A novel experiment, coordinated by the University of Newcastle and Rutherford Appleton Laboratory, was conducted to demonstrate the feasibility of measuring relative height differences using transponder altimetry. This technique used the radar altimeter onboard the first European Remote-sensing Satellite, ERS-1, in conjunction with active ground-based 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 which provided reliable ERS-1 orbits. Coincident observations at both transponder sites were made for a total of five overpasses.

These measurements were subsequently compared with a height difference determined using the Global Positioning System (GPS). The experiment has estimated the precision of the transponder derived height difference by differencing the observed and computed transponder ranges at both transponder sites. The precision of one transponder-derived height difference, based on four of the recorded passes, is estimated as ¿3.5 cm and the 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 network was ¿3.1 cm and thus the ground-based transponders have performed, as a height difference measurement system, to a comparable accuracy to that of GPS. Ground-based transponders do, however, have the significant practical advantage of being able to be operated in a simpler, automatic, and possibly more economical mode of operation. Data collection takes place onboard the space vehicle and power requirements can be made such that the equipment could be left unattended for many months. This makes the equipment especially suitable for hostile environments, for example, measuring the vertical precursory motion of volcanoes and for situations where long-period observations are needed such as the monitoring of subsidence or sea level changes. ¿ American Geophysical Union 1995