CSR Release-06.1-LRI GRACE-FO Level-2 Data Products
Updated: 2023-05-26
Note to users: Most of the documentation referenced in this page is currently being migrated from the PO.DAAC Drive to the PO.DAAC Earthdata Cloud. As a result, the links to the documentation may not work. We apologize for the inconvenience.
Thank you for your interest in the latest, Release-06.1-LRI GRACE-FO Level-2 data products.
This page describes the RL06.1 time-series of monthly time-variable gravity field estimates obtain using the satellite-2-satellite ranging observation from the Laser Ranging Interferometer instrument. The description on this page is presently intended for users already familiar with the nomenclature and use of GRACE and GRACE-FO Level-2 data products.
Product Availability
The CSR RL06.1-LRI Level-2 data products are available for June 2018 onwards. Due to interruption in the data collection, the RL06.1-LRI products do not exist for the following months: August 2018 through November 2018, February and March 2019, July and August 2022, January and February 2023, July 2023 to present. The RL06.1-LRI data products include the GSM, GAC and GAD data products. The GAC and the GAD products may vary from the ones delivered as part of the RL06.1 products (KBR) because the days included in either set of product may differ.
Data Access
The CSR RL06.1-LRI Level-2 data products can be downloaded here.
Information Content in RL06.1-LRI GSM Files
The information content in RL06.1-LRI GSM files is the same as in the RL06 (and RL06.1) GSM files from KBR. That is, the RL06.1-LRI GSM files represent the total gravity variability due to land surface hydrology, cryospheric changes, episodic (earthquake) processes, glacial isostatic adjustment (GIA), and corrections to the background models for atmospheric and oceanic processes. The RL06.1-LRI GSM products can be used in the same way as RL06 and RL06.1 GSM products.
Level-2 Data Products
For each calendar month, in general four product files are available. For example, for the month of June 2018, the available products are:
GAC-2_2018165-2018181_GRFO_UTCSR_EC01_0601
GAD-2_2018165-2018181_GRFO_UTCSR_EC01_0601
GSM-2_2018165-2018181_GRFO_UTCSR_EA01_0601
GSM-2_2018165-2018181_GRFO_UTCSR_EB01_0601
GSM Products
The product files GSM-2* contains the coefficients for the Earth gravity field for the indicated span. The monthly gravity field solutions are unconstrained where the unconstrained gravity parameters are estimated seperately from other orbit related parameters. ONLY the formal error standard deviations (sigmas) of the estimated geopotential harmonics are contained in the GSM-2* products. The use of a suitable truncation or smoothing technique is recommended. Examples are available in the literature. The selection of the smoothing technique and the highest degree are left to the user's discretion. Users desirous of replacing the C20 and C30 values of the GSM file with improved estimates are offered the option of using SLR-determined C20 and C30 values given in GRACE Technical Note - 14.
GAC Products
The product file GAC-2 contains the average of the Release-06 AOD1B product for the ensemble of days as used in creating the monthly solution. Although this file contains the degree 0 and degree 1 harmonics for completeness, these harmonics were NOT used in Level-2 data processing. The averaging is carried using the whole days - regardless of whether full or partial day's data were used in creating the monthly solution.
GAD Products
The product file GAD-2 represents the mean ocean bottom pressure. Please review the AOD1B Product Description Document for its complete definition and usage guidelines. Over the oceans, maps of GAC and GAD products will look the same only if degree 0 and degree 1 terms are included while drawing both the maps. Note, however, that degree-0 and degree-1 of the AOD products (from which GAC is derived) were not used in Level-2 data processing. The GAD* products were not used in GRACE data processing, and are provided only as a service to the user community.
The fields definition of the Level-2 filenames have comply with GRACE-FO filename conventions. Please refer to the Level-2 handbook and RL06.1 Release Notes for more details.
