April 19, 2020 marks the 25th anniversary of the “launch” of the first satellite of the GFZ German Research Centre for Geosciences. 'GFZ-1' was placed in its orbit in a very unusual way, which is why the word “launch” is written in quotation marks: The bowling ball-sized mini-satellite was ejected from an airlock at the Russian space station 'Mir' shortly after 9pm CEST (19:12 UTC). GFZ-1 not only founded the satellite-based research at GFZ, but also holds an important scientific record: It was the lowest geodynamic satellite ever measured with lasers.
The passive satellite (it had no propulsion and no on-board electronics) had 60 retro-reflectors on its surface. These were used to reflect laser beams from the ground through the global network of the Satellite Laser Ranging (SLR) system. The SLR stations picked up the light reflected from space and thus were able to determine its exact position in orbit. Measurements from stations distributed around the world also allowed conclusions to be drawn about the Earth's gravitational field.
Today, GFZ, together with the US space agency NASA, operates a satellite mission dedicated to measuring the Earth's gravitational field, which enabled several scientific breakthroughs: GRACE (2002 to 2017) and GRACE Follow-On (since 2018). For the first time it was possible to quantify the loss of ice mass of the large ice sheets on Greenland and over Antarctica. In addition, the mission makes it possible “to track how water behaves underground by observing mass displacements”, says Reinhard Hüttl, Chairman of the Board and Scientific Director of the GFZ German Research Centre for Geosciences. “This area was previously inaccessible to space-based remote sensing”, says Hüttl, “and this has opened up new opportunities for us to observe and quantify climate change”.
GRACE and GRACE FO are the so far highlight of a series of missions that started with GFZ-1, at that time affectionately called “Space Trabi” (as a reference to the famous small German car called “Trabant”). In addition, there is the modern SLR station on Telegrafenberg, which is part of a worldwide network. The Potsdam laser ranging station can measure the distance to satellites in orbits of 400 to 25,000 kilometres above the Earth with an accuracy of about one centimetre.
GFZ-1 was also used for “training” the Potsdam SLR station. GFZ-1 demonstrated both the possibilities and the difficulties of tracking such low-altitude targets with the most modern SLR systems. One of the great possibilities was the particularly precise measurement of the Earth's gravitational field. The lower the satellite orbits, the more it is exposed to the effects of gravity. However, the problem there is that solar storms disturb the upper layers of the atmosphere, and the turbulent “space weather” makes the orbit of the passive satellite irregular. Therefore, the closer the satellite is to Earth, the more difficult it is to track it exactly with laser telescopes. GFZ-1 initially orbited around the globe at an altitude of almost 400 km. The last measurement was made from Australia at an altitude of 230 km. Afterwards the “Space Trabi” burnt up on 23. June 1999 at 1am UTC.
For GFZ numerous research areas are linked to this mission. Besides the measurement of the Earth's gravitational field and the SLR station, also the exact orbit determination of global navigational satellites, the determination of the water vapour content of the atmosphere (important for weather services), the investigation of “space weather” and its influences on satellites as well as the measurements of ground water, soil moisture and ice mass loss. Several spin-offs of the GFZ are related to these activities. DiGOS, for example, designs and builds SLR stations and was recently awarded the Berlin-Brandenburg Innovation Prize.
Another GFZ start-up, which is currently being founded, aims to supply agricultural enterprises, reinsurers or water suppliers with data on water resources.