Today various satellite techniques (GNSS, SLR, LLR, DORIS, Satellite Altimetry) are used for a variety of geodetic applications, for example navigation, determination of Earth rotation parameters or the realization 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.
GNSS (Global Navigation Satellite System)
The active Global Navigation Satellite Systems GPS, GLONASS, Galileo and BeiDou 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.
SLR (Satellite Laser Ranging) / LLR (Lunar Laser Ranging)
SLR (Satellite Laser Ranging) and LLR (Lunar Laser Ranging) measure 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 (Doppler Orbitography and Radio-positioning Integrated by Satellite)
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 and 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 between geoid and the reference ellipsoid (geoid undulation).
Gravity field missions
The gravity-field missions GRACE and GOCE are/were 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 450 (GRACE) and 250 (GOCE) km are determined out of GPS-observations. Because of inhomogenities of the Earth's crust and in the upper mantle the attraction of the Earth to the spacecraft-tandem of GRACE is varying. This can be monitored in disturbances of microwave range measurements between the two satellites, from which the gravity-field of the Earth can be reconstructed. A follow-on mission GRACE-FO is planned in late 2017. By contrast, GOCE carries highly sensitive gravity gradiometers consisting of three pairs of accelerometers which measure gravitational gradients along three orthogonal axes. The mission ended in November 2013, nevertheless the data is still used for research.