Reduced-power GPS-based system for tracking multiple objects from a central location

• US 5,752,218 A
• Filed: 08/25/1997
• Issued: 05/12/1998
• Est. Priority Date: 05/31/1995
• Status: Expired

• Current Assignee: Asset Intelligence LLC (PowerFleet, Inc.)
• Original Assignee: General Electric Company
• Inventors: Pradeep, Anantha Krishnan, Harrison, Daniel David, Hladik, Stephen Michael, Brooksby, Glen William
• Primary Examiner(s): NGUYEN, TAN QUANG
• Application Number:
  US08/924,478
• Time in Patent Office: 260 Days
• Field of Search: 701/207, 701/213, 701/214, 701/215, 701/216, 701/223, 701/300, 342/357, 342/356, 342/358, 342/450, 342/457
• US Class Current: 701/207
• CPC Class Codes: B61L 2205/04 Satellite based navigation systems, e.g. global positioning system [GPS]
• CPC Class Codes: B61L 25/025 Absolute localisation, e.g. providing geodetic coordinates
• CPC Class Codes: G01S 19/09 providing processing capability normally carried out by the receiver
• CPC Class Codes: G01S 19/34 Power consumption
• CPC Class Codes: G01S 5/0027 of actual mobile position, i.e. position determined on mobile
• CPC Class Codes: G01S 5/0036 of measured values, i.e. measurement on mobile and position calculation on base station
• CPC Class Codes: G01S 5/10 Position of receiver fixed by co-ordinating a plurality of position lines defined by path-difference measurements , e.g. omega or decca systemsG01S5/12 takes precedence ; beacons and receivers cooperating therewith G01S1/306, G01S1/308
• CPC Class Codes: G01S 5/14 Determining absolute distances from a plurality of spaced points of known location
• CPC Class Codes: G08G 1/20 Monitoring the location of vehicles belonging to a group, e.g. fleet of vehicles, countable or determined number of vehicles

Abstract

Location of an object to be tracked is determined by measuring, at a receiver situated at the object, the propagation time differences between the signals from a plurality of GPS satellites, each of which is received by the receiver situated at the object. The measured propagation time difference values are transmitted to a central station where the location of the object to be tracked is calculated based upon the propagation time differences of the signals transmitted from the satellites and data derived from a receiver apart from the object but also receiving signals from the satellites.

Claims

1. A method for identifying location of an object to be tracked, comprising:

measuring data related to propagation time differences between signals transmitted from a plurality of GPS satellites and received at said object to be tracked, said data comprising code word phase measurements μ

_i for a satellite at a time t_R, where μ

₁ =γ

_i /T_i^C, and defined as time elapsed to time t_R from the beginning of a code word in the signal from satellite i in which t_R falls, T_i^C being defined as the code period for satellite i at time t_R in the signal received from satellite i, said code word phase measurements being simultaneously derived from the signals transmitted from said plurality of satellites and received at the object to be tracked;

transmitting said data to a central station; and

calculating at said central station the location of said object to be tracked based upon the transmitted data and data derived from at least one receiver apart from said object to be tracked receiving said signals from said plurality of satellites.

2. The method of claim 1 wherein the data transmitted to the central station includes satellite identification data so that the step of calculating the location of said object to be tracked is thereupon based further upon the satellite identification data.

3. The method of claim 1 wherein the step of calculating the location of said object comprises calculating a point of intersection of curves defined by said propagation time differences.

4. The method of claim 1 including the step of transmitting time signals to said object to be tracked over a separate channel so as to maintain clock accuracy at said object to be tracked.

5. A method for identifying location of an object to be tracked comprising:

measuring data related to propagation time differences between signals transmitted from a plurality of GPS satellites and received at said object to be tracked, said data comprising bit phase measurements μ

_i for a satellite i at a time t_R, where μ

_i =β

_i /T_i^B, β

_i being the receiver bit-time offset for satellite i and defined as time elapsed to time t_R from the beginning of a code word in the signal from satellite i in which t_R falls, T_i^B being defined as the bit period for satellite i at time t_R in the signal received from satellite i, said bit phase measurements being simultaneously derived from the signals transmitted from said plurality of satellites and received at the object to be tracked;

transmitting said data to a central station; and

calculating at said central station the location of said object to be tracked based upon the transmitted data and data derived from at least one receiver apart from said object to be tracked receiving said signals from said plurality of satellites.

6. The method of claim 5 and further including:

recording, at said object, the time at which the data are simultaneously derived; and

transmitting the recorded time to said central station.

7. The method of claim 5 and further including:

measuring, at said object to be tracked, delay between the time at which the data are recorded and the time when the data are transmitted to the central station; and

transmitting the measured delay to said central station.

8. The method of claim 5 wherein the data transmitted to the central station includes satellite identification data and the step of calculating the location of said object to be tracked is additionally based upon the satellite identification data.

9. The method of claim 5 wherein the step of calculating the location of said object comprises calculating a point of intersection of curves defined by said propagation time differences.

