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GGOS as an observing system has five major levels of instrumentation and objects that actively perform observations, are passively observed, or both. These levels are:

  • Level 1: the terrestrial geodetic infrastructure;
  • Level 2: the LEO satellite missions;
  • Level 3: the GNSS and the Lageos-type SLR satellites;
  • Level 4: the planetary missions and geodetic infrastructure on Moon and planets;
  • Level 5: the extragalactic objects.

These five levels of instrumentation and objects, independent of whether they are active or passive, receivers or emitters or both, are connected by many types of observations in a rather complex way to form the integrated GGOS observing system. In this system, the major observation types at present are:

  1. observations of the microwaves emitted by GNSS satellites at the ground and at the LEO satellites;
  2. laser ranging to LEOs, dedicated laser ranging satellites, GNSS satellites and the Moon;
  3. microwave observation of extragalactical objects (quasars) by VLBI;
  4. instrumentation onboard the LEO satellites measuring accelerations, gravity gradients, satellite orientation, etc.;
  5. radar and optical observations of the Earth's surface (land, ice, glaciers, sea level, ect.) from remote sensing satellite;
  6. distance measurements between satellites (K-band, optical, interferometry, etc.).

In the future, new measurement techniques will evolve and be included into the system. Different parts of the overall system are cross-linked through observations and inter-dependent. All these techniques are affected by and measure the "output" of the same unique Earth system, that is, the various geodetic fingerprints induced by mass redistribution and changes in the system's dynamics. Therefore, consistency of data processing, modeling, and conventions across the techniques and across the "three pillars"is mandatory for maximum exploitation of the full potential of the system.


Contributing infrastructure Levels of infrastructure


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