Precise absolute GPS positioning with and without selective availability

Abstract

This paper presents the method and the results of investigations to determine the position vector of a static or moving vehicle using the Global Positioning System (GPS) in the absolute (point) positioning mode with and without selective availability. Starting from the basic positioning scheme with code observation, the most accurate positioning strategy using the phase is described. The main strategy for the highest accuracy absolute positioning is to estimate GPS satellite clock errors independently, thus obviating the between-station differencing. The GPS clock errors are estimated at 30-second intervals using International GPS Service (IGS) orbits and stations. The clock error estimates are then used in an absolute positioning algorithm to determine the coordinates without any other reference site. Static and kinematic GPS data at 1-second sampling rate were processed and compared with the known values and the corresponding DGPS solutions. For the static case, an IGS station was assumed as unknown and its coordinates were estimated. For the kinematic case, the aircraft survey observations were tested. The positions were compared with the corresponding DGPS solutions for each case. Since Selective Availability (SA) is now turned off, its effect on the clock estimation as well as on the position is compared and emphasized.