A.3
NGGM and Bridging the Gap

A.3

NGGM and Bridging the Gap
Orals
| Thu, 20 Oct, 14:15–16:57 (CEST)|Lecture Hall, Building H
Posters
| Attendance Wed, 19 Oct, 16:15–17:15 (CEST)|Foyer, Building H

Orals: Thu, 20 Oct, 14:15–16:57 | Lecture Hall, Building H

14:15–14:27
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GSTM2022-27
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On-site presentation
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Mass Change as a core element of NASA’s Earth System Observatory: update and progress on pre-formulation activities 
David Wiese, Charley Dunn, Michael Gross, Frank Webb, Neil Dahya, Andre Girerd, Srinivas Bettadpur, Bernard Bienstock, Brent Ware, Carmen Boening, Jonathan Chrone, Bryant Loomis, Scott Luthcke, Matthew Rodell, Jeanne Sauber, Nicole Herrmann, and Lucia Tsaoussi

The 2017-2027 US National Academy of Sciences Decadal Survey (DS) for Earth Science and Applications from Space classified mass change as one of five Designated Observables having the highest priority in terms of Earth observations required to better understand the Earth system over the next decade.  In response to this designation, NASA initiated multi-center studies with an overarching goal of defining observing system architectures for each Designated Observable.  The Mass Change Designated Observable study concluded in 2021, after identifying a small subset of high value observing system architectures for further study during Pre-Phase A formulation.  Mass Change is in the process of concluding Pre-Phase A activities and transitioning to Phase A, after successfully passing a Mission Concept Review in June 2022.  The baseline architecture is a partnership between NASA and DLR with an architecture design similar in nature to GRACE-FO.  This concept meets the primary goal of maintaining continuity in the mass change data record.  In parallel, ESA is continuing their Phase A studies focused on launching an inclined pair of satellites to complement this baseline architecture which would result in a demonstration of the Bender constellation and thus enhance the overall science and applications value of the observing system.  In this talk, an overview of Mass Change pre-formulation activities will be provided, as well as a status update on next steps and associated milestones.

How to cite: Wiese, D., Dunn, C., Gross, M., Webb, F., Dahya, N., Girerd, A., Bettadpur, S., Bienstock, B., Ware, B., Boening, C., Chrone, J., Loomis, B., Luthcke, S., Rodell, M., Sauber, J., Herrmann, N., and Tsaoussi, L.: Mass Change as a core element of NASA’s Earth System Observatory: update and progress on pre-formulation activities, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-27, https://doi.org/10.5194/gstm2022-27, 2022.

14:27–14:39
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GSTM2022-13
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On-site presentation
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GRACE-I: A joint US-German mission for continued mass transport monitoring and enabling global biodiversity monitoring 
Frank Flechtner, Christoph Dahle, Markus Hauk, Josefine Wilms, Michael Murböck, Michael Nyenhuis, and Peter Schaadt

NASA´s Earth Science Decadal Survey Report highlights mass transport monitoring as one of top priorities in Earth Observation for the next decade. To realize such a Mass Change Mission (MCM), NASA is seeking international partnership. Based on the large success of the GRACE and GRACE-FO missions and their contributions to climate change research, there is a large interest in Germany to continue mass change measurements.

GFZ and the German Space Agency at DLR have suggested a “GRACE-I” mission which is based on a GRACE-like concept combined with an optional ICARUS (International Cooperation for Animal Research Using Space) payload. In continuation of a 9 months Phase 0 study in 2021 this concept is currently investigated in Phase A (April – September 2022) with significant support of JPL/NASA as a future continuation of the very successful US-German GRACE/GRACE-FO technological and scientific partnership.

