In the past twenty years, gravimetry missions have demonstrated a unique capability to monitor not only major climate-related changes of the Earth directly from space like quantifying the melting of large glaciers and ice sheets, global sea level rise, continental draught, major flooding events, but also effects of large earthquakes and tsunamis. To respond to the increasing demand of the user community for sustained mass change observations at higher spatial and temporal resolution, ESA and NASA are coordinating their activities and harmonizing their cooperation scenarios in an implementation framework, called MAGIC (MAss change and Geosciences International Constellation). This builds upon the heritage from the GOCE, GRACE and GRACE-FO missions as well as on-going pre-developments on laser–ranging interferometry in preparation for the Next Generation Gravity Mission (NGGM). The new Laser Tracking Instrument (LTI) is being developed by the industrial lead SpaceTech GmbH with scientific lead at Albert Einstein Institute under contract to ESA. To consolidate the performance of the mission concept and the technological and programmatic feasibility of the entire mission, technology risk-retirement activities will be conducted to achieve Technology Readiness Level (TRL) 5/6 for the LTI at the end of Phase B1 and TRL 6 at the Instrument Unit level at the end of Phase A. Additional presentation content can be accessed on the supplemental content page.
In the past twenty years, gravimetry missions have demonstrated a unique capability to monitor major climate-related changes of the Earth directly from space – among others quantifying the melt of large glaciers and ice sheets, global sea level rise, continental draught and major flooding events. A Quantum Space Gravimetry (QSG) mission will provide corresponding Essential Climate Variables (ECV) with unprecedented quality compared to the initially demonstrated and already very successful missions like GOCE and GRACE (FO). To respond to the increasing demand of the user community for sustained mass change observations at higher spatial and temporal resolution, ESA and NASA are coordinating their activities and harmonizing their cooperation scenarios in an implementation framework, called MAGIC (MAss change and Geosciences International Constellation). In a future post-MAGIC mission, classical sensors can be combined with a Cold Atom Interferometry (CAI) instrument, or at a later stage a full quantum sensor could be employed. These Quantum Missions for Climate will reach sensitivities, which enable many applications addressing user needs with respect to water management and hazard prevention among others. Several studies related to these new sensor concepts were initiated at ESA, including technology development for different instrument configurations and validation activities. A new study has been initiated, the Quantum Space Gravimetry for Earth Mass Transport (QSG4EMT), with the focus on both, QSG mission architectures for monitoring of Earth's mass transport processes and the development of QSG user requirements.
Additional presentation content can be accessed on the supplemental content page.
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 temporal (down to 3 days) and spatial (100 km) resolution. Such variations carry information about mass change 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; ultra-precise accelerometers measure the nongravitational accelerations to correct the gravity signal in the data processing. The optimal satellite system comprises two pairs of satellites on low (between 396 and 488 km) circular orbits, at 220 km separation, one pair quasi-polar and the other around 65°-70° inclination. The satellite-to-satellite tracking technique for detecting the temporal variations of gravity was established by GRACE (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 (upper MBW). At 150 km spatial resolution, mass change would become observable in 80% of all significant river basins, against 10% achieved with GRACE. High temporal resolution will reveal large-scale sub-weekly mass variations, with applications in water and emergency management. NGGM is a candidate Mission of Opportunity for ESA-NASA cooperation in the framework of MAGIC. The paper focusses on the on-going Phase A system design and technology pre-development activities.
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.
Measurement of the static and temporal variation of Earth’s gravity field yields important information on water storage, seasonal and sub-seasonal water cycles, their impact on water levels and delivers key data to Earth’s climate models. The satellite mission GOCE (ESA) and GRACE (US-GER) resulted in in a significant improvement on our understanding of the system Earth. On GRACE and GRACE Follow-On two satellites are following each other on the same orbit with approx. 200 km distance to each other. A microwave inter-satellite ranging system measures the variation of the intersatellite distance from which the gravity field is derived. In addition, on GRACE Follow-On, which has been launched May 22nd 2018, a laser interferometer is added as an experiment to demonstrate the capability of this system to improve the ranging accuracy by at least one order of magnitude. To significantly improve the gravity field measurement accuracy, ESA is investigating the concept of a ‘Next generation gravity mission’ (NGGM), consisting of two pairs of satellites and a laser interferometer as the sole inter-satellite ranging system. Based on the heritage of the development of the laser ranging interferometer for GRACE Follow-On and the former and ongoing studies for NGGM, several concepts for the laser metrology instrument (LMI) for NGGM, namely the on- and off-axis variants of the transponder and the retroreflector concept have been investigated in detail with respect to their application for an inter-satellite distance of approx. 100 km. This paper presents the results of the detailed tradeoff between different concepts, including laser link acquisition, ranging noise contributors, instrument performance analyses, technology readiness levels of the individual instrument units and an instrument reliability assessment.
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