Satellite antenna phase center offsets and scale errors in GPS solutions

Abstract.

 ITRF2000 solutions have shown that there are parts-per-billion (ppb)-level scale differences between GPS and other techniques, as well as among various GPS analysis centers (ACs). The trends of the scale differences reach 0.2 ppb per year. It is demonstrated that the uncertainty in the satellite antenna phase center offset (z-direction) is one of the major reasons for the scale differences. The scale change caused by an error in the z_offset can be approximately expressed as: scale change (in ppb)=7.8 × error of z_offset (in meters). Changing the z_offset value of the BLOCK II and IIA satellites from 102 to 95 cm could produce a 0.5-ppb scale variation. For BLOCK IIR satellites the uncertainty of the z_offset is much larger. The number of these satellites increases each year. The accuracy of IGS products could be significantly affected if this problem is not properly solved. Besides scale, satellite antenna phase center uncertainties have perceptible effects on also the clock, zenith path delay, and other solved-for parameters.