The performance of Sentinel-3 will be validated against other altimeter missions (i.e., Jason-3/Jason-CS reference missions, CryoSat-2, HY-2B and its follow on, etc.). This is carried out on land and at open seas employing the transponder and sea-surface calibration techniques.
Multi-mission crossover analysis constitutes an alternative approach to estimate the relative bias between two (or more) satellite altimeters. This analysis involves the examination of the sea-surface heights, as estimated by each satellite altimeter when they happen to fly over the same location at sea within a short period of time (i.e., 1-3 days) (Fig. 1). Note that CryoSat-2 ground-tracks are not presented in Fig. 1, because of its drifting orbits.
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| Fig. 1. Crossover locations at sea and on land, over the Permanent Facility for Altimetry Calibration, for the Sentinel-3 missions and other altimeters. |
Implementation of crossover analysis at sea does not require any in-situ measurements. This is not the case for transponder calibrations. Crossover analysis over land may be employed only when two satellites overpass the transponder within a short period of time (i.e., a few seconds up to a few hours).
The following crossover locations of satellite altimetry missions have been identified in west Crete, Greece.
| Crossover Location | Altimetry Missions | Local Cal/Val site |
| Lat: 35.142690° N Lon: 23.367572°E |
Sentinel-3B Pass No.14 HY-2B Pass No. 278 |
CRS1 |
| Lat: 34.642546° N Lon: 23.983581° E |
Sentinel-3A Pass No. 14 Jason-3 Pass No. 18 |
Gavdos |
| Lat: 34.809865° N Lon: 24.092945° E |
Sentinel-3A Pass No. 335 Jason-3 Pass No. 109 |
Gavdos |
| Lat: 35.163577° N Lon: 24.296527° E |
Sentinel-3B Pass No71 Jason-3 Pass No. 109 |
RDK1 |
| Lat: 35.337911° N Lon: 23.779521° E |
Sentinel-3A Pass No. 14 Sentinel-3B Pass No. 335 Jason-3 Pass No. 18 |
CDN1 |
The main advantage of the sea-surface crossover analysis is that it can be employed globally as it only requires the sea-surface heights as measured by each altimeter. No permanent Cal/Val site is necessary. Without doubt, the presence and continuous operation of the sea-surface and transponder infrastructure in this PFAC enhance accuracy, robustness, redundancy and reliability of the Cal/Val results.
The main advantage of the PFAC’s geographical location of ESA is that it allows calibration of Sentinel-3A and Sentinel-3B with diverse and independent calibration techniques. Let us examine, for example, Sentinel-3A Pass No.14. This is an ascending orbit. As the satellite approaches the calibration facility from the south, the sea-surface calibration is employed with the Gavdos Cal/Val site. Nine seconds later, the transponder calibration takes place at the CDN1 Cal/Val site on the mountains of west Crete at an elevation of 1100m.
For the descending Sentinel-3B Pass No. 335, a similar procedure takes place but with a reverse order, that is transponder first and a few seconds later sea-surface calibrations. To strengthen and safeguard results of sea-surface calibrations, a new sea-surface Cal/Val site (“SUG1”) in the south coast of Crete has been established. This site has been operational as of 5-Mar-2021 (see also Cal/Val Techniques: Sea-Surface).
Sea-surface calibration is also carried out for Sentinel-3A Pass No. 335 (with the Gavdos Cal/Val site) and for the Pass No. 278 (with the CRS1 Cal/Val site). Also, Sentinel-3B Pass No. 71 and Pass No. 14 are also calibrated using ground-truth measurements based upon the RDK1 and CRS1 Cal/Val sites, respectively.
The following transponder calibrations have been carried out so far at the CDN1 Cal/Val site for Sentinel-3A and Sentinel-3B:
The Gavdos and Crete Cal/Val Permanent Facility for Altimetry Calibration has been operating and providing absolute biases for altimetry satellites for more than a decade. It was established in 2001 and has been on continuous operation as of 2004. It provides calibration/validation (Cal/Val) for all international altimetric missions (i.e., Sentinel-3, Sentinel-6, Jason series, HY-2, SARAL/AltiKa, CryoSat-2, etc.). It includes a major set of permanent facilities, prototype scientific equipment (transponder), while at the same time it collects archives, analyses, interprets and disseminates scientific data internationally. It has thus established an international reputation, and standing underpinned by consistency, accuracy and reliability in calibration results.
