Evaluation of absolute positioning using the Defense Mapping Agency's GASP program
Clynch, James R.
Titus, Harold A.
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The Geodetic Absolute Sequential Positioning (GASP) program, as utilized by the Defense Mapping Agency (DMA), processes static GPS measurements collected with the TI 4100 GPS receiver to estimate geodetic point (absolute) positions. In this thesis, the GASP program is modified to accept data from different receiver types, the estimated point positions are compared to positions produced by the Transit Doppler positioning system, the between-receiver estimates are compared, the difference between estimates using the broadcast and the precise are examined, and the effects of selective Availability assessed. During the Monterey Bay Precision Positioning Experiment (MBPPE), conducted in the Winter of 1990-91, a large set of static GPS positioning data was collected with four types of GPS receiver; the TI 4100, the Trimble 4000ST, the Ashtech LD XII, and the Magnavox MX4200. Additional static GPS measurements were obtained with the TI 4100 receiver at a reference site established to support the experiment. A third data set was collected after activation of Selective Availability. Measurements collected with the TI 4100, Trimble and Ashtech receivers were subsequently processed with GASP using broadcast and precise ephemerides to product point position estimates. In order for GASP to accept the data from the Ashtech and Trimble receivers, the program had to be modified. The positioning results are analyzed for accuracy and precision. the accuracy of the GASP GPS estimates is determined by the comparison to independent estimates obtained by the Transit Doppler positioning system. Precision or repeatability (i.e., consistency of the estimated positions) is also examined. Analysis of the accuracy and repeatability reveals little difference between the positions computed for the three receivers using the precise ephemeris and that all three provide good agreement to the Transit Doppler positions. It is also clearly demonstrated that the precise ephemeris does produce a more accurate, higher precision solution than the broadcast ephemeris. The activation of Selective Availability has substantially degraded position solutions available from the broadcast ephemeris.
Approved for public release; distribution is unlimited
NPS Report NumberNPS-OC-91-006
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