GRACE Science Team Meeting :: Joint Session A.2+A.4 GRACE Follow-On and Data Continuity

Posters

Performance Analysis of Satellite Constellations for the Next Generation of Gravity Missions
(Dahle, Raimondo, Flechtner, Loescher, Kusche)

A coherent ocean reflection experiment for GRACE Follow-On
(Beyerle, Semmling, Grunwaldt, Wickert, Flechtner)

Session: A.2 - GRACE-Follow On
Title: Recent Studies on Future Gravity Field Missions in Europe: e.motion vs. NGGM
First Author: Thomas Gruber
Presenter: Thomas Gruber
Co-Authors: e.motion Team; NGGM Team

Abstract: In the recent years several studies on future gravity field missions were conducted in Europe. In answer to ESA's Earth Explorer 8 Mission Call a proposal named e.motion (Earth System Mass Transport Mission) was submitted in May 2010 as one of 30 competing proposals. In October 2010 the evaluation was finalized and e.motion was not selected to proceed to the next phase even if rated very positive from a scientific point of view. In parallel from 2009 to late 2010 two ESA studies on Next Generation Gravity Field Missions (NGGM) were performed by independent science and industrial teams. Both studies came to similar conclusions about scientific requirements and potential mission layouts. The paper will give an overview of these studies and specifically compares the identified science requirements and their implications on the mission design and instrument performance. The paper shall be an initial step towards a consolidated view of science requirements for a future gravity field mission to be launched in the timeframe 2020 to 2025.

Session: A.2 - GRACE-Follow On
Title: Overview of the laser ranging instrument for GRACE follow-on
First Author: Benjamin Sheard
Presenter: Benjamin Sheard
Co-Authors: et al.

Abstract: For the GRACE follow-on mission in addition to the main microwave ranging instrument it is planned to include a demonstration instrument to test interferometric laser ranging. Interferometric laser ranging is of interest for future geodesy missions due to the higher ranging precision achievable with laser interferometry. The implementation of this demonstration instrument is complicated by the fact that it must be adapted to an existing proven design without imposing large changes. An overview of the planned implementation of this laser ranging instrument will be given.

Session: A.2 - GRACE-Follow On
Title: Laser Ranging Demonstrator for GRACE Follow-on Mission: US Contribution
First Author: Michelle Stephens
Presenter: Michelle Stephens
Co-Authors: J. Leitch; R. Pierce;G. deVine; W. M. Folkner; W. M. Klipstein; K. McKenzie; R. Spero; R. Thompson; N. Yu; M. Watkins

Abstract: The GRACE follow-on mission currently planned for launch in 2017 minimizes risk and time to launch through a baseline design that is a close rebuild of GRACE. However, in order to demonstrate the potential to improve spatial resolution through more accurate ranging using interferometric methods, a laser ranging demonstrator is also included. The laser ranging demonstrator will be a collaboration between U.S, German, and Australian institutions. This talk describes the work performed at JPL and Ball Aerospace that has advanced the technology readiness level of the laser frequency stabilization and the phase measurement systems.

Session: A.2 - GRACE-Follow On
Title: Improvment of the thermal characteristics of the GRACE-FO accelerometer
First Author: Bernard Foulon
Presenter: Bernard Foulon
Co-Authors: B. Christophe; D. Boulanger; F. Liorzou; V. Lebat

Abstract: The accelerometer proposed for the GRACE Follow-On mission is very similar to the SuperSTAR models operating since nine years on board the twin GRACE satellites.

However, lessons learned from the GRACE mission and also from the six GRADIO models composing the three axis gravity gradiometer of the GOCE ESA mission, lead to minor modifications in the packaging and some electronics circuits in order to improve the thermal behaviour of the instrument. The presentation will present the gain, in terms of thermal stability, expected from these improvements.

Session: A.2 - GRACE-Follow On
Title: Proposal For a GNSS Reflectometry Payload on GRACE Follow-on
First Author: G-FO team CNES
Presenter: François Soulat
Co-Authors: Richard BIANCALE (CNES/GRGS), Jean-Damien DESJONQUERES (CNES), Steven HOSFORD (CNES), André LAURENS (CNES), Laurent LESTARQUIT (CNES), Felix PEROSANZ (CNES/GRGS), Nadine POURTHIÉ (CNES), Antoine RICHARD DE LATOUR (CNES), Lionel RIES (CNES), François SOULAT, (CLS), Jean-Claude SOUYRIS (CNES).

Abstract: Several GNSS constellations are presently operational or under development, including the pioneering US Global Positioning System (GPS) and the forthcoming European system, Galileo. These all-weather, long-term, stable and precise L-band signals can be used for bistatic remote sensing of the ocean surface and beyond, an emerging concept known as GNSS Reflectometry (or GNSS-R).

Among several applications, three have been emphasized by the GNSS-R community: soil moisture measurements, sea-surface altimetry, and sea-surface roughness, related to the statistical properties of sea surface gravity wave slopes.

Thanks to its passive multistatic character, GNSS-R clearly holds the potential to provide a spatio-temporal sampling of the ocean surface that can be of high interest for oceanographic applications. The expected spatial and temporal measurements can certainly provide relevant measurements as a complement to existing sensors. For instance, and to follow successful scatterometer measurements, GNSS-R can complement ocean winds and wave models. Being rain immune, such new data could help quantify atmosphere-ocean coupling, including momentum and energy fluxes under extreme conditions for hurricane modeling.

