The Geodetic Infrastructure in Europe - today and tomorrow

Transcription

1 The Geodetic Infrastructure in Europe - today and tomorrow THE GEODETIC INFRASTRUCTURE BEHIND THE NEW CONCEPTS OF REFERENCE SYSTEMS João Agria Torres SPUIAGG IAG Sub-commission 1.3: Regional Reference Frames jatorres@iol.pt

2 CONTENT 1. Review of the INSPIRE IR on CRS 2. A brief look into the past 3. Space geodetic observing system 4. Geodetic reference systems and frames 5. ITRS/ITRF ETRS89/ETRF 6. The role of EUREF 7. Summary

3 1- Review of the INSPIRE IR on CRS INSPIRE INfrastructure for SPatial InfoRmation in Europe Directive 2007/2/EC of the European Parliament and of the Council of 14 March 2007 establishing INSPIRE was published in the official Journal on the 25th April 2007 The INSPIRE Directive entered into force on the 15th May 2007 (

4 1- Review of the INSPIRE IR on CRS Community policy on the environment must aim at a high level of protection taking into account the diversity of situations in the various regions of the Community. Moreover, information, including spatial information, is needed for the formulation and implementation of this policy and other Community policies, which must integrate environmental protection requirements in accordance with Article 6 of the Treaty. In order to bring about such integration, it is necessary to establish a measure of coordination between the users and providers of the information so that information and knowledge from different sectors can be combined. The Infrastructure for Spatial Information in the European Community (Inspire) should assist policy-making in relation to policies and activities that may have a direct or indirect impact on the environment.

5 1- Review of the INSPIRE IR on CRS the infrastructures for spatial information in the Member States should be designed to ensure that spatial data are stored, made available and maintained at the most appropriate level it is possible to combine spatial data from different sources across the Community in a consistent way and share them between several users and applications it is possible for spatial data collected at one level of public authority to be shared between all the different levels of public authorities spatial data are made available under conditions that do not restrict their extensive use it is easy to discover available spatial data, to evaluate their fitness for purpose and to know the conditions applicable to their use

6 1- Review of the INSPIRE IR on CRS SPATIAL DATA THEMES The INSPIRE Directive addresses 34 spatial data themes needed for environmental applications. These themes are subdivided in the three annexes of the directive. D2.3_Definition_of_Annex_Themes_and_scope_v3.0.pdf

7 1- Review of the INSPIRE IR on CRS Annex I 1. Coordinate reference systems 2. Geographical grid systems 3. Geographical names 4. Administrative units 5. Addresses 6. Cadastral parcels 7. Transport networks 8. Hydrography 9. Protected sites Annex II 1. Elevation 2. Land cover 3. Orthoimagery 4. Geology Annex III 1. Statistical units 2. Buildings 3. Soil 4. Land use 5. Human health and safety 6. Utility and Government services 7. Environmental monitoring facilities 8. Production and industrial facilities 9. Agricultural and aquaculture facilities 10. Population distribution demography 11. Area management / restriction / regulation zones & reporting units 12. Natural risk zones 13. Atmospheric conditions 14. Meteorological geographical features 15. Oceanographic geographical features 16. Sea regions 17. Bio-geographical regions 18. Habitats and biotopes 19. Species distribution 20. Energy resources 21. Mineral resources

8 1- Review of the INSPIRE IR on CRS ANNEX I SPATIAL DATA THEMES ON CRS AND CGS 1. Coordinate reference systems Systems for uniquely referencing spatial information in space as a set of coordinates (x, y, z) and/or latitude and longitude and height, based on a geodetic horizontal and vertical datum. 2. Geographical grid systems Harmonised multi-resolution grid with a common point of origin and standardised location and size of grid cells.

9 1- Review of the INSPIRE IR on CRS 14 December 2009 Approval of the IR by INSPIRE Committee Draft COMMISSION REGULATION implementing Directive 2007/2/EC of the European Parliament and of the Council as regards interoperability of spatial data sets and services D en.doc (pp 18-20)

10 1- Review of the INSPIRE IR on CRS EN ISO19111 Coordinate Reference System Datum Coordinate System Geodetic Vertical Engineering

11 1- Review of the INSPIRE IR on CRS EN ISO19111 Compound Coordinate Reference System Reference System 1 Reference System 2 Datum 1 Coordinate System 1 Datum 2 Coordinate System 2 The horizontal and vertical components to describe a position come from different reference systems (it s the case of heights related to the mean sea level)

