TRANSPONDER ALTIMETRY - PRECISE HEIGHT MEASUREMENTS OVER LAND

A novel experiment, coordinated by the University of Newcastle and Rut herford Appleton Laboratory, was conducted to demonstrate the feasibil ity of measuring relative height differences using transponder altimet ry. This technique used the radar altimeter onboard the first European Remote-sensing Satellite, ERS-1, in conjunction with active ground-ba sed microwave transponders. Two transponders were placed 127 km apart, one in the foothills close to Venice and the second, 1000 m higher in the Austrian Alps. The sites were chosen to coincide with the ground track of ERS-1 and also to be within the altimeter calibration zone wh ich provided reliable ERS-1 orbits. Coincident observations at both tr ansponder sites were made for a total of five overpasses. These measur ements were subsequently compared with a height difference determined using the Global Positioning System (GPS). The experiment has estimate d the precision of the transponder derived height difference by differ encing the observed and computed transponder ranges at both transponde r sites. The precision of one transponder-derived height difference, b ased on four of the recorded passes, is estimated as +/- 3.5 cm and th e standard deviation of the mean is +/- 1.7 cm. This implies that the standard deviation of one transponder range at a single site is +/- 2. 5 cm. The precision of the height difference derived from the GPS netw ork was +/- 3.1 cm and thus the ground-based transponders have perform ed, as a height difference measurement System, to a comparable accurac y to that of GPS. Ground-based transponders do, however, have the sign ificant practical advantage of being able to be operated in a simpler, automatic, and possibly more economical mode of operation. Data colle ction takes place onboard the space vehicle and power requirements can be made such that the equipment could be left unattended for many mon ths. This makes the equipment especially suitable for hostile environm ents, for example, measuring the vertical precursory motion of volcano es and for situations where long-period observations are needed such a s the monitoring of subsidence or sea level changes.