The toolbox of Geodesy comprises different sensors and instruments on the Earth (land and oceans), in the air, and in space, which together compose one large, comprehensive “geodetic instrument” for monitoring the System Earth in a wide range of spatial and temporal scales. The various types of observation techniques used in Geodesy can be classified in five major levels:
These five levels are connected by many types of observations in a complex way to form the integrated geodetic observing system. Space techniques dominate global and regional observations, while terrestrial techniques are mainly used for interpolation in space and time, and at registering specific local features.
The major observation types at present are:
– Microwave observations of extragalactic objects (quasars) by VLBI
– Laser ranging to LEO satellites, GNSS satellites, and to the Moon
– Microwave observations of GNSS satellites from the ground and from LEO satellites
– Radar and optical observations of the Earth’s surface (land, ice, glaciers, ocean, etc.) from remote sensing satellites
– Distance measurements between satellites (K-band, laser interferometry, optical methods, etc.)
– Measurement of gravity acceleration and gradients with sensors on board LEO satellites
– Absolute and relative gravity measurements with sensors on or near the ground
Novel observation types in development and increasing application are:
– GNSS Reflectometry, GNSS Radio Occultation
– Quantum gravimetry in space and on ground
– Optical Atomic Clocks comparisons to measure gravity potential differences
The ground-based geodetic infrastructure consists of all terrestrial networks of geodetic stations contributing to the geodetic reference frames or to Earth system monitoring:
- The global network of GGOS ground sites, coordinated with the GGOS-BNO, includes:
- Core sites with all of the major observation space techniques: VLBI, SLR, GNSS, DORIS (where available);
- Co-location sites with two or more of the major observation space techniques, but less than the full core site complement;
- Newly added requirement for core and co-location sites are absolute and superconducting gravimetry; and
- A variety of additional sensors and instruments (e.g. meteorological sensors, water vapour radiometers, etc.).
- The co-location of the different space geodetic techniques allows not only the integration of individual technique-specific networks into a unique terrestrial reference frame, but also validation of the results. The combination allows us to exploit the strengths of each technique and mitigate some of the weaknesses of others.
- The integration of the individual technique specific networks is coordinated by the International Earth Rotation and Reference Systems Service (IERS) and the International Gravity Field Service (IGFS).GGOS Core Sites
- The global network of radio telescopes is used for the microwave observation of extragalactic objects (quasars) by VLBI. The operation of the VLBI global network is coordinated by the International VLBI Service for Geodesy and Astrometry.
- The global network of satellite and lunar laser ranging stations is coordinated by the International Laser Ranging Service.
- The global network of GNSS stations is coordinated by the International GNSS Service, providing precise GNSS satellite orbits, satellite clock synchronisation and reference products for the optimal application of GNSS positioning, navigation and time transfer.
- The global network of DORIS stations is operated by the of the International DORIS Service.
- The global network of absolute gravity stations recorded by the International Gravimetric Bureau (BGI) and the global network of superconducting gravimeters, tiltmeters, strainmeters and other geodynamic sensors are inventoried by the International Geodynamics and Earth Tide Service (IGETS).
- The global network of tide gauge records are managed by the Permanent Service for Mean Sea Level (PSMSL).
The global networks, in particular the GNSS and gravity station networks, are extended by regional densifications, which allow the accessibility to the reference frames at local levels. These networks are coordinated by the regional sub-commissions of the International Association of Geodesy for reference frames and gravity field modelling. The regional networks are further extended by national reference networks, including levelling networks, which are determined and maintained by the national bodies responsible for the geodetic infrastructure.
Very Long Baseline Interferometry (VLBI) observations to extragalactic radio sources (quasars: quasi-stellar radio sources, radio galaxies) and tracking of MEO and GEO satellites are fundamental to the maintenance and improvement of the geodetic reference frames and the determination of Earth orientation. The combination of VLBI and satellites tracking provides the connection between the celestial and terrestrial reference frames.
Satellites used for geodetic observations differ in design, equipment and orbital parameters according to the mission purpose. Satellites can be regarded as moving targets at high altitude and then used for positioning and navigation. Since the satellite orbits are sensitive to the gravitational field of the Earth, the satellites also serve as a sensor for gravitation.
Space geodetic techniques are essential for providing accurate and long-term stable global reference frames and to observe and understand Earth dynamics. Each space geodetic technique is sensitive to different Earth signals, conducts observations with different resolutions in space and time, and provides geodetic products with different latency and quality.
Satellites at high altitudes (MEO and GEO) are preferred for positioning, as they are less influenced by gravitational and air drag perturbations. Satellite-based geodetic techniques that play an essential role in the precise satellite orbit determination, the establishment and maintenance of global reference frames, the determination of Earth orientation and the modelling of the long-wave part of the Earth’s gravity field include Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR) and Doppler orbitography and radio positioning integrated by satellite (DORIS).
Geodetic LEO satellites are preferred for determining the Earth’s gravity field (as they are more sensitive to short-wavelength gravity components) and to map the Earth’s surface (topography, oceans, ice caps, lakes, rivers, and soil moisture). Satellites have the big advantage that they collect data homogeneously and consistently over large parts of the Earth’s surface. They also allow the collection of data that cannot be recorded at the Earth’s surface. Observations based on LEO satellites are essential to monitor sea level, ice sheets, water storage on land, atmospheric water content, high-resolution surface motion and variation in the Earth’s gravity field.
Space geodetic techniques for the determination of global reference frames and Earth orientation:
Space geodetic techniques based on LEO satellite missions:
- Satellite-based gravity field observations
- Satellite altimetry
- GNSS Radar Occultation
- Interferometric Synthetic Aperture Radar (InSAR)
Terrestrial geodetic techniques observes the Earth surface and its changes, sea level, gravity field and the height by sensors on or near to the Earth surface. Such terrestrial techniques are:
- Tide Gauge measurements to measure sea surface heights at the coast
- Absolute and relative gravity measurementson ground
- Quantum gravimetry on ground
- GNSS Reflektometry
- Optical Atomic Clocks comparisons to measure gravity potential differences
- Geometric angle- and distance-measurements with Tachymeter
- Geometric height measurements by Leveling