Faculty of Applied Engineering and Urban Planning

1. Faculty of Applied Engineering and Urban Planning Civil Engineering Department Introduction to Geodesy and Geomatics Position, Positioning Modes, and the Geodetic Models

2. Spatial reference systems and frames • The geometry and motion of objects in 3D Euclidean space are described in a reference coordinate system. • A reference coordinate system is a coordinate system with well-defined origin and orientation of the three orthogonal, coordinate axes. We shall refer to such a system as a Spatial Reference System (SRS).

3. Spatial reference systems and frames • A spatial reference system is a mathematical abstraction. • It is realized (or materialized) by means of a Spatial Reference Frame (SRF). • We may visualize an SRF as a catalogue of coordinates of specific, identifiable point objects, which implicitly materialize the coordinate axes of the SRS. • Object geometry can then be described by coordinates with respect to the SRF.

4. Spatial reference systems • Several spatial reference systems are used in the Earth sciences. • The most important one for the GIS community is the International Terrestrial Reference System (ITRS). The ITRS has itsorigin in the centre of mass of the Earth. The Z-axis points towards a mean Earth north pole. The X-axis is oriented towards a mean Greenwich meridian and is orthogonal to the Z-axis. The Y -axis completes the right-handed reference coordinate system

5. Spatial reference systems • The ITRS is realized through the International Terrestrial Reference Frame (ITRF), a catalogue of estimated coordinates (and velocities) at a particular epoch of several specific, identifiable points (or stations). • These stations are more or less homogeneously distributed over the Earth surface. They can be thought of as defining the vertices of a fundamental polyhedron , a geometric abstraction of the Earth’s shape at the fundamental epoch.

6. Spatial reference systems • Maintenance of the spatial reference frame means relating the rotated, translated and deformed polyhedron at a later epoch to the fundamental polyhedron. • Frame maintenance is necessary because of geophysical processes (mainly tectonic plate motion) that deform the Earth’s crust at measurable global, regional and local scales. • The ITRF is ideally suited to describe the geometry and behaviour of moving and stationary objects on and near the surface of the Earth.

7. Spatial reference systems and frames • Global, geocentric spatial reference systems, such as the ITRS, became available only recently with advances in extra-terrestrial positioning techniques. • The centre of mass of the Earth is directly related to the size and shape of satellite orbits (in the case of an idealized spherical Earth it is one of the focal points of the elliptical orbits), observing a satellite (natural or artificial) can pinpoint the centre of mass of the Earth, and hence the origin of the ITRS.

8. Modern Implementation of the ITRF in a region • It means: • First, a regional densification of the ITRF polyhedron through additional vertices to ensure that there are a few coordinated reference points in the region under consideration. • Secondly, the installation at these coordinated points of permanently operating satellite positioning equipment and communication links.

9. Modern Implementation of the ITRF in a region The ITRF continuously evolves as new stations are added to the fundamental polyhedron. As a result, we have different realisations of the same ITRS, hence different ITRFs. A specific ITRF is therefore codified by a year code. One example is the ITRF96. ITRF96 is a list of geocentric coordinates (X, Y and Z in metres) and velocities (δX/δt, δY/δt and δZ/δt in metres per year) for all stations, together with error estimates. The station coordinates relate to the epoch 1996.0. To obtain the coordinates of a station at any other time (e.g., for epoch 2000.0) the station velocity has to be applied appropriately.

10. International Earth Rotation and Reference Systems Service (IERS) Established in 1987 to realize/maintain/provide: • The International Celestial Reference System (ICRS) • The International Terrestrial Reference System (ITRS) • Earth Orientation Parameters (EOP) • Geophysical data to interpret time/space variations in the ICRF, ITRF & EOP • Standards, constants and models (i.e., conventions) http://www.iers.org/

11. International Terrestrial Reference System (ITRS) Adopted by IUGG in 1991 for all Earth Science Applications • Realized and maintained by ITRS Product Center of the IERS • Its Realization is called International Terrestrial Reference Frame (ITRF) • Set of station positions and velocities, estimated by combination of VLBI, SLR, GPS and DORIS More than 800 stations located on more than 500 sites Available: ITRF88, 89,…,97, 2000 Latest: ITRF2005 http://itrf.ensg.ign.fr/GIS/index.php

