Next Generation Gravity Mission Research Articles

The European way to gravimetry: From GOCE to NGGM

The European gravity mission GOCE (Gravity field and steady-state Ocean Circulation Explorer), which flew between 2009 and 2013 with extraordinary success, was the result of more than 20years of system studies and technology developments directed by the European Space Agency (ESA). Already while GOCE was being developed, ESA started preparatory studies for a Next Generation Gravity Mission (NGGM). While GOCE aimed to provide a high resolution static map of the Earth’s gravity, the objective of NGGM is long-term monitoring of the time-variable gravity field with high temporal and spatial resolution, by satellite-to-satellite tracking, a technique flown with the US–German mission GRACE for the first time. With respect to GOCE, the new mission implies new measurement techniques and instrumentation, a new mission scenario and different spacecraft design drivers. Despite the differences, however, several achievements of GOCE (e.g. demonstration of long-duration wide-band drag compensation, ultra-sensitive accelerometers, stable non-cryogenic temperature control in low Earth orbit) stand as the basis upon which NGGM is being defined. Technologies and techniques specific of the new mission (laser metrology, multi-task control architectures, high performance propulsion) are undergoing parallel developments in anticipation of the NGGM implementation phase. The paper recalls the GOCE mission and heritage, and introduces the objectives, scenarios, concepts and challenges of the NGGM, highlighting commonalities and differences between the two missions.

What Can be Expected from the GRACE-FO Laser Ranging Interferometer for Earth Science Applications?

The primary objective of the gravity recovery and climate experiment followon (GRACE-FO) satellite mission, due for launch in August 2017, is to continue the GRACE time series of global monthly gravity field models. For this, evolved versions of the GRACE microwave instrument, GPS receiver, and accelerometer will be used. A secondary objective is to demonstrate the effectiveness of a laser ranging interferometer (LRI) in improving the satellite-to-satellite tracking measurement performance. In order to investigate the expected enhancement for Earth science applications, we have performed a full-scale simulation over the nominal mission lifetime of 5 years using a realistic orbit scenario and error assumptions both for instrument and background model errors. Unfiltered differences between the synthetic input and the finally recovered time-variable monthly gravity models show notable improvements with the LRI, on a global scale, of the order of 23 %. The gain is realized for wavelengths smaller than 240 km in case of Gaussian filtering but decreases to just a few percent when anisotropic filtering is applied. This is also confirmed for some typical regional Earth science applications which show randomly distributed patterns of small improvements but also degradations when using DDK4-filtered LRI-based models. Analysis of applied error models indicates that accelerometer noise followed by ocean tide and non-tidal mass variation errors are the main contributors to the overall GRACE-FO gravity model error. Improvements in these fields are therefore necessary, besides optimized constellations, to make use of the increased LRI accuracy and to significantly improve gravity field models from next-generation gravity missions.

The measurement of Earth’s gravity field after the GOCE mission

GOCE ( Gravity Field and Steady-State Ocean Circulation Explorer), the first Earth Explorer Core Mission in ESA’s Living Planet programme, has been launched on March 17th 2009. Employing an ultra-sensitive gradiometer on a very low altitude orbit in along-track drag-free condition, within about 2 years GOCE will provide a global model of the Earth’s gravity field and of the geoid to unprecedented spatial resolution and accuracy. Meanwhile, the requirement for monitoring the temporal variations of gravity with a spatial resolution similar to GOCE, but over much longer time periods than the lifetime of GOCE, is emerging. It is driven by the need to detect long-term periodicities and trends in geophysical phenomena that involve large mass redistributions (e.g. melting of the polar ice sheets, changes in continental water storage, etc.), which are extremely valuable for a better understanding of climate change. From a series of preparatory studies led by the European Space Agency (ESA) since 2003, it turned out that the most suitable technique for such a next-generation gravity mission is the so-called Low-Low Satellite-to-Satellite Tracking based on laser distance metrology. A first design of this mission and of its laser metrology system has been defined in the studies performed for ESA the team led by Thales Alenia Space Italia (TAS-I).

Project SEE (Satellite Energy Exchange): aninternational effort to develop a space-based mission for precisemeasurements of gravitation

Project SEE (Satellite Energy Exchange) is an international effort todevelop a space-based mission for precise measurements of gravitation.Gravity is the missing link in unification theory. Because of the uniquepaucity of knowledge about this, the weakest of all known forces, andbecausegravity must have a key role in any unification theory, many aspects ofgravity need to be understood in greater depth. A SEE mission wouldextendour knowledge of a number of gravitational parameters and effects, whichare needed to test unification theories and various modern theories ofgravity.SEE is a comprehensive gravitation experiment. A SEE mission would testfor violations of the equivalence principle (EP), both byinverse-square-law (ISL) violations and by composition dependence (CD),both at ranges ofthe order of metres and at ranges on the order of RE. A SEEmissionwould also determine the gravitational constant G, test for timevariation of G, and possibly test forpost-Einsteinian orbital resonances. The potential finding of a non-zerotimevariation of G is perhaps the most important aspect of SEE. A SEEmission will also involve a search for new particles with very lowmasses,since any evidence of violations of the EP would be analysed in terms of aputative new Yukawa-like particle.Thus, SEE does not merely test for violations of general relativity (GR);SEE is a next-generation gravity mission.