GFO-1 Geophysical Data Record and Orbit Verifications for Global Change Studies

This final report summarizes the research work conducted under NASA's Physical Oceanography Program, entitled, GFO-1 Geophysical Data Record And Orbit Verifications For Global Change Studies, for the investigation time period from December 1, 1997 through November 30, 2000. The primary objectives of the investigation include providing verification and improvement for the precise orbit, media, geophysical, and instrument corrections to accurately reduce U.S. Navy's Geosat-Followon-1 (GFO-1) mission radar altimeter data to sea level measurements. The status of the GFO satellite (instrument and spacecraft operations, orbital tracking and altimeter) is summarized. GFO spacecraft has been accepted by the Navy from Ball Aerospace and has been declared operational since November, 2000. We have participated in four official GFO calibration/validation periods (Cal/Val I-IV), spanning from June 1999 through October 2000. Results of verification of the GFO orbit and geophysical data record measurements both from NOAA (IGDR) and from the Navy (NGDR) are reported. Our preliminary results indicate that: (1) the precise orbit (GSFC and OSU) can be determined to approx. 5 - 6 cm rms radially using SLR and altimeter crossovers; (2) estimated GFO MOE (GSFC or NRL) radial orbit accuracy is approx. 7 - 30 cm and Operational Doppler orbit accuracy is approx. 60 - 350 cm. After bias and tilt adjustment (1000 km arc), estimated Doppler orbit accuracy is approx. 1.2 - 6.5 cm rms and the MOE accuracy is approx. 1.0 - 2.3 cm; (3) the geophysical and media corrections have been validated versus in situ measurements and measurements from other operating altimeters (T/P and ERS-2). Altimeter time bias is insignificant with 0-2 ms. Sea state bias is about approx. 3 - 4.5% of SWH. Wet troposphere correction has approx. 1 cm bias and approx. 3 cm rms when compared with ERS-2 data. Use of GIM and IRI95 provide ionosphere correction accurate to 2-3 cm rms during medium to high solar activities; (4) the noise of the GFO altimeter data (uncorrected SSH) is about 15 mm, compared to 19 min for ERS-2, and 12 min for TOPEX. It is anticipated that the operational GFO-1 altimeter data will contribute to a number of researches in physical oceanography. A list of relevant presentations and publications is attached.