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 the German Research Centre for Geosciences (GFZ). Thereby, internal validation and comparison of the science products are assured and in addition, backup capabilities concerning data processing and archiving are present.
A detailed description of the GRACE-FO SDS including the roles and responsibilities of the SDS institutions, the data flow and the products and documents to be provided will be written down in a 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. During the first step, the binary encoded Level-0 measurements are converted to engineering units. Subsequently, the resulting Level-1A data are correctly time tagged and their sampling rate is reduced to obtain the Level-1B products. The essential Level-1B data are the K/Ka-Band Microwave Link and the Laser Ranging Interferometer measurements and the GPS, accelerometer and star camera observations. The GRACE-FO Level-1 processing will be performed by JPL with GFZ as backup capability.
Level-1 processing also includes the generation of atmosphere and ocean de-aliasing products (AOD1B) which will be done by GFZ.
Level-2 Processing
Using Level-1B data, 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 standard Level-2 products are due 60 days after Level-0 data acquisition; besides, operations of low latency NRT (“Near Real Time”) products will also be investigated. GRACE-FO Level-2 products will be generated by all three SDS partners.
As additional Level-2 products, monthly means of the AOD1B de-aliasing products (so-called GAx-products) are generated and provided in order to offer users to restore the corresponding atmospheric and/or oceanic signals.
Level-3 Processing
In addition to the Level-2 products, more user-friendly Level-3 products are generated by transforming the spherical harmonic coefficients to gridded geopotential functionals (e.g. water equivalent mass change). Level-3 products also comprise ancillary data like time-series of geocenter motion or SLR derived C20 (describing the Earth’s oblateness) which need to be applied for correct geophysical interpretation of the Level-2 products. A limited number of official GRACE-FO Level-3 products will be generated by JPL; in order to expand the variety of offered products and to appeal to as much users as possible, GFZ is planning to provide additional own Level-3 products.
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 one archive is a mirror of the other, both archives will be harmonized regularly.