GRACE-I will be a single satellite pair based on a fully redundant Laser Ranging Interferometer on a polar orbit at 500 km altitude. Launch shall be not later than 2027 to guarantee data continuity w.r.t. GRACE-FO. GRACE-I could be a first component (P1) of a hybrid Bender constellation if combined with an inclined MAGIC pair (P2). The realization of this Mass-change And Geoscience International Constellation is currently discussed between ESA and NASA. P2 will fly on a lower orbit than P1 and will be based on advanced instrumentation. Therefore, Phase A also investigated the option to add one or two adapted MicroStar accelerometers to the baseline GRACE-FO like accelerometer on each P1 satellite as a technology demonstrator for P2.

At the time of writing this abstract the main focus was on the final steps to refine the technical design and to select the final payload configuration for a US/German MCM/GRACE-I mission. We will present the proposed mission architecture and will discuss further steps towards realization of MCM/GRACE-I.

How to cite: Flechtner, F., Dahle, C., Hauk, M., Wilms, J., Murböck, M., Nyenhuis, M., and Schaadt, P.: GRACE-I: A joint US-German mission for continued mass transport monitoring and enabling global biodiversity monitoring, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-13, https://doi.org/10.5194/gstm2022-13, 2022.

14:39–14:51
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GSTM2022-42
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On-site presentation
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Towards Laser Ranging in Future Gravimetric Missions 
Vitali Müller, Malte Misfeldt, Anne Feiri, Kolja Nicklaus, Kai Voss, and Gerhard Heinzel

The next generation of gravimetric satellite missions will probably utilize a single laser-based instrument to track distance variations between the satellites in a pair. Mission studies and technology developments are ongoing at NASA/USA, DLR/Germany and at the European Space Agency (ESA) in order to advance the successful technology demonstrator aboard GRACE-FO, the Laser Ranging Instrument (LRI), to a primary instrument with appropriate redundancy. The new instruments should of course incorporate learned lessons from the development as well as in-orbit operation of the instrument on GRACE-FO.

The new generation of instruments is expected to have similar noise requirements as in GRACE-FO, since laser ranging observations are usually not limiting the monthly gravity field maps. Design changes in the future LRI are carefully assessed in order to ensure that the actual in-flight precision can reach the same level as in the LRI aboard GRACE-FO, which has shown at high frequencies a noise of 200 pm/√Hz, i.e. is able to resolve changes in the 200 km distance as small as single atoms over short time scales. Efforts focus in particular on an improved knowledge of the LRI scale factor, i.e. the absolute laser frequency, because the current approach of correlating KBR and LRI range can not be employed in future missions.

In this presentation we address some of the trade-offs that have been performed in the design of future instruments in the context of the above studies, discuss the limiting performance aspects for tone errors and noise and summarize the learned lessons and their potential relevance for future missions.

How to cite: Müller, V., Misfeldt, M., Feiri, A., Nicklaus, K., Voss, K., and Heinzel, G.: Towards Laser Ranging in Future Gravimetric Missions, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-42, https://doi.org/10.5194/gstm2022-42, 2022.

14:51–15:03
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GSTM2022-33
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Virtual presentation
Absolute Laser Frequency Readout of Cavity for Next Generation Geodesy Mission 
Emily Rose Rees, Andrew Wade, Andrew Sutton, and Kirk McKenzie

We demonstrate the absolute frequency calibration of a laser using a free spectral range cavity readout designed for next generation geodesy missions.

The Gravity Recovery and Climate Experiment (GRACE) missions rely on inter-satellite interferometry to measure changes in the local gravity of the Earth. These measurements are compared over seasons and years, providing a critical tool for the understanding of large-scale mass transport, in particular the movement of water and ice.  The GRACE Follow-On mission launched in 2018 and included a Laser Ranging Instrument (LRI) as a technology demonstration. The LRI demonstrated performance two orders of magnitude better than the equivalent Microwave Instrument (MWI). As such, laser interferometry is expected to be relied upon as the primary instrument for the next generation of GRACE-like missions.

