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

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.

Description

[0001]This application is a continuation of application Ser. No. 08 / 465,229 filed May 31, 1995 now abandoned.FIELD OF THE INVENTION[0002]This invention relates to object tracking systems used to locate and track multiple terrestrial (including water-based) objects and, more particularly, to power efficient object tracking systems based upon information obtained from satellites.BACKGROUND OF THE INVENTION[0003]The tracking and location of assets such as railcars, shipping or cargo containers, trucks, truck trailers, automobiles, etc. can be highly advantageous in commerce. Precise tracking of such vehicles and objects can facilitate their being allocated and positioned in an efficient manner, and can provide for immediate, accurate localization of lost, delayed or damaged assets. The space-based global positioning system (GPS) implemented by the United States Department of Defense constitutes a convenient instrumentality for determining geographical position in real time.[0004]The GP...

AI Technical Summary

Benefits of technology

[0025]Utilizing the present invention, power consumption and calculation complexity at the tracked object are reduced relative to that for a standard GPS receiver. Arrival time differences between satellite signals are measured at the tracked object and this information is relayed to the central station via the separate communications link. Satellite data streams need not be decoded at the tracked object.

[0026]The central station thereby necessarily determines the location of the object to be tracked. Because the receiver front end and data processor use significant power only when processing, the receiver power can be dramatically reduced by being “active” only long enough to get accurate time-difference measurements. This can be less than one second and requires no GPS data-frame synchronization because of the nature of the signals. For example, assuming that the tracked object is a railcar, new railcar locations typically are needed only as frequently as 15 minutes. Thus the receiver energy used is reduced in direct proportion to the reduction of “active” receiver time. Moreover, receiver complexity and cost can be reduced by replacing the advanced microprocessor employed in current GPS receivers with a simpler one that is matched to the arrival-time differencing tasks.

[0028]Another object is to provide a GPS-based asset tracking system which requires minimal energy at the tracked assets.

Problems solved by technology

With receiver noise and imperfect knowledge of satellite positions, the receiver-satellite ranges can only be estimated.

Any error in satellite and receiver clock synchronization leads to proportional range errors.

However, the full position determination is not made at the railcar.

This consumes significant power.

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