The Navy’s Geosat Follow-On (GFO) Mission, launched on Feruary 10, 1998, is the latest in a series of altimetric satellites which include Seasat, Geosat, ERS-1, and TOPEX/POSEIDON (T/P). Data derived from these missions has and will lead to vast improvements in our knowledge of ocean circulation, ice sheet topography, and climate change. In order to capture the maximum amount of information from the altimetric data, accurate altimeter calibrations are required for the civilian data set which NOAA will produce. NASA/Goddard Space Flight Center (GSFC) has provided these products for the Geosat and T/P missions and will do the same for GFO.
GFO has the same 17-day repeat period orbit as the Geosat “Exact Repeat Mission” (800 km altitude). Unlike Geosat, GFO is tracked by two extremely accurate tracking systems: satellite laser ranging (SLR) and the Global Positioning System (GPS). Accordingly, the spacecraft carries a laser retro-reflector array and Turbo-Rogue GPS receivers. These tracking data, especially in combination, can support GFO orbit accuracies approaching those achieved on T/P despite the lower altitude and increased atmospheric drag modeling errors.
Given precise knowledge of the spacecraft’s position, it is imperative that the altimeter’s performance be calibrated and monitored if the data are to be used for precision oceanographic applications and ice sheet mapping. Unlike T/P, the GFO altimeter is single frequency. Therefore, an ionospheric correction must be modeled or computed. Additionally, pre-launch analysis of temperature and voltage dependencies and return waveform effects as well as post-launch determination of corrections for seastate and attitude based on the calibrated waveform are required. Post-launch instrument-related drifts must also be determined. The Navy applications for the GFO data do not require an absolute range calibration and they will not perform one. However, the Navy has agreed to provide the required GFO altimeter calibration data to other agencies interested in absolute range applications.
The Observational Science Branch (Code 972) of GSFC Wallops Flight Facility (WFF) has provided altimeter calibration for all U.S. altimeter missions including Skylab, GEOS-3, Seasat, Geosat, and T/P. WFF has provided the pre-launch algorithms necessary to correct the TOPEX altimeter instrument data. These corrections have proven to be better than 1 centimeter after three years of scientific applications. Post-launch range and sigma-0 measurement drift values are known to within a few millimeters and 0.03 dB levels, respectively. The oceanographic community would not have been able to produce the acclaimed considerable body of results from T/P without the altimeter calibration.
Mr. Robert Cheney’s group at NOAA will produce the GFO geophysical data records (GDR) for distribution to the scientific community via CD-ROM. As is currently done for Geosat, the NASA Ocean Altimeter Pathfinder group will provide algorithm support for the GDR generation as well as producing higher level data sets. Similarly, the NASA Ice Sheet Altimetry Pathfinder group will evaluate the GFO over-ice altimeter waveforms and provide measurements of ice sheet topography to the polar science community.
Altimeter Calibration Overview
WFF processed all the TOPEX altimeter pre-launch test data and verified the altimeter calibrations and corrections. This led to the development of an exact tracker model used to compute range corrections. The Automatic Gain Control (AGC) calibration provided spacecraft temperature-corrected sigma-0 to better than 0.05 dB. Consequently, the electromagnetic bias estimate, which is based on the windspeed derived from sigma-0, reflected subcentimeter temperature effects. This pre-launch effort allowed the TOPEX/POSEIDON Ground System to produce useful TOPEX data from the first turn-on. The algorithms have proven to be quite robust and no changes have been required. In the post-launch phase, WFF’s complete understanding of the altimeter and its data allowed for quick modeling of the TOPEX waveform leakages and update to the corrections. WFF continually performs calibration monitoring for the TOPEX altimeter and has created databases useful for trend analyses. These detailed performance studies have resulted in corrections which have proven to be better than 1 centimeter after three years of scientific applications. A similar activity will be performed for the GFO radar altimeter. WFF will:
- Perform analysis on GFO pre-launch test data to characterize the altimeter’s performance and to obtain pre-launch calibration data needed for absolute range applications.
- Perform analysis on GFO inflight data to characterize the altimeter’s performance and to provide post-launch instrument calibrations needed for absolute range applications.
- Perform trend analysis for the calibration of range to correct for instrument drifts over the life of GFO.
WFF has been working with the Navy GFO program to define the pre-launch data and testing needed to provide for the calibration of the altimeter instrument. David Hancock of WFF has been invited to be a member of the Navy pre- and post-launch GFO altimeter calibration team.
Pre-launch Altimeter Calibration
Pre-launch, WFF worked with the Applied Physics Lab of Johns Hopkins University (JHU/APL) to analyze GFO test data to determine any dependence of science parameters on the altimeter characteristics. The pre-launch test data was used by the Navy to determine GFO altimeter initial calibrations and data corrections. This phase included helping to define pre-launch test requirements, receiving test data, and performing test data analysis. The GFO altimeter is much like the TOPEX Ku altimeter. WFF has converted pre-launch TOPEX processing software to ingest GFO data to perform similar analyses and to create similar databases. The waveform modeling will be modified to the GFO parameters and tracker to provide the needed tracker bias range corrections. WFF performed analysis on a limited set of pre-launch GFO data (for TOPEX, extensive analysis was performed on all test data) to determine instrument effects on the science data (range, SWH, AGC). The results have been provided to the GFO project and other users.
As part of the Navy team, WFF visited Ball Aerospace System Division (BASD) to witness testing and collect data. While working with the Navy and BASD, WFF assisted in collecting a number of one-time data products needed by other non-Navy users such as the POD team; measurement of center of mass and of spacecraft dimensions are among these one-time data products. Data was made available as requested to other users to support their modeling and software development. A set of pre-launch instrument calibrations was made available. WFF converted some of their TOPEX inflight processing to ingest GFO data and create databases.
Post-launch Altimeter Calibration
In the initial post-launch phase, WFF will examine the GFO data to insure that engineering corrections were applied correctly and to validate the pre-launch instrument calibrations. WFF will perform analysis of inflight data to develop waveform-based seastate and attitude corrections to be provided to the GFO project and the user community. WFF will continue to work as a member of the Navy calibration team and will request that a few special tests that were found useful in the calibration of the TOPEX altimeter be performed. The first few months of inflight GFO altimeter data will be used for a more intensive study period to finalize the tracker corrections and provide an assessment of instrument performance. A set of post-launch instrument calibrations will be made available.
Throughout the remainder of the mission, WFF will continue to perform monitoring of the GFO altimeter to determine the corrections for any instrument-related range and sigma-0 measurement drifts. Altimeter performance databases similar to those used for the TOPEX altimeter will be maintained. WFF plans to continue to be an active member of the Navy calibration team. Updates to correct the altimeter calibration for changes/drifts during the mission will be provided as required to the Navy, NOAA GFO GDR production, NASA Altimeter Pathfinder programs, and other interested users.