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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.

 

Another objective of the SeRAC project is to refurbish and upgrade the existing ESA sigma-0 transponder for calibrating now the backscatter coefficient of Sentinel-3 SRAL instruments. That old ESA sigma-0 transponder had been used in the past to calibrate the Envisat Radar Altimeter 2(RA-2) (https://doi.org/10.1016/j.asr.2012.12.014).

   
Fig.1: Left: The sigma0 transponder of ESA in June 2019. Right: The transponder’s modes diagram.

Refurbishment has been carried out by the Radio Analog Micro Electronics (RAME, https://ramesrl.it), Rome, Italy and completed in 2019 and included, among others:

  • New transponder configuration for ensuring its operation in both Low-Resolution-Mode (with only one antenna) and Synthetic-Aperture-Radar (SAR) (two antennas) for Sentinel-3A and Sentinel-3B satellite altimetry missions;
  • Assessment of new time domain of calibration sequences, both relative and absolute in test-bench environment with the developed board;
  • Development of new software for on-site and remote operations and control of the refurbished transponder;
  • Characterization of the receive and transmit antennas using outdoor test range facilities;
  • Factory Acceptance Tests for the Radio-Frequency components (carried out in May 2019 in RAME, Rome, Italy).
   
Fig.2: Left: The ESA sigma-0 transponder consisted from five boards. Right: After its refurbishment, only one board is used to provide the advanced operating features.

In order to evaluate the performance of the refurbished transponder under real-life conditions for both Sentinel-3A and Sentinel-3B several field tests have been scheduled and carried out and results delivered. A crossover location of Sentinel-3A Pass No. 193 and Sentinel-3B Pass No. 214 had been identified in Tuscany, Italy (Fig. 3).

 

   
Fig.3: The location of the crossover of Sentinel-3A and Sentinel-3B for the field testing of the sigma0 transponder of ESA in Tuscany, Italy. Test sites are indicated by SP1A and SP1B.

A series of six Sentinel-3A (29/1/2020, 25/2/2020, 12/6/2020, 9/7/2020, 1/9/2020 and 28/9/2020) and two Sentinel-3B (23/6/2020 and 20/7/2020) follow-on field tests have been carried out in the period January – September 2020.

The Sentinel-3A signal as recorded by the sigma-0 transponder in September 2020 is shown in Fig. 4.

   
Fig.4: Left: The received power of Sentinel-3A as captured during the 28-Sept-2020 field testing (credit: RAME). Right: fully-focused radargram around the transponder location Radio-Frequency input power (credit: isardSAT).

The Sentinel-3 backscatter coefficient bias is determined employing two diverse processing methods: Delay Doppler and Fully Focused. First preliminary results are expected to be delivered and published in the next few months.

The aim of the SeRAC project is to support the absolute calibration of the altimeter onboard the Sentinel-3A and Sentinel-3B satellites using the Permanent Facility for Altimetry Calibration (PFAC) in west Crete, Greece.

Absolute calibration of satellite altimeters, during their lifetime, by external and independent facilities, such as the PFAC, is a prerequisite for a continuous, homogenous and reliable monitoring of the Earth, its oceans and its climate change. Altimetry system’s responses have to be continuously monitored and controlled for their quality, biases, errors, drifts, although relations among different missions have to be established on a common and reliable Earth-center reference system, maintained over a long period of time.

In PFAC, calibration of the multi-mission satellite altimeters takes place employing several absolute and relative calibration and validation techniques both at sea but also on land using a prototype microwave transponder (Mertikas et al., 2018). The latter operates at the CDN1 Cal/Val site that has been designed, constructed and maintained by the European Space Agency (link).

The European Space Agency’s strategy for Fiducial Reference Measurements (FRM) constitutes an effort to achieve reliable, long-term and consistent satellite Earth observations and products, via undisputable calibration and based upon metrology standards. In this context, an Action Plan towards FRM for Altimetry has been recently published (Mertikas et al., 2019).

According to this action plan, the use of a microwave profiler, to be placed at Cal/Val sites of the PFAC would be an advantage for attaining the FRM status and also an asset for wet troposphere delay (WTD) estimation that is currently carried out via dedicated GNSS (Global Navigation Satellite System) processing. This is because it constitutes an alternative and independent technique that is expected to lead us more close to the true WTD at the time of satellite overpass.

The European Space Agency provided the Radiometrics MP-3000A Radiometer to be installed at the CDN1 transponder Cal/Val site. Several preparatory works had to be performed before the instrument’s installation such as: (1) construction of a concrete base to support the instrument, (2) excavation of underground cannel to route the power and data cables from the instrument to the CDN1 equipment room, (3) upgrade the power supply and communications link systems, and (4) purchase lightning protection equipment for the instrument and the underground cables.

Funded by the EU and ESA