To enable the use of laser interferometry as the primary science measurement, laser frequency stability is important at two time scales; short timescales (10-1000 seconds) to measure the local gravity, and long timescales (months and years) to enable the comparison of these gravity measurements over time.

Short term laser frequency stability is provided by stabilizing the laser to an optical cavity using the Pound-Drever-Hall method, however, a new technique will be required to provide long term laser frequency stability.

We have previously demonstrated a simple phase modulation scheme that is able to measure laser frequency change over long timescales using measurements of the optical cavity's free spectral range [1]. More recently we have calibrated the technique to absolute frequency by comparing with an atomic reference and have also validated an approach for on-ground calibration to allow the absolute frequency to be determined in orbit [2].

[1] E.R. Rees, A. R. Wade, A. J. Sutton, R. E. Spero, D. A. Shaddock, and K. Mckenzie, ‘Absolute frequency readout derived from ULE cavity for next generation geodesy missions’, Opt. Express, OE, vol. 29, no. 16, pp. 26014–26027, Aug. 2021, doi: 10.1364/OE.434483.

[2] E.R. Rees, A. R. Wade, A. J. Sutton, and K. McKenzie, ‘Absolute Frequency Readout of Cavity against Atomic Reference’, Remote Sensing, vol. 14, no. 11, p. 2689, Jun. 2022, doi: 10.3390/rs14112689.

How to cite: Rees, E. R., Wade, A., Sutton, A., and McKenzie, K.: Absolute Laser Frequency Readout of Cavity for Next Generation Geodesy Mission, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-33, https://doi.org/10.5194/gstm2022-33, 2022.

15:03–15:15
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GSTM2022-80
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On-site presentation
A Simplified Gravitational Reference Sensor for NASA’s Mass Change Mission and Beyond 
John Conklin and the Simplified Gravitational Reference Sensor team

The University of Florida, is leading a team that includes Caltech/JPL, Ball Aerospace, Fibertek, Inc, CrossTrac Engineering, Texas A&M University, and the University of Central Florida to develop a Simplified Gravitational Reference Sensor (S-GRS), an ultra-precise inertial sensor optimized for future Earth geodesy missions. Inertial sensors like the S-GRS are used to measure or compensate for all non-gravitational accelerations of the host spacecraft so that they can be removed in the data analysis to recover spacecraft motion due to Earth’s gravity field, the main science observable. Low-low satellite-to-satellite tracking missions like GRACE-FO that utilize laser ranging interferometers are technologically limited by the acceleration noise performance of their electrostatic accelerometers, as well as temporal aliasing associated with Earth’s dynamic gravity field. The S-GRS is estimated to be at least 40 times more sensitive than the GRACE accelerometers and more than 500 times more sensitive if operated on a drag-compensated platform. The S-GRS concept is a simplified version of the flight-proven LISA Pathfinder GRS. Our performance estimates are based on models vetted during the LISA Pathfinder flight and the expected Earth orbiting spacecraft environment based on flight data from GRACE-FO. The improved performance is enabled by removing the small grounding wire used in the GRACE accelerometers and replacing it with a UV photoemission-based charge management system, enabling more massive test masses and larger gaps between the test mass and its housing. We have shown that the increased S-GRS performance allows future missions to take full advantage of the improved sensitivity of the GRACE-FO Laser Ranging Interferometer (LRI) over microwave ranging systems in the gravity recovery analysis. A specific version of the S-GRS, optimized for NASA’s Mass Change Mission, is also under study as part of that mission’s Phase A development. This presentation will describe the S-GRS, its development timeline and performance estimates. 

How to cite: Conklin, J. and the Simplified Gravitational Reference Sensor team: A Simplified Gravitational Reference Sensor for NASA’s Mass Change Mission and Beyond, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-80, https://doi.org/10.5194/gstm2022-80, 2022.

