Absolute Calibration of the European Sentinel-3A Surface Topography Mission over the Permanent Facility for Altimetry Calibration in west Crete, Greece  
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1,*  
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2 European Space Agency/European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands; [EMAIL_ADDRESS] (C.D. ); [PERSON][PERSON]  
3 European Space Agency/European Space Research Institute (ESA/ESRIN), Via Galileo Galilei, I-00044 Frascati, Italy; [PERSON]  
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2 European Space Agency/European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands; [EMAIL_ADDRESS] (C.D. ); [PERSON][PERSON]  
5 European Space Agency/European Space Research Institute (ESA/ESRIN), Via Galileo Galilei, I-00044 Frascati, Italy; [PERSON]  
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5 European Space Agency/European Space Research Institute (ESA/ESRIN), Via [PERSON], I-00044 Frascati, Italy; [PERSON]  
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6 Space Geomatica P.C., Xanthoudidou 10A, GR-73132 Chania, Greece; [EMAIL_ADDRESS] (D.G. ); admin@  
protection. It is the world's largest single EO program, a huge European investment that, although not fully operational, already produces a clear economic impact. For every 1 Euro that is spent in the Copernicus program a total of 1.39 Euro of added value is produced in the European economy [2]. The Sentinel satellites are the means to provide the foreseen set of observations for the Copernicus program. They consist of six different satellite families (Sentinel-1 to Sentinel-6 and, in the future, to be extended up to Sentinel-9 series) designed to serve different Copernicus services (atmosphere, land, climate change, emergency management, security, marine environment). The Sentinel-3 family contributes to the Copernicus Marine Services by measuring variables such as sea-surface topography, ocean and land color, and temperature to support operational oceanography and environmental and climate change monitoring. Sentinel-3A was launched on 16 February 2016 and its twin satellite Sentinel-3B was put in orbit on 25 April 2018. This Sentinel-3 family is expected to grow soon with two more satellites, i.e., Sentinel-3C and Sentinel-3D, to ensure continuity of service until 2030 [3]. The Surface Topography Mission (STM) element of Sentinel-3 consists of a dual-frequency (Ku-and C-band) Synthetic Aperture Radar Altimeter (SRAL), a Microwave Radiometer (MWR) and a system for Precise Orbit Determination (POD) that includes a Global Navigation Satellite System (GNSS), a DORIS (Determination d'Orbite et Radiopositionnement Integre par Satellite) system, and a Laser Retro Reflector. The main objective of Sentinel-3 STM is to deliver sea-surface heights with an accuracy of 3.5 cm [4]. This shall be considered as a combination of contributions arising from the SRAL, MWR and POD instruments. To reach this goal and ensure long-term confidence, accuracy and quality in Sentinel-3 data products, calibration and validation (Cal/Val) activities have to be carried out during all mission phases, i.e., pre-launch, commissioning, and operations. This work presents the latest results of the external altimeter calibration of the Sentinel-3A STM with in-situ data at the dedicated Permanent Facility for Altimetry Calibration (PFAC) in west Crete, Greece. This is a Cal/Val facility that provides external range transponder calibration, cross-calibration with other altimetry missions, and sea-surface height calibration with in-situ data, primarily in the Gavdos island, south of Crete [5]. In addition, validation of the atmospheric delays provided by the MWR instrument is carried out using observations from a network of Continuously Operating Reference GNSS Stations established in the broader PFAC area. In the following, Section 2 presents the PFAC infrastructure and instrumentation setting. Then, Section 3 describes the diverse methodologies applied for the calibration of the altimeter products, along with the dataset used.