One year of time-variable CHAMP-only gravity field models using kinematic orbits

Sneeuw, Nico; Gerlach, Christian; Földváry, Lóránt; Gruber, Thomas; Peters, Thomas; Rummel, Reiner; Švehla, Dražen

doi:10.1007/3-540-27432-4_49

Cited by 6 publications

(5 citation statements)

References 5 publications

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“…This strategy has been used for gravity field recovery based on the energy integral (see Sneeuw et al 2005 or Han et al 2003), where as a consequence of treating the field as being static within the chosen interval, observation eqs and have been applied. This might seem acceptable at first sight, as in the case of a stationary field the line integral can theoretically be replaced by potential differences.…”

Section: Gravity Field Recoverymentioning

confidence: 99%

“…Han et al 2002, Gerlach et al 2003 or Howe & Tscherning 2003). The approach has been used for time‐variable gravity field recovery of CHAMP (Sneeuw et al 2005) and GRACE (Han 2004) and is currently used to determine the long‐wavelength static field of GOCE (Pail et al 2011). The method is based on the energy conservation law, which states that in a conservative field, the sum of potential energy and kinetic energy is constant.…”

Section: Introductionmentioning

confidence: 99%

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Time-variable gravity field from satellite constellations using the energy integral

Wang

Gerlach

Rummel

2012

Geophysical Journal International

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SUMMARY The magnetic field mission Swarm, expected to be launched in 2012, comprises a constellation of three satellites. As all of them are equipped with GPS receivers and accelerometers, they can be used for gravity field recovery. We study the capability of a Swarm‐like constellation for (time‐variable) gravity field recovery and compare it with a gravity field tandem mission of GRACE‐type. Due to the lower accuracy of the GPS measurements compared with GRACE low–low satellite‐to‐satellite tracking (SST), the accuracy of a Swarm derived gravity field cannot compete with the state‐of‐the‐art GRACE models. However, unlike the GRACE mission, Swarm allows for the derivation of GPS‐baselines between the satellites in directions other than purely along‐track. This makes Swarm an interesting case for studying the error structure of gravity field models derived from various constellation geometries. Therefore, one focus of this study is the general analysis of different baseline constellations independent of the observation accuracy, that is, we do not restrict ourselves to just the actual Swarm case of pure GPS‐baselines. To make the results comparable to GRACE, we explicitly study the error behaviour of the different Swarm baseline geometries assuming GRACE‐type K‐band ranging (KBR) links. This gives an indication of candidate constellations for future missions; at least in regards to the general error structure. To study the potential of Swarm for recovering time‐variable components of the gravity field, we have set up a 2‐yr simulation to recover annual and semi‐annual components of continental hydrology. We show that Swarm has the potential to recover the annual signal up to spherical harmonic degree 6. This is of interest should there be a gap between the end of the GRACE mission and the launch of a follow‐on mission. All our simulations make use of the energy integral method. This method is usually (implicitly) formulated for static potential fields. Therefore, it is necessary for our study to investigate the properties of this method when applied to the analysis of time‐variable fields.

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Section: Gravity Field Recoverymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Time-variable gravity field from satellite constellations using the energy integral

Wang

Gerlach

Rummel

2012

Geophysical Journal International

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“…The dedicated gravity mission GRACE (Gravity Recovery and Climate Experiment, Tapley et al, 2004) provides monthly gravity-field solutions throughout the mission's lifetime for the purpose of detecting temporally varying geophysical phenomena. To detect these signals precisely, we should characterize the errors which degrade the quality of the gravity-field solutions.…”

Section: Introductionmentioning

confidence: 99%

“…Although the main purpose of the CHAMP mission is the detection of a static gravity field, Sneeuw et al (2003) attempted to detect time-varying signals from monthly CHAMP solutions. They proved that the error level was too large to detect time-varying signals and stated that the ground track variation of the CHAMP satellite is one of the main contributors.…”

Section: Introductionmentioning

confidence: 99%

A simulation study of effects of GRACE orbit decay on the gravity field recovery

2005

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The effects of satellite ground track changes of GRACE on monthly gravity field recoveries are investigated. In the case of a gravity field recovery using a relatively short period of a month or so, the variation of ground tracks affects the precision of the gravity field solutions. It is a serious problem when the solutions are employed for detecting temporal gravity changes which are almost at their detection limits. In this study, the recoveries of four-weekly gravity fields are simulated and the relation between the recovery precision and the ground track is investigated. The result shows that the GRACE ground track of the year 2003 was in good condition for four-week gravity field recovery, but it will sometimes appear as worse cases as the orbit altitude decays. In those cases, the global standard deviations of geoid height errors will be about one order worse than the best case. From our simulation, ground tracks of around altitudes of 473, 448, 399, 350 and 337 km give insufficient spatial resolutions, even for gravity field recovery up to degree 30.

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“…Several publications showed that this causes a severe quality degradation of the monthly gravity field solutions, e.g. Sneeuw et al (2003Sneeuw et al ( , 2005; Weigelt et al (2009). As a consequence, the sampling at the equator and in the longitude direction is of primary concern in the following.…”

Section: Theoretical Aspectsmentioning

confidence: 99%

An improved sampling rule for mapping geopotential functions of a planet from a near polar orbit

Weigelt

Sneeuw

Schrama

et al. 2012

J Geod

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One of the limiting factors in the determination of gravity field solutions is the spatial sampling. Especially during phases, when the satellite repeats its own track after a short time, the spatial resolution will be limited. The Nyquist rule-of-thumb for mapping geopotential functions of a planet, also referred to as the Colombo-Nyquist rule-ofthumb, provides a limit for the maximum achievable degree of a spherical harmonic development for repeat orbits. We show in this paper that this rule is too conservative and solutions with better spatial resolutions are possible. A new rule is introduced which limits the maximum achievable order (not degree!) to be smaller than the number of revolutions if the difference between the number of revolutions and the number of nodal days is of odd parity and to be smaller than half the number of revolutions if the difference is of even parity. The dependence on the parity is reflected in the eigenvalue spectrum of the normal matrix and becomes especially important in the presence of noise. The rule is based on applying the Nyquist sampling theorem separately in North-South and East-West direction. This is only possible for satellites in highly inclined orbits like e.g. CHAMP and GRACE. Tables for these two satellite missions are also provided which indicate the passed and (in case of GRACE) expected repeat cycles and possible degradations in the quality of the gravity field solutions.

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One year of time-variable CHAMP-only gravity field models using kinematic orbits

Cited by 6 publications

References 5 publications

Time-variable gravity field from satellite constellations using the energy integral

Time-variable gravity field from satellite constellations using the energy integral

A simulation study of effects of GRACE orbit decay on the gravity field recovery

An improved sampling rule for mapping geopotential functions of a planet from a near polar orbit

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