Coffee break
15:45–15:57
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GSTM2022-77
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On-site presentation
Optomechanical accelerometers for geodesy 
Felipe Guzman

Gravitational acceleration provides unique measurement opportunities to identify natural and man-made phenomena at global scales with signatures that are extremely difficult to mask due to their nature. Such gravitational observations are currently conducted with commercial gravimeters and gravity gradiometers that consist of complex mechanical structures operating large, inertially sensitive test masses and cumbersome displacement readout systems.

We are currently developing highly compact, portable, and cost-effective optomechanical inertial sensors of high sensitivity, building upon recent advances in the area of optomechanics. These technologies consist of low loss and highly stable monolithic mechanical oscillators that we combine with miniaturized laser interferometric displacement sensors, enabling us to achieve extremely high performances in acceleration sensing in small form factors.

To this end, it is necessary to develop various subsystems that are building blocks, each contributing to the implementation of these kinds of instruments. We will discuss our work on the fabrication of low-loss mechanical resonators, and the development of compact monolithic laser interferometer heads, fiber-based multi-color fiber interferometers, and new measurement concepts targeting smaller footprints and higher integration into photonic platforms. We will primarily report on the progress of our novel low-frequency optomechanical accelerometers for geodesy.

How to cite: Guzman, F.: Optomechanical accelerometers for geodesy, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-77, https://doi.org/10.5194/gstm2022-77, 2022.

15:57–16:09
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GSTM2022-58
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On-site presentation
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Closed Loop Simulations Evaluating the Resolvability of Climate-Related Mass Transport Signal in Current and Next-Generation Satellite Gravity Missions 
Marius Schlaak, Roland Pail, and Annette Eicker

The existing observation record of satellite gravity missions spans more than two decades and is already closing in on the minimum time series of 30 years needed to decouple natural and anthropogenic forcing mechanisms according to the Global Climate Observing System (GCOS). Next Generation Gravity Missions (NGGMs) are expected to be launched within this decade, setting high anticipation for an enhanced monitoring capability that will improve the spatial and temporal resolutions of gravity observations significantly. They will allow an evaluation of long-term trends in the Terrestrial Water Storage (TWS) signal. The observations might therefore be used to verify climate projections and give additional inputs to the climate modelling community. This contribution shows numerical closed-loop simulation results of a GRACE-type in-line single-pair missions and Bender double-pair missions with realistic noise assumptions for the key payload and ocean-tide background model errors. The gravity signal in the simulations is based on modeled mass transport time series of components of the TWS, obtained from future climate projections until the year 2100 following the shared socio-economic pathway scenario 5-8.5 (SSP5-8.5). It evaluates different parameter models, among them the recoverability of long-term climate trends, annual amplitude, and phase of the TWS employing closed-loop numerical simulations of different current and NGGM concepts. Special emphasis shall be given on the robustness of the estimated TWS long-term-trend for different parameter models applied in different simulation scenarios, systematic changes, as well as on the methodology of the simulation themselves.

How to cite: Schlaak, M., Pail, R., and Eicker, A.: Closed Loop Simulations Evaluating the Resolvability of Climate-Related Mass Transport Signal in Current and Next-Generation Satellite Gravity Missions, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-58, https://doi.org/10.5194/gstm2022-58, 2022.

16:09–16:21
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GSTM2022-64
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On-site presentation
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Impact of tone errors on gravity field solutions in NGGM concepts 
Nikolas Pfaffenzeller, Roland Pail, Thomas Gruber, Thomas Usbeck, and Domenico Gerardi