The purpose of the present proposition is to offer the opportunity, by embarking a GNSS-R demonstrator as an auxiliary payload on GRACE Follow-On satellites, to demonstrate the use of the GNSS Reflectometry technology for Earth Observation applications.

Given the state-of-the-art in GNSS-R altimetry - with the expected resolutions (few km) while keeping inside reasonable hardware constraints (compatibility to what can be expected for an auxiliary payload) - it is possible to access the sea surface roughness characterization from the bistatic scattering mechanism.

After presenting the opportunity of such a demonstration, the communication will expose the principles and potentials applications of such an experiment, and then will sketch a payload architecture and propose an acquisition strategy

Session: A.2 - GRACE-Follow On
Title: Possible Use of Pacific/Antarctic Mass Variation Information in Future GRACE-Type Missions
First Author: Peter L. Bender
Presenter: Peter L. Bender
Co-Authors: C. R. Betts, J. Flury, F. G. Lemoine, B. Loomis, S. B. Luthcke, P. N. A. M. Visser, D. N. Wiese

Abstract: Corrections for differential satellite accelerations for the GRACE mission usually are made about every two orbital revolutions. Models from other data sources are used for the mass distribution variations in making these corrections. Thus errors in the mass distribution variation models affect the GRACE results through the so-called empirical parameter corrections. This limitation probably will still be important for future drag-free GRACE-type missions.

In view of this, it was pointed out in a poster at GSTM2010 that such errors might be reduced if the correction procedure were based on a priori mass variation models only for regions where the variations are known substantially better than for the whole globe. One such region is believed to be the central Pacific, within perhaps 30 deg of the equator and well away from boundary currents. However, it was recognized that the inclusion of an additional region, the Antarctic, would be highly desirable. Thus a brief investigation has been made of the availability of surface atmospheric pressure data from automatic weather stations in the Antarctic. The results indicate that data from at least 11 sites within 15 deg of the pole and at altitudes of 500 m or more could be obtained regularly and used to determine changes in the geopotential height over the polar region fairly accurately.

Based on the assumption of being able to use mass variation model data from both the central Pacific and the South Pole, simulations of a possible differential satellite acceleration correction proceedure for future GRACE-type missions have now been started. For satellites in polar orbits, there are two periods each day when the ground tracks pass across the central Pacific for 4 successive orbits. During these times, changes in the satellite separations can be fit to the calculated changes from the mass variation models in the Pacific and Antarctic to determine the corrections for the effects of spurious accelerations. The main limitation is that there would be a gap of about 240 deg during each revolution when data for the correction would not be available. The principal improvements hoped for are in Europe and Africa, which are crossed by the orbits during the roughly 6 hour correction period. However, frequent comparisons between the South Pole and different locations in the Pacific during this period would be valuable also.

Session: A.4 - Bridging the Gap
Title: Supplementing the GRACE Mission with Time Variable Gravity Variations from SLR
First Author: Minkang Cheng
Presenter: Minkang Cheng
Co-Authors: John Ries and Byron D. Tapley

Abstract: Monthly estimates of the geocenter and the long-wavelength gravity variations have been determined from analysis of the Satellite Laser Ranging (SLR) tracking of five geodetic satellites, which have been an important supplement to GRACE for the determination of the entire mass redistribution signal. In particular, the rate of change in J2 is observed to have decreased significantly during the last decade or so, presumably due to accelerated ice mass loss at the higher latitudes. The estimate of the change in the J2 rate could provide a validation of the GIA models and estimate of the mass component of global sea level rise. We will also present the weekly and monthly geocenter solutions from SLR and compare them to the solution from ITRF2008 and a global inversion using GPS, GRACE and ocean bottom pressure. The time series of the lower degree portion of the time-varying gravity field from SLR can be a part of a possible bridge through the anticipated gap between GRACE and the planned GRACE Follow On mission.

Session: A.2 - GRACE-Follow On
Title: Performance Analysis of Satellite Constellations for the Next Generation of Gravity Missions
First Author: Christoph Dahle
Presenter: Christoph Dahle
Co-Authors: J.C. Raimondo; F. Flechtner; A. Loecher; J. Kusche

Abstract: The GOCE and GRACE gravity missions have dramatically improved the knowledge of the Earth's static and time-variable gravity field due to their highly precise on-board instrumentation. This resulted in new information about the mass distribution and transport within or around the Earth system to be used in solid Earth geophysics, oceanography and sea level studies, hydrology, ice mass budget investigations and geodesy.

Several studies have already been conducted in order to develop a concept for a future gravity mission based on low-low satellite-to-satellite tracking, realized with laser metrology. In our poster we summarize the performance of different mission scenarios through full-scale simulations and their capacity to reach the science objectives.

Session: A.2 - GRACE-Follow On
Title: A coherent ocean reflection experiment for GRACE Follow-On
First Author: Georg Beyerle
Presenter: Frank Flechtner
Co-Authors: M. Semmling; L. Grunwaldt; J. Wickert; F. Flechtner

Abstract: The Coherent Ocean Reflection Experiment on GRACE Follow-On ("choralGRACE") is a GNSS reflection experiment proposed for the GRACE Follow-On satellite mission. The experiment is a low-cost extension to the GNSS receiver instrument designated for precise orbit determination and radio occultation measurements. Its main objectives are:

Improve the knowledge on coherent GNSS reflections as observed from a LEO platform.
Allow for estimates on altimetric height profiles from ocean and ice/snow surfaces.
"choralGRACE" is implemented through a limited set of receiver firmware modifications; additional hardware resources are not required.

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