GRACE Science Team Meeting

Session A.2: GRACE Follow-On

Michael Watkins

(Spurious trends in basin-wide water storage and the duration of GRACE)
Ming Fang

(GRAF/3M4C - Two GRACE Follow-On Mission Feasibility Studies )
F. Flechtner

(Technology Development for a Follow-on Gravity Mission to GRACE)
Michelle Stephens

(Recovering hydrology and ice mass loss using a polar pair coupled with a moderately inclined pair of satellites)
David Wiese

(A Short Wavelength Argument for a Future Moderate Inclination Mission to Supplement GRACE-2)
Peter L. Bender

(A roadmap for future gravity satellite missions)
Hans-Peter Plag

Session: A.2 - GRACE-Follow On
Title: Spurious trends in basin-wide water storage and the duration of GRACE
First Author: Ming Fang
Presenter: Ming Fang
Co-Authors: Bradford H. Hager

Abstract: The successful capture of variability in regional and continental water storage by GRACE demands deeper understanding of the physical nature of the water storage in the context of our climate system. If irregular fluctuations in the precipitation rate are to taken as a stochastic process, then irregular physical variations in water storage are correlated, generating spurious trends in GRACE observations for durations not long enough to overcome the correlated irregularities. To explore the connection between trend estimates and duration of observation, we give four definitions for "storage-trend" based on different considerations of uncertainties. We call a storage-trend obtained without considering its consistency with the properties of its rate function an apparent trend, and a storage-trend taking into account this consistency physical trend. An analytical analysis of the apparent trend is conducted based on a simple white noise fluctuation in its rate function. Using the uncertainty induced by random observation errors in a storage-trend derived directly from GRACE observations as the reference level for gauging the systematic biases, we find that it takes a minimum of 18 years to bring the systematic bias in a long-term apparent trend to the reference level, and it takes a minimum of 38 years to bring the systematic bias in a long-term physical trend to the reference level. Numerical simulations based on the observed precipitation rates over the Ob and Lena watersheds in Siberia show that even a weak redness in the rate function could produce very significant short-period apparent trends.

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Session: A.2 - GRACE-Follow On
Title: GRAF/3M4C - Two GRACE Follow-On Mission Feasibility Studies
First Author: F. Flechtner
Presenter: F. Flechtner
Co-Authors: F. Flechtner (1), K.H. Neumayer (1), B. Doll (2), J. Munder (2), Ch. Reigber (2), J.C. Raimondo (2), M. Herding (2)

(1) German Research Centre for Geosciences - GFZ, Helmholtz Centre Potsdam,Germany
(2) SpaceTech International GmbH -STI, Immenstaad, Germany

Abstract: After more than 7 years of very successful operation in orbit, the US-German GRACE mission has demonstrated in a very impressive way its outstanding capability to monitor mass motions in the Earth system with unprecedented accuracy and temporal resolution. These results have stimulated many novel research activities in hydrology, oceanography, glaciology, geophysics and geodesy, which also indicate that long term monitoring of such mass motions, possibly with improved spatial and temporal resolution is a must for further understanding of various phenomena.

GRACE mission life time, originally designed for 5 years, will likely end around 2013 to 2015, depending on solar activity, thruster actuations or battery status. Considering this, GFZ Potsdam has finished recently two short R&D studies with Space Tech International GmbH (STI) as industrial partner to investigate the feasibility/boundaries of a follow on mission, taking into account satellite and instrument system, cost, programmatic and schedule aspects.

As a result, attractive improvements through a mix of measures (lower orbital height, constellation design (pendulum), satellite design, or/and SST instrument performance by a laser interferometer) seem to be feasible with a launch date around 2014/15 mostly based on German technology. The presentation will focus on the main targets and results of the studies and on programmatic issues, to realize a short-term GRACE gap filling mission to be launched already in 2013/14. For this, a coordination meeting with US GRACE team members has already taken place at GFZ just recently.

