The Modern Geodesy

The primary mission of Modern Geodesy is the definition and maintenance of precise geometric and gravimetric reference frames and models, and the provision of high accuracy positioning techniques for users in order to connect to these frames.

The International Association of Geodesy (IAG) has established services for all the major satellite geodesy techniques:

International Global Navigation Satellite System (GNSS) Service (IGS);

International Laser Ranging Service (ILRS);

International Very Long Baseline Interferometry Service (IVS);

International Doppler Orbitography and Radio positioning Integrated by Satellite (DORIS) Service (IDS);

International Gravity Field Service (IGFS); and others.

These services generate a wide range of products, including precise satellite orbits, ground station coordinates, Earth rotation and orientation values, gravity field quantities and atmospheric parameters, all of which are vital to the definition of the terrestrial and celestial reference systems. These reference systems are the foundation for all operational geodetic applications associated with mapping and charting, navigation, spatial data acquisition and management, as well as support for the geosciences.

The International Celestial Reference System (ICRS) forms the basis for describing celestial coordinates, and the International Terrestrial Reference System (ITRS) is the foundation for the definition of terrestrial coordinates to the highest possible accuracy. The definitions of these systems include the orientation and origin of their axes, scale, physical constants and models used in their realization, e.g., the size, shape and orientation of the reference ellipsoid that approximates the geoid and the Earth’s gravity field model. The coordinate transformation between the ICRS and ITRS is described by a sequence of rotations that account for variations in the orientation of the Earth’s rotation axis and its rotational speed.

Global Geodetic Observing System
In order to address the ever increasing performance requirements for global change monitoring, the IAG established in 2007 the Global Geodetic Observing System (GGOS).
GGOS’s goal is to coordinate all of the geodetic services and provide high-level products through a single portal. It will also fuel the next revolution in modern geodesy – the unified analysis of all geodetic data through common models – so as to drive an order of magnitude improvement in geodetic accuracy (reference frame stability, quality of geodetic products and models, etc.). In other words, GGOS will promote the development of tools and observing systems to ensure the ITRF can be defined and monitored to millimetre accuracy, with stability at the mm/yr level.

The mission of GGOS is:
(1) to provide the observations needed to monitor, map and understand changes in the Earth’s shape, rotation and mass distribution;

(2) to improve the quality of the global reference frames so that they may provide the fundamental backbone for measuring and interpreting key global change processes; and

(3) to support a variety of applications in geoscience and society for precise positioning, gravity field mapping and modeling.

The resultant improvement in reference frame accuracy and stability will not only benefit critical scientific studies such as measuring sea level rise, but also will provide a stronger framework for precise (centimeter-level) GNSS-enabled positioning in national, regional or global datums.

At a practical level the integration of the outputs of all the IAG services implies a coordinated upgrade of the ground station infrastructure (the stations in the IGS, ILRS, IVS and IDS networks); an increase in the number of co-located stations of the different space geodesy techniques; and steady improvement and continuity of a number of critical geodetic satellite missions (altimetric, GNSS, gravity mapping, etc). Under the GGOS initiative increased investment in geodetic infrastructure such as GNSS permanent ref- erence stations is strongly encouraged.

Reference Frames in Practice Manual Commission 5 Working Group 5.2 Reference Frames
Editor: Graeme Blick
INTERNATIONAL FEDERATION OF SURVEYORS (FIG)