... 8, ``Celestial Mechanics'', 2002 First results of GPS orbit determination with GRAPE package using a square--root information filter M. O. Keshin Institute of Applied Astronomy, St. Petersburg, Russia The zero--differenced carrier phase measurements contain the distance be­ tween the phase centres of receiver and transmitter antennas biased by unknown integer number of cycles (phase ambiguities). ... In the case of GPS the problem of solar radiation pressure modelling is of primary importance. ...