## 1 Introduction
Although there is observational evidence that GMSL is rising, considerable debate still remains as whether the rate of sea level rise is currently increasing as a result of human activity and, if so, by how much (Brazil, 2016). To answer this question the sea-level variability over the short-term has to be removed from the climate records (Brazil, 2016), while instrumental drifts have to be carefully scrutinized as they may distort the GMSL observations (Brazil, 2016). Climate change monitoring requires longstanding observations of several decades; a duration going beyond the typical observation length and lifetime for a satellite altimeter of about 5-7 years. In addition, the sound identification of sea-level signals connected to long-term climate changes still remains a challenge, although current altimeters provide remarkable accuracies (Brazil, 2016) (also Table 1). Consequently, to support a seamless, reliable, and objective monitoring for sea level and inland waters tied to an inertial reference system (Brazil, 2016), diverse satellite altimeters have to be calibrated and also cross-calibrated against each other continuously and in an absolute sense using ground-truth research infrastructures. Absolute calibration of satellite altimeters by external, permanent, and independent to satellite facilities is a prerequisite for a continuous, homogenous, and reliable monitoring of the Earth, its oceans, and climate change. These calibration/validation (Cal/Val) facilities on the Earth's surface ensure that altimetry observations are free of errors and biases, uninterrupted, and tied from one mission to the next in an objective and absolute sense. Altimeter systems' responses have to be, thus, continuously monitored and controlled for their quality, biases, errors, drifts, etc. Relations also among different missions have to be established on a common and reliable Earth-center reference system, maintained over a long period of time. Up to now, absolute calibration of satellite altimetry is primarily provided by permanent ground calibration and validation facilities. These are located either offshore or on land (e.g., microwave transponders) exactly under (absolute direct Cal/Val) or adjacent to satellite's ground track on nearby coasts to ensure monitoring of uncontaminated satellite observations (absolute indirect Cal/Val). Relative calibration of satellite measurements is also performed using either multi-mission crossover analysis between reference altimeters and other missions (relative direct Cal/Val) or distributed tide-gauge networks (relative indirect Cal/Val). Today, there exist four absolute, permanent, and historic such Cal/Val facilities in the world: one is operated by CNES (French Space Agency) in Corsica, France [21], one run by the Jet Propulsion Lab/NASA in California, USA [22], one managed by The University of Tasmania in Bass Strait, Tasmania, Australia [23], and one operated by the Technical University of Crete in western Crete and Gavdos in Greece [24]. Each calibration method presents advantages and disadvantages. Nonetheless, there exist several international groups dedicated to satellite altimetry calibration, while periodic campaigns had been conducted in the past to support mission-specific Cal/Val services [6; 25; 26; 27; 28; 29]. With the recent advent of diverse satellite altimeters along with advanced measuring techniques (Nadir, Delay-Doppler altimeters, wide swath, Ku-band, and Ka-band frequencies), it has become mature and is high time to maintain absolute reference Cal/Val sites to regularly monitor any altimeter but based upon fundamental and undisputable reference and metrology standards, i.e., speed of light, absolute time, etc. This fiducial reference measurement (FRM) concept had been introduced by the European Space Agency and ESA plans to calibrate all its present and future altimeters in that manner.