The main function of the SDS is the processing of the GRACE-FO science data. This includes the generation of GRACE-FO Level-0 to Level-3 products as well as their distribution and archiving. Just as the whole mission, the GRACE-FO SDS is a joint US/German cooperation consisting of the Jet Propulsion Laboratory (JPL), the University of Texas Center for Space Research (UTCSR) and GFZ. This ensures, internal validation and comparison of the science products and in addition, also provides backup capabilities for data processing and archiving.
A detailed description of the GRACE-FO SDS including the roles and responsibilities of the SDS partners, the data flow and the products and documents to be provided has been detailed in the Science Data System Development Plan. In the following, the key aspects of the SDS are described very briefly.
Level-1 Processing
Starting from Level-0 data, i.e., the raw telemetry data received from the two GRACE-FO satellites, the Level-1 processing comprises two steps. In the first step, the binary encoded Level-0 measurements are converted to engineering units. The resulting Level-1A data are then tagged with correct time stamps and their sampling rate is reduced to obtain the Level-1B products. The most important Level-1B data include measurements from the K/Ka-band microwave instrument and the Laser Ranging Interferometer as well as GPS, accelerometer and star camera observations. The GRACE-FO Level-1 processing is carried out by JPL.
Level-1 processing also includes the generation of atmosphere and ocean de-aliasing products (AOD1B) which is done by GFZ.
Level-2 Processing
Using Level-1B data, background models (e.g., to correct for gravitational forces acting on the satellites induced by tides) and other ancillary data (e.g., Earth Rotation Parameters), spherical harmonic coefficients representing the Earth’s gravitational potential are estimated. These coefficients are typically based on monthly batches of Level-1B data, so that the resulting Level-2 products build a time-series of monthly global gravity field models. The GRACE-FO Level-2 products are generated by all three SDS partners with a latency requirement of 60 days after Level-0 data acquisition.
As additional Level-2 products, monthly means of the originally three-hourly AOD1B de-aliasing products (so-called GAx-products) are generated and provided in order to offer users the possibility to restore the corresponding atmospheric and/or oceanic high-frequency mass signals.
Level-3 Processing
In addition to the Level-2 products, user-friendly “ready-to-use” Level-3 products are generated by transforming the spherical harmonic coefficients to gridded geopotential functionals (e.g., water equivalent height change). Level-3 products also comprise ancillary data like time-series of geocenter motion or SLR-derived C20 coefficients (describing the Earth’s oblateness) which need to be applied for correct geophysical interpretation of the Level-2 products. Official GRACE-FO Level-3 products are generated by JPL. In order to expand the variety of offered products and to appeal to as much users as possible, GFZ, in cooperation with Alfred Wegener Institute (AWI) and TU Dresden, are providing additional Level-3 products which are visualized at the Gravity Information Service (GravIS) portal and can be downloaded from the ISDC .
Archiving
All GRACE-FO SDS Level-0 to Level-3 products as well as the corresponding documents will be archived at JPL/NASA's Physical Oceanography Distributed Active Data Center (PODAAC) and at GFZ’s Information System and Data Center (ISDC). To guarantee that both archives mirror each other, they are regularly harmonized.