Technological context and background

The determination of global mass transport phenomena via gravity field monitoring from satellites is an essential method to tackle the environmental and societal challenge of climate change. Indeed, previous and current space missions (e.g. GRACE, GOCE and GRACE-FO) revolutionised the understanding of mass transport in the Earth system, enabling for the first time the recovery of a global time-varying gravity field. The data obtained in missions so far provides unique and invaluable contributions for understanding climate change processes, such as the melting of the glaciers and ice sheets, sea-level rise, regional droughts, and flooding, and potentially allow for the early warning of such events. The largest error contributor of state-of-the-art missions is linked to the effect of aliasing which results from their incomplete observation geometry. The next generations of gravimetry missions will improve the spatiotemporal sampling of the gravity signal using well-designed satellite constellations, but remain limited in accuracy and resolution. To go one step further, a technological breakthrough involving novel sensors is at hand. It will provide a significant step forward in accuracy. This improvement would pave the way for a global and repetitive remote sensing of essential climate variables related to ocean, cryosphere, atmosphere and land hydrology and monitoring geodynamics phenomena related to earthquakes and volcanic eruptions.

Classical electrostatic accelerometers used so far in gravimetry missions present increased noise at low frequency and long-term drifts that limit the ability to reconstruct the Earth’s gravity field at long wavelength and to accurately model its temporal fluctuations. These limitations have generated a broad interest in disruptive technologies, based on Cold Atom Interferometry (CAI), that measure the acceleration of freely falling independent atoms by manipulating them with laser light. These systems, free from most systematic errors that affect classical systems, provide higher sensitivity at long wavelength, drift-free measurements, and higher absolute accuracy, leading to accurate long-term measurements and comparisons. On the Earth, state-of-the-art CAI has reached an accuracy in the low nano-g range for absolute gravimeters and a differential acceleration sensitivity of order of the nano-g/m for gravity gradiometry. In space, where the quantum superposition in CAIs may be maintained for seconds well beyond what can be reached on ground, the sensitivity at long wavelength is expected to be several orders of magnitude higher, thus outperforming the best classical devices while contributing to improved measurements of climate change-related mass transport products. In particular, the improved long-term stability of quantum sensors is promising for significantly more accurate understanding of mass transport processes at large scales. Despite its recent progress, this innovative technology is currently not considered mature enough , i.e. a low Technology Readiness Level (TRL), to be ready for operation in space at its best potential level of performance. Indeed, operating a quantum sensor in space represents a major scientific and technical challenge. First, it is necessary to understand and simulate the performance of these instruments in the context of space gravimetry missions to fully benefit from their performance. Second, the operation of these instruments on a satellite requires specific development to be adapted to microgravity conditions and a dedicated qualification for the space environment.