Geoscience Reference
In-Depth Information
knowledge on oceanographic, geodetic, and solid earth issues such as oceanic
heat flux, change of sea level, ocean currents, precise positioning, orbit de-
termination, and leveling.
The GRACE concept can be regarded as a one-dimensional gradiometer
with a very long baseline of 220 km (Seeber 2003: p. 480). In contrast to this
concept, GOCE uses very short baselines (50 cm) in three directions.
7.6.5
The GOCE mission
The main sources of this section are www.esa.int/export/esaLP/goce.html,
ESA (1999), Muller (2001), Drinkwater et al. (2003), and Pail (2003).
The gravity field and steady-state ocean circulation explorer (GOCE)
mission is a Core Mission of the ESA Living Planet Programme. The primary
objectives of the GOCE mission are to measure the earth's stationary gravity
field and to model the geoid with extremely high accuracy. More specifically:
•
to determine the gravity anomalies with an accuracy of 1 mgal,
•
to determine the geoid with an accuracy of 1-2 cm,
•
to achieve these results at a spatial resolution better than 100 km.
According to the above mission requirements, GOCE is intended for a
representation of the gravity potential by spherical harmonics complete at
least to degree and order 200 (corresponding to the spatial resolution of
100 km), but 250 is envisaged.
From the
geodetic point of view
, a global geoid of 1-2 cm accuracy and
a gravity field model accurate to 1 mgal at about 100 km spatial resolution
may be used - among many other important applications - for the following
purposes:
•
Control (or replacement) of traditional leveling by leveling with GPS.
In Sect. 4.6 we have learned the basic equation (4-72),
H
=
h
N
,
relating the orthometric height
H
(above the geoid), the ellipsoidal
height
h
(above the ellipsoid), and the geoidal undulation
N
.With
N
accurately known from GOCE and
h
measured by GPS (Sects. 5.5,
5.6.1), the orthometric height
H
is readily obtained.
−
•
Worldwide unification of height systems so as to refer to one height
datum which allows for comparison of different sea levels (e.g., in the
North Sea and in the Mediterranean) and sea-level changes (which
may be caused by melting continental ice sheets). Remember that the
geoid is defined as an equipotential surface which follows a hypothetical
ocean surface at rest (in the absence of tides and currents and other
smaller influences). Consequently, a precise geoid is crucial in deriving
accurate measurements of ocean currents and sea-level changes.
