Goddard Trajectory Determination System

The mathematical specifications for the Goddard Trajectory Determination System (GTDS) were originally written in 1976. Since then the software has evolved into a robust accurate orbit determination (OD) option for a wide variety of missions. GTDS has supported most NASA Low Earth Orbit missions launched since the mid-1980’s including HST, GRO, COBE, ERBS and EUVE. It has also been used to support lunar missions (Clementine and Lunar Prospector) and libration point missions (WIND, ACE, SOHO, and MAP). GTDS has been shown to give orbit solutions accurate to within 10 cm for the TOPEX mission.

GTDS currently resides on both Unix and PC platforms. GTDS is used by either Consolidate Space Operations Contract personnel in the Flight Dynamics Facility or by FDAB-trained Flight Operations Team members using GTDS in the Mission Operations Center. GTDS is a Bayesian weighted least squares estimator. It is capable of estimating spacecraft states (including relay satellites), measurement biases, station locations, drag and solar radiation correction factors, and thrust scale factors. GTDS is keyword driven via ascii user input files. GTDS provides detailed statistics regarding the differential correction process as well as the spacecraft ephemeris. The ephemeris can then be used in numerous other pieces of software (i.e. Satellite Tool Kit) to generate required mission products.

GTDS currently accepts formatted data from the following tracking systems DSN, UTDF, STDN, TDRSS, NORAD B3, NORAD 46c, C-band, and laser tracking. Additional formats can easily be added. The measurement model includes most forms of spacecraft ground station tracking including 1-, 2-, and 3-way range & Doppler, XY angles, azimuth & elevation, and TDRSS 1- and 2-way range & Doppler.