## 2 The Permanent Facility for Altimeter Calibration
### Infrastructure and Instrumentation
#### 2.1.4 The CDN1 Transponder Cal/Val Site  
A permanent Sentinel-3 altimeter site of the European Space Agency has been established in western Crete for calibrating both Sentinel-3A and Sentinel-3B at the same location. During early surveys for field reconnaissance in 2013, the location was looked after to be able to calibrate other altimeters as well, and particularly the baseline/reference missions of Jason (i.e., Jason-2, Jason-3, and Jason-CS/Sentinel-6). A thorough examination of several sites has taken place in the region and a site had been identified and finally selected at a mountainous location in western Crete. That was named the CDN1 Cal/Val site. It has been picked to be a triple crossover point between Sentinel-3A, -3B, and Jason; actually, it is a quadruple crossover place, as it is also only 2 km away from the ground track of SARAL/AltiKa (previously of Envisat).  
In the sequel, a signal analysis of the altimeter satellite at the proposed site was performed by CNES, France. The investigation revealed that altimetry signals from Jason-2 scattered from the surrounding surface at CDN1 were sound and perfectly compatible with the needs and the requirements set for transponder calibration.  
Meanwhile, a prototype microwave transponder had been constructed by the Technical University of Crete, Greece [33]. It took almost three years (2008-2011) to manufacture it and during this period ESA provided specifications and technical support. This transponder had been fully characterized for 4 months (March-July 2012) at the Compact Payload Test Range facilities in ESA/ESTEC, the Netherlands. (Figure 12). This instrument has been setup at the CDN1 Cal/Val site and has been operational as of 2015.  
The transponder Cal/Val site is located on a mountainous area at about 1000 m altitude. Control and access to the instrumentation is achieved remotely via central and backup communications links. The instrumentation at this altitude needs to be, and certainly is, protected from extreme weather conditions (snow, wind, lightning, etc.). The external GNSS antennas and the meteorological sensors are built to operate under a harsh environment in the field. Therefore, no special protection has been necessary for these instruments. This is not however the case for the transponder, the GNSS receivers, the communications links, and the power supply units (wind generator, solar panels, inverters, etc.).  
Along those lines, special protective housing for the transponder and the instrumentation had been designed and constructed. This is because if the transponder's antennas are exposed to, for example, strong winds, then any minor alterations in their orientation (even in sub-mm level) may lead to erroneous observations. Several solutions for the transponder housing had been evaluated given certain constraints regarding construction material, operating environment, etc. In the end, it has been decided that the transponder be protected by a sliding roof that will move and leave the instrument free of any signal interference during calibrations (Figure 13). In summary, the CDN1 Cal/Val site has been fully operational as of September 2015. It provides external, independent post-launch calibration services for all Ku-band altimeters that fly over the area.  
Figure 12: The prototype transponder (**left**) built by the Technical University of Crete. It was fully characterized at the Compact Payload Test Range facilities in ESA/ESTEC, the Netherlands, for four months (March–July 2012) (**right**).  
Figure 13: _Cont._