## 2 The Permanent Facility for Altimeter Calibration
### Infrastructure and Instrumentation
#### 2.1.3. The RDK1 Cal/Val Site  
The RDK1 Cal/Val site is located in a small private fishing harbor south of Crete and is exactly under the Jason No. 109 ascending orbit. It was established in February 2009. The Jason satellite moves  
Figure 10.— The CRS1 Cal/Val facility is expected to provide calibration services for Sentinel-3A (Pass No. 278) and Sentinel-3B (Pass No. 14).  
Figure 9.— The CRS1 calibration facility has been operational since March 2008 (**left**). The (**right-most**) picture shows the established instruments at the site. The (**upper**) image shows an overview of the CRS1 site with the mountains in its background of West Crete. (**Lower**) images show the housing with is communication links and solar panels on its roof, its west view with the tide gauges as connected to sea, and the operating instrumentations inside this CRS1 Cal/Val site.  
up from south of Gavdos, intersects the island, and then continues to meet the RDK1 Cal/Val site in southern Crete. The segment of the Jason orbit (No. 109), south of Gavdos, has already been used for sea-surface calibration for more than a decade. The RDK1 site comes to fill in the gap and strengthen the calibrating region from south to the north, astride Gavdos, and certainly restores confidence for the results produced in the southern part. The RDK1 Cal/Val site not only happens to serve the Jason calibration but it presents itself to calibrate Sentinel-3B as it lies along its ascending Pass No. 71. In addition, the Sentinel-3A Pass No. 335 descends from RDK1 (9 km west of it) towards Gavdos and completely converges to Jason groundtrack when it comes nearer to Gavdos. Therefore, this RDK1 can serve as a testing area for different altimetric missions, but also ascending and descending orbits could be compared against each other.  
The sea that extends between Crete and Gavdos is deep enough with a depth of about 1400 m. Thus, it is suitable for satellite calibration over a stretch of 20 km with uncontaminated observations. The area has been experiencing strong west to east currents, so the dynamic topography has to be carefully examined. Over the period of 1990-2008, ocean circulation has been thoroughly investigated (Sanchez-Crete and Gavdos, 2014). At the same time, several underwater sensors are regularly deployed at various locations in this water channel. These instruments are operated by government institutes to monitor different geophysical parameters (e.g., wind, pressure, temperature, wave height, salinity, currents, turbidity, etc.), and thus a good picture of what is happening in terms of the ocean currents and circulation is known.  
At the moment, the RDK1 Cal/Val site is equipped with a radar tide gauge, a GNSS receiver and a set of communication links with a solar power system (Figure 11). It is planned that another tide gauge is to be setup there to increase confidence in water level observations. All instrumentation is under either lab or factory testing and characterization.  
Figure 11: The RDK1 Cal/Val site located exactly under the Jason ascending Pass No. 109 (**lower left**) and equipped with a radar tide gauge and a GNSS receiver (**lower right**). The site will be used not only for Jason calibration, but also for Sentinel-3A (Pass No. 335, descending, 9 km west) and for Sentinel-3B (Pass No. 71, ascending) (**upper**).