Geoscience Reference
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calculation. The most important impact generally comes from the Earth's gravity
field. Since each satellite is only particularly sensitive to one certain subset of
spherical harmonic coefficients, it is an effective method to develop a particular
gravity model for the observed quantity of a particular satellite. For instance, use of
the gravity model GEM10 in the altimetry satellite GEOS-3 improves the accuracy
of the satellite's orbit from 10 m to 1-2 m. The tracking system is the second
important factor to affect orbit accuracy. To acquire a high-precision orbit, orbit
determination using on-board GPS is required, and the accuracy of the coordinates
has reached several centimeters. Even so, the remaining orbit error is still much
greater than the altimeter's accuracy. Consequently, we have to improve the orbit
determination models, adopt some non-dynamic methods, etc.
The effects on the signal path can be categorized into instrumental errors and
propagation errors. The major instrument effects include the distance between the
phase center of the radar antenna and the satellite's center of mass, the propagation
delay in the electronic circuit of the altimeter, and the timing error in the measure-
ment system. In the manufacture of altimeters, these effects can be reduced to a
minimum and can be estimated. All the effects of instrumental errors should be
determined and kept under control when calibrating an altimeter in the test area
with measured accuracy. The signal propagation error caused by ionospheric
refraction is about 5-20 cm and depends on the ionization intensity, whose effects
can be corrected by dual frequency. The effect of tropospheric refraction is about
2.3 m. Since only the observed quantity in the vertical direction is adopted, the
effect can be well corrected by a proper atmospheric refraction model to reach an
accuracy of several centimeters. Propagation errors also include the impact of
actual sea conditions on the reflected signals.
The discrepancy between the instantaneous sea surface and the geoid can be
divided into a time-invariant part H and a time-dependent part
H. Before deter-
mining the mean sea level using the altimetry observed quantity, the time-
dependent component should be corrected. The wave-induced sea level change,
which has been smoothed out in the altimeter's observation, can be negligible.
Therefore, the correction contributor to be considered is mainly the sea level change
induced by tides and changing atmospheric pressure fields.
The resolution and accuracy of the sea surface height measured directly from
satellite altimetry can reach 5 km and 5 cm, respectively. However, under the effect
of the sea surface topography, the tides, and errors in the environmental correction
model, the accuracy of the sea geoid can rarely be better than
ʔ
10 cm.
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