Gravity Research Missions reviewed in light of the indirect
ocean tide potential.
Introduction
E.J .0. Schrama
Faculty of Geodestic Engineering, section FMR,
Thijsseweg 11, 2629 JA, The Netherlands.
December 14, 1995
e-mail: schrama@geo.tudelft.nl
In the last 15 years a number of proposals have been made for the realization of a
dedicated gravity research mission, hereafter referred to as a GRM. We deliberately
avoid a lengthy summary of all the different concepts. It is sufficient to mention
that gravity gradiometry and tracking of the orbiter have been proposed for mapping
the detailed structures of the Earth's gravitational field. The current shortcomings
in global gravity models are still the main motivation for flying a gravity research
mission. Terrestrial data is too heterogeneous with still many areas not sufficiently
covered, cf. Rapp (1993). Airborne gravimetry, cf. (Brozena,1991), may help to map
poorly accessible areas but is still far from filling in the gaps in current terrestrial
gravity datasets. Satellite altimetry is only effective over the oceans and rather helpful for mapping the detailed structures of the marine gravity field. For the longer
wavelengths, ie. up to degree and order 10 or so, altimetry can distinguish between
the quasi stationary sea topography and the geoid, thereafter it is hardly possible to
separate both fields, cf. (Schrama,1989). The technique that comes closest to what
could be called a global gravity model is based on the concept of tracking spacecrafts
which fundamentally provides use with the long wavelength features of the field, cf.
(Tapley,1989). Satellite gravity models still show a heterogeneous error pattern which
is especially the case where terrestrial gravity data and direct satellite altimeter data
are included in the solution, cf. (Tsaoussi and Koblinski,1994).
In the design of a GRM gravity gradiometry is a key technique. The intent is to
measure some or all components of a tensor of gravity gradients. Several designs
have been suggested where in the simplest case a gradiometer consists of a number of
single axis accelerometers with their sensitive axis lined up. In the most favorable case
three axis accelerometers are placed on the corners of a cube enabling to measure all
components of the tensor. The quality of a gravity gradiometer is mostly determined
by the common mode rejection of the instrument. This means that the sensitive axes
need to be aligned to very high accuracy and that the accelerometers themselves need
to be calibrated, cf. (Paik,1986). Common mode rejection is crucial for avoiding nongravitational accelerations mapping into the gradiometer. Another quality criterion
is the attitude control/reconstruction of the gradiometer, cf. (Rummel,1986). A full
tensor gradiometer can help to estimate rotational gradients from gravity gradients.
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ocean tide potential.
Introduction
E.J .0. Schrama
Faculty of Geodestic Engineering, section FMR,
Thijsseweg 11, 2629 JA, The Netherlands.
December 14, 1995
e-mail: schrama@geo.tudelft.nl
In the last 15 years a number of proposals have been made for the realization of a
dedicated gravity research mission, hereafter referred to as a GRM. We deliberately
avoid a lengthy summary of all the different concepts. It is sufficient to mention
that gravity gradiometry and tracking of the orbiter have been proposed for mapping
the detailed structures of the Earth's gravitational field. The current shortcomings
in global gravity models are still the main motivation for flying a gravity research
mission. Terrestrial data is too heterogeneous with still many areas not sufficiently
covered, cf. Rapp (1993). Airborne gravimetry, cf. (Brozena,1991), may help to map
poorly accessible areas but is still far from filling in the gaps in current terrestrial
gravity datasets. Satellite altimetry is only effective over the oceans and rather helpful for mapping the detailed structures of the marine gravity field. For the longer
wavelengths, ie. up to degree and order 10 or so, altimetry can distinguish between
the quasi stationary sea topography and the geoid, thereafter it is hardly possible to
separate both fields, cf. (Schrama,1989). The technique that comes closest to what
could be called a global gravity model is based on the concept of tracking spacecrafts
which fundamentally provides use with the long wavelength features of the field, cf.
(Tapley,1989). Satellite gravity models still show a heterogeneous error pattern which
is especially the case where terrestrial gravity data and direct satellite altimeter data
are included in the solution, cf. (Tsaoussi and Koblinski,1994).
In the design of a GRM gravity gradiometry is a key technique. The intent is to
measure some or all components of a tensor of gravity gradients. Several designs
have been suggested where in the simplest case a gradiometer consists of a number of
single axis accelerometers with their sensitive axis lined up. In the most favorable case
three axis accelerometers are placed on the corners of a cube enabling to measure all
components of the tensor. The quality of a gravity gradiometer is mostly determined
by the common mode rejection of the instrument. This means that the sensitive axes
need to be aligned to very high accuracy and that the accelerometers themselves need
to be calibrated, cf. (Paik,1986). Common mode rejection is crucial for avoiding nongravitational accelerations mapping into the gradiometer. Another quality criterion
is the attitude control/reconstruction of the gradiometer, cf. (Rummel,1986). A full
tensor gradiometer can help to estimate rotational gradients from gravity gradients.
131