Accurate baseline determination by radio interferometry on NAVSTAR GPS satellite transmissions

The vector components of several baselines shorter than 100 meters have been measured by interferometric processing of NAVSTAR GPS satellite C/A-code broadcasts to an accuracy better than one centimeter, and a repeatability on the order of a few millimeters. Interferometric observables were extracted by two processing schemes. The first involved broadband cross correlation of the modulated GPS signals received by the antennas that define the unknown baselines. The second approach uses GPS receivers at each antenna to reconstruct the satellite carrier from which phase differences can be measured. In both cases the interferometric baseline determinations were compared in a double-blind test with a conventional survey that was believed to be accurate to 2mm (one-sigma) and 3 arcseconds. Multipath was shown to be a significant effect only to the extent that reflecting structures might block the visibility of useful satellite signals. At the present time, the limit on extending the technique to precisely measure baselines exceeding 100 km is uncertainty in GPS satellite ephemerides. For baselines of intermediate length, the measurement performance is limited by uncertainties in tropospheric propagation delays. The results of the experiments plus trends toward miniaturizing GPS equipment complete the demonstration that short baselines can be measured to subcentimeter accuracy using highly portable equipment.