**3.1 Sea-level change**

*Geodetic Sciences - Theory, Applications and Recent Developments*

for applications to inland water monitoring as well as the applications in cryosphere studies, such as measuring the ice sheet elevation change and sea ice freeboard.

*Comparison of the principles of the conventional and SAR altimetry (adapted from the [7]).*

Altimeter data are available at different levels of complexity through different platforms and for various purposes. AVISO (Archiving, Validation, and Interpretation of Satellite Oceanographic Data) for instance offers gridded and along-track multi-mission altimeter data products (not) corrected for the geophysical effects and for different purposes [35]. Besides, AVISO offers access to Basic Radar Altimetry Toolbox (BRAT) software as well as the tools such as Marine Geospatial Ecology Tools (MGET). On top of that, through the Live Access Server (LAS), AVISO offers on-the-fly data visualization, metadata access, and quick comparisons of the measurements. For geodetic purposes, AVISO's most valuable products are related to the SSH, often upgraded to show ocean variability or

Near-real-time along-track satellite altimeter data are available also through the Jet Propulsion Laboratory (JPL) PO.DAAC Drive system (Physical Oceanography Distributed Active Archive Center). The data are delivered as a map or digital data, focusing on the SSH, wind speed, wave heights, and geostrophic velocity vectors [36]. Different products are also available from Copernicus Marine Service [16], which offers complete studies on sea-related topics. That includes original measure-

Finally, all the georeferenced source altimeter measurements and many corrections for the measurements are available through the RADS (Radar Altimeter Database System) [37]. RADS provides harmonized, validated, and cross-calibrated sea level altimeter data for the desired area and period of the observations, and it is probably the best place to start with the altimetry for the geodetic studies. Also, RADS offers data preprocessing and processing steps integrated within the system

**2.5 Altimeter data download and processing**

ments, sea-level-related maps, and sea-level forecasts.

and available through the additional tools.

**84**

cryosphere changes.

**Figure 4.**

Sea level change is studied as the global and local phenomena (**Figure 5**). Today, the global sea-level change is routinely computed from the altimetry for the period from 1992 onwards by AVISO, Commonwealth Scientific and Industrial Research Organization (CSIRO), University of Colorado Boulder (CU), NASA - Goddard Space Flight Center (GSFC), The National Oceanic and Atmospheric Administration (NOAA), and others. All the global research studies agree on the current sea level linear trend of approx. 3.2 mm/yr. although the processing methods could differ slightly. The estimates on the global sea-level change trends from satellite altimetry are regularly reported within the IPCC (The Intergovernmental Panel on Climate Change) reports that provide policymakers with regular scientific assessments on climate change. Several studies reported on the regional and local sea-level change, e.g., [39] consolidated the trends and expected sea-level change globally and for the ocean regions, [40] reported on the projections of the regional sea level for the 21st century, [41, 42] recomputed all satellite altimeter data to get more pronounced sea-level change estimates and a better perspective on the impact of future sea-level rise.

The satellite altimetry enabled finer detection of the current acceleration of global and regional sea-level rise. E.g., [38] reported on the climate-change-driven acceleration in sea level rise over the altimeter era, [43] investigated the regional sea-level rise during the altimeter era with previous studies done on uncovering the anthropogenic influence on the sea level rise in some regions [44]. With the climate change acceleration, monitoring of the sea-level change and its variation is going to be even more important. A future perspective on gauging the sea-level change and the needed improvements, both for the satellite observations and the terrestrial (tide gauge and other) measurements, is summarized in [45].

**Figure 5.** *Global Sea level trends (data downloaded from [38]).*

The mean sea surface and its change are one of the bases for vertical height system modeling and implementation. A wide initiative on unifying the vertical height reference systems (for details see [46–48]) most usually encompasses absolute sealevel modeling from satellite altimetry extended for the tide gauge measurements at the coast (see e.g., [49]) along with the extensive analysis of vertical land movements, GNSS measurements, gravity estimations, etc. For such purposes, further progress in coastal altimetry and altimetry, in general, is crucial.
