Dynamic orbit determination and gravity field model improvement from GPS, DORIS and Laser measurements on TOPEX/POSEIDON satellite F. Perosanz, J.C. Marty, G. Balmino CNES/GRGS, 18 Avenue Edouard Belin, F-31055 Toulouse Cedex, France Received 30 March 1995; Accepted 19 September 1996 Summary. In the framework of the GRIM series of gravity field models, the CNES/GRGS GINS precise orbit determination software has been adapted to dynamic GPS data processing. That is simultaneous processing of all available observables (i.e. GPS, DORIS, Laser) and all available satellite orbits (i.e. GPS, TOPEX/POSEIDON) can now be performed. The TOPEX/POSEIDON (T/P) mission satellite is equipped with a GPS receiver, a DORIS receiver and a laser reflector that represents an unprecedented oppor- tunity to compare and combine these three tracking systems to estimate orbital parameters and gravity field coefficients. Different combinations including : GPS + DORIS, DORIS + LASER, GPS + DORIS + LASER, have been tested and have shown small but systematic improvement in T/P orbit accuracy when GPS and DORIS data were processed simultaneously. Five tuned gravity field models have been generated by accumulating different combinations of T/P normal equations associated to the GPS, DORIS and Laser data. GPS data have a greater dynamic impact on gravity field spherical harmonics coefficient determina- tion than DORIS and Laser data. Furthermore, the results obtained with the solutions including GPS DORIS and GPS DORIS Laser T/P normal equations suggest that indeed these different tracking systems are somehow complementary in a dynamic sense. 1. Introduction For more than 20 years GRGS has been involved in the problem of gravity field model improvement that is still required for many geophysical and oceanographic purposes. Satellite tracking data obviously play an essential role in the determination of these models and important progress is still expected from a future satellite geodetic mission. Such a project of low altitude satellite(s) will certainly be based on one (or a combination) of the powerful tracking systems which exist today (Laser, DORIS, GPS, ) and/or Satellite to Satellite Tracking (SST) technique. In this context, we decided to undertake an important step forward in the capabilities of our Precise Orbit Determination (POD) software, named GINS (Ge ´ode ´sie par Inte ´grations Nume ´riques Simultane ´es) by implementing GPS data processing and multi-satellite orbit determination cap- abilities. The TOPEX/POSEIDON (T/P) mission satellite (Fu, et al. 1994) was equipped with a GPS receiver, a DORIS receiver and a laser reflector. This represented an unprecedented opportunity to validate the new capabil- ities of GINS and also to compare and combine these three tracking systems by estimating orbital parameters and gravity field coefficients from different data sets. The complete processing realised in this context is presented in this article. After a first part describing data pre-processing and processing characteristics, a chapter on GPS and T/P orbit quality assessment compares several kinds of combination of DORIS, GPS and Laser tracking data. Before concluding, a last chapter presents and discusses gravity model tuning from the accumulation of different DORIS, GPS and Laser normal equation systems. 2. Data pre-processing and processing characteristics DORIS and Laser tracking data are delivered to the scientific community in a form which is directly usable in such a way that no pre-processing is needed. DORIS and Laser systems and data processing have been described in several works (Dorrer, et al. 1991), (Degnan 1985), (Tapley, et al. 1994a), (Nouel, et al. 1994) and will not be repeated here. GPS ground and T/P data are distributed in the standard RINEX format (Gurtner, et Correspondence to: F. Perosanz Journal of Geodesy (1997) 71: 160–170