GRACE satellite mission

Availability of good quality data is essential to deduce meaningful information in all disciplines of science and technology. This statement is especially relevant in climate research, which is under constant scrutiny from sceptics. In order to better understand the Earth system response to the ongoing anthropogenic global warming, we require global observations of climate indicators at very high spatiotemporal resolutions. In a perfect world, every country provides a dense observation network and shares its data openly for research. Yet political and financial constraints prohibit this reality.

Hence, the era of satellite-based Earth observation has been a revolution for geosciences. The launch of satellites that observe our planet since the 1970s, such as optical remote sensing satellites, satellite altimetry launched in the early 1990s followed by the gravity recovery and climate research (GRACE) missions in 2002, provided a much needed momentum to climate change research. Most importantly, GRACE has been a game changer. It consists of two satellites, separated by 220 km with one behind the other in the same near-polar orbit, and by measuring changes in the distance between the two satellite with micro-meter precision, we are able to map the changes in the gravity field of the Earth from one month to the other. These very small changes in local gravity field are driven by mass redistribution in the Earth system. For example, if it rains heavily and the water is collected in an adjacent (sub-surface or on surface) water body, an increase in mass is recorded in the gravity data from the satellite.  Similarly, for droughts or ice-sheet melting, a mass loss perturbs the gravity field and will be recorded by the satellite mission. As a result, GRACE data has been used in nearly all the disciplines of Earth sciences where we are concerned with mass redistribution. However, the impact of GRACE on hydrology is unmatched.