Time variable gravity from satellite laser ranging (SLR)

Technical Note 14 C20/C30 product download (GRACE/GRACE-FO data span; Loomis et al., 2020):


C20 product download (GRACE data span; Loomis et al., 2019):



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 observe global mass change 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 both missions, SLR-derived C20 estimates have been used to replace the values obtained with GRACE/GRACE-FO. Additionally, evidence has emerged that GRACE/GRACE-FO estimates of C30 are problematic when operating in single-accelerometer mode, which requires the use of a transplant accelerometer product in the Level 1B data processing (Loomis et al., 2020). The use of transplant data begins after August 2016 for GRACE and currently spans the full GRACE-FO mission. Fortunately, C30 is well-observed by SLR beginning with the launch of LARES in 2012. GRACE-FO Technical Note 14 contains the SLR C20/C30 values that we recommend for replacing the GRACE/GRACE-FO GSM spherical harmonic coefficients. We recommend the replacement of C20 for the span of both missions and the replacement of C30 beginning after August 2016. Due to their relatively large magnitude, the replacement of these coefficients can have a significant impact on the scientific interpretation of regional mass changes derived from GRACE/GRACE-FO data products.

Background models used for SLR data processing:

  • Static gravity: GOCO-05S with epoch of 2008.0
  • Time-variable gravity: Trend and periodic terms to 10x10 determined from GRACE/GRACE-FO
  • Atmosphere and ocean de-aliasing: AOD1B RL06
  • Tide model: GOT4.7 to degree/order 90


Contact: Bryant Loomis: Bryant.D.Loomis@nasa.gov


Loomis, B. D.Rachlin, K. E., & Luthcke, S. B. (2019).  Improved Earth oblateness rate reveals increased ice sheet losses and mass-driven sea level riseGeophysical Research Letters466910– 6917https://doi.org/10.1029/2019GL082929


Loomis, B. D.Rachlin, K. E.Wiese, D. N.Landerer, F. W., & Luthcke, S. B. (2020).  Replacing GRACE/GRACE-FO C30 with satellite laser ranging: Impacts on Antarctic Ice Sheet mass changeGeophysical Research Letters47, e2019GL085488. https://doi.org/10.1029/2019GL085488