RA-2 Bias Determination Using a Transponder
Elena Cristea(1) , Philip Moore(2) , Walter Hausleitner(1) , and Peter Pesec(1)
Austrian Academy of Sciences,
(2) University of Newcastle upon Tyne, Cassie Building, NE1 7RU Newcastle, United Kingdom
The usual technique of the radar altimeter range calibration is to compare the altimeter-observed sea surface height with the accurate ground truth referred to as in situ data sets, observed independently by tide gauges and GPS buoys.
The use of a transponder is a different and convenient technique to determine the altimeter bias. A transponder is a device that receives the signal from the satellite, amplifies it and re-sends it back. In contrast to the ocean surface, a transponder represents a stable and very precise reflective reference point (few millimeters in height), which allows a very precise determination of the vertical distance between the satellite and the transponder. The accuracy of such determined ranges depends on the ability to estimate the path delays caused by the atmosphere, the precision of the orbit and the GPS height positioning of the transponder.
When deployed within the footprint of the altimeter and if the altimeter range window set to the estimated range, the transponder will appear in the data with a significant waveform power. Also, the corresponding waveforms differ both in power and shape from the other waveforms caused by natural targets. By means of transponder signature analysis, the time of the closest approach of the satellite to the transponder is determined and the range is hence calculated and further used to determine the altimeter bias.