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 allow for the determination of position, velocity and time within an Earth-fixed or quasi-inertial coordinate reference system. The realized accuracy in positioning can vary from the millimeter-level (post-processing, phase-observations, baseline approach) up to meters (real-time, just code-observations, single-point approach), depending on the application. Simultaneous measurements to at least four satellites are required.
The IGS (International GNSS Service), starting its service on 01.01.1994, is a voluntary federation of worldwide GNSS station providers, research institutes and federal mapping agencies. All GNSS-users can download observation data of the IGS station network and further products like precise satellite orbits, clock corrections, atmospheric delay information and Earth rotation parameters 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. Depending on the orbital height of target satellite, SLR observations are 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 about 50 globally distributed stations. The IDS (International DORIS Service) coordinates the DORIS activities and is responsible for data-storage and -analysis.
Satellite Altimetry allows observing the distance between the sea-surface and a satellite with a very high accuracy of about 1cm. As a result one can gain a very exact determination of the mean sea 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 were used to study the Earth’s gravity-field with a very high spatial and temporal resolution. The GRACE mission focused mainly on the temporal variation of the gravity field (monthly solutions), while GOCE focused on a static but very-high resolution of the gravity field (up to degree n= 250). The orbital height of these LEO- satellites was about 450 km for the GRACE satellites and about 250km for GOCE. The satellite trajectories were determined by means of GPS-observations at the cm-level. Because of density inhomogeneities within the Earth's crust and in the upper mantle the gravity attraction of the Earth on the spacecraft-tandem of GRACE is varying. This could be monitored in variations of microwave range measurements between the two satellites, from which the gravity-field of the Earth could be reconstructed. A follow-on mission GRACE-FO is planned to be launched in early 2018. By contrast, GOCE carried 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.