The GRACE-FO Mission Operations System (MOS) and Ground Data System (GDS) were initially financed by GFZ for the nominal mission duration of five years after the launch. In the meantime, GFZ has agreed to continue funding mission operations beyond this period. Contract negotiations are currently underway for a continuation until the end of 2029, with mission operations being subcontracted to the German Space Operation Centre (GSOC) of the DLR (German Aerospace Centre) in Oberpfaffenhofen. GFZ provides the Operations Mission Manager (OMM), validated flight procedures and the Ny-Ålesund/Spitzbergen (NYA) ground station complex.
The MOS/GDS consists of the people, processes and procedures, facilities, and ground hardware and software required to operate the GRACE-FO Flight Segment (FS). These capabilities are used to monitor and control the satellite, perform initial processing of the telemetry data, and to deliver all data to the Science Data System (SDS) for further processing and generation of science products. The combination of the ground hardware and software is the Ground Data System (GDS).
The GDS is composed of the:
- Ny-Ålesund/Spitzbergen (NYA) ground station complex operated by GFZ,
- Weilheim/Germany (WHM) ground station complex operated by DLR,
- Neustrelitz/Germany (NSG) ground station complex operated by DLR,
- Mission Control Center (MCC) located at DLR’s German Space Operations Center (GSOC) in Oberpfaffenhofen/Germany, and
- Raw Data Center (RDC) operated by DLR at Neustrelitz/Germany.
Additionally, MOS/GDS will make use of the following stations (if required):
- O’Higgings/Antarctica (OHG) ground station complex operated by DLR
- Inuvik/Canada (INU) ground station complex operated by DLR.
The five distinct GRACE-FO mission phases can be described as follows:
Pre-Launch (Test and Integration)
The Pre-Launch Phase C/D (Design & Development) started about 3.5 years before launch. During this phase schedules were negotiated and the GRACE-FO mission was designed and developed. The phase began with building and integrating of subsystems into the spacecraft. In a process called ATLO (Assembly, Test, and Launch Operations) the spacecraft was assembled, integrated, and tested in a simulated space environment. Additionally, ground systems to support the mission were developed in parallel with the satellites, and were exercised along with the spacecraft during tests.
Launch and Early Operations Phase
The GRACE-FO satellites were launched together on a single launchwith a Space-X/Falcon-9 rocket. Once in the desired orbit, the satellites were simultaneously released from the upper stage of the launch vehicle , separated and started transmitting health data to the ground. During this Launch and Early Operations Phase (LEOP), which took only 5 days, the basic satellite and instrument functions were checked. At the end of LEOP, the satellites were in safe, stable orbits with a nominal separation distance of 220 km ± 50 km using nominal attitude control, and nominal uplink and downlink communications were achieved.
In-Orbit Checkout Phase
The following In-orbit Checkout (IOC) Phase should have lasted another 85 days and focused on the final power-up, evaluation and calibration of the science instruments. Due to problems with the Instrument Processing Unit (IPU) on the second GRACE-FO satellite (GF2), finally resulting in a necessary switch to the IPU backup unit on GF2, IOC officially ended on January 28, 2019.
Science Phase
In the ongoing Science Phase, the instrument data are routinely gathered and processed. This phase will continue until the end of the mission, and will include brief interruptions for orbit maneuvers and instrument re-calibrations.
Decommissioning Phase
The Decommissioning Phase is the final phase handling the end of mission. In this phase the satellites will be passivated followed by disposal to atmospheric reentry. Emphasis will be given to limit the generation of orbital debris, to limit the risk to the public and to comply with NASA policy directives as well as U.S. national space policy.
