Monitoring Displacements by GPS: A Calibration Test

Abstract

In April 1988, the position of three stations located near Paris were monitored with single frequency GPS SERCEL receivers. The three base lengths were 30.030 km, 28.168 km and 2.715 km. One station was equipped with an antenna mounted on a stage and free to move in three directions. The displacements were known with a precision of one millimeter through the stage micrometers. Repeated measurements were done, first without displacements, then by steps varying from 1 to 50 cm. A priori coordinates were obtained from the pseudo-range data. Large differences between results obtained with or without satellite 8 are probably due to satellite 8 clock problems.

Baseline vector was determine by phase inversion. Comparaisons between GPS and stage measurements show that the inversion result quality is function of two parameters: the r.m.s. of the residuals after inversion and the number of data taken into account during the inversion. When the displacements are computed from GPS baseline vector components, the scatter reach 10 ppm. Repeatability baseline measurements have a scatter of 1–3 ppm on the baseline lengh and reach 5 ppm on the baseline vector components. However, the r.m.s. on the repeatability measurements is less than one ppm on the baseline length and varies up to 2 ppm on the baseline vector components.