In order to meet the upcoming challenges of climate change, it is necessary to understand the different Earth system processes in more detail and to monitor them. With satellite gravity missions it is possible to continuously measure and observe mass transport processes on Earth globally.  For the current study on Next Generation Gravity Missions (NGGMs) which is called Mass Change and Geosciences International Constellation study (MAGIC) various parameters are subject of the investigation. One aspect is the instrument behavior and its impact on gravity field performance. Besides stochastic noise sources, deterministic errors are present in the instrument measurements, mainly caused by environmental variations on the satellites. These disturbances are generated by temperature variations that occur when passing through sun illuminated and shadow areas, as well as magnetic field variations. As a consequence, periodic errors occur at integer multiples of the orbital frequency.           
In this contribution we want to analyze the impact of the errors on gravity field solutions and provide the findings generated within the MAGIC study. In detail we want to present the results for GRACE-like and Bender-type constellation for scenarios considering an instrument only-case and a full-noise case including temporal gravity field. Depending on the amplitude of the tone errors, they can affect the complete spherical harmonic spectrum of the solutions. To mitigate the effect of tone errors, it is possible to encounter them in data processing e.g. by applying notch filters on the integer multiple of the orbital frequency. As a result of this approach, tone errors affect only the very low harmonic coefficients, but the remaining coefficients can be estimated with good quality.

How to cite: Pfaffenzeller, N., Pail, R., Gruber, T., Usbeck, T., and Gerardi, D.: Impact of tone errors on gravity field solutions in NGGM concepts, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-64, https://doi.org/10.5194/gstm2022-64, 2022.

16:21–16:33
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GSTM2022-55
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On-site presentation
Enhanced near-real time gravity field retrieval for NGGM 
Petro Abrykosov and Roland Pail

The main goal of a next generation gravity mission (NGGM) is to improve the observation of mass transport processes primarily through an enhanced sampling with respect to space and time. Thus, high-resolution gravity products can be retrieved over short time periods which are highly valuable for geophysical applications like the monitoring and forecast of extreme climate events such as e.g. floods and droughts.

In this regard, a near-real-time (NRT) processing strategy was proposed in Purkhauser et al. (2020) where the so-called Wiese approach (co-parametrization of short-periodic low-resolution gravity fields and a long-period high-resolution one) was combined with a sliding window averaging. However, in recent studies some drawbacks of the Wiese scheme could be identified and partially rectified by a so-called data-driven multi-step self-de-aliasing approach (DMD).

In this contribution, the original NRT scheme is revised and re-run on the basis of the DMD method within numerical closed-loop simulations. The added value of DMD over the Wiese approach within the NRT processing is quantified and presented, and drawbacks and advantages of either methodology are discussed.

How to cite: Abrykosov, P. and Pail, R.: Enhanced near-real time gravity field retrieval for NGGM, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-55, https://doi.org/10.5194/gstm2022-55, 2022.

16:33–16:45
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GSTM2022-1
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On-site presentation
SLR Simulations to Improve Time-Variable Gravity: Evaluating the Impact of Combination Solutions and a Future Satellite 
Evan Tucker, R. Steven Nerem, and Bryant Loomis

The Gravity Recovery and Climate Experiment (GRACE) has been known to poorly recover the low degree zonal coefficients of the gravity field. Towards the end of its life, GRACE operated with only a single accelerometer, which further degraded these coefficients. The recently launched GRACE Follow-On (GRACE-FO) continues to observe Earth’s mass change, providing critical measurements of time-varying geophysical signals. However, one of the GRACE-FO satellites suffers from an underperforming accelerometer and a transplant algorithm is used to map data from the functioning instrument. This has created an additional challenge for estimating the low degree zonal coefficients.

Satellite Laser Ranging (SLR) data have long supplemented GRACE gravity estimates. These passive spherical satellites are covered in reflectors to allow laser ranging measurements from a global network of ground stations. On its own SLR can recover low-degree gravity fields which have been used to support the GRACE estimates. The 2012 launch of the Laser Relativity Satellite (LARES) led to large improvements in SLR’s ability to estimate C3,0. Conventionally, these SLR-derived estimates are substituted directly into the GRACE estimates. This approach neglects the influence of higher order terms due to correlated errors in the SLR solution. In this work we simulate SLR and GRACE data to investigate a combined solution at the normal equation level. Motivated by LARES’s effect on the solution, we also simulate a potential new SLR satellite and search for an optimal orbit to improve the gravity recovery. We present results demonstrating the improvements from a combined SLR and GRACE solution as well as the impact of a hypothetical SLR satellite on gravity field recovery.

