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

• https://archive.podaac.earthdata.nasa.gov/podaac-ops-cumulus-docs/gracefo/open/docs/TN-14_C30_C20_GSFC_SLR.txt

Weekly 5x5+C₆₁/S₆₁ product download (Jan 2000 – Sept 2024; Loomis et al., 2020):

• GSFC_SLR_5x5c61s61.txt

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

• GSFC_SLR_C20_GSM_replacement.txt

Overview:

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

References:

Loomis, B. D., Rachlin, K. E., & Luthcke, S. B. (2019).  Improved Earth oblateness rate reveals increased ice sheet losses and mass-driven sea level rise. Geophysical Research Letters, 46, 6910– 6917. https://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 change. Geophysical Research Letters, 47, e2019GL085488. https://doi.org/10.1029/2019GL085488