## 2 The Permanent Facility for Altimeter Calibration
### Infrastructure and Instrumentation
#### 2.1.1 The Gavdos Cal/Val Site
The development timeline of the Gavdos Cal/Val instrumentation is presented in Figure 4 and its present setup at the harbor is shown in Figure 5.  
At the outset in 2001, there was no mains power on the island of Gavdos. This forced us to depend upon renewable energy sources, i.e., solar panels and wind generators, to charge batteries at first, and those in turn to supply continuous and stable power to all scientific instruments operating in the field. It was only in 2010 when the mains power was supplied by government on the island. Since then, a hybrid system (mains and renewable power supply) has been implemented in Gavdos for safety and continuity of operations at the Cal/Val facility. Figure 4: The operational status of the main instrumentation (tide gauges and GNSS receivers) at the Gavdos Cal/Val site as of 2004 till present. Figure 5: The current setup of the “Karave” Cal/Val site in Gavdos harbor for sea-surface calibrations. When running scientific instruments in the field on remote islands, access, operations, and control of them becomes a difficult issue. At the early establishment of Gavdos Cal/Val, the island lacked decent and stable telephone and communications links. Therefore, several alternate channels in communications were sought and examined, e.g., UHF, GPRS, and satellite links (see also Figure 6) in an effort to ensure that observations were always accessed by the Operations Control Center (OCC) at the Technical University of Crete on the mainland of Crete. After several trials and errors, it has been evident that satellite communications had been the most reliable means, especially after their recent reduction in service cost. These satellite links serve as the essential communications carrier for the PFAC at the moment. By following the same example as done regarding the power supply, two independent communication channels (e.g., satellite and GPRS links) have been put into action to safeguard access to the scientific instruments, and to download and archive field observations immediately and automatically onto the Operations Control Center in Crete. Gavdos Cal/Val has been providing absolute calibration over sea surface for all altimeters of the Jason series (Jason-1, Jason-2, and Jason-3) along their descending and ascending passes No. 18 and No. 109, in conjunction with transponder calibration at the crossover point over the land of Gavdos (DIAS Cal/Val site). Later in 2013, this Gavdos Cal/Val site could calibrate SARAL/AltiKa using its ascending Pass No. 571 as this groundtrack intersects the island from the south to northwest and finally reaches southwest Crete (Figure 7). As of 2016, the European Sentinel-3A has been calibrated by the Gavdos Cal/Val using its ascending Pass No. 14 at sea, as well as its descending Pass No. 335. This last pass of S3-A has been used for sea-surface calibration north and south of Gavdos, and could be simultaneously used for land calibration, plus it crosses the DIAS Cal/Val site of the transponder on Gavdos (Figure 7). Figure 6: The UHF communications link with relay stations used to connect the Operations Control Center in Chania city, Crete, and Gavdos bypassing and going around the high mountains ridge, having an altitude of more than 2000 m. (**a**) shows the established network of installed UHF sites to link Gavdos Cal/Val with the central facility at the University in Chania city, Crete. (**b**) shows the final end-point UHF station in Gavdos, while (**c**) depicts the middle relay UHF station in Sougia.