How to cite: Tucker, E., Nerem, R. S., and Loomis, B.: SLR Simulations to Improve Time-Variable Gravity: Evaluating the Impact of Combination Solutions and a Future Satellite, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-1, https://doi.org/10.5194/gstm2022-1, 2022.

16:45–16:57
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GSTM2022-46
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On-site presentation
Comparison of gap-filling temporal methods to improve GRACE and GRACE-FO time series 
Hugo Lecomte, Severine Rosat, and Mandea Mioara

The GRACE and GRACE Follow-On missions are separated by an 11-month gap between 2017 and 2018 and contain 22 more missing months. These gaps in the time series lead to a difficult recovery of gravity variation signals with pluri-annual temporal scales. In this context, various studies proposed machine learning approaches and decomposition techniques to predict the missing values.

 

This study summarizes the different approaches that we have implemented and compares their results. We consider both grid and spherical harmonics at global scales. Some gap-filling solutions use an extrapolation of the GRACE products and some others propose to use Swarm gravity field products to reduce the missing data. We tested several methods in terms of their capacity to predict signals on monthly or annual periods, randomly chosen between 2005 and 2010. The Root-Mean Square Error between the predictions and the original solution gives an estimation of the uncertainty associated with each method.

 

We show that simple methods like « Constant, Trend, Annual and Semi-annual fit » do not deliver the complexity of the original signal. We finally conclude that the Singular Spectrum Analysis (SSA) and Multivariate SSA produce the best results at large spatial scales.

How to cite: Lecomte, H., Rosat, S., and Mioara, M.: Comparison of gap-filling temporal methods to improve GRACE and GRACE-FO time series, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-46, https://doi.org/10.5194/gstm2022-46, 2022.

Posters: Wed, 19 Oct, 16:15–17:15 | Foyer, Building H

P11
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GSTM2022-85
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On-site presentation
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Advancing Inter–Spacecraft Laser Interferometry for Future Gravity Missions 
Sariga Sachit, Timm Wegehaupt, Jens Große, Vitali Müller, and Gerhard Heinzel

              A decisive step for future space missions in the field of next-generation gravity missions was taken by launching the first and only inter-satellite interferometer aboard the GRACE-FO mission. Improving the LRI for NGGM is the next goal.  Both microwave and LRI instruments provide ‘biased’ ranges with an unknown offset. Thus, absolute ranging that resolves the bias may open new avenues for orbit determination and data processing. Laser ranging involves precise pointing due to the narrow width of the laser beams. This complicates the initial link acquisition which requires the simultaneous alignment of two optical beams with respect to the line of sight, as well as matching the laser frequencies to enable interferometry. Even small changes in the pointing angle lead to enormous movement in the beam spot at the opposite spacecraft several 100 km away. In the LRI, the link acquisition procedure consists of synchronized spatial scans (two per spacecraft) performed by a steering mirror on each spacecraft together with a frequency scan of the laser on the transponder spacecraft. The whole procedure takes about nine hours.

               We investigate alternative strategies to improve link acquisition speed, robustness, autonomy, and compatibility with redundancy schemes. In particular, we evaluate the use of a dedicated non-coherent acquisition sensor, its capabilities and its interaction with the other optical elements. The sensor, likely an InGaAs CMOS camera, will measure the tilt of the incoming beam significantly reducing the initial acquisition time.

