The Next Generation Gravity Mission (NGGM) is an ESA mission concept, designed as a follow-up to GOCE/GRACE, with the goal of observing long-term time variations of Earth’s gravity field with higher accuracy and related mass-redistribution processes. The measurement is based on Low-Low Satellite-to-Satellite Tracking: two satellites fly in a low, near-polar orbit, in an in-line formation with a separation of about 100 km. A laser interferometer measures tiny changes in their inter-satellite distance caused by anomalies in Earth’s gravity field. To correctly recover the gravity signal, it is necessary to estimate and strongly reduce non-gravitational accelerations (atmospheric drag, radiation pressure, etc.).