The CDN1 transponder Cal/Val site has been established on the mountains of west Crete under a multiple cross-over point of Sentinel-3A & 3B, Jason series and that of Sentinel-6/Jason-CS.
A prototype microwave range transponder, a microwave radiometer (Radiometrics MP3000A), two Global Navigation Satellite System (GNSS) stations (Leica GR10 with a Leica AR25.R4 antenna and Trimble NetRS with a Trimble Zephyr Geodetic Antenna), and two meteorological stations (Vaisala WXT520 and Vaisala WXT534) comprise the main instrumentation of the CDN1 Cal/Val site. Hybrid power supply systems (solar, wind, diesel generator and batteries) provide continuous, safe and stable power to these instruments.
Microwave transponders have been applied as alternative tool for satellite altimetry calibration. A microwave transponder is an electronic equipment which receives the pulsed radar signal transmitted by the altimeter, amplifies it and retransmits it back to the satellite, where it is there recorded. The flight time of the radar signal is measured on the satellite, from which the absolute range between the transponder and the satellite can be deduced. The main advantage of this transponder technique, compared to the conventional sea‐surface calibration, stands for the fact that no ocean dynamics errors are involved in this calibration. To process transponder data, atmospheric corrections are needed. At first, those are determined through dedicated GNSS data processing. Delays caused by the ionosphere and the troposphere (wet & dry) are estimated using the propagation properties of the GNSS signal, and then reductions are made for altimetry signals.
A water vapor radiometer (WVR) constitutes another independent, and alternative to GNSS data processing technique for the determination of tropospheric delays at the Cal/Val transponder site. Customarily, a WVR uses two microwave frequencies: one at ~22.2 GHz (sensitive to water vapor in the atmosphere) and one at ~30 GHz (sensitive to liquid water). By measuring the power of thermal radiation in the atmosphere, the respective signal delays arising from water vapor and liquid water can be determined. Thus, the wet troposphere delays of altimetry signals can be determined (Teke et al., 2011).
The CDN1 Cal/Val site instrumentation is fully compliant with the FRM4ALT requirement of putting into use redundant and complimentary observing systems. For example, two diverse and independent approaches are used to derive atmospheric delays during a satellite overpass: (a) microwave radiometer and (b) GNSS-derived delays. Independent GNSS processing is also applied for not only the estimation of atmospheric delays, but also for the determination of absolute coordinates of the Cal/Val site.
The CDN1 transponder Cal/Val site has been operational since September 2015. It has been established initially to support absolute calibration of the Sentinel-3 satellite altimeters. In order to cross-calibrate Sentinel-3 with other altimetry missions, the transponder is also activated for other European (i.e., Sentinel-6, CryoSat-2) and international missions (i.e., Jason-2 and Jason-3). The CDN1 Cal/Val site operators have reached an agreement with the agencies operating these missions: EUMETSAT and CNES, respectively.
Till 30-Nov-2021, 441 transponder calibrations had been taken place for Sentinel-3A Pass No.14 (75 calibrations), Sentinel-3B (45 calibrations), CryoSat-2 (51 calibrations) and Jason-3 Pass No.18 (234 calibrations). On 18-Dec-2020, the first Sentinel-6 transponder calibration was carried out at the CDN1 Cal/Val during its tandem phase with Jason-3. Out of these 441 calibrations, 411 were successful whereas 30 had been cancelled mainly because of extreme weather conditions that could have put the transponder’s safety at risk.