12 1- Review of the INSPIRE IR on CRS Coordinate Reference Systems Concept GEO-SPATIAL DATA SETS COORDINATES 3D/2D CRS 1D CRS ATTRIBUTES / PARAMETERS CARTOGRAPHIC PROJECTION ELLIPSOID X,Y,Z φ,λ,h φ,λ H x,y H 3D 2D + 1D ITRS WGS84 ETRS89 National Global None EVRS National pressure depth time

13 INSPIRE Specification on Coordinate Reference Systems

14 1- Review of the INSPIRE IR on CRS Datum for three-dimensional and two-dimensional coordinate reference systems For the three-dimensional and two-dimensional coordinate reference systems and the horizontal component of compound coordinate reference systems used for making spatial data sets available, the datum shall be the datum of the European Terrestrial Reference System 1989 (ETRS89) in areas within its geographical scope, or the datum of the International Terrestrial Reference System (ITRS) or other geodetic coordinate reference systems compliant with ITRS in areas that are outside the geographical scope of ETRS89. Compliant with the ITRS means that the system definition is based on the definition of the ITRS and there is a well documented relationship between both systems, according to EN ISO

15 INSPIRE Specification on Coordinate Reference Systems

16 1- Review of the INSPIRE IR on CRS Compound Coordinate Reference Systems For the horizontal component of the compound coordinate reference system, one of the coordinate reference systems specified in section shall be used. For the vertical component, one of the following coordinate reference systems shall be used: For the vertical component on land, the European Vertical Reference System (EVRS) shall be used to express gravity-related heights within its geographical scope. Other vertical reference systems related to the Earth gravity field shall be used to express gravity-related heights in areas that are outside the geographical scope of EVRS. For the vertical component in the free atmosphere, barometric pressure, converted to height using ISO 2533:1975 International Standard Atmosphere shall be used.

17 CONTENT 1. Review of the INSPIRE IR on CRS 2. A brief look into the past 3. Space geodetic observing system 4. Geodetic reference systems and frames 5. ITRS/ITRF ETRS89/ETRF 6. The role of EUREF 7. Summary

18 2- A brief look into the past ELLIPSOIDAL REFERENCE SYSTEMS 8Conceived to support the implementation of Geodetic Networks 8System of axes tri-orthogonal (direct) 8Origin coincident with the centre of the revolution ellipsoid 8Axes parallel to the ones of the Conventional Terrestrial System defined by the BIH (Bureau International de l Heure)

19 2- A brief look into the past 8(ϕ 1G, λ 1G ) fix one normal line 8(ξ 1, η 1 ) fix this normal wrt the Earth (vertical) 8(N 1 ) fixes the separation ellipsoid - geoid 8(Laplace equation) fixes the orientation DATUM PARAMETERS

20 2- A brief look into the past ED50

21 2- A brief look into the past NEWTON - CASSINI N? N O O S S

22 2- A brief look into the past GRAVITY a N = GM / R 2 N r a c a c = ω 2 r EQUADOR a N R a g O S GRS80 a g equator = ms -2 a g pole = ms ,5%

23 2- A brief look into the past THE GRAVITY FIELD AND THE EARTH S SHAPE NEWTON Philosophiæ naturalis principia mathematica rotational ellipsoid as figure of equilibrium in a homogeneous, fluid and rotating Earth flattening (shape parameter) f = 1 - a/b Proposition XVIII. Theorem XVI The axes of the planets are shorter than the diameters perpendicular to the axes f = 1/230

24 2- A brief look into the past MEASUREMENT OF ARCS OF MERIDIAN French Royal Academy of Sciences decides to solve the question through an experiment Maupertuis departs to Lapland and Bouguer arrives at Quito Maupertuis presents the results Bouguer presents the results Vous avez confirmé dans ces lieux pleins d ennui Ce que Newton connut sans sortir de chez lui Voltaire

25 2- A brief look into the past

26 Friedrich Georg Wilhelm Struve ( ) Struve Geodetic Arc main stations km - 10 countries

27 2- A brief look into the past 1861: General Baeyer presented a report aiming at the cooperation of the central Europe states for the measurement of the Earth s shape and dimensions (Mitteleuropäische Gradmessung) 1864: 1 st CREATION OF THE IAG International Geodetic Conference at Berlin 1867: 2 nd International Geodetic Conference (Europäische Gradmessung); Portugal and Spain join the organization 1885: end of General J.J. Baeyer s presidency 1887: creation of the International Association of Geodesy (Internationale Erdmessung) integrated by 20 states

28 CONTENT 1. Review of the INSPIRE IR on CRS 2. A brief look into the past 3. Space geodetic observing system 4. Geodetic reference systems and frames 5. ITRS/ITRF ETRS89/ETRF 6. The role of EUREF 7. Summary