12. DORISSateDopplerOrbitography and Radio-positioning Integrated by Satallite Principle • A so called beacon is installed on the ground and emits a radio signal, which is received by the satellite. A frequency shift of the signal occurs that is caused by the movement of the satellite (Doppler effect). From this observation satellite orbits, ground positions, as well as other parameters can be derived. Organization • DORIS is a French system which was initiated and is maintained by the French Space Agency (CNES). It is operated from Toulouse, where a master beacon ensures the communication with the satellites Ground network • There are about 50-60 stations equally distributed over the earth and ensure a good coverage for orbit determination. For the installation of a beacon only electricity is required because the station only emits a signal but does not receive any information. Therefore it is possible to install beacons in remote areas such as the Mount Everest base camp. Satellites • The best known satellites equipped with DORIS are the two altimetry satellites TOPEX/Poseidon and Jason. They are used to observe the ocean surface as well as currents or wave heights. DORIS contributes to their orbit accuracy of about 2 cm. Other DORIS satellites are the ERS, Envisat and SPOT satellites. Positioning • Apart from orbit determination the DORIS oberservations are used for positioning of ground stations. The accuracy is a bit lower than with GPS, but it still contributes to the International Terrestrial Reference Frame (ITRF).

13. Doppler effect The Doppler effect (or Doppler shift), named after Austrian physicist Christian Doppler who proposed it in 1842, is the change in frequency of a wave for an observer moving relative to the source of the wave

14. VLBI Very Long Baseline Interferometry, or VLBI, is a geodetic technique that determines the positions of observing stations (sites) on the Earth by measuring the time it takes a radio wavefront from a quasar (source) to reach pairs of sites. For each pair, as the wavefront travels from the source at a constant rate in all directions, the front will arrive at each site at a different time, yielding differences in position, or distance, between the sites. As these distances are measured over time for a network of sites, site positions and position changes over time can be measured. Other factors must be applied to model complications in observing -- for example, changes in the atmosphere that slow the wavefront down.

15. The SLR Technique Satellite Laser Ranging program began in March 1973 with the signing of a NASA-Division of National Mapping agreement under the USA-Australia Hornig Treaty for cooperation in Science The primary function of Satellite Laser Ranging (SLR) is the measurement of precise distances between laser telescope and reflectors on passing satellites. This is done by firing extremely short pulses of laser light at the reflectors, and measuring the time it takes for the light to return to Earth. Because the speed of light is known, the distance travelled by the light in that time can be calculated.

16. GPS The Global Positioning System (GPS) is a U.S. space-based radionavigation system that provides reliable positioning, navigation, and timing services to civilian users on a continuous worldwide basis -- freely available to all. For anyone with a GPS receiver, the system will provide location and time. GPS provides accurate location and time information for an unlimited number of people in all weather, day and night, anywhere in the world. • Structure: • The GPS is made up of three parts: • satellites orbiting the Earth; • control and monitoring stations on Earth; • GPS receivers owned by users. • GPS satellites broadcast signals from space that are picked up and identified by GPS receivers. Each GPS receiver then provides three-dimensional location (latitude, longitude, and altitude) plus the time

17. GPS • The U.S. Air Force develops, maintains, and operates the space and control segments • The space segment is composed of 24 to 32 satellites in medium Earth orbit and also includes the boosters required to launch them into orbit. • The control segment is composed of a master control station, an alternate master control station, and a host of dedicated and shared ground antennas and monitor stations. • The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial, and scientific users of the Standard Positioning Service

18. Current Space Geodesy Networks (1999.0 onward)

19. Current Co-locations (1999 onward) (8) (59) (16) (2)

20. Future ITRF solutions Based on Time Series of Station Positions : • Daily (VLBI) • Weekly (GPS, SLR & DORIS) and Earth Orientation Parameters: Polar Motion (xp, yp) Universal Time (UT1) (Only from VLBI) Length of Day

21. ITRF2005 Co-locations

22. ITRF2005 results • Polar Motion • Origin (Geocenter) and Scale time variations • Geophysical results • Plate motions • Post Glacial Rebound • Geocenter Motion • Surface Loading ==> Seasonal variation

23. Polar Motion Y(mas) X (mas)

24. Polar motion differences with IERS C04

25. ITRF2005 and Plate motion:Horizontal Site velocitiesσ<3mm/yr

26. Vertical Velocities

27. World Geodetic System 84 (WGS 84) • Collection of models including Earth Gravity model, geoid, transformation formulae and set of coordinates of permanent GPS monitor stations • WGS 60…66…72…84 • Originally based on TRANSIT satellite DOPPLER data

28. WGS 84 • Recent WGS 84 realizations based on GPS data: - 1994 - 1996 • TRS of GPS Broadcast Ephemerides • Coincides with any ITRF at 10 cm level • For most applications “WGS 84 = ITRF”, but ITRF is better realized (a few mm).