Regularly updated data products:

Satellite laser ranging overview:

For almost 50 years, satellite laser ranging (SLR) tracking measurements have been used to monitor very large-scale changes in Earth’s gravity field. The largest and longest observed component of the time variable gravity field is the dynamic oblateness, described by the spherical harmonic coefficient C₂₀, or its unnormalized form, J₂, which is equal to –C₂₀√5. Dedicated “cannonball” SLR satellites equipped with laser retroreflectors commonly used for time variable gravity estimation include Starlette (1975 Launch), LAGEOS‐1 (1976), AJISAI (1986), LAGEOS‐2 (1992), Stella (1993), Larets (2003), LARES (2012), and LARES‐2 (2022). 

SLR contributions to the GRACE & GRACE-FO missions:

The GRACE (2002–2017) and GRACE-FO (2018–Present) gravimetry missions have 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, it was determined that GRACE/GRACE-FO estimates of C₃₀ can be problematic when operating in single-accelerometer mode and requiring 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. We note that the GRACE-FO C30 estimates have significantly improved since the publication of (Loomis et al., 2020) through the development of improved transplant products. 

Long-term estimates of C₂₀:

Loomis et al. (2019) demonstrated the importance of utilizing the GRACE/GRACE-FO time variable gravity information when processing the SLR tracking data to optimize the accuracy of the recovered C₂₀ estimates, particularly the long-term trend. This result called into question the validity of the pre-GRACE SLR estimates of C₂₀ (1976–2002) when no time variable gravity information is available. Loomis et al. (2025) developed a novel method for the accurate recovery of C₂₀ from 1976–Present without the need for GRACE-derived time variable gravity information. This is accomplished by only using the highest altitude SLR satellites (LAGEOS-1, LAGEOS-2, LARES-2) and applying a Truncated Singular Value Decomposition Mixed-Model (TSVD MM) algorithm for the estimation of C₂₀. 

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

Loomis, B. D., Sabaka, T. J., Rachlin, K. E., Croteau, M. J., Lemoine, F. G., Nerem, R. S., & Bellas-Manley, A. (2025). “Optimized J2 recovery for multi-decadal geophysical studies,” Geophysical Research Letters, 52. https://doi.org/10.1029/2024GL114472 

Products from publications (not updated):

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

• GSFC_SLR_C20_GSM_replacement.txt

Long-term C₂₀ (Jun 1976 – Jul 2024; Loomis et al., 2025)

• GSFC_SLR_C20_Long_Term_Loomis2025.txt