The General Question
The spatial and temporal variations of the Earth’s gravitational field mirror the continuous redistribution of mass within the Earth system due to processes in land hydrology, in the cryosphere, in the oceans, in the atmosphere, and within the solid Earth. The gravitational field is full of signals from processes that are highly relevant for understanding the dynamics of the Earth system.
In the past, the gravitational signals from most of these processes could not be observed because of the limited capabilities and precision of measurement instruments. In recent years, big steps have been achieved in satellite gravimetry, with ground-breaking new information: Greenland ice mass loss, ground water loss in India, the mass variation due to the Scandinavian land uplift, and mass changes related to earthquakes such as the event in Japan in 2011 are just a few examples.
Yet, for many questions on mass variations and geophysical processes the current knowledge of gravity variations is not accurate enough, and maps of mass variations derived from gravitational observations are too diffuse. In addition, new and very tough challenges rise with the need to monitor and understand the global and regional processes of climate change.
The scientific challenges are threefold:
- to determine and monitor global and regional gravity and mass variations from processes that cannot be resolved with the accuracy of current gravitational measurement techniques,
- to determine gravity variations with the spatial resolution that is needed for a detailed understanding of mass redistribution and for the separation of sources and mechanisms,
- to provide an accurate gravity reference for monitoring processes over long time scales as a basis to reliably quantify both long-term change and rapid variations.
Our Approach
To meet these challenges and future needs, a further significant step in precision and accuracy of gravitational measurements and modelling is required. New concepts based on quantum metrology for observing mass variations have to be elaborated. Thus, geo-Q has been successfully initiated, where expertise from physics and geodesy has been integrated in a unique constellation. For the first time, geo-Q brought together three highly topical and active fields of physics for the benefit of geodetic Earth observation:
- ultra-precise optical clocks connected by optical networks
- atomic interferometry,
- and ultra-precise laser interferometry
geo-Q directs the investigation of measurement techniques towards three core goals that contribute to integrated gravity modelling.
First, we are designing measurement systems for monitoring global gravity and mass change, to be used on next generation satellite missions. Second, we develop atomic gravity sensors providing new opportunities for dedicated regional and local monitoring of geophysical and hydrological processes. Third, we will use ultra-precise optical clocks as atomic frequency standards for relativistic geodesy – the relativistic determination of the gravitational potential and physical heights – with the perspective of contributing to a homogeneous dynamic reference frame for the quantities of the gravitational field.
