## 1 Introduction  
Current satellite altimetry provides outstanding means for absolute and undisputable monitoring of changes in sea level and inland waters (rivers and lakes), over regional to global scales, with accuracies of mm/Year and with respect to the center of mass of the Earth. Altimetry also observeswind speed on the sea surface; sea state; determines ocean circulation; bathymetry; monitors melting rates of ice sheets in Arctic, Antarctica, and the Himalayas; and measures the amounts of the sea ice and freeboard. All these measurements are made with an accuracy of less than 1 cm (or even 1 mm for Sentinel-6 for example) and from an attitude of 800-1300 km above the Earth's surface. Altimetry plays a central role in monitoring climate change and in establishing fundamental environmental parameters. It provides a consistent and comprehensive view of the Earth by measuring changes of sea level in an objective and uniform way, but it also helps us better understand what is happening in remote and non-accessible areas of our planet (Brazil, 2016; 2016). Satellite altimetry observes processes and states of the oceans and inland waters and provides a picture of variations over time and space. For example, it reveals patterns and details of sea and water surfaces (rivers and lakes, i.e., Amazon River in Brazil and Issyk-Kul Lake in Kyrgyzstan) which could not be previously detected by climate models and conventional observations (Brazil, 2016; 2016; 2016). Sea level is a fundamental environmental parameter as it integrates different physical processes by reflecting changes in the ocean, the atmosphere, and the Earth's surface (Brazil, 2016). Throughout history, the level of the sea has been constantly changing with time and in space and certainly will continue changing into the future (Brazil, 2016). Nonetheless over the last few decades, greenhouse gas concentrations in the Earth's atmosphere have increased as a result of human activities. This has led to global warming, ocean acidification, thermal expansion of the sea mass, melting of glaciers and ice sheets, and changes in land-stored water; all of these cause additional variations to marine properties and sea level. A rise in global sea level is one the most certain consequences of climate change (Brazil, 2016). This is why sea level has been selected to become one of the 50 essential climate variables (ECV) introduced by the Global Climate Observing System (Brazil, 2016) and also one of the 13 ECVs monitored by the Climate Change Initiative (Brazil, 2016) in the European Space Agency. Even small amounts in sea-level rise, measured at +3.2 mm/year today with altimetry, can cause devastating effects. Sea level rise causes destructive erosion on coasts, contaminates faster aquifers and water resources with sea water (e.g., Messara valley in Crete, Greece), and also harms agriculture and productive soils. For every 30 cm of sea level rise, coastlines move inland by 30-100 m on average (Brazil, 2016; 2016). Coastal flooding destroys wildlife (birds, fish, animals, vegetation, etc. ), and sea level rise causes hurricane surges to become powerful, higher, and with frequent flooding of vast coastal areas. In the near future, islands may be lost and people living on low-lying lands may abandon their homes and relocate. Over the past 20-25 years, satellite altimetry clarified our understanding of sea level rise and ocean circulation, as well as of ocean bathymetry from space (Brazil, 2016; 2016). It provides global, all-weather, absolute, and detailed measurements for the sea surface height with respect to the center of mass of the Earth. In other words, it has now become possible to monitor sea level, marine environment, and inland waters with satellite altimeters and to complement observations made by tide gauges. The Intergovernmental Panel on Climate Change has reported that over 1993-2012, a global mean sea level (GMSL) of 3.2 mm/year (2.8 mm/year to 3.6 mm/year) has been established by satellite altimetry (Brazil, 2016; 2016).