Session: C - GRACE Follow-On and Continuity
(Convener: )

Title: ONERA contribution to GRACE-FO accelerometers post processing
Presenter: Bruno, Christophe
Co-Authors: J. Bergé, D. Boulanger, R. Chhun, M. Dalin, B. Foulon, E. Hardy, P-A. Huynh, V. Lebat, F. Liorzou, M. Rodrigues

Abstract: The GRACE FO mission, led by the JPL (Jet Propulsion Laboratory) and GFZ (GeoForschungsZentrum), is an Earth-orbiting gravity mission, continuation of the GRACE mission, which will produce an accurate model of the Earth’s gravity field variation providing global climatic data during five years at least.

For that the accelerometer data shall be post-processed by suppressing the perturbations (twangs,…), by correcting some default thanks to calibration or by using other outputs (angular acceleration, sensor temperature, …).

The presentation will focus on how ONERA can participate to the improvement of the post processing:

1. In CNES MICROSCOPE mission, for detecting a violation of the Einstein equivalence principle, ONERA is in charge of the mission scientific center. The experience acquired with MICROSCOPE as well as the tools developed for this scientific center can profitably be used for GRACE-FO.
2. Moreover, as manufacturer of the accelerometer, ONERA has a perfect knowledge of the accelerometer and can simulate its behavior with respect to any perturbation (external or internal). Such simulation can help to verify different post-processing methods and their capability to retrieve the original signal or to suppress the default of the instrument

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Title: Robust attitude regulation for spacecraft with hybrid actuation
Presenter: Pylorof, Dimitrios
Co-Authors: S. Bettadpur; E. Bakolas

Abstract: The scientific measurements and, in a broader sense, the mission outcome of Earth observation spacecraft can be influenced by the spacecraft’s rotational motion along the orbit and the attitude regulation process. Actuation schemes and measurement availability are, among others, two factors of significance for this problem. Using the GRACE mission and spacecraft architecture as our starting points, we outline recent results and ongoing work to develop attitude control solutions which are more tailored to spacecraft of this class and their mission. Output feedback robust control designs for spacecraft with hybrid attitude actuation are discussed, while numerical simulations show the potential for obtaining a smoother attitude trajectory and acceptable pointing errors under less overall thrusting, which is inherently impulsive and can interfere with various scientific measurements performed onboard. Our approach and its results are contrasted with earlier results from the literature and the spacecraft engineering practice.

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Title: Exploring the sensitivity of satellite gravimetry to temporal aliasing errors
Presenter: Wiese, David
Co-Authors: . McCullough, E. Fahnestock, F. Webb

Abstract: Studies have shown that future satellite gravimetry missions utilizing low-low satellite-to-satellite tracking (LL-SST) comprised of either a single pair of satellites or two pairs of satellites, will be limited by temporal aliasing errors, even when improved onboard measurement systems relative to GRACE are used (i.e. laser interferometer, improved accelerometers). Temporal aliasing errors are introduced via deficiencies in models of high frequency mass variations that are required for the data processing due to the limited temporal sampling of the mission. In this study, we probe the spatio-temporal characteristics of temporal aliasing errors to understand their impact on satellite gravity retrievals using high fidelity numerical simulations. We find that while temporal aliasing errors are dominant at long wavelengths and multi-day timescales, improving knowledge of high frequency mass variations at these resolutions translates into only modest improvements (i.e. spatial resolution/accuracy) in the ability to measure temporal gravity variations at monthly timescales. These findings highlight the difficult nature of the convolution between temporal aliasing errors and achieving improved estimates of mass flux in the Earth system using satellite gravimetry measurements.

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Title: Initial placement of GFO’s ascending node: Tidal considerations
Presenter: Ray, Richard
Co-Authors: B. Loomis

Abstract: If the GFO launch window, in local time, is flexible, then it might be useful to consider whether there is benefit to having GFO’s ascending node somehow complementary to GRACE’s node. In particular, this might benefit future tidal inversions of the combined datasets. This consideration is surely worthwhile if GRACE were still operational, but it could conceivably still be worthwhile even if GRACE isn’t.

Inversion of GRACE data for the solar semidiurnal tide, S2, has been problematic because (a) real variability occurs at the S2 alias period and leaks into the S2 solutions and (b) errors associated in any way with the beta-prime orbit period also leak into the S2 solutions. If GRACE and GFO data are collected concurrently over sufficient time, problem (a) can be mitigated by placing the GFO node at 90 degrees from GRACE’s node. Could the problem be mitigated or at least reduced even if the data will not be concurrent? We have conducted some simple simulations of tidal inversions with various orbit scenarios, using existing time-variable gravity models as the potential aliasing error. Results of these simulations will be described.

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Poster Title: Error Reduction Analysis and Optimization of Varying GRACE-Type Micro-Satellite Constellations
Presenter: Widner, Maxon
Co-Authors: S. Bettadpur, F. Wang

Abstract: The Gravity Recovery and Climate Experiment (GRACE) mission has been a principal contributor in the study and quantification of Earth’s time-varying gravity field. Both GRACE and its successor, GRACE Follow-On, are limited by their paired satellite design which only provide a full map of Earth’s gravity field approximately every thirty days and at large spatial resolutions of over 300 km. Micro-satellite technology has presented the feasibility of improving the architecture of future missions to address these issues with the implementation of a constellations of satellites having similar characteristics as GRACE. To optimize the constellation’s architecture, several scenarios are evaluated to determine how implementing this configuration affects the resultant gravity field maps and characterize which instrument system errors improve, which do not, and how changes in constellation architecture affect these errors.

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Poster Title: The GRACE Missions Applications Plan: Status and Implementation
Presenter: Srinivasan, Margaret
Co-Authors: J.T. Reager, M. F. Jasinski, B. Doorn

Abstract: The NASA Applied Sciences Program supports the GRACE and Follow On (GRACE-FO) projects in the development and implementation of a mission Applications approach designed to highlight existing and potential applied uses and user communities of existing and future GRACE missions data products.

The GRACE Applications Working Group (GAWG) is engaging with current and potential GRACE missions user and modeling communities to explore collaboration opportunities. We seek to highlight innovative uses and practical benefits of GRACE data and information products, as well as technologies, to promote the use of the data by federal, research, educational, public, and private-sector groups, and enable support of decision-making activities to increase the benefits of GRACE to society.

After launch, GRACE-FO will join 19 operating and several planned NASA satellite missions in providing Earth observations to improve understanding of Earth systems and the climate implications of a warming world. The applied science community can be an important element of these missions to demonstrate the high value of their science and data products and address societal issues and benefits that stem from them.

GAWG activities focus on implementing the GRACE Missions Applications Plan and developing a systematic framework to evaluate opportunities in the science and operational communities using GRACE data, identifying future applications potential, and working with the GRACE Science Team to develop training opportunities (webinars, tutorials, focus sessions and workshops) to support and implement knowledge transfer and capacity building.

GRACE hydrology applications include measurements of storage and trends in surface and subsurface water. Examples of water resources monitoring, assessment of hydrologic extremes (flood and drought), and fluxes at the land-ocean-atmosphere boundaries will be presented. GRACE-FO will extend this valuable time series into the next decade.

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