## 2 The Permanent Facility for Altimeter Calibration
### Infrastructure and Instrumentation
#### 2.1.5 The GNSS Monitoring Network  
A network of continuously operating GNSS reference stations [34] has been established by the Technical University of Crete in western Crete as early as 2001. This supports the PFAC operations. A map with the various locations of the GNSS monitoring network as distributed in western Crete and some of site pictures are shown on Figure 13. This permanent array has been providing absolute determination for the coordinates of the Cal/Val sites, as well as keeping track of ionosphere and wet troposphere delays of satellite signals in the region. It also acts as a geodetic infrastructure for monitoring tectonic deformation in this earthquake prone part of Crete. Three GNSS stations also operate inside the campus of the Technical University of Crete: (a) The \"TUC2\" GNSS station (Figure 14 Upper), continuously operating as of June 2004. This was officially the first station in Greece to be part of the European Reference Frame Permanent Network. (b) The \"TUC3\" station (Figure 14 Lower Left) established in February 2013 as part of the EGNOS (European Geostationary Navigation Overlay System) Resources Real Time Performance Array. (c) A BeiDou Chinese station (called \"TUC4\") (Figure 14 Lower Central and Right) set up in May 2012 (Figure 15). Figure 13: (**a**) The site as is located on top of Crete mountains at an elevation of about 1000 m. (**b**) The initial setting of the CDN1 site as was inspected by ESA in June 2014 with no construction at that stage. (**c**) The site as it exists today and fully operational for transponder calibrations. (**d**) An overview of the site as seen in May 2017 from a higher altitude where groundtracks of Sentinal-3A and Sentinel-3B, as well as SWOT, are depicted. The red ground track extends south to Gavdos island where sea-surface calibrations complement and cross-examine the transponder Cal/Val results. Processing of this network data from those permanent GNSS sites is carried out using three different types of scientific software: GAMIT [35], Bernese [36], and GIPSY [37]. The coordinates and their velocities for each GNSS site are estimated along with the ionospheric and tropospheric signal delays. These atmospheric signal delays are also used to provide ground truth verification for the radiometer on board the satellite altimeter. Tables 2 and 3 show the latest results for the coordinates, velocities, and their processing uncertainties of 11 such GNSS stations in the ITRF 2008 reference system. Independent positioning systems and processing techniques have been applied to achieve FRM standards for the determination of geodetic coordinates and results. For example, an inter-comparison and cross-examination of tropospheric delays, as determined by GPS [called \"GVD0\"] in Gavdos but also with a collocated DORIS beacon (less than 50 m away, called \"GAVB\") (Figure 16), have been carried out [38]. This DORIS site had been installed on 27 September 2003, as part of the permanent  
Figure 14: Location of the geodetic GNSS network in support for the operations of the Permanent Altimeter Calibration Facility. Example photos of the permanent GNSS stations are from the MEN2 (**lower right**), SUGI (**Lower central**), and SEL1 (**Lower left**) in western Crete. The (**upper**) images refer to CDN1 (**upper right**) and SUGI sites. Figure 15: (**Upper**) The “TUC2” permanent GNSS station on University Campus. This has been the first Greek site to be part of the EUREF permanent network as of 2004. (**Lower Left**) The “TUC3” established in February 2013 as part of the EGNOS Resources Real Time Performance Array. (**Lower Central** and **Right**) The Chinese BeiDou station (UR240-CORS Receiver), called “TUC4”, installed on campus as of May 2012.  
facility for satellite altimetry calibration. Both of these sites are less than 1.5 km away from the harbor Cal\(/\)Val site in Gavdos where the main tide gauges are located.