Today various Satellite Techniques (GNSS, SLR, LLR, DORIS, Satellite Altimetry) are used for a variety of geodetic applications, for example navigaton, determination of Earth rotation parameters or the realisation of global coordinate systems. Moreover the combination of different space geodetic techniques can improve the accuracy of certain applications and determined parameters but also allow to identify systematic errors of the individual techniques. Read further for a short description of the most important Satellite Techniques!
The active GNSS-satellitesystems (GPS, GLONASS), as well as the planned GALILEO, serve for the determination of the position and the velocity of a user in a reference coordinate system. The realised accuracy in positioning can vary from the millimeter-level (post-processing, phase-observations,baseline approach) up to several meters (real-time,code-observations,single-point approach), dependending on the application. To gain useful observation data it is necessary to measure the signals of at least four satellites simultaneously.
The IGS (International GNSS Service), starting its service on 01.01.1994, is a voluntary federation of worldwide GNSS stations and agencies. All GNSS-users can download observations of the IGS station network and precise satellite orbits for free. The IGS is organized into different compenents.
SLR (Satellite Laser Ranging) and LLR (Lunar Laser Ranging) measures the distance between a reference station on the Earth's surface and the satellite, respectively the moon:
SLR-observations are, depending on the orbital height of target satellite, very important input data for the determination of the ITRF (International Terrestrial Reference Frame), for observing geocenter motion, for orbit control and for the determination of the Earth's gravity field. The ILRS (International Laser Ranging Service) coordinates globally the Laser Ranging activities.
DORIS uses, like GNSS, radio signals. With dual frequency transmitters and -receivers (400 MHz and 2 GHz) the velocities of the DORIS-satellites can be observed with respect to an continuously operated network of ca. 50 globally distributed stations. The IDS (International DORIS Service) coordinates the DORIS activities and is responsible for data-storage and -analysis.
Satellite Altimetry allows to observe the distance between the sea-surface an a satellite with a very high accuracy. As a result one can gain a very exact determination of the mean see level above the geoid (Sea Surface Topography), as well as the distance betwenn geoid and the reference ellipsoid (geoid undulation). The map below shows an example of the derived Sea Surface Topography:
The gravity-field missions CHAMP and GRACE (see picture) are used for the study of the gravity-field of the Earth in a very high resolution in space and time. The orbits of the satellites in a height of about 400 km are determined out of GPS- and SLR-observations. With accelerometers the effects of non-gravitational forces attracting the satellites can be eliminated. Because of inhomogenities of the Earth's crust and in the upper mantle of the Earth the attraction of the Earth to the satellites is varying. This can be monitored in disturbances of the satellites' orbits and the gravity-field of the Earth can be reconstructed. In 2007 another mission to observe the Earth's gravity-field, GOCE, is planned.