Filename Convention
A GRACE-FO Level-2 gravity field product is a set of spherical harmonic coefficients of the exterior geopotential. A product name is specified as
PID-2_YYYYDOY-YYYYDOY_dddd_sssss_mmmm_rrvv
| Field | Meaning |
|---|---|
| PID | 3-character product identification mnemonic |
| -2 | Denotes a GRACE Level-2 product |
| YYYYDOY-YYYYDOY | Specifies the date range (in year and day-of-year format) of the data used in creating this product |
| dddd | Specifies the gravity mission |
| sssss | Institution specific string |
| mmmm | 4-character mnemonic used to identify the characteristics of the gravity solution |
| rrvv | 2-digit release number and 2-digit version number |
Product Identifier Mnemonic
| Position | Code | Meaning |
|---|---|---|
| 1st Character | G | Geopotential coefficients |
| 2nd Character | S | Estimate made from only GRACE-FO data |
| A | Average of any background model over a time period | |
| 3rd Character | M | Estimate of the Static field |
| C | Combination of non-tidal atmosphere and ocean (for details of combination see AOD1B Description Doc) | |
| D | Bottom-pressure over oceans, zero over land (for details of combination, see AOD1B Description Doc) |
Mission and Institution Codes
| Field | Code | Meaning |
|---|---|---|
| dddd | GRFO | Gravity Recovery and Climate Experiment Follow-On |
| sssss | UTCSR | The University of Texas at Austin Center for Space Research |
Gravity Solution Characteristics
The ‘mmmm’ string is a 4-character mnemonic used to characterize the gravity solution. The first character is used to identify the primary observation type used in the gravity solution (Note: MWI = Microwave Instrument). The second character defines the size of the spherical harmonic expansion in the file. The third and fourth characters are used to represent other characteristics of the gravity solution, including the type of basis function used, whether it is an unconstrained or constrained solution, and the type of windowing function used. For any files that describe the average of a background model (where ‘A’ is the 2nd character in the PID), only the 2nd character in the ‘mmmm’ string is defined/applicable. The 1st, 3rd, and 4th characters are set to be equivalent to the corresponding gravity solution.
| Position | Code | Meaning |
|---|---|---|
| 1st Character | E | LRI range-rate data |
| 2nd Character | A | 60 x 60 spherical harmonic expansion |
| B | 96 x 96 spherical harmonic expansion | |
| 3rd and 4th Characters | 01 | Unconstrained spherical harmonic solution with a boxcar windowing function |
The ‘rrvv’ string indicates the release (rr) and version (vv) of the solution. The release number is tied to a specific set of background force models, and indicates consistency between solutions across different missions. The version number indicates the version of the solution under a specific release.
Recommendations
Geocenter
Consistent with GRACE, GRACE-FO is not sensitive to degree 1 harmonics (geocenter). GRACE/GRACE-FO Technical Note TN-13 contains geocenter estimates using the methods of Swenson et al. [2008] and Sun et al. [2016], and is updated in synch with Level-2 monthly releases. These have been reprocessed for the entire GRACE and GRACE-FO time span to be consistent with the below-mentioned TN-14, so users need to replace the entire TN-13 time series. It is recommended to augment the GRACE and GRACE-FO geocenter with this product for surface mass change estimation.
C20
Consistent with the GRACE SDS recommendations, GRACE-FO SDS recommends the replacement of the native GRACE-FO C20 coefficient with that from SLR using the GRACE/GRACE-FO Technical Note TN-14. GRACE/GRACE-FO Technical Note TN-14 contains both C20 and C30 estimates derived from SLR and using Level-2 RL06 standards and is updated in synch with Level-2 monthly releases. It is recommended to replace/substitute the native GRACE and GRACE-FO C20 coefficients with this product [Loomis et al., 2019] for all months.
C30
The GRACE-FO SDS has determined that the C30 coefficient in GRACE-FO shows comparatively more variability relative to the long-term climatology derived from the GRACE C30 coefficient. Therefore, SDS recommends that users assess the impact on regional mass budgets of substituting the GRACE-FO C30 coefficient with one derived from SLR (similar to the C20 approach). GRACE/GRACE-FO Technical Note TN-14 is now provided and contains both C20 and C30 estimates derived from SLR and using Level-2 RL06 standards, updated in synch with Level-2 monthly releases. It is recommended to replace/substitute the native GRACE and GRACE-FO C30 coefficients with this product [Loomis et al., 2020] from 08/2016 onwards.
Citation
When using the CSR RL06.1-LRI products, please acknowledge that the data was downloaded from doi:10.5067/GFL20-LC061 and cite:
Pie, N., Bettadpur, S. V., Tamisiea, M., Krichman, B., Save, H., Poole, S., et al. (2021). “ Time variable Earth gravity field models from the first spaceborne laser ranging interferometer”. Journal of Geophysical Research: Solid Earth, 126, e2021JB022392. https://doi.org/10.1029/2021JB022392.
Feedback is Requested
The GRACE-FO project SDS is looking for feedback from the Science Team and wider community on the these new RL06.1 gravity fields derived from the LRI ranging observation. Note that due to interruption in the LRI data collection a few monthly RL06.1-LRI solutions are expected to perform poorer than their KBR couterparts. For more details, see Pie, et al (JGR, 2021) and Pie, et al (AGU Fall 2022).
If you are encoutering issues accessing the data, please contact us.
Further Details on CSR Level-2 Processing Standards
Please revisit these pages for further information.