Time variable gravity from satellite laser ranging (SLR)

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

GSFC_SLR_C20_GSM_replacement.txt

 

SLR C20/C30 product download (GRACE/GRACE-FO data span):

TN-14_C30_C20_GSFC_SLR.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 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.

Due to its relatively large magnitude and impact on the scientific interpretation of the GRACE products, an accurate SLR C20 solution continues to be of great importance. Here we present a new SLR-derived C20 product from NASA GSFC that modifies the trend significantly from the CSR TN11 SLR product most commonly applied for replacing the GRACE C20 values. In the figure below, we show that our C20 trend agrees well with other previously-published SLR C20 estimates. (Note that the AIUB and DGFI solutions are not available for the full GRACE mission span). We recommend using the GSFC product for replacing the C20 values of the current RL06 GRACE GSM data products available at: https//podaac-tools.jpl.nasa.gov/drive/files/allData/grace/L2/, to ensure proper scientific application and interpretation of the GRACE products. We note that using the GSFC product modifies the mass component of global mean sea level by +0.08 mm/yr and the mass trend in the Antarctic Ice Sheet by –15.4 Gt/yr as compared to CSR TN11.

Evidence has emerged that GRACE/GRACE-FO estimates of C30 are problematic when operating in single-accelerometer mode. For GRACE this began after August 2016, and GRACE-FO Level-1B data might also be processed in a similar manner. Beginning with the launch of LARES in 2012, C30 is well-observed by SLR. The provided C20/C30 data product is a 5-satellite (LAGEOS-1/2, Stella, Starlette, AJISAI) solution prior to the launch of LARES, and is a 7-satellite (LARES and Larets added) solution afterwards. For both 5 and 7-satellite configurations, we estimate the spherical harmonic coefficients 5x5+C61/S61.

 

[Loomis et al., 2019]


(Left) C20 trends over the time span common to all available datasets: Sep 2002 – June 2013. (Right) C20 for the new recommended GSFC product (green), the GSFC solution that uses the same setup as CSR TN11 (blue), and CSR TN11 (orange) over the GRACE mission span.

 

 

Background models:

  

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

 

References:

GSFC: Loomis, B. D., K. E. Rachlin, and S. B. Luthcke (2019) Improved Earth oblateness rate reveals increased ice sheet losses and mass-driven sea level rise.Geophysical Research Letters, doi:10.1029/2019GL082929

CSR: Cheng, M. & Ries, J. (2017) The unexpected signal in grace estimates of C20. Journal of Geodesy, 91 (8), 897–914. doi:10.1007/s00190-016-0995-5

AIUB: Sosnica, K., Jaggi, A., Meyer, U., Thaller, D., Beutler, G., Arnold, D., & Dach, R. (2015) Time variable earth’s gravity field from SLR satellites. Journal of Geodesy, 89(10), 945–960. doi:10.1007/s00190-015-0825-1

DGFI: Bloßfeld, M., Muller, H., Gerstl, M., Stefka, V., Bouman, J., Gottl, F., & Horwath, M. (2015) Second-degree stokes coefficients from multi-satellite SLR. Journal of Geodesy, 89(9), 857–871. doi:10.1007/s00190-015-0819-z

GRGS: Lemoine, J.-M., Bourgogne, S., Biancale, R., & Bruinsma, S. (2018) RL04 monthly gravity field solutions from CNES/GRGS. GRACE/GRACE-FO Science Team Meeting, Potsdam, Germany. http://presentations.copernicus.org/GSTM-2018-42/presentation.pdf