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SteveM
Thought it would be interesting to look at what's turning up by monitoring the Earth using gravity rather than electromagnetic sensors.

Here are two examples related to global warming:

NASA's Grace Finds Greenland Melting Faster.... (Dec 20 2005)

NASA Mission Detects Significant Antarctic Ice Mass Loss (March 02, 2006)
ljk4-1
General Relativity and Quantum Cosmology, abstract
gr-qc/0605144

From: Lorenzo Iorio [view email]

Date: Tue, 30 May 2006 15:01:45 GMT (16kb)

On the possibility of measuring the PPN parameters beta and gamma in the Earth's gravitational field to a 10^-5-10^-6 accuracy

Authors: Lorenzo Iorio

Comments: LaTex2e, 18 pages, no figures, no tables

The current bounds on the PPN parameters gamma and beta are of the order of 10^-4-10^-5. Various missions aimed at improving such limits by several orders of magnitude have more or less recently been proposed like LATOR, ASTROD, BepiColombo and GAIA. They involve the use of various spacecraft, to be launched along interplanetary trajectories, for measuring the effects of the solar gravity on the propagation of electromagnetic waves. In this paper we explore the possibility of measuring the combination nu=(2+2gamma-beta)/3 of the post-Newtonian gravitoelectric Einstein perigee precession of a test particle to an accuracy of 10^-5-10^-6 with a pair of drag-free spacecraft in the Earth's gravitational field. It turns out that the latest gravity models from the dedicated CHAMP and GRACE missions would allow to reduce the systematic error of gravitational origin just to this demanding level of accuracy. In regard to the non-gravitational errors, the spectral noise density of the drag-free sensors required to reach such level of accuracy would amounts to 10^-8-10^-9 cm s^-2 Hz^-1/2 over very low frequencies. Although not yet obtainable with the present technologies, such level of compensation is much less demanding than those required for, e.g., LISA. As a by-product, an independent measurement of the post-Newtonian gravitomagnetic Lense-Thirring effect with a 0.9% accuracy would be possible as well. The forthcoming Earth gravity models from CHAMP and GRACE will further reduce the systematic gravitational errors in both of such tests.

http://arxiv.org/abs/gr-qc/0605144
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