The establishment of new permanent calibration and validation (Cal/Val) facilities is necessary to insure efficient monitoring of satellite altimeter’s performance. At the moment, the Permanent Facility for Altimetry Calibration (PFAC) in Crete, Greece consists of four sea-surface and two transponder Cal/Val facilities (Fig. 1).

Fig.1: The ground tracks of current and past satellite altimetry missions, the sea-surface Cal/Val sites (Gavdos, RDk1, CRS1, SUG1) and the transponder Cal/Val sites (CDN1, GVD1) in west Crete, Greece

The “SUG1” Cal/Val facility constitutes the newest member of the PFAC for sea-surface. It was established, with the support of the SeRAC project, on the south coast of Crete. It supports absolute calibration of Sentinel-3A, Sentinel-3B, Jason-3 and Sentinel-6A satellite altimeters (Fig. 2).

Fig.2: A new sea-surface Cal/Val site named SUG1 in south-west Crete (left) has been established to support Cal/Val activities for Sentinel-3A, Sentinel-3B and Sentinel-6 altimeters (right-up). The scientific instrumentation and associated infrastructure are installed at a small fishing harbor (right-down).

In this short story the scientific reasons behind such an investment and the infrastructure and instrumentation installed are given.

Scientific Justification

The PFAC is currently one of the very few satellite altimetry calibration facilities, fully equipped relying on a number of different coastal and land sites implementing diverse methodologies and settings. It supports sea-surface calibration at four strategic locations and land calibrations with two microwave transponders.

Why then a new (fourth) sea-surface Cal/Val site needs to be established in this geographic region? The answer is twofold: First, to upgrade and secure calibration services immediately after the CDN1 transponder calibration and within two seconds of satellite fly time, and second, to prepare the ground for the upcoming calibration of wide swath altimeters, such as the Surface Water and Ocean Topography and Quanlan.

But how are the existing PFAC Cal/Val services going to be upgraded? The “SUG1” Cal/Val is located a few km south of the “CDN1” transponder Cal/Val site, on the mountains. Thus, through the operations of the “SUG1” Cal/Val a unique opportunity is raised for direct and immediate comparison of the altimeter bias by two independent techniques (land and sea) (Fig. 3). This is in line with the European Space Agency strategy for Fiducial Reference Measurements which promotes, among others, redundancy in scientific instrumentation, and diversity in Cal/Val techniques employed.   

Figure 3: Simultaneous calibration of Sentinel-3A Pass No.14 but also Sentinel-3B Pass No.335 (not shown) using two diverse techniques: sea-surface using the new “SUG1” Cal/Val and transponder employing the “CDN1” Cal/Val complies with the European Space Agency strategy for Fiducial Reference Measurements for Altimeter (FRM4ALT).

The launch of the new interferometric satellite altimeters, such as the SWOT mission, are to be launched in November, next year. Thus, the PFAC shall be prepared to support such type of calibration for wide-swath altimeters. The main difference between SWOT and the currently operating satellite altimeters is that, due to the wide swath (±10 to 60 km on each side of the nadir track) of its Ka-Band Radar Interferometer (KaRIn) instrument, calibration in two dimensions is required. The “SUG1” Cal/Val in conjunction with the other PFAC sea-surface Cal/Val sites enables such two-dimensional sea-surface Cal/Val services.  

Figure 4: Up: The SWOT ascending Pass No. 58 and descending Pass No. 349 nadir track (blue) and swath (white) with the Sentinel6-MF/Jason-CS (red), Senitnel-3A (green), Sentinel-3B (light blue) and HY-2B (pink) missions. Relative direct calibration with all these missions is possible in west Crete, Greece. Down: The PFAC geographical areas that will provide valid ground truth observation for the SWOT mission sea-surface calibration from its current (Gavdos-GVD8, CRS1, RDK1, SUG1) and future (KLN1) sites. Each square is 10 km × 10km. Shaded areas are more than 30 km from Cal/Val sites thus less accurate (but not invalid) calibrating regions. The blue line gives the SWOT nadir track and the green lines its swath. Left: descending Pass No. 349., Right: ascending Pass No. 58.

"SUG1" Infrastructure & Instrumentation

According to the FRM4ALT strategy of ESA, any sea-surface Cal/Val site shall be equipped with at least: one GNSS station, one meteorological station and one tide gauge. This scientific instrumentation shall be continuously operational, protected against weather conditions and vandalism while remote access and control to their operation shall be ensured.

Apart from this minimum instrumentation, the “SUG1” site also fulfills the FRM4ALT recommendation for redundancy in water level determination. This is accomplished through the operation of two tide gauges which employ different measuring principles: one radar and one pressure (Fig. 5).

Figure 4: Up: The SWOT ascending Pass No. 58 and descending Pass No. 349 nadir track (blue) and swath (white) with the Sentinel6-MF/Jason-CS (red), Senitnel-3A (green), Sentinel-3B (light blue) and HY-2B (pink) missions. Relative direct calibration with all these missions is possible in west Crete, Greece. Down: The PFAC geographical areas that will provide valid ground truth observation for the SWOT mission sea-surface calibration from its current (Gavdos-GVD8, CRS1, RDK1, SUG1) and future (KLN1) sites. Each square is 10 km × 10km. Shaded areas are more than 30 km from Cal/Val sites thus less accurate (but not invalid) calibrating regions. The blue line gives the SWOT nadir track and the green lines its swath. Left: descending Pass No. 349., Right: ascending Pass No. 58.