MOCAST+ (MOnitoring mass variations by Cold Atom Sensors and Time measures) is an on-going study of a quantum technology mission for the gravity field mapping to monitor mass transport and mass variations above and below the Earth surface. The study, which is the continuation of the MOCASS project, is jointly carried out by several Italian research groups and is funded by the Italian Space Agency (ASl).
The study considers the integration of two different technologies, based on atomic interferometry gravity sensors and on the time/frequency measurement with optical clocks. This integration requires an evolution in the technology for the gravity measurement, where the atomic interferometer is based on strontium atoms instead of rubidium.
The goal is to investigate whether, in reasonably realistic scenarios for some mission to be launched in the near future, the use of atomic clocks could improve the estimation of time-variable gravity models even at low harmonic degrees, with consequent advantages in the modelling of mass transport and its global variations.
The studied mission configurations include “nominal”, “improved” and “Bender” cases. In the nominal configuration (similar to GRACE and GRACE-FO) we have a pair of spacecrafts flying in an in-line formation in a polar orbit, both having on board an atomic clock and a single-axis cold atom gradiometer. The two clocks provide measurements of potential differences, while the gradiometers can provide gravity gradient observations, assuming that they are oriented along different directions on board different spacecrafts. A strategy based on the use of three clocks in combination with an optical link in heterodyne has been identified to better mitigate baseline fluctuations. Several mission scenarios, e.g., considering different noise levels, orbit altitudes, inter-satellite distances and sampling rates, have been processed, thus defining the “improved” configuration. Finally, two pairs of spacecrafts have also been considered, in the so called “Bender” configuration (similar to NGGM-MAGIC). The data processing has been performed by exploiting the so-called space-wise approach, which in this case consists in estimating the long wavelengths of the field from the potential differences and then using this estimation to reduce the already filtered gravity gradients. The residuals are then processed by a local collocation gridding procedure in order to retrieve the shorter wavelengths. The conversion from gridded values to spherical harmonic coefficients is finally performed by discretization of the quadrature formulas, and the error budget is evaluated by Monte Carlo simulations.
Comparisons of the results of the simulations are carried out against the performance of GRACE, GRACE-FO and the expected performance of the NGGM-MAGIC mission, with the aim of investigating if and for which harmonic degrees a mission like MOCAST+ might provide additional information on time-variable gravity field models.