GPS-based centralized tracking system with reduced energy consumption

• US 6,104,978 A
• Filed: 04/06/1998
• Issued: 08/15/2000
• Est. Priority Date: 04/06/1998
• Status: Expired due to Term

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Harrison, Daniel David, Welles, Kenneth Brakeley II
• Primary Examiner(s): NGUYEN, TAN QUANG
• Application Number:
  US09/055,336
• Time in Patent Office: 862 Days
• Field of Search: 701/207, 701/213, 701/214, 701/215, 701/216, 701/223, 701/210, 342/357, 342/457, 340/992, 340/989, 470/316
• US Class Current: 701/207
• CPC Class Codes: G01S 19/09 providing processing capability normally carried out by the receiver
• CPC Class Codes: G01S 19/243 Demodulation of navigation message
• CPC Class Codes: G01S 19/25 involving aiding data received from a cooperating element, e.g. assisted GPS
• CPC Class Codes: G01S 19/34 Power consumption
• CPC Class Codes: G01S 5/0036 of measured values, i.e. measurement on mobile and position calculation on base station

Abstract

A communication method reduces both the asset energy requirements and communication channel occupancy in a reduced order GPS asset tracking system. The asset is provided with an indication of its position so as to allow detection of substantial movement of the asset at the asset. Reduced order GPS data derived at the asset, such as a railcar, is compressed to allow for shorter railcar data transmission times. This, in turn, reduces both the transmitter energy requirements and communication-channel occupancy, while allowing substantial railcar position changes to be conveniently detected at the railcar. This capability can be used to save further substantial railcar energy by, for example, reporting to the central station less frequently when the railcar position is unchanging.

Claims

1. A method for identifying location of an asset to be tracked comprising the steps of:

receiving signals transmitted from a plurality of Global Positioning System (GPS) satellites at said asset;

detecting a GPS telmetry word preamble (TWP) and measuring data related to propagation differences between signals transmitted from GPS satellites and received at said asset;

deriving satellite identifications for each of said GPS satellites;

maintaining an auxiliary GPS (AGPS) data base and a GPS hand-over word (HOW) table to determine TWP start time at said asset;

computing an estimated location of said asset from said measured data, AGPS and HOW data and a clock in approximate synchronization with GPS time;

encoding and transmitting to a central station the satellite identifications and the computed estimated position;

decoding, at said central station, data received from said asset; and

calculating, at said central station, a corrected location of said asset based upon the transmitted data and data derived from at least one receiver spaced apart from said asset.

2. The method recited in claim 1 wherein the steps of encoding and decoding include, at the asset, differential encoding of an absolute coordinate value of the computed estimated location of the asset to generate a relative coordinate value which is transmitted to said central station and, at the central station, differential decoding of the relative coordinate value to generate an absolute coordinate value corresponding to the computed estimated location of the asset.

3. The method recited in claim 1 including, prior to the step of detecting a GPS telemetry word preamble, the step of determining a time window during which the GPS TWP is expected.

4. The method recited in claim 3 wherein the steps of encoding and decoding include, at the asset, differential encoding of an absolute coordinate value of the computed estimated location of the asset to generate a relative coordinate value which is transmitted to said central station and, at the central station, differential decoding of the relative coordinate value to generate an absolute coordinate value corresponding to the computed estimated location of the asset.

5. A method recited in claim 3 wherein the step of determining a time window during which the GPS TWP is expected includes:

specifying a sub-frame index for a scheduled asset position measurement;

predicting the associated TWP transmission time for each expected receivable satellite signal;

estimating the associated TWP reception times at the asset;

processing received signals for sufficient time to reliably complete detection, identification and acquisition of marker timing for four satellite signals before the TWPs are received;

choosing, as a reference satellite signal, the satellite signal having the lowest GPS satellite index; and

setting a TWP detection-gate time so that the correct TWP will be detected.

6. The method recited in claim 1 wherein the step of calculating, at said central location, a corrected location of said asset, is additionally based upon stored data representing geographical features.

