Geoscience Reference
In-Depth Information
7 Space methods
7.1
Introduction
The subject of this chapter is the use of satellite observations for determining
features of the gravity field and the figure of the earth. Only the barest
essentials can be presented within the scope of a chapter. The reader will
find more information in special textbooks such as Hofmann-Wellenhof et
al. (2001), Montenbruck and Gill (2001), and Seeber (2003).
Historical remarks
Immediately after the first launch of artifical satellites (Sputnik 1957, Ex-
plorer 1958), their use for geodetic purposes was initiated, and by now the
Global Positioning System (GPS) has become the most important method
for a fast and precise determination of geodetic positions (see Sect. 5.3).
Historically, the first observational methods were intended to determine the
spatial direction and the distance to the satellite. Most of these methods are
now obsolete, but some principles may be still useful.
Directions
They may be measured by photographing the satellite against the back-
ground of stars, or by means of radio waves transmitted from the satellite,
using the principle of interference. Photography can only achieve an accuracy
of about 0.2 arc seconds and is not used any more in its original sense. The
principle of the photographic method was as follows. On the photographic
plate, the image of the satellite is surrounded by images of stars. The direc-
tions to the surrounding stars are defined by their right ascensions
α
and
declinations
δ
, which are known from astronomy. Therefore, by interpolation
we find the right ascension and declination of the satellite representing the
desired direction. This technique is now obsolete.
Ranges
They are measured by radar or by laser. Radar is used for measuring ranges
to space probes orbiting in the solar system, which is important to space
sciences rather than to geodesy. Lunar Laser Ranging (LLR) and Satellite
Laser Ranging (SLR) are useful for determining the earth rotation param-
eters because of their high (subcentimeter) accuracy; however, their use is
restricted to a limited number of fundamental stations.
