Geoscience Reference
In-Depth Information
(b)
Figure 5.4.
(
cont.
)
were launched - GEOSAT in 1985, ERS-1 in 1991, TOPEX/POSEISDON in
1992 and ERS-2 in 1995. This radar altimetry has defined the marine geoid to
within better than 10 cm. The gravimetrically defined geoid and the geoid defined
by radar altimetry agree well, indicating that the theories and assumptions are
correct.
The shape of the geoid can be determined from gravity observations, but, to
determine the geoid at a point, we effectively need worldwide gravity observa-
tions. Since it is difficult to measure gravity at sea due to accelerations of the ship,
prior to the satellite era we had a relatively poor knowledge of the geoid, par-
ticularly over the oceans. Within weeks of the launch of the first Earth-orbiting
satellite by the U.S.S.R. in 1957, geophysicists had begun to re-evaluate the
geoid.
Altimetry satellites, of course, measure the geoid only over the oceans; thus,
within months of the launch, we had much more detailed information on the
geoid over oceans than over land, a reversal of the previous situation. Satellites
now measure the geoid so accurately that we can detect the slow rebound of the