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Session: A.2 - GRACE-Follow On
Title: Technology Development for a Follow-on Gravity Mission to GRACE
First Author: Michelle Stephens
Presenter: Michelle Stephens
Co-Authors: P. Kaptchen; J. Leitch;R. Pierce; W.M. Folkner; W.M. Klipstein; D. Shaddock; R. Spero; R. Thompson; N. Yu; M. Watkins

Abstract: The success of the GRACE mission has led to great interest in a follow-on mission. Using interferometric satellite-to-satellite ranging to improve spatial resolution through more accurate ranging performance in a follow-on mission has been proposed. An interferometric ranging system could be included as a technology demonstration on a follow-on mission that includes the microwave ranging capability currently on GRACE, or it could be part of an independent mission that uses only interferometric ranging in conjunction with a drag-free spacecraft. Regardless of the implementation of the interferometric ranging, several common components are required. These include 1) a stable optical transmit/receive system, 2) a laser locking system, 3) a phasemeter system, 4) a continuous wave laser, and 5) a laser frequency stabilization system. The first component has been built and tested to NASA Technology Readiness Level (TRL) 6 and the next two to TRL 3 as part of technology d' NASA’s Earth Science and Technology Office Instrument Incubator Program. Lasers that meet the requirements for a GRACE follow-on mission have been built and flown in space and are considered to be at the highest TRL, i.e. TRL 9. The technological component still needing development for a successful interferometric ranging mission is a flight-qualified laser frequency stabilization system. We report on an effort, again supported by NASA’s Earth Science and Technology Office Instrument Incubator Program, to demonstrate a laser frequency stabilization system at TRL 6 and to increase the TRL of the phasemeter and laser locking electronics. Once this effort is successfully completed, all of the components necessary to provide an interferometric ranging system with improved performance for GRACE follow-on will be proven for flight.

The overall performance of an interferometric ranging system for a GRACE follow-on mission will be limited at low frequencies by the accelerometer noise and at higher frequencies by the laser frequency noise. Such a system on-orbit with a >200 km interferometer arm requires highly frequency stabilized lasers, with fractional frequency noise ideally < 10-14; however, even 10-13 stability would still give a substantial improvement in measurement accuracy. Frequency stabilization to this level has been demonstrated in the laboratory but requires careful design for flight. For example, the optical system and optical alignment must survive launch vibrations and flight thermal environments, and the electronics and electro-optic modulators must also survive the harsher radiation environment. We have developed a design for a frequency-stabilization system that uses a Pound-Drever-Hall lock to an ultra-stable optical cavity that meets flight requirements and will provide a fraction frequency stability of better than 10-13. We describe the design and show our status in the construction and test of this design.

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Session: A.2 - GRACE-Follow On
Title: Recovering hydrology and ice mass loss using a polar pair coupled with a moderately inclined pair of satellites
First Author: David Wiese
Presenter: David Wiese
Co-Authors: R. S. Nerem, P. N. A. M. Visser

Abstract: Should a follow-on mission to GRACE be flown drag-free and with a laser interferometer as the primary instrument rather than a microwave ranging system, while maintaining the current two-satellite collinear formation, it has been shown that temporal aliasing errors from atmospheric and ocean models are the dominant error source in recovering hydrological and ice mass loss signals. Thus, this study focuses on reducing temporal aliasing errors by increasing the sampling frequency of the mission using multiple pairs of satellites. Several architectures are considered, each consisting of two collinear pairs of GRACE satellites with an altitude of approximately 300 km: one in a 5-day repeating polar orbit, and the other with an inclination of 65 deg in a 10-day, 15-day, or 23-day repeating groundtrack. Global spherical harmonic solutions are made out to degree and order 60 for the respective duration of the repeating groundtrack (10-day, 15-day, or 23-day). Furthermore, an alternate processing methodology is explored which estimates daily low degree and order gravity fields in an effort to reduce the temporal aliasing errors from atmospheric and ocean models. These low degree and order gravity fields are used to correct the final multi-day solution. Results from this methodology show improvement at higher degrees and a reduction in the aliasing-induced longitudinal striations seen in the gravity solutions. The multiple satellite pairs show a substantial reduction in the level of error in recovering a hydrological signal when compared with a single polar pair of collinear satellites. Specifically, the 15-day repeating groundtrack decreases the spatial RMS of the error in recovering hydrology from 8.2 cm to 1.9 cm. Recovering ice mass loss from Greenland and Antarctica reduces the spatial RMS of the error from 7.0 to 4.9 cm. Furthermore, the longitudinal striping and striations are reduced due to the added east-west sensitivity in the observable combined with estimating daily low degree and order gravity fields.

