On 19 April 1995, the football-sized “GFZ-1” marked the start of the first satellite mission of the GFZ Helmholtz Centre for Geoscience – and thus the era of successful satellite-based research at the GFZ. One of the mission objectives at the time was to measure the Earth's gravitational field. 

The view of the Earth and near-Earth space from space is essential for the geosciences: for measuring the Earth and understanding many of the interrelationships on our planet – from the positioning of the Earth in space and its application in satellite navigation systems to the measurement of the Earth’s magnetic field, the effects of space weather and changes in the Earth’s external appearance at both global and local levels – for example, through earthquakes and land subsidence – to the condition of soil, water bodies and vegetation and the analysis of mineral deposits.

History and function of the pioneer “GFZ-1”

The football-sized mini-satellite “GFZ-1” was launched into orbit 30 years ago in an unusual way: After being transported from Baikonur to the Russian space station MIR on 9 April by the PROGRESS M-27 space transporter, it was ejected into space from an airlock on the space station shortly after 9 p.m. CEST (7:12 p.m. UTC) on 19 April. The satellite, weighing just under 21 kg, initially orbited the Earth at a relatively low altitude of 400 km.

The passive satellite had no propulsion system or on-board electronics of its own, but was at the mercy of the Earth's gravitational field. Measuring this field was one of its mission objectives.

For this purpose, GFZ-1 was equipped with 60 retro-reflectors on its surface. These were targeted by laser beams sent by satellite laser ranging (SLR) stations around the world as GFZ-1 orbited the Earth. The light reflected from the satellite in space was captured again by the respective SLR station. The position of the satellite could be determined precisely from the analysis of the light’s transit time.

The continuous position measurements from stations distributed around the world in turn allowed conclusions to be drawn about the Earth's gravitational field. In order to achieve the best possible results by maximising the influence of gravity on the satellite’s orbit, GFZ-1 was placed in a relatively low Earth orbit. At 400 kilometres, GFZ-1 also holds an important scientific record: it was the lowest geodynamic satellite ever measured with lasers.

The satellite, affectionately nicknamed “Space Trabi” at the time, orbited the Earth almost 24,000 times over a period of four years and 64 days before burning up in the upper atmosphere at 01:00 UT on 23 June 1999, bringing its mission to an end.

GFZ research in various fields related to this mission

In addition to measuring the Earth's gravitational field, further science include the precise determination of the orbits of navigation satellites, the determination of water vapour content in the atmosphere (important for weather services), research into ‘space weather’ and its influence on satellites, and the measurement of groundwater, soil moisture and ice mass loss from gravitational data.

Further satellite missions of the GFZ

Over the years, the GFZ has been involved in numerous other satellite missions or has provided scientific leadership for them. Following on from GFZ-1, CHAMP enabled the measurement of the Earth's gravity field and magnetic field from 2000 to 2010, and since 2013, the ESA mission SWARM has been measuring the Earth’s magnetic field. Since 2022, the EnMAP environmental satellite has been providing hyperspectral data in more than 250 colours, which provide information about the condition of soils, water bodies and vegetation, as well as mineral deposits.

Continuation of the gravity field mission by GRACE

The increasingly precise determination of the Earth’s gravity field has remained an important focus, with enormous progress being made thanks to new measurement and modelling methods. Since 2002, the GFZ has been operating the GRACE (2002-2017) and GRACE Follow-On (since 2018) satellite missions, which specialise in the Earth's gravity field, in collaboration with the US space agency NASA. Each mission is made up of twin satellites flying at an altitude of 500 km, with their distance from each other measured ultra-precisely, varying by around 200 km depending on gravity. This allows conclusions to be drawn about changes in the Earth’s mass and thus about changes in the water systems.

The aim of the GRACE missions is to continuously monitor the effects of the diverse and complex feedback loops of human activities on the global water cycle, sea level rise and the climate system. They have enabled various scientific breakthroughs. For example, the ice mass loss of the large ice sheets on Greenland and over Antarctica could be quantified for the first time.

The GRACE missions provide important climate data for the reports of the Intergovernmental Panel on Climate Change (IPCC) and are among the most frequently cited missions in these reports. Thousands of scientific publications are based on data from the two satellite pairs. The third generation of the successful twin satellites, GRACE-C (Continuity), is scheduled to be launched into polar orbit at the end of 2028.

The website www.globalwaterstorage.info provides the latest news and background information on global water data and the GRACE missions.

SLR: Satellite ranging measurement on the Telegrafenberg

In addition to satellite research, the development of the SLR station on Telegrafenberg also benefited from GFZ-1. Satellites have been tracked from here since 1974 for the purpose of position measurement using laser beams. The SLR station on Telegrafenberg is part of a global network of several dozen stations. The third generation is now in operation and the fourth is in the planning stage. Over the decades, precision has continued to increase: the Potsdam laser station can currently measure the distance to satellites in orbits ranging from 400 to 25,000 km above the Earth with an accuracy of less than 1 cm, compared to metres in the early days.

With its low orbit, GFZ-1 demonstrated both the possibilities and the difficulties of tracking such low targets with the most advanced SLR systems available at the time. One of the possibilities is the particularly accurate measurement of the Earth’s gravitational field. The lower the satellite flies, the more it is exposed to the effects of gravity. The problem here is that solar storms disrupt the upper layers of the atmosphere, and the turbulent ‘space weather’ makes the orbit of the unpowered satellite irregular. This means that the closer the satellite is to Earth, the more difficult it is to track it with laser telescopes.

Satellite research fosters entrepreneurial spirit

Over the decades, satellite research at the GFZ has also led to various spin-offs. For example, the company DiGOS has been designing and building SLR stations worldwide since 2014 and was awarded the Berlin-Brandenburg Innovation Prize in 2019.

Founded in 2022, Leomagnetics GmbH offers consulting services for all aspects related to geomagnetism and space weather. Whether radio communication, ensuring the reliable operation of drones or autonomous vehicles, or predicting satellite orbits, the consulting services cover all activities that may be affected by the effects of solar storms.

Also founded in 2022, maRam UG designs, builds and services customised Global Navigation Satellite Systems (GNSS) sensors. Possible applications for the tinyBlack GNSS data logger include monitoring dams and volcanoes or as a base station for drones.