29 3- Space geodetic observing system VLBI (Very Long Baseline Interferometry) (accuracy better than 1 cm/5000 km or 0,002 ppm)

30 3- Space geodetic observing system VLBI Global Network (

31 3- Space geodetic observing system ILRS Contributions study of the solid Earth - atmosphere hydrosphere - cryosphere fundamental support to altimetric satellites contribution to the monitoring of the sea and ice levels long-term dynamics of the solid Earth, oceans and atmosphere study of tectonic motions contribution for the research in fundamental Physics

32 3- Space geodetic observing system ILRS Global network (

33 3- Space geodetic observing system IGS International GNSS Service Created from a IUGG resolution in 1991 (Vienna) Activities started officially in 1994 SET OF GPS (+GLONASS) PERMANENT STATIONS (presently more than 380, some inactive)

34 3- Space geodetic observing system IGS Global network (

35 3- Space geodetic observing system DORIS Doppler Orbitography and Radiopositioning Integrated by Satellite Doppler System for satellite tracking precise orbit determination precise location on the terrain altimetric satellites Jason-1 and ENVISAT (TOPEX/POSEIDON) remote sensing satellites SPOT-2, SPOT-4 and SPOT-5 (SPOT-3)

36 3- Space geodetic observing system IDS - International DORIS Service (

37 3- Space geodetic observing system

38 3- Space geodetic observing system Geodetic techniques Earth rotation Reference systems Gravity field (CNES/NASA)

39 3- Space geodetic observing system THE IUGG Non-governmental scientific organization founded in 1919 Member of the ICSU (International Council for Science) Integrates 8 Associations IAG Geodesy IASPEI Seismology and Physics of the Earth s Interior IAVCEI Volcanology and Chemistry of the Earth s Interior IAGA Geomagnetism and Aeronomy IAMAS Meteorology and Atmospheric Sciences IAHS Hydrological Sciences IAPSO Physival Sciences of the Oceans IACS Cryospheric Sciences

40 3- Space geodetic observing system IAG STRUCTURE Approved in Budapeste, 2001 (IAG Scientific Assembly) Ratified in Sapporo, 2003 (IUGG General Assembly) Services Commissions Inter commission committees IAG Projects Communication and Outreach

41 3- Space geodetic observing system SERVICES IERS (International Earth Rotation and Reference Systems Service) IGS (International GNSS Service) ILRS (International Laser Ranging Service) IVS (International VLBI Service for Geodesy and Astrometry) IGFS (International Gravity Field Service) IDS (International DORIS Service) BGI (International Gravimetric Bureau) IGES (International Geoid Service ) ICET (International Centre for Earth Tides) PSMSL (Permanent Service for Mean Sea Level) BIPM (Bureau International des Poids et Measures - time section) IBS (IAG Bibliographic Service)

42 3- Space geodetic observing system COMMISSIONS Commission 1 Reference Frames Commission 2 Gravity Field Commission 3 Earth Rotation and Geodynamics Commission 4 Positioning and Applications INTER COMMISSION COMMITTEES Inter commission committee on Theory (ICCT) Inter commission committee on Geodetic Standards (ICCGS) Inter commission committee on Planetary Geodesy (ICCPG)

43 CONTENT 1. Review of the INSPIRE IR on CRS 2. A brief look into the past 3. Space geodetic observing system 4. Geodetic reference systems and frames 5. ITRS/ITRF ETRS89/ETRF 6. The role of EUREF 7. Summary

44 4- Geodetic reference systems and frames REFERENCE SYSTEMS: TERMINOLOGY Ideal Reference System theoretical definition (not accessible to the users) Reference Frame Set of physical objects with their coordinates Realization of an Ideal Reference System Accessible to the users Coordinate System cartesian (X,Y,Z), geographic (λ, φ, h),...

45 4- Geodetic reference systems and frames TRS - TERRESTRIAL REFERENCE SYSTEM Spatial reference system co-rotating with the Earth in its diurnal motion in space Mathematical model for a physical Earth in which point positions are expressed and have small temporal variations due to geophysical effects (Plate motion, Earth tides, etc.) Mathematical and physical definition Tridimensional system of axes defined by an origin, a unit of lenght and an orientation Associated physical constants: time, speed of light in vacuum, GM

46 4- Geodetic reference systems and frames TRF - TERRESTRIAL REFERENCE FRAME Realization of the reference system Set of geodetic references (physical points) Coordinates estimation based on space geodetic techniques Each technique and data analysis realizes its own TRS Multitude of TRF exist

47 CONTENT 1. Review of the INSPIRE IR on CRS 2. A brief look into the past 3. Space geodetic observing system 4. Geodetic