The Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission is a successor to the current GRACE mission. The main objectives are:
Abbildung 4: Logo of GRACE follow-on Mission.
To maintain continuity of measurements to accurately determine Earth's gravity field and its changes (primary objective)
Two satellites orbit Earth within 90 minutes. During this process, their nominal separation distance of approximately 200 kilometres varies minimally due to different mass distributions on Earth. The causes of these differences are numerous, including mountains, groundwater and glaciers. By measuring the changes caused by these distributions over a longer period of time, statements can be made regarding climate change.
The current GRACE mission measured the changes in distance using microwave links, which enabled resolutions in the order of micrometres (µm; one millionth of a metre). The follow-on mission by GRACE-FO is set to use a laser for measuring the distances.
The two satellites for this German-US mission are being constructed at Airbus DS in Friedrichshafen. The Laser Ranging Instrument is being provided by the company SpaceTech in Immenstaad (STI) and will be integrated into the satellites. The mission was launched in May 2018.
Laser Ranging Instrument – LRI
The LRI is being constructed under the leadership of STI, which is working in collaboration with, among others, the NASA Jet Propulsion Laboratory (JPL), the DLR Institute of Optical Sensor Systems (DLR-OS, Berlin-Aldershof) and the DLR Institute of Space Systems (DLR-RY in Bremen). The German contribution, also known as LRI-D, includes the Optical Bench Assembly (OBA), the Triple Mirror Assembly (TMA) and the Optical Bench Electronic (OBE). The main contributions made by the US partners – known as LRI-U – include the laser, an optical frequency reference (Cavity) and the Laser Ranging Processor (LRP).
First, the sub-units of LRI-D (TMA, OBA and OBE) will be constructed and tested as an engineering model (EM) and a qualification model (QM). Even at this early stage, it is necessary to have the corresponding test equipment (OGSE – optical ground support equipment) in place to test the hardware parameters.
Following the successful completion of the tests, the flight model can be manufactured and subsequently tested. These tests will later be referred to as Sub-unit tests.
The next stage will be to test how the sub-units interact with one another. TMA, OBA and OBE will be connected to other sub-units via a Main Equipment Platform (MEP) for LRI-D before being tested, which will be extended during the next stage when the US sub-units are integrated. This process will be carried out for both LRI structures and following this, a third (spare) LRI will be constructed. Extensive testing of LRI-1 and LRI-2 will be conducted. During subsequent testing, these initial tests will be named unit level or LRI level tests.
Both LRIs will be integrated by Airbus DS (Friedrichshafen) – one for each satellite. During and following the integration process, a considerable amount of testing will be carried out again in order to ensure that full functionality has not been affected.After successful integration, both satellites will be taken to IABG (Ottobrunn) in 2016, where they will undergo thermal testing. Some tests will also be carried out here on site. Subsequently, the satellites will be returned to Airbus DS. These tests will later be referred to as S/C level tests.
The next stage of the project will involve the satellites being tested together, rather than on their own. During this stage, it is essential to couple the laser beams transmitted by one satellite with the other, and vice versa. The tests carried out on the entire system will be called S/C to S/C leveltests, and should demonstrate the full functionality of laser-based distance measuring. During launch preparation, further ‘health checks’ will be carried out on the satellites, where necessary.
Optical ground support equipment – OGSE
To be able to test all the properties of the LRIs from the sub-unit level to the S/C to S/C level, various optical structures are needed, which are collectively referred to during the project as optical ground support equipment (OGSE). Not all testing is required throughout each stage. Some are only needed at the sub-unit level; others have to be adapted for use at the next level. The design and implementation of tests, as well as carrying out and analysing the relevant measurements is the responsibility of the DLR-RY and all are key to the success of the mission.