Short-Period Mass Variations and the Next Generation Gravity Mission

At the time that the 2017-2027 Decadal Survey for Earth Science and Applications from Space was released, there was a strong emphasis on reducing the possibility of a substantial gap between the GRACE Follow-On mission and a successor mission. This has led to the subsequent rapid development of a successor mission in partnership between NASA and DLR, GRACE-Continuity (GRACE-C), expected to launch in 2028, to continue the timeseries of Earth system mass change established by GRACE and GRACE-FO. In parallel, ESA continues development of a pair of satellites called Next Generation Gravity Mission (NGGM), targeted for an inclination between 65°-75° to complement GRACE-C, launching in the early 2030s. NGGM offers the possibility for reduced noise in measuring short-period variations in the satellite separation using an improved accelerometer relative to what is flying on GRACE-C. One pathway for this is by using a simplified version of the Gravitational Reference Sensors demonstrated on the LISA Pathfinder Mission in 2016. And, if the measurement accuracy is much improved, it appears desirable to fly NGGM with a fixed ground track and an approximately five-day orbit repeat period.