This paper presents the design and breadboarding of the proof of concept demonstrator for the so called retro-reflector interferometer scheme in off-axis configuration for the ‘Next Generation Gravity Mission’ (NGGM) studied at the European Space Agency (ESA). This configuration can offer benefits in terms of overall satellite configuration compared to the transponder scheme, which is currently flying on board of GRACE-FO. However, it relies on very low received laser signal levels due to the fact that the laser light is travelling about 100 km from the master satellite to the remote satellite and is reflected back to the master satellite by a retro-reflector. In comparison to the transponder scheme, where the signal is amplified on the remote satellite using a laser, which is optically phase locked to the laser signal of the master spacecraft, this reflection does not amplify the signal. Thus, even with higher emitted laser power, instead of some nanowatt, only a few picowatt are available on the according science detector. Therefore, less than a femtowatt of straylight within the detectable heterodyne frequency and angular range is allowed on the detector to fulfil the ranging noise requirement. The paper gives insights into the main opto-mechanical design topics of the Optical Bench Assembly (OBA). It includes the optical analysis results as well as mechanical design to suppress straylight below the required limit. The optomechanical design of the OBA is complemented by the opto-mechanical design of the test setup and by the electro-optical design of the phase read-out chain. Finally, preliminary results from the test campaign are presented.
The objective of ESA’s Next Generation Gravity Mission (NGGM) is long-term monitoring of the temporal variations of Earth’s gravity field at high resolution in time (down to 3 days) and space (100 km). Such variations carry information about mass transport induced by the water cycle and the related mass exchange among atmosphere, oceans, cryosphere and land, and will complete our picture of Global Change with otherwise unavailable data. The observable is the variation of the distance between two satellites measured by a laser interferometer; accelerometers measure the non-gravitational accelerations to be separated from the gravity signal in the data processing. The optimal satellite system comprises two pairs of satellites on low (around 340 km) circular orbits, at 100 km mutual distance, one pair near-polar and the other around 65° inclination. The technique of satellite-to-satellite tracking for detecting the temporal variations of gravity was established by GRACE, which reached 300-400 km spatial resolution at monthly intervals, using tracking in the microwave band. Today, GRACE is being continued by GRACE-Follow-On, with similar objectives, where the laser interferometry has improved the measurement resolution by a factor of 100 (in the upper MBW) which however cannot be fully exploited due to other system limitations. At 150 km spatial resolution, mass change would become observable in 80% of all significant river basins, up from 10% achieved with GRACE. High temporal resolution will reveal large-scale daily mass variations, with applications in water management and operational prediction. Currently, the NGGM is a candidate Mission of Opportunity for ESA-NASA cooperation. Over the last decade, numerous system and technology activities have advanced the maturity of the system and the key subsystems, and the mission can now be proposed for launch around 2028. The paper focusses on the latest design and test achievements, with a discussion on alternative drag compensation scenarios.
The objective of ESA’s Next Generation Gravity Mission is long-term monitoring of the temporal variations of Earth’s gravity at high time (3 days) and space (100 km) resolution. Such variations carry information about mass transport in the Earth system produced by the water cycle and the related mass exchange among atmosphere, oceans, cryosphere and land, and will complete our picture of Global Change with otherwise unavailable data. The basic datum is the distance variation between two satellites measured by a laser interferometer; as a necessary complement, accelerometers measure the non-gravitational accelerations, to be separated from the gravity signal in the data processing. The optimal satellite formation comprises two pairs of satellites, at 100 km mutual distance, on low (≈340 km) circular orbits with 89° and 70° inclination. The NGGM is a candidate Mission of Opportunity of ESA’s Earth Observation programme. Studies and technology development activities have advanced the maturity of the system concept and of the key subsystems (attitude and drag control, proportional thrusters, laser optics and electronics) for the mission to be proposed for adoption in 2022 and launch in the 2026-2028 time frame. The latest stand of the ESA studies is illustrated, concerning both the platform (featuring drag-free control, high-stability temperature control, drawing on the heritage of GOCE) and the laser interferometer instrument, for which two designs have been extensively studied, “Transponder” and “Retro-Reflector”, one of which will be selected for flight. A hybrid breadboard of the “off-axis” Retro-Reflector concept is being built and tested.
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