GPS relative positioning at a precision level of one part per billion

Six days of data from the GIG'91 experiment have been analyzed with a fiducial strategy. The results obtained with our orbital software GEOSAT, show daily horizontal and length repeatabilities at the level of 1 part in 10⁹ plus 2 mm for baseline lengths up to 4000 km. A direct comparison with results from the GIPSY software shows, with some exceptions, mean differences at the sub-cm level. After transformation to ITRF'90 the rms of the coordinate differences is 14.8 mm. Studies of orbital predictions and comparisons with external high precision orbits indicate a mean orbit precision and accuracy of around 35 cm in each cartesian coordinate. Correlations between the GEOSAT and GIPSY solutions indicate some common model deficiencies.