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Harmonised Radar Altimeter Data | ADS

The graph shows the globally averaged sea level between 1993 and the end of 2024. During this period, it will rise by almost 11 cm, which corresponds to an average rise of 3.5 mm per year. The rise is not linear, but accelerated, amounting to 2.6 mm per year in the first 10 years and 3.8 mm per year in the last 10 years. In addition to the increase, an annual variation with amplitudes of approx. 5 mm with maxima in the northern fall can be observed. There are also multi-year fluctuations of around 5 mm, e.g. 1997-2000, 2011-2014, 2016-2018 and 2024.
Time series of the global mean sea level (GMSL) between 60°S and 60°N based on the Topex/Poseidon, Jason-1, Jason-2, Jason-3 and Sentinel-6MF missions. The data are smoothed over time and are available with (light blue) and without annual variation (dark blue). The uncertainty of the trends is 0.3 mmy-1. (Data: ADS, Figure: S. Esselborn, GFZ)
High-rate water level anomalies along 12 satellite tracks are shown in the area of the western Baltic Sea and the Belt Sea. The values range from -40 cm in the southeast off the German coast and +40 cm in the Kattegat north of the Danish island of Zealand.
High-rate multi-mission measurements of water level anomalies in the western Baltic Sea during an inflow event on two days between December 18 and 20, 2023 (Data: ADS, Figure: S. Esselborn, GFZ)
The figure shows the water level at the Kachowka reservoir since 2018. The water level of the reservoir is about 15 meters higher than that of the Dneper downstream. A drop of 14 meters in the water level of the reservoir and a flooding of the region below the dam by 5 meters is clearly visible in June 2023.
Water level series from altimetry for the Kachowka reservoir (orange) and the Dneper River below the reservoir in Ukraine. The dam breach in June 2023 is clearly visible. (Data: SDSS, Figure: T. Schöne, GFZ)
The figure shows the water level in the western, northern and central Aral Sea. The central reservoir has been dry since around 2009, as can be seen from the flat water level curve. The western part, which is deeper, continues to lose water. The northern part shows only slight variations in the water level.
The Aral Sea is a prominent example of the increasing drought in Central Asia. The Aral currently consists of two water areas (Aral West and Aral North). Since around 2009, the central Aral Sea has disappeared and only the western and northern parts remain. (Data: SDSS, Figure: T. Schöne, GFZ)

Satellite-based radar altimeters actively measure the water level of the ocean and inland waters along their ground tracks. In addition, they record the wind and significant wave heights. Nineteen radar altimeter missions have been launched into orbit since the mid-1980s. Most of these missions use different data formats and are available with different processing standards.

In order to generate long and consistent time series of water levels, the Altimeter Database and Processing System (ADS)was developed at the GFZ, which combines all missions and enables uniform processing. In addition, the stability and consistency of the individual missions is regularly checked. Apart from the actual altimeter data, ADS also contains current correction models for the sensors, the satellite orbits and the environmental influences on the measurements. For example, the influence of orbit modelling on the water level measurements can be quantified and external models can also be validated. With the help of the ADS, data processing can be optimized for the respective region and scientific question.

ADS currently includes data from the following completed missions:

Geos-3 (1975-1978), Geosat (1985-1989), ERS-1 (1991-1996), Topex/Poseidon (1992-2005),  ERS-2 (1995-2011), GFO-1 (2000-2008), Jason-1 (2002-2013), Envisat (2002-2012), ICESat (2003-2009), Jason-2  (2008-2019), and HY-2A (2011-2020),

and the following still active missions:

Cyrosat-2 (2010), Saral  (2013), Jason-3  (2016), Sentinel-3A/B  (2016/2018), HY-2B (2018), HY-2C (2020), Sentinel-6MF (2020), HY-2D  (2021), and SWOT (2022).

Altimetry over the Ocean

Classic application examples for the harmonized data are the observation of the global mean sea level and the regional distribution of the rise. The radar altimeter measurements show a global rise of approx. 11 cm since 1993. The average rise is 3.5 ± 0.3 mmy-1, with a clear acceleration being observed. As a result of the superposition of various processes in the solid earth, in the ocean and in the atmosphere, the sea level rise is not evenly distributed across the oceans.

The sensors, the correction models and the analysis strategies are continually being further developed, whereby the accuracy increases and processes in direct coastal proximity and in inland waters can be examined to an increasing extent. On the basis of the high-rate data, which are also contained in ADS, water levels on the North Sea coast and exchange processes between the North Sea and the Baltic Sea are examined, for example.

Altimetry over Inland Waters

Another possible application of harmonized radar altimetry is the investigation of inland waters. Our methods can be used to observe and analyze the filling levels of dams, changes in lake levels and the rise and fall of rivers. The time series, which date back to 1993, enable us to observe and analyze changes in the continental water cycle. We are also working on methods that allow us to forecast water availability for short-term periods. This work contributes to important projects  such as CAWa  and GreenCentralAsia. The time series are made freely accessible via the SDSS  for use by the general public and political decision-makers. 

Project partners:

  • AWI, CAIAG, Hereon, Geomar
  • CLS, DTU, TUD, University of Bonn and TUM
  • Second Institute of Oceanography, State Oceanic Administration (SOA), Chinese Academy of Surveying and Mapping
  • MARUM - Zentrum für Marine Umweltwissenschaften, Universität Bremen, ZMT- Leibniz-Zentrum für Marine Tropenforschung

Project duration:

  • since 2000

Budget:

  • Strategie Fonds Helmholtz Gemeinschaft: 2000-2003 (SEAL)
  • BMBF, Geotechnologien: 2005-2008 (SEAVAR)  
  • ESA: 2009-2010 (REAPER)
  • ESA: 2010-2013 (ESA-CCI sea level)
  • DFG, SPP 1889 SeaLevel: 2016-2023 (CoRSEA)
  • BMBF, FONA3: 2017-2020 (SEAHAP) 

  • POF IV: 2021-2027

Project-related publications:

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