  Radio occultation events recorded on 28-29 July 2004 by a GPS receiver aboard
the GRACE-B satellite are analyzed. The stability of the receiver clock allows
for the derivation of excess phase profiles using a zero difference technique,
rendering the calibration procedure with concurrent observations of a reference
GPS satellite obsolete. 101 refractivity profiles obtained by zero differencing
and 96 profiles calculated with an improved single difference method are
compared with co-located ECMWF meteorological analyses. Good agreement is found
at altitudes between 5 and 30 km with an average fractional refractivity
deviation below 1% and a standard deviation of 2-3%. Results from end-to-end
simulations are consistent with these observations.