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Session: A.2 - GRACE-Follow On
Title: A Short Wavelength Argument for a Future Moderate Inclination Mission to Supplement GRACE-2
First Author: Peter L. Bender
Presenter: Peter L. Bender
Co-Authors: David N. Wiese

Abstract: A GRACE-2 mission hopefully will be flown soon after the expected end of the GRACE-1 mission lifetime, and in a polar orbit. However, there appear to be important scientific benefits to be obtained if a second pair of GRACE-type satellites can be flown soon afterward in a moderate inclination orbit (I about 50 to 65 deg). As an example, it is assumed here that the 2nd pair would be drag-free, with 360 km altitude, I = 55 deg, 100 km separation, and a 12.8 day repeat ground track.

For this example, the crossing angles for the SW-NE and NW-SE arcs will be fairly large over most of the globe. Thus the short wavelength variations in the satellite separation (< about 1000 km) over a particular location of interest during 12.8 days can be combined efficiently to give the E-W and N-S variations in the geopotential at satellite altitude. It appears that this approach would prevent atmospheric mass distribution uncertainties at longer wavelengths from having a strong effect on the results. However, simulations to check on this and on ways to combine the short wavelength results with those from other approaches at longer wavelengths certainly are needed.

An additional benefit of a moderate inclination pair of satellites is that the band of latitudes within a few deg of the maximum latitude will be covered quite thoroughly each day. Thus one day solutions for the geopotential variations at altitude in this limited region potentially can be solved for.

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Session: A.2 - GRACE-Follow On
Title: A roadmap for future gravity satellite missions
First Author: Hans-Peter Plag
Presenter: Hans-Peter Plag
Co-Authors: Roland Pail, Michael Watkins, Roger Haagmans

Abstract: During the Workshop "Towards a Roadmap for Future Satellite Gravity Missions", which was held on September 30 - October 2, 2009 at the Graz University of Technology, Graz, Austria, 55 experts from twelve countries developed a roadmap for future gravity satellite missions. In addition, the participants agreed on a declaration, which has the aim to bring the roadmap to the attention of the Plenary meeting of the Group on Earth Observations (GEO), which will take place on November 17-18, 2009, in Washington, D.C. Taking into account the nature of GEO as an intergovernmental organization focusing on decision support through Earth observations, the declaration emphasizes the important role gravity missions can play in informing decision makers about mass redistribution particularly in the water cycle, and asks the GEO Plenary to take action in order to facilitate the implementation of the roadmap. The roadmap formulates a strategic target (see below) and identifies four main activities that would lead from the current situation to reaching this target by 2020. These activities concern (1) Science developments; (2) Technology developments; (3) Mission implementation; and (4) Processing, modeling and applications. The roadmap recognizes the importance of a long sequence of satellite gravity missions, and emphasizes that a follow-on mission for GRACE needs to be decided on, planned and implemented in a timely manner to avoid an undesirable gap in the record of Earth gravity changes. Furthermore, current and emerging requirements for science and applications require considerable science and technology developments to build improved satellite gravity missions that would push the accuracy, spatial and temporal resolution, and latency of the results to new levels. Efficient use of resources and timely progress necessitate coordinated and focused research and technological developments (including processing, modeling, applications as well as principles, sensors, mission design). Support and coordination across national borders is needed, and coordination of missions, including the development of multi-mission plans, is essential. GEO may provide the framework to facilitate an effort in which both geo-scientists and state-of-the-art technology developers from research institutes and industry could work closely together towards the implementation of the roadmap.

Strategic Target: A multi-decade, continuous series of space based observations of changes in the Earth's gravity field begun with the GRACE mission, and leading, before 2020, to satellite systems capable of global determination of changes in the Earth's gravity field from global down to regional spatial scales and on time scales of two weeks or shorter as a contribution to an integrated sustained operational observing system for mass redistribution, global change and natural hazards, and to support global water management, the understanding of climate variations, and the characterization and early detection of natural hazards.

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