              Lastly, laser interferometry can be combined with techniques from telecommunications. With minimal extra hardware, auxiliary functions can be added to the existing laser link. These include the measurement of pseudo-ranges, i.e. combinations of absolute range and clock offsets that can be disentangled to measure the absolute range to centimeter accuracy and clock offsets to nanosecond accuracy. Both may be useful for gravity field recovery data processing. In addition, a data stream of up to dozens of kilobits per second can be transmitted in parallel with the same modulation. This would provide extra redundancy for the ground contacts and/or simplify ground operations, or maybe even enable additional operational features in real-time. Since noise sources and performance limitations directly depend on laser performance and stabilization technique, advanced frequency stabilization based on molecular iodine references may be integrated into the test bed in the future.

           In this poster, we show a LRI test bed comprising two hexapods capable to simulate satellite rotations, initially built to test different acquisition procedures. We address the methodology to improve the 5D link acquisition with realistic hardware and introduce a 3-stage control system for the FSM. Furthermore, we evaluated different optical layouts for NGGM and provide trade-off.

How to cite: Sachit, S., Wegehaupt, T., Große, J., Müller, V., and Heinzel, G.: Advancing Inter–Spacecraft Laser Interferometry for Future Gravity Missions, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-85, https://doi.org/10.5194/gstm2022-85, 2022.

P12
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GSTM2022-10
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On-site presentation
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Another Option for the Next Generation Gravity Mission Orbit Configuration 
Peter Bender

ANOTHER OPTION FOR THE NEXT GEN. GRAVITY MISSION ORBIT CONFIGURATION

For the polar pair of a next generation gravity mission, an attractive feature would be to keep the repeat period for the ground track quite short.  The satellites hopefully will include a much-simplified version of the Gravitational Reference Sensors demonstrated on the LISA Pathfinder Mission, as well as laser interferometry between the two satellites.  A short period repeat option for the orbits is 107 satellite revolutions in 7 sidereal days, at an altitude of 468 km.  Because of the fairly high altitude, the amount of propulsion needed for drag-free operation would be relatively small.

With this geometry, the upward passes across the equator will be separated by 3.4 deg in longitude, and will be followed 3.5 days later by downward passes at the same longitudes.  In the northern hemisphere, the upward and downward passes also will cross at latitudes of 25.7 and 51.4 degrees.  At all latitudes, the maximum gaps in longitude between upward and downward passes for 3 successive days will be 6.7 deg. or less.  In analysing the 7-day data sets, the maximum ground track separation in longitude would be 378 km.  Thus, for a point mass half way between the nearest ground tracks, the minimum distance to the satellites would be increased only from 468 km to 505 km at the most. 

A recent study by R. Spero (Adv. Space Res. 67 (2021) 1656-1664) addressed the question of how accurately rapid changes in a local mass concentration could be measured with earth gravity change mission measurements.  One case that was included had about the same measurement capability as the next generation gravity mission (NGGM) assumed here.  For this case, the reduction in the instrumental measurement uncertailty was dramatic.  However, the actual usefulness of the results would be strongly limited by the a priory uncertainty in our ability to understand the geophysical sources of the geopotential variations.  Thus, while our uncertainty in the geopotential variations at satellite altitude would be reduced at almost all frequencies, it probably would be a long time before our understanding of the geophysical sources of the variations becomes good in most regions at frequencies below about 60 cycles/rev.

Fortunately, this limitation will be less severe in regions where one source of geopotential variation is dominant, such as river basins where the dominant variation is in the near-surface stored water level.

How to cite: Bender, P.: Another Option for the Next Generation Gravity Mission Orbit Configuration, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-10, https://doi.org/10.5194/gstm2022-10, 2022.

P13
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GSTM2022-92
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On-site presentation
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Enhanced Optical Frequency References for Next Generations of Gravity Missions 
Timm Wegehaupt, Sariga Sachit, Gerhard Heinzel, Claus Braxmaier, and Jens Grosse

Aboard GRACE-FO, the first and up to now only intersatellite laser ranging interferometer (LRI) was launched in 2018. Developed as a technology demonstrator, it exceeds the performance of the actual main instrument, which is based on microwave ranging (MWI), by several orders of magnitude. Therefore, the LRI will most likely be the prime technology for next generations of gravity missions, opening up further optimization opportunities, but also new challenges.

