The CDDIS was established in 1982 as a dedicated data bank to archive and distribute space geodesy related data sets. Today, the CDDIS archives and distributes mainly Global Navigation Satellite Systems (GNSS, currently Global Positioning System GPS and GLObal NAvigation Satellite System GLONASS), laser ranging (both to artificial satellites, SLR, and lunar, LLR), Very Long Baseline Interferometry (VLBI), and Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) data for an ever increasing user community of geophysists.
Since the beginning of the GRACE mission it has been standard practice to apply atmosphere and ocean de-aliasing products when processing the Level 1B data in order to directly remove these high-frequency signals from the inter-satellite measurements and the distributed gravity solutions.
For more than four decades, satellite laser ranging (SLR) measurements have been monitoring changes in Earth’s dynamic oblateness, C20, which is the largest component of Earth’s time-variable gravity field. The launch of GRACE in 2002, and subsequent launch of GRACE-FO in 2018, has revolutionized the ability to track mass fluxes at a spatial resolution of 300-500 km. Early in the mission, however, GRACE-derived estimates of C20 were deemed unreliable, as evidenced by a non-geophysical 161-day periodic signal and trend estimates that differed significantly from those determined with SLR. Throughout the mission, SLR-derived C20 estimates have been used to replace the values obtained with GRACE.