7. A reduced order global positioning system (GPS) asset tracking system comprising:

a tracking unit on an asset to be tracked for receiving signals from a plurality of GPS satellites, determining a time window during which a GPS telemetry word preamble (TWP) is expected, detecting the TWP and measuring data related to propagation differences between signals received from a plurality of GPS satellites deriving satellite identifications for each of said GPS satellites, maintaining an auxiliary GPS (AGPS) data base and a GPS hand-over word (HOW) table to determine a TWP start time, computing an estimated position from each of the measured, AGPS and HOW data and from a clock in approximate synchronization with GPS time, and encoding and transmitting the satellite identifications and the computed estimated position, and

a central station adapted to decode received data from said asset and to calculate a corrected location of said asset based upon the transmitted data and data derived from at least one receiver spaced apart from said asset and adapted to receive said signals from said plurality of GPS satellites.

8. The reduced order global positioning system (GPS) asset tracking system recited in claim 7 wherein said tracking unit comprises:

a GPS receiver for receiving signals from a plurality of GPS satellites;

a GPS signal processor for receiving output signals from said GPS receiver and generating said satellite identifications for each of said GPS satellites;

a command receiver for receiving command and AGPS data signals from said central station;

a data base for storing said AGPS data, including ephemerides data, from said command receiver and maintaining a HOW, said data base being adapted to receive HOW updates from said GPS signal processor;

a clock in approximate synchronization with GPS time;

a satellite position calculator for receiving said satellite identifications from said GPS signal processor, a time signal from said clock, and ephemerides and HOW data from said data base and for calculating satellite positions and frame-time offsets;

an asset position calculator for receiving the calculated satellite positions and frame-time offsets and for calculating said estimated position; and

a transmitter coupled to said GPS signal processor and said asset position calculator for encoding and transmitting said satellite identifications and calculated estimated position to the central station.

9. The reduced order global positioning system (GPS) asset tracking system recited in claim 8 further comprising, at the central station:

a second GPS receiver for receiving signals from a plurality of GPS satellites;

a master GPS data base adapted to be periodically updated by new data from said second GPS receiver;

an asset AGPS data base and HOW tables maintained as a copy of the AGPS data base and HOW tables used at the asset;

a satellite time and position calculator for receiving accurate ephemerides and times from the master GPS data base and approximate ephemerides and times from the asset AGPS data base and HOW tables and for calculating accurate satellite position and transmission times associated with an asset transmission;

a report receiver for receiving the encoded satellite identifications and calculated estimated position transmitted from the asset; and

an asset position calculator for receiving from the satellite time and position calculator the calculated accurate satellite position and transmission times associated with an asset transmission and satellite identifications and calculated estimated position from the report receiver and for calculating an accurate asset position.

10. The reduced order global positioning system (GPS) asset tracking system recited in claim 8 and further comprising, at the asset, a differential encoder for differentially encoding an absolute coordinate value of the calculated estimated position of the asset to generate a relative coordinate value for transmission to said central station, and a differential decoder at the central station for differentially decoding the relative coordinate value transmitted by the asset to generate an absolute coordinate value corresponding to the calculated estimated position of the asset.

11. A method of determining, at the tracked asset, an approximate position of the asset comprising the steps of:

energizing a receiver for a time interval during which a Global Positioning System (GPS) telemetry word preamble (TWP) is expected;

processing a GPS signal received during said time interval to acquire code and data-bit timing for four GPS satellite signals;

recognizing a TWP starting edge in each satellite signal;

choosing a signal with a lowest GPS index as a reference signal and establishing bit or codeword edges as markers in each signal;

maintaining an auxiliary GPS (AGPS) data base and a GPS hand-over word (HOW) table to determine TWP start time at the asset;

reading a marker count from the AGPS data base during a measurement instant selected to be interger marker periods after the starting edge of the TWP in the reference signal;

measuring marker counts and phases for a measurement instant, the counts being measured with respect to the starting edge of the TWP in each satellite signal;

reading a pre-determined transmitter marker period and frame-time differences from the AGPS data base and then calculating propagation time differences;

reading a HOW time value from the reference signal and calculating the reference signal transmission time;

calculating remaining transmission times;

using the calculated transmission times with approximate ephemerides from the AGPS data base to determine satellite locations; and

using the determined satellite locations to calculate the asset position.

12. The method of claim 11 wherein the measurement instants for each of the reference signal and the satellite signals are identical.