It is capable of calibrating ascending and descending orbits of the same altimeters in the open ocean around Crete. Ocean tides at this location are very small (a few cm), however the ocean circulation, and the reference models are well established.
The Gavdos and RDK1 Cal/Val facilities are to be used for sea-surface calibration of Sentinel-3A & -3B but also Jason satellites. Under the framework of the SeRAC project, a new sea-surface Cal/Val site called "SUG1", at the south west Crete has been established. This SUG1 Cal/Val site will calibrate S3A, S3B and Jason series satellites using the established sea-surface methodology.
At least two tide gauges, one GNSS and one meteorological station shall be continuously operating at each sea-surface Cal/Val site.
The CDN1 transponder Cal/Val site has been established on the mainland of west Crete, Greece under a triple cross-over of Sentinel-3A & 3B, but also of Jason and subsequently of Sentinel-6/Jason-CS series.
A prototype microwave range transponder, a microwave radiometer (Radiometrics MP3000A), two Global Navigation Satellite System (GNSS) stations (Leica GR10 with a Leica AR25.R4 antenna and Trimble NetRS with a Trimble Zephyr Geodetic Antenna), and two meteorological stations (Vaisala WXT520 and Vaisala WXT534) comprise the main instrumentation of the CDN1 Cal/Val site. Hybrid power supply systems (solar, wind, diesel generator and batteries) provide continuous, safe and stable power to these instruments.
Microwave transponders have been applied as alternative tool for satellite altimetry calibration. A microwave transponder is an electronic equipment which receives the pulsed radar signal, transmitted by the altimeter, amplifies it and retransmits it back to the satellite, where it is recorded. The flight time of the radar signal is measured on the satellite, from which the absolute range between the transponder and the satellite can be deduced. The main advantage of this technique, compared to the conventional sea‐surface calibration, stands for the fact that no ocean dynamics errors are involved in this calibration.
To process transponder data, atmospheric corrections are needed. At first, those are determined through dedicated GNSS data processing. Delays caused by the ionosphere and the troposphere (wet & dry) are estimated using the propagation properties of the GNSS signal, and then reductions are made for altimetry signals.
A water vapor radiometer (WVR) constitutes another independent, and alternative to GNSS data processing, technique for the determination of tropospheric delays at the Cal/Val transponder site. Customarily, a WVR uses two microwave frequencies: one at ~22.2 GHz (sensitive to water vapor in the atmosphere) and one at ~30 GHz (sensitive to liquid water). By measuring the power of thermal radiation in the atmosphere, the respective signal delays arising from water vapor and liquid water can be determined. Thus, the wet troposphere delays on altimetry signals can be determined (Teke et al., 2011).
The CDN1 Cal/Val site instrumentation is fully compliant with the FRM4ALT requirement of putting into use redundant and complimentary observing systems. For example, two diverse and independent approaches are used to derive atmospheric delays during a satellite overpass: (a) microwave radiometer and (b) GNSS-derived delays. Independent GNSS processing is also applied for not only the estimation of atmospheric delays but also for the determination of absolute coordinates of Cal/Val site.
The CDN1 transponder Cal/Val site has been operational since September 2015. It has been established mainly to support absolute calibration of the Sentinel-3 satellite altimeters. In order to cross-calibrate Sentinel-3 with other altimetry missions, the transponder is also activated for other European (i.e., CryoSat-2) and international missions (i.e., Jason-2 and Jason-3). The CDN1 site operators have reached an agreement with the agencies operating these missions: EUMETSAT and CNES respectively.
Till 31-Mar-2020, 285 transponder calibrations had been taken place for Sentinel-3A Pass No.14 (54 calibrations), Sentinel-3B (23 calibrations), CryoSat-2 (36 calibrations) and Jason Pass No.18 (172 calibrations). From these calibrations, 261 were successful whereas 24 had been cancelled mainly because of extreme weather conditions to protect the transponder.
