# _remote sensing_
## 1 Introduction  
Satellite altimeters provide accurate sea surface height (SSH), wind speed (WS), and significant wave height (SWH) measurements [1]. Such measurements are great contributions for technology and research of oceans and inland waters by providing reliable observations of global oceans, sea ice, ice sheets, lakes, and rivers [2]. The time series of accurate altimeter data covers more than 35 years, starting with TOPEX/POSEIDON (T/P), and presently supported satellites include Jason-3/CS, SARAL/AltiKa, Sentinel-3A/B, HY-2B/C/D, CryoSat-2, and Surface Water and Ocean Topography (SWOT). The requirements of the accuracy and stability of the altimeter system are rather stringent for its data applications. The Global Climate Observing System (GCOS) stated that to address questions of climate science, the sea level records at scales of 50-100 km should have an accuracy of order 10 mm, with the accuracy of the global average (on a weekly or 10-day basis) being 2-4 mm and with a stability of 0.3 mm/year. Therefore, calibration and validation (Cal/Val) of satellite altimeters are crucial [3]. Although there are often major Cal/Val campaigns at the start of a satellite mission, it is also essential that such work continues throughout the lifetime of the spacecraft, because an instrument's behavior may change due to continued usage and long space exposure. Furthermore, long-term validation series are not only important for reducing the uncertainty in any bias, but also for allowing the estimation of gradual drifts in the measurements of the altimeters. To accomplish the Cal/Val of the altimeters in different regions around the world to comprehensively evaluate their performance and keep the consistency and continuity of the measurements, scientists have established several calibration sites, including Lampedusa in Italy [4], Senetosa in southwest Corsica in France and Ibiza Island in Spain [5], Harvest Oil Platform in America [6], Bass Strait in Austrilia, Gavdos in Greece [7], Kavaratti in India [8], and Wanshan calibration site (WSCS) in China (Figure 1). Dedicated infrastructure such as tide gauges, permanent GNSS stations (PGSs), dedicated GNSS buoys (DGB), meteorological instruments, and auxiliary data/models are employed in such sites. Repeat Cal/Val campaigns have been performed in these sites for long-term exposure. Also, the lake Issykkul in Kyrygzstan has been identified as a suitable location for calibration of water surface height of altimeters over land water bodies. As well as these in situ sites, many campaigns have been conducted to evaluate the performance of the altimeters (blue dots in Figure 1). The scientists also use globally or regionally distributed tidal stations to assess the relative accuracy of the altimeters. The preliminary task for the Cal/Val of altimeters was to set the unified datum of the site or campaign. This is achieved using the GNSS data of several PGSs placed in the calibration sites or campaigns [4; 5; 6; 7; 8]. The GNSS data make combined progress with nearby GNSS stations using GAMIT/GLOBK, Berenese, GIPSY, or precise point positioning (PPP) software or methods. The accuracy of the height datum can reach to less than 5 mm [9; 10]. After that, there are many methods to accomplish the Cal/Val of altimeters, such as the DGB, tide gauges, transponder, moored oceanographic instruments, etc. ; the tide gauges are the most commonly used method [5; 7; 9; 11]. In this method, the absolute datum of the tide gauge should be determined using the PGSs plus leveling, kinematic GNSS measurements, or DGBs [12]. Moreover, the tide gauge is often placed on land/an island, while the altimeter signals are contaminated in such an area [11]. There is a distance of about 15-30 km from the tide gauge location to the non-contaminated altimeter footprints. The mean sea surface (MSS) and tidal corrections must be taken into consideration [11; 12]. The MSS gradient can reach up to 10 cm/km in some places, which is the key correction element for altimeter calibration [1].