Support the absolute altimeter calibration during the Operational Phase of the Sentinel-3A & -3B missions, including the Commissioning Phase of S-3B for the surface topography element, by maintaining and operating the CDN1 Sentinel-3 Altimeter Calibration Site in West Crete, Greece.
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CAL/VAL Techniques

Absolute calibration of satellite altimeters, during their lifetime, by external and independent facilities, is a prerequisite for a continuous, homogenous and reliable monitoring of the earth, its oceans and climate change.
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SeRAC project's partners.
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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.


News and Events


Proceedings of the FRM4ALT International Review Workshop

The SeRAC team is pleased to announce the release of the Proceedings of the International Review Workshop on Satellite Altimetry Cal/Val Activities and Applications. The workshop was held in Chania, Crete, Greece, 23-26 April 2018.


Sentinel-3B Transponder Calibration

A successful transponder calibration of Sentinel-3B Pass No.335, Cycle 37 was carried out at the CDN1 Cal/Val site in west Crete, Greece on 12 April 2020.


Crete@Sentinels 2020 Calendar

The 2020 Copernicus Sentinels calendar focuses on European islands. Crete the largest Greek island is November 2020. Thank you ESA!!! The full calendar is available here.


Sigma-0 Transponder Field Tests Cancellation

In line with the decree released by the Italian Prime Minister and related ESA instructions related to the spread of the Coronavirus, all deployment of the S3 Sigma0 Transponder will be suspended until further notice.


Intl. Workshop

International Review Workshop on Satellite Altimetry Cal/Val Activities and Applications

An International Review Workshop on Satellite Altimetry Cal/Val Activities and Applications is to be held in Chania, Crete, Greece, 23-26 April 2018.

Read more: http://frm4alt.eu/int-cal-val-review 

Funded by the EU and ESA