The nucleus for Earth parameter estimation and orbit determination is our EPOS software system, a collection of tools around the core module “OC” (Orbit Computation). EPOS comprises modules for data pre-processing, orbit integration, orbit improvement, orbit predictions, orbit analysis, normal equation handling and solution, and simulation of observations. The EPOS software package is used for operational Precise Orbit Determination (POD) for various past, present and future satellite missions such as Meteor-3,  Mir, Space Shuttle, GFZ-1, ERS-1/2, LAGEOS-1/2, CHAMP, GRACE-1/-2, GOCE, GRACE-FO, SWARM, TerraSAR-X, TanDEM-X, GEOSAT, TOPEX, GFO, ENVISAT, Jason-1/-2, Sentinel-3A/3B, GPS, Galileo etc. exploiting SLR, GNSS, DORIS, radar altimeter, PRARE, GRACE K-Band-Ranging, accelerometer, gradiometer, star camera etc. observation data.

Additionally, different pre-CHAMP GRIM (GRgs and geodetic Institute Munich model) and CHAMP, GRACE and GOCE based EIGEN (European Improved Gravity field of the Earth by New techniques) gravity field solutions have been computed during the past 4 decades (for details see Topic 1 and Topic 2).

The satellite orbits and Earth’s gravity field models are calculated with the so called “classical or dynamic approach” based on the analysis of orbit perturbations from artificial Earth satellites. Based on the numerical integration of the equations of motion, the dynamic approach allows for the solution of various system parameters of the Earth, provided a sensitivity of the observation material is given. By the dynamic method, an optimal approximation of the satellite’s trajectory at the level of observational accuracies can be achieved and the method can be adapted to virtually any observation type of satellite geodesy. Since the orbit dynamics are modeled,  the dynamic approach is ideally suited for the exploitation of tracking data distributed irregularly in space and time.

EPOS is written in FORTRAN90 and consists of more then 100,000 lines of code. The load module runs on different UNIX workstations and LINUX clusters, as well as on various other platforms. EPOS-OC has been installed also out of house at the University of Rome “La Sapienza” for the project VESTA (“Validation/Evaluation of SLR Tests and Applications in fundamental physics” see "Testing GR Principles") and at the Technical University of Berlin for the project GAGNOS (“GNSS Applications in Geophysical Networks – from the Observation to the Solution”) on the basis of mutual agreements between the universities and GFZ.