JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. C3, PAGES 2887-2898, MARCH 15, 1990 Precision Orbit Determination for theGeesat ExactRepeat Mission C. K. SHUM, D. N. YUAN, J.C.RIES, J.C. SMITH, I B. E. SCHUTZ, AND B. D. TAPLEY Center for Space Research, The University of Texas at Austin Precise ephemerides have been determined for the Navy's Geodesy Satellite (Geosa0 Exact Repeat Mission (ERM) using an improved gravityfield model,PTGF-4A (Shumet al., 1989). The Geesat orbitswere computed in a terrestrial reference system which istied tothe reference system defined by satellite laser ranging (SLR) to LAGEOSthrough a survey between the TRANET Doppler receiver andthe SLR system located at Wettzell, Federal Republic of Germany. The remaining Doppler tracking station coordinates wereestimated simultaneously withthegeepotential in the PTGF-4A solution. In this analysis, three continuous 17-day Geesat orbits, which were computed using the 46-station TRANET dataandglobal altimeter crossover data, have a crossover residual root-mean-square (rms) of 20 cm,indicating thattheGeesat radial orbit error is of the order of 20 cm. The orbits computed based on data collected by a 7-station OPNETtracking network andcrossover data have the same levelof accuracy. Collinear sea surface height analysis using the improved orbits for two Geesat 17-day repeat cycles yields anrms height difference of 16cm. Spectral analysis ofthe sea surface height difference indicates thatthe long-wavelength orbiterror has been reduced significantly. Finally,the Geesat altimeter timetag bias, based on 54 days of theGeesat ERM altimeter crossover data, is 5 q- 3 ms. INTRODUCTION Operational Network (OPNET) and the Defense Mapping The Geesat (Geodesy Satellite) mission was conceived by the Agency's (DMA) Transit Network (TRANET) Doppler tracking U.S. Navy in late 1978 as a means of improving the Earth's systems. OPNET consists of three sites located in the continental gravitational model. The objective of the 18-month primary U.S. and one site located in Hawaii. The TRANET system mission of Geesat was to complete the high-resolution mapping of consists of 48 globally distributed stations. Under agreement with the marine geoid initiated bythe Seasat mission, which ended DMA, 80 days of TRANET data were made available forthe prematurely in !ate !978 because of a power system failure. assessment of the TRANET tracking system to perform precision (leosat carries a single-frequency radar altimeter, which measures orbit determination forthe planned TOPEX/POSEIDON (Ocean the distance from the satellite orbit tothe subsatellite point on the Topography Experiment) Mission [TOPEX Science Working ocean surface with aprecision of a few centimeters. Geesat was Group, 1981]. The U.S.Navy Astronautics Group performs launched from Vandenberg Air Force Base on March 12, 1985, by operational orbit determination for the Geesat ERMusing the an At!as-E launch vehicle intoa near-circular retrograde orbit OPNET tracking data.The Geesat ephemerides arecomputed identical to the Seasat orbit altitude (800 kin) and inclination using the Goddard Earth Model-10 (GEM-10) [Lerch eta!.,1981] (108ø). and are provided tothe Naval Ocean Research and Development The primary mission was completed onSeptember 30, 1986, Activity (NORDA) (now Naval Oceanographic and Atmospheric and during October 1 through November 8, 1986, the spacecraft Research Laboratory) and the National Oceanic and Atmospheric performed a series of maneuvers to change into an exact repeat Administration (NOAA).NOAA produces theERM altimeter orbit. The extended mission is referred to astheGeesat Exact geophysical data records (GDR), which are distributed to the Repeat Mission (ERM) and isdedicated to the study ofmesoscale scientific community through NOAA's National Oceanographic (50- to 1000-kin wavelength) oceanographic features [McConathy Data Center (NODC). The GDR contains altimeter data, inferred and Kilgus, 1987]. The 17-day exact repeat orbit for Geesat ERM seasurface height, measurement corrections, orbitheights was designed such that theground tracks would lie within determined by the Navy Astronautics Group, fides, significant approximately 10 kmof the Seasat ground tracks atthe equator wave heights, and geoid and other quantities. [Born et al., !987]. Errors inthe knowledge ofthe global geoid cause errors inthe The Geesat ERM orbit characteristics are listed in Table1. A inferred sea height measurements. An exact repeat orbitallows frozen orbit, in which eccentricity and argument of periapsis long-term averaging togive an accurate mean surface along the changes due to the geepotential are nearly eliminated, was ground tracks togreatly reduce the effect ofgeoid error. The selected to maintain astable altitude time history and to eliminate accuracy ofthe mesoscale oceanic topography determined during lateral variations inground tracks associated with an eccentric the first 18 months ofthe primary mission was limited by the orbit having a circulating line ofupsides [Born etal., 1987]. geoid error, since the orbit was not inan exact repeat orbit and Maneuvers were performed about once per month in order to maintain the satellite ground tracks towithin 1km for every 17.05 days (244orbital revolutions). During the ERM mission, Geesat was tracked by theNavy's 1Now at Jet Propulsion Laboratory, Pasadena, California. Copyright 1990 by the American Geophysical Union. Paper number 89JC03268. 0148-0227/90/89JC.03268505.00 TABLE 1. Geosat ERM Orbit Parameters Semimajor axis ..... Eccentricity Inclination Longitude (east) of node Periapsis altitude Period Exact repeat period Node rate Perigee rate Specification. "".."'." 7162.6 km 0.0008 108 ø 1705 + n x 17475 (n = 0-244) 778.7 km 6035.6 s (1.68 hours) 244 revolutions (17.05 days) 2.0 øday-• 0.0 oday -t 2887