The Gavdos/Crete Permanent Facility for Altimeter Calibration: Absolute and relative calibration of satellite altimeters at ESA Fiducial Reference Measurement Standards

Author:

Stelios P. Mertikas - (Technical University of Crete)

Co-Investigator(s):

Costas Kokolakis (Space Geomatica P.C.)
Panos Partsinevelos (Technical University of Crete)
Ilias Tziavos (Aristotle University of Thessaloniki)

Abstract:

To establish a continuous, homogenous and reliable monitoring of the ocean and its changes, altimetry observations have to be free of errors and biases, uninterrupted, but also tied from one mission to the next in an objective and absolute sense. Altimetry system’s responses have to be, thus, 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.

This research proposal aims at continuing that long‐term service for calibration of existing and future satellite altimeters using the Permanent Facility for Altimeter Calibration (PFAC) in Gavdos and Crete, Greece. These Cal/Val facilities have been established in 2001 and have been seamlessly operating and providing absolute reference results for all world satellite altimeters. The location of the Gavdos and Crete Island, in the center of East Mediterranean, constitutes a strategic location for the calibration of satellite altimeters on a world level, and also for monitoring absolute sea level and climate change on a continuous and long‐term basis.

Calibration in Gavdos and Crete will be carried out using the following diverse, independent but complementary techniques: (1) absolute direct range calibration using two Ku-band transponders on land at the CDN1 and GVD1 Cal/Val facilities on the mountains in west Crete and Gavdos respectively, (2) relative direct sea-surface height calibration using four coastal Cal/Val sites on both islands of Gavdos and Crete, (3) backscatter coefficient calibration using active and passive instrumentation, and (4) validate the performance of altimeters’ water vapour corrections employing ground instruments (i.e., microwave radiometer) as well as other remote sensing techniques. The PFAC calibration facilities will provide reference measurements to attain Système international d'unités (SI) traceability for the calibration values, along with their uncertainty budget for the produced Cal/Val results for all altimeter satellites.

In other words, this Cal/Val infrastructure will provide fiducial reference measurements for the absolute calibration for the ascending and descending orbits of baseline missions, such as those of Jason satellites (i.e., Jason‐3, Sentinel‐6/Jason‐CS, etc.), ESA missions (CryoSat‐2, Sentinel‐ 3, etc.), as well as for the relative calibration against other altimetric missions (e.g., the Chinese HY‐2B, HY-2C, HY-2D, etc.). It will also make preparations for the future altimetric missions, such as that of SWOT, the Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL), the Chinese Quanlan, and the Japanese COMPIRA, operating in different measuring modes and frequencies. Finally, satellite radiometric measurements will be monitored at this infrastructure against atmospheric path delays determined by a regional, continuously operating network of 17 permanent GNSS sites as well as by microwave radiometers.

This fiducial reference site in Gavdos/Crete for satellite altimeters will consistently and reliably determine (a) absolute altimeter biases and their drifts; (b) relative bias among diverse missions; but also (c) continuously and independently connect different missions, on a common and reliable reference. Results from this fiducial reference site should be based on historic Cal/Val site records, and would be the yardstick for building up capacity for monitoring the climate change records.