Design, testing, and performance analysis of transponders for use with satellite altimeters

The primary objective of this research is to develop a theoretical formulation for altimeter transponder performance and to establish the inherent measurement capabilities in practical field and spaceflight environments. Two types of transponders are studied: a conventional bent-pipe transponder and the Active Transponder for Altimetry Calibration (ATAC). A bent-pipe transponder was deployed during several ERS-1 overflights near Santa Barbara, California. The experiment yielded a number of data sets, which show that subcentimeter and one hundred fifty microsecond epoch measurement precisions are obtainable for the range satellite altimeter/transponder point of closest approach. The ATAC is a range-rate compensated transponder designed for use with the TOPEX altimeter. The instrument was developed under two phases of a NASA Small Business Innovative Research (SBIR) contract by Interferometrics, Inc. and the Colorado Center for Astrodynamics Research, University of Colorado, Boulder.; Within the next few years, spaceborne radar altimeters will be able to measure Earth ocean topography with a precision of better than one centimeter. Utilizing this unprecedented level of measurement precision as accuracy will require very accurate system calibration and satellite orbit determination. The use of transponders for calibration of satellite altimeters is a relatively new field, in which, there is little published research. Transponders provide a predictable Earth-fixed reference within the satellite altimeter data record. With a range measurement precision equal to or better than the actual satellite altimeter measurement precision, the applications of transponders in satellite altimetry are very extensive: precision orbit determination, range and time bias estimation, and electromagnetic bias estimation are a few examples.