording means such that signals from the control means cause the recording means to record GPS signals at preset intervals. 30. A mobile equipment location system as claimed in claim 19, wherein the mobile equipment further comprises control means coupled to the recording means such that signals from the control means cause the recording means to record GPS signals at preset intervals. 31. A mobile equipment location system as claimed in claim 17, wherein the mobile equipment further comprises control means coupled to the recording means such that signals from the control means cause the recording means to record GPS signals at preset intervals.