## 2 The Permanent Facility for Altimeter Calibration
### Infrastructure and Instrumentation  
#### 2.1.1 The Gavdos Cal/Val Site  
Preparation plans, procedures, and works to establish an absolute sea-level monitoring and altimeter calibration facility on the island of Gavdos were initiated in late 2001, and have been in continuous operation since 2004. The island of Gavdos occupies a place just in the center of eastern Mediterranean. It is a strategic location for the calibration of satellite altimetry missions, as south of it extends a deep ocean in an over 500 km stretch till Africa with no islands in between to contaminate satellite signals. Ocean tides at this location are small (a few cm); however, ocean circulation, and reference models (gravity models, mean dynamic topography, ocean circulation, bathymetry, etc.) are well established. The location was exactly under a crossing point of the ground tracks of the Jason series satellites (TOPEX/Poseidon, Jason-1, Jason-2, Jason-3, and Jason-CS), and adjacent to an Envisat pass [30] (Figures 1-3). Thus, this Cal/Val site has been capable of calibrating ascending and descending orbits of the Jason series. Gavdos is an isolated island about 40 km south of the mainland of Crete in Greece with only 30-50 permanent residents and sparse boat connections, especially during the winter time when extreme weather conditions prevail for its most part. The infrastructure to accommodate the scientific instruments was built up at three different locations: the \"Karave\" port (the harbor of the island where boats are berthed), the \"Theophilos\" site (a University owned land bought in 2001 to host the central Cal/Val site), and the \"DIAS\" crossover site for the deployment of a microwave transponder (Figures 2 and 3). Figure 2: The initial motivation for the establishment of the Gavdos permanent altimeter calibration facility was that a crossover of Jason satellite (Pass No. 18 and No. 109) and adjacent to the Envisat Pass No. 571. (**a**) shows the crossover location of Jason over Gavdos island, and the adjacent Envisat ground track. (**b**) presents a general overview of the Jason and Sentinel-3 cross over locations over west Crete, along with the Chinese HY-2 ground tracks and associated crossover locations. Figure 3: Cont. Figure 3: **(a)** An aerial view of the Gavdos island. Photo was taken by the aircraft used for the airborne gravimetry in 2001. The initial setting at the Gavdos island at the “Karave” harbor in 1999 and the first installations of the tide gauge and GPS stations. The right photo shows where the “DIAS” mail Cal/Val site is located with respect to the harbor. **(b)** Installations of the first tide gauge and GPS site at the Gavdos harbor by NOAA (USA) personnel. The right picture shows the harsh conditions prevailing over that winter of 2001. **(c,d)** Depiction of the gradual evolution from a stainless-steel equipment box to a concrete housing structure for the equipment in the summer of 2001. **(e)** The present situation with the new jetty built at the Gavdos harbor and the old site, as well as the new site in relation to the harbor. The right photo is the DIAS Cal/Val site. **(f)** This is the DIAS location (cross over location of Jason satellites) in the south part of the island where the first transponder was set up in 2003. During the initial setup of the Gavdos Cal/Val facility, several instruments were installed: two GNSS receivers, two tide gauges (one acoustic and one pressure sensor), a DORIS beacon, a microwave transponder, and meteorological sensors. Throughout its 15 years of operation, several instruments have been replaced at times either because they were damaged by extreme weather or substituted after some period of regular \"wear and tear\". At times, parts of infrastructure have been expanded or upgraded to meet new altimetry measuring techniques and accommodate different satellites, but also to gain confidence in establishing the uncertainty budget of the Cal/Val results.