Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
1999-11-11
2001-09-25
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C701S213000
Reexamination Certificate
active
06295022
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention is related in general to the field of satellite receivers. More particularly, the invention is related to an apparatus and a method for determination of a receiver position, using GPS (global positioning system).
BACKGROUND OF THE INVENTION
Global positioning system (GPS) has been used for navigation and survey purposes for civilian and military applications. Global positioning system uses a constellation of orbiting satellites or space vehicles (SVs) that send radio signals to earth. The current constellation includes at least 24 operational satellites, which provides between five to eight space vehicles visible from any point on earth. Using the signals received from at least four of the visible space vehicles (the ensemble), an earth position (X,Y,Z) and time of a user can be computed. Typical applications of a global positioning system includes navigation aide and surveying.
There are two global positioning system services available. The Precise Positioning Service (PPS) is intended for military and select government agency users. The accuracy of the Standard Positioning Service (SPS), designated for civilian applications, is intentionally degraded to provide predictable accuracies of only 100 meters in the horizontal plane. The mechanism by which this intentional degradation is achieved is called selective availability (SA). Selective availability has two components: satellite clock dither (&dgr;) and manipulation of the broadcast ephemeris data (&egr;). The selective availability bias on each satellite signal is different, resulting in a position solution that is a function of the combined selective availability bias from each space vehicle in the ensemble. Selective availability constitutes the largest component of error in the position solution.
Differential global positioning systems have been used to correct selective availability errors to improve the accuracy of the position solution. The differential technique uses bias errors measured by a receiver at a known position to correct the bias errors at a receiver with an unknown position. Therefore, differential global positioning system removes correlated errors measured by two or more receivers from the same ensemble of space vehicles. This differential technique is known as the local area differential global positioning system (LADGPS). However, local area differential global positioning system is useless when the user is remotely located from reference receivers.
When accuracy is required for users over a large region, a second differential technique known as the wide area differential global positioning system (WADGPS) may be used. This technique uses a network of intercommunicating reference stations that work in conjunction to overcome errors originating from sources such as selective availability. However, the control and communication schemes for the network of reference stations are complex and costly to operate.
SUMMARY OF THE INVENTION
Accordingly, there is a need for a receiver or method to accurately determine the position solution with the errors introduced by selective availability substantially eliminated without requiring the use of differential global positioning systems.
In accordance with the present invention, a system and method of determining an accurate fixed position are provided which eliminate or substantially reduce the disadvantages associated with prior systems and methods.
In one aspect of the invention, a method of accurate determination of a fixed position using global positioning system uses pseudo-range output over a predetermined measurement period from a stand-alone GPS receiver located at the fixed position, the pseudo-range output being measured from an ensemble of space vehicles at each time t over the measurement period, and historical ephemerides of the ensemble of space vehicles over the same measurement period to eliminate errors caused by selective availability. The historical ephemerides of the ensemble of space vehicles corresponding to each measurement time t are determined, and using the pseudo-range output and the precise ephemerides, a position solution at each time t is computed. The accurate position in earth-centered earth-fixed coordinates is then the average of all the position solutions at each time t over the measurement period.
The aspect of the invention is a stand-alone global positioning system (GPS) receiver having a post-processor for accurate determination of a fixed position is provided. The post-processor is adapted to receive a plurality of pseudo-range measurements over a predetermined measurement period (the pseudo-range measurements being determined from an ensemble of space vehicles at each time t over the measurement period), obtain historical ephemerides of the ensembles of space vehicles over the measurement period (the historical ephemerides being free of the selective availability broadcast ephemeris error), determine the historical ephemerides of the ensemble of space vehicles corresponding to each time t in response to the obtained historical ephemerides, use the plurality of pseudo-range measurements and the historical ephemerides to determine a position solution at each time t, and average the position solutions over the measurement period.
A technical advantage of the present solution is the provision of a post-processing system and method that substantially eliminates the broadcast ephemeride errors, &egr;, and reduces the space vehicle clock dither, &dgr;, through averaging of the position solution data without the use of costly DGPS and WADGPS.
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Baker & Botts L.L.P.
Phan Dao
Raytheon Company
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