International Terrestrial Reference Frame (ITRF)
A realisation of the International Terrestrial Reference System (ITRS) - a world spatial reference system with its origin at the centre of the Earth. Numbers following the designation ITRF specify the last year from which data was used in the datum definition e.g. ITRF 97. A GNSS or other radio-navigation system may be referenced to a specific ITRF designation so that it can be precisely related to the physical world.
Source: Nick Ward, Vicechair, IALA e-Nav Committee, March 2009
acceleration due to gravity (m/s2)
Global Navigation Satellite System
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM