There are four main techniques used in space geodesy :

  1. Very Long Baseline Interferometry (VLBI)
  2. Satellite Laser Ranging (SLR) and Lunar Laser Ranging (LLR)
  3. Global Navigation Satellite System (GNSS), including the U.S. Global Positioning System (GPS), the Russian GLObal NAvigation Satellite System (GLONASS), and future systems such as Galileo, Beidou/Compass, and QZSS
  4. Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS)


(VLBI, SLR, GNSS and DORIS respectively in the image)

The VLBI technique has been employed in geodesy for more than 40 years; it measures the time difference between the arrival at two Earth-based antennas of a radio wavefront emitted by a distant quasar. Using large numbers of time difference measurements from many quasars observed with a global network of antennas, VLBI determines the inertial reference frame defined by the quasars and simultaneously, the precise positions of the antennas. Because the time difference measurements are precise to a few picoseconds, VLBI determines the relative positions of the antennas to a few millimeters and the quasar positions to fractions of a milliarcsecond. Since the antennas are fixed to the Earth, their locations track the instantaneous orientation of the Earth in the inertial reference frame. Relative changes in the antenna locations from a series of measurements indicate tectonic plate motion, regional deformation, and local uplift or subsidence.

Comments are closed.