Ocean dynamics in general and sea level variation in particular are important key factors for climate change processes. Current and predicted rates of sea level rise give a picture of serious potential threats. These assumptions largely rely on a long history of numerous ocean monitoring altimeter missions such as ERS-1/2, Envisat, Topex, Jason-1/2, etc. Hence, a precise calibration and validation of these missions and related products is essential for providing reliable results. One method to perform a direct absolute altimeter calibration is by means of a dedicated microwave transponder.
A transponder is an electronic device that receives, amplifies and re-transmits the radar pulse emitted by the satellite back to the altimeter in the zenith direction. It operates at a centre frequency of 13.7 GHz to be compatible with the altimeter instrument and achieves a total signal gain of 77 dB. The transponder owned and serviced by the Academy is located at the Cal/Val-site on Gavdos (a small island 40 km south of Crete) which is operated by the Technical University of Chania. Besides various other sensors such as GPS, DORIS, tide gauges and meteo-station, the transponder is located directly beneath a cross-over point of the Jason orbit. Due to the orbit geometry Jason passes over this site every five days.
The main advantage of the transponder calibration compared to other standard methods is that it is not affected by any influences related to the ocean dynamics, i.e. sea state bias, ocean tides, etc. The actual observation is simply the range between the altimeter and the transponder, rTRP , which is compared to the corresponding calculated value resulting from the satellite’s orbital height, hSAT, and the ellipsoidal height of the transponder, hTRP, measured by means of GPS.
During an over-flight of the spacecraft (currently OSTM Jason-2 at an orbital height of 1340 km) the transponder echo return is received, sampled into 3.125 ns bins and recorded on-board the satellite. The corresponding data product is the transponder waveform data provided by CNES. In a post-processing analysis the range observations are obtained and corrected for numerous environmental and geophysical effects (dry/wet troposphere, ionosphere, solid Earth tides, etc.) which sum up to a total of about 2.5 m. Considering also a number of electronic and geometric corrections (with a total of >10 m) the ranging observation and subsequently the instrument bias is determined with a few centimetres accuracy.
|
