## 1 Introduction
It promotes evaluations for the uncertainty of altimetry observations but also to trace uncertainties for water level observations to undisputable standards. It thus aims at achieving reliable, long-term and consistent satellite Earth observations and products, via reliable calibration and standards. It provides a procedure-based service to support the long-term calibration and validation of satellite multi-mission altimetry, and provides independent FRM-class data for use in harmonization of long-term multi-satellite observation of the Earth system, and comprehensive and integrated comparisons and assessments [11]. The Gavdos/Crete facilities in west Crete, Greece have been providing altimetry Cal/Val services since 2004. In 2015, the European Space Agency declared Gavdos facilities as the ESA Permanent Facility for Altimetry Calibration (PFAC) in support of Copernicus altimetry and ESA CryoSat mission calibration and validation activities. Over the past 5 years, the facility has pioneered FRM standards and procedures for altimetry calibration products. The PFAC Cal/Val infrastructure consists of one transponder Cal/Val site for external range calibration that is complemented by three permanent sea-level-surface Cal/Val sites. A second transponder (providing range and sigma-0 retrieval functionality) is being developed for operational redundancy in support of the Copernicus Sentinel-6 mission. The infrastructure that is permanently installed and maintained at the PFAC is used routinely for multi-mission altimeter calibration activities and additional relative calibration at crossover locations. The PFAC is designed to allow assessment of different Cal/Val techniques (i.e., sea-surface heights versus transponder) and to perform calibration activities using satellite measurements taken only a few seconds apart along the same orbit. For example, the Sentinel-6 mission will fly 30 s in time apart from Jason-3 along the same nominal ground track for a 12-month tandem flight designed to maintain the stability of the mean sea level trend time series. The ability to perform cross-comparison by diverse and redundant Cal/Val techniques at different settings (at 1050 m, 160 m altitude and at sea level, for example) is required to establish confidence in the calibration results that are derived. The revisit cycle of Sentinel-3A and -3B is every 27 days and that of Jason-3/Sentinel-6 is every 10 days, with ascending and descending orbits crossing over PFAC Cal/Val transponder infrastructure. This allows the PFAC to support annually about 50 calibrations for each Sentinel-3A and Sentinel-3B satellite and more than 70 calibrations per year for Jason-3 (and subsequently Sentinel-6). It can be seen from Figure 1 that the PFAC calibrates three Sentinel-3A Passes (No. 14, No. 278, and No. 335), three Sentinel-3B Passes (No. 14, No. 71, and No. 335) and two Jason-3 Passes (No. 18 and No. 109). Figure 1: _Cont._  
This work presents the latest calibration results for Sentinel-3A, Sentinel-3B and Jason-3 as determined at the PFAC, making use of three independent Cal/Val techniques: transponder, sea-surface and crossover analysis. A short description of the PFAC infrastructure and instrumentation is given in Section 2, together with a summary of the three calibration methods employed. The procedures to ensure the Cal/Val measurements (compliant with the FRM standards), and the uncertainty analysis for the sea-surface and transponder calibrations, are described in Section 3. Section 4 presents the absolute Cal/Val results for each satellite (i.e., Sentinel-3A, Sentinel-3B and Jason-3), along with relative calibration results of Sentinel-3A and Sentinel-3B with respect to Jason-3 at crossover locations over the sea surface. Section 5 discusses single-mission results and evaluates performance using each Cal/Val site. Inter-validation of calibration results obtained by diverse Cal/Val methods is also reported. Section 6 summarizes calibration results and the performance of Sentinel-3A, Sentinel-3B and Jason-3 obtained by the PFAC.