Improvements in DORIS processing for ITRF2013 at the GSC Analysis Center

DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) is one of the fundamental geodetic techniques that contributes to the International Terrestrial Reference Frame (ITRF). The data from DORIS also find a strong application in Precision Orbit Determination (POD) for altimeter satellites such as TOPEX/Poseidon, Jason-1, Jason-2, Envisat and Cryosat-2. At GSFC we have processed DORIS data for POD, and for the ITRF, routinely submitting SINEX solutions to the IDS Combination Center in Toulouse. In the context of the preparation for the next ITRF (ITRF2013), we have conducted an intensive effort to improve the DORIS processing and deliver a new complete SINEX series based on processing data from most DORIS satellites from 1993 to 2013. In this paper we discuss the improvements that have been implemented in the GSC analysis center processing, which have been undertaken under the aegis of the DORIS Analysis Working Group (AWG). These improvements have included the following: (1) Application of improved models of time-variable gravity; (2) Derivation and application of improved macromodels to characterize the nonconservative forces on the DORIS satellites; (3) Better modeling of the frequency changes at DORIS sites; (4) application of a phase law for the DORIS antennae as derived from anachoeic chamber measurements by the CNES for the Starec antennae, and from manufacturer specifications for the Alcatel antennae. We summarize the improvements in the DORIS processing since the last complete SINEX delivery of GSC for ITRF2008, and we characterize the impact on the POD, and on the intrinsic products of interest to the derivation of the IDS technique combination.