Interplay of altitude, ground track coverage, noise and regularisation in the spatial resolution of GRACE gravity field models

Models of the temporal gravity field derived from space gravity missions are typically produced with monthly temporal resolution and ~300 km spatial resolution. However, variations in instrument performance and altitude of the GRACE mission impact the spatial resolution that can be achieved month-to-month. As the altitude of the orbits of the twin spacecraft vary throughout the mission, so does the ability of the observations to recover certain components of the temporal gravity field. The spatial resolution of GRACE observations should increase as the altitude decreases throughout the mission because the reduced altitude intensifies the gravity signals acting on the satellites. Simulations using actual GRACE altitude and ground track coverage and realistic noise levels confirm this predicted influence of the altitude of the satellites on the accuracy of the estimated solutions. Solutions with larger mass concentration elements (mascons) are more numerically stable as the satellite altitude decreases but they suffer from greater error caused by the inability to properly represent spatial variations of signals within mascons, referred to as intra-mascon variability. Mascons as small as ~150 x 150 km (i.e. ~1.5 arc-degree) reduce the intra-mascon variability and, with appropriate regularisation, yield the most accurate solutions, especially during the low-altitude periods of the GRACE mission. Importantly, unlike spherical harmonic solutions, regularised mascon solutions are not degraded during resonant orbit months, and are of comparable quality to months with full ground track coverage.