10. The method of claim 5 wherein the signals from said GPS satellites are received at said object to be tracked, and including the step of transmitting time signals to said object to be tracked over a separate channel so as to maintain clock accuracy at said object to be tracked.

11. A method for identifying location of an object to be tracked comprising:

measuring data related to propagation time differences between signals transmitted from a plurality of satellites and received at said object to be tracked, said data comprising receiver code-time offsets for a satellite i and defined as time elapsed to a time t_R from the beginning of a code word in the signal from satellite i in which t_R falls, and code periods in the signal received from satellite i in which time t_R falls, said plurality of satellites comprising GPS satellites, and including the additional step of simultaneously deriving said receiver code-time offsets and code periods from signals received from the plurality of satellites at said object to be tracked;

transmitting said data to a central station; and

calculating at said central station the location of said object to be tracked based upon the transmitted data and data derived from at least one receiver apart from said object to be tracked receiving said signals from said plurality of satellites.

12. A method for identifying location of an object to be tracked, comprising:

measuring data related to propagation time differences between signals transmitted from at least four GPS satellites and received at said object to be tracked, said data related to propagation time differences comprising bit phase measurements simultaneously derived from said signals;

transmitting said data, including satellite identification data, to a central station;

measuring, at said object to be tracked, delay between the time at which the data are recorded and the time when the data are transmitted to the central station;

transmitting the measured delay to said central station; and

calculating at said central station the location of said object to be tracked based upon the transmitted data, the satellite identification data, and data derived from at least one receiver apart from said object to be tracked receiving said signals from said plurality of satellites, the calculating step comprising;

assuming a feasible value for a communication time delay required for a signal transmitted from said object to be tracked to reach the central station;

calculating the location of said object to be tracked based upon the satellite identification data and the assumed value of said communication time delay;

calculating a new value for said communication time delay based upon the calculated location of said object to be tracked; and

calculating a corrected location of said object to be tracked based upon the calculated new value for said communication time delay.

13. The method of claim 12 including the additional step of:

iteratively repeating the calculating steps until little change in location of said object to be tracked is observable.

14. A system for identifying location of an object to be tracked, comprising:

means for measuring data related to propagation time differences between signals transmitted from a plurality of GPS satellites and received at said object to be tracked, each of said signals identifying an associated satellite, said object to be tracked including;

receiver means for receiving signals from at least four GPS satellites; and

first processor means for processing data from the receiver means at predetermined time intervals in synchronism with received signal events, said data being related to propagation time differences for said signals;

receiver means apart from said object for receiving said signals transmitted from said plurality of satellites;

a central station for calculating the location of said object based upon the measured data, data derived from said receiver means apart from said object, and the satellite identification data; and

transmission means for transmitting the processed data to said central station;

said system further including;

second processor means at said central station for determining the location of said object based on the data received from said transmission means and data derived from said receiver means apart from said object.

15. The system of claim 14 wherein said signal events comprise a telemetry-word preamble signal event in a GPS signal.

16. The system of claim 15 wherein said first processor means further comprises means for decoding a satellite time stamp from a predetermined one of the received GPS signals, based upon the telemetry-word preamble signal event.

17. A method for identifying location of an object to be tracked comprising:

means for measuring data related to propagation time differences between signals transmitted from a plurality of GPS satellites and received at said object to be tracked, each of said signals identifying an associated satellite, said object to be tracked including;

receiver means for receiving signals from at least four GPS satellites; and

first processor means for calculating a receiver bit phase for each of said satellites, said bit phase for any satellite i at a time t_R being defined as β

_i /T_i^B, β

_i being the receiver bit-time offset for satellite i and defined as time elapsed to time t_R from the beginning of a code word in the signal from satellite i in which t_R falls, and T_i^B being defined as the bit period for satellite i at time t_g in the signal received from satellite i;

receiver means apart from said object for receiving said signals from said plurality of satellites;

a central station; and

transmission means for transmitting the calculated bit phases to said central station;

said system further including;

second processor means at said central station for determining signal propagation times between said plurality of satellites and said object and for determining location of said object based upon the bit phases transmitted by said transmission means and data derived from said receiver means apart from said object.

18. A system for identifying location of an object to be tracked, comprising:

means for measuring data related to propagation time differences between signals transmitted from a plurality of GPS satellites and received at said object to be tracked, each of said signals identifying an associated satellite, said object to be tracked including;

receiver means for receiving signals from at least four GPS satellites, and

first processor means for calculating a bit-time offset for each of said satellites and for determining a bit period for each signal received from said satellites, said bit-time offset for a satellite i being defined as time elapsed to a time t_R from the beginning of a code word in the signal from satellite i in which t_R falls, said bit period for satellite i being determined at time t_R in the signal from satellite i;

receiver means apart from said object for receiving said signals from said plurality of satellites;

a central station; and

transmission means for transmitting time stamps, the calculated bit-time offsets and bit periods, and satellite identification data, to said central station;

said system further including;

second processor means at said central station for determining signal propagation times between said plurality of satellites and said object and for determining location of said object based upon the bit-time offsets and periods, time stamps, satellite identification data transmitted by said transmission means, and data derived from said receiver means apart from said object.