In a future mission, the central axis will not be occupied by a microwave system, allowing new interferometer layouts as e.g. an on-axis design. Beside the layout, the frequency reference of the laser plays a crucial role. Currently, optical cavities that provide high frequency stability are used, while at the same time only small portion of the available size- mass- and power-budget is required for the laser stabilization unit. However, relative frequency references also bring some disadvantages that make absolute references a possible alternative, especially when the MWI is no longer available. Relative references are dependent on external environmental influences, which can be suppressed on short timescales by a well-designed setup, while on long timescales they become apparent and influence the so-called scale-factor. The scale-factor corresponds to the absolute frequency of the laser and affects the conversion of the phase measurement into a length change and therefore the performance of the LRI. Absolute frequency references based on molecular iodine or hybrid references consisting of a cavity and a molecular iodine spectroscopy unit would suppress these dependencies and will simultaneously ease up the initial acquisition process of the LRI.

We will discuss possible alternatives to optical cavities for next generation gravity missions and will outline the advantages and disadvantages of the different technologies. In addition, we analyze proposed technologies that would allow a readout of the absolute frequency of optical cavities in space and underline these with first measurement results from the laboratory.

How to cite: Wegehaupt, T., Sachit, S., Heinzel, G., Braxmaier, C., and Grosse, J.: Enhanced Optical Frequency References for Next Generations of Gravity Missions, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-92, https://doi.org/10.5194/gstm2022-92, 2022.

P14
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GSTM2022-40
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On-site presentation
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Kinematic orbits and their contribution to monitoring Earth's gravity field 
Barbara Suesser- Rechberger, Torsten Mayer-Guerr, Sandro Krauss, Patrick Dumitraschkewitz, Felix Oehlinger, and Andreas Kvas

For the observation of temporal mass variations on Earth, the determination of the Earth's gravity field is of great importance. Therefore, additional concepts are required to bridge possible gaps occurring within dedicated gravity field missions such as CHAMP, GRACE and GOCE or between consecutive gravity field missions i.e., GRACE and GRACE Follow-On. Gravity field solutions based on kinematic orbits of low earth orbiting (LEO) satellites are a common practice. Our working group use an in-house developed approach based on an iterative least-squares adjustment utilizing raw GNSS observations for the kinematic orbit determination. Since varying algorithms, models, and processing strategies applied by different institutions can lead to inconsistencies, which affects the performance we now use GNSS products which are consistently processed with our in-house software package GROOPS.  As a result, improved kinematic orbit and subsequently gravity field solutions could be achieved due to this consistency. Up to now, we reprocessed the kinematic orbits of 19 LEO satellite missions, including non-gravity missions like Sentinel 1A/B, Sentinel 3A/B, Swarm A/B/C, TerraSAR-X, TanDEM-X, MetOp A/B, and Jason 1/2/3. On our website we subsequently publish these kinematic orbit solutions together with further satellite orbit products like the reduced dynamic orbit, the attitude, and the accelerations due to non-conservative forces.  Based on these kinematic orbits a time series of individual monthly gravity field solutions has been determined. Additional to more precise orbits, enhanced non-gravitational force models based on satellite macro models contribute to more accurate gravity field solutions. The time series spans meanwhile 20 years without gaps, starting in January 2002. For this time span, we will show the mass variations for regions such as Greenland, the Amazon basin and other large river basins and compare the results derived from the kinematic orbits with those from COST-G Swarm and COST-G GRACE/GRACE-FO.

How to cite: Suesser- Rechberger, B., Mayer-Guerr, T., Krauss, S., Dumitraschkewitz, P., Oehlinger, F., and Kvas, A.: Kinematic orbits and their contribution to monitoring Earth's gravity field, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-40, https://doi.org/10.5194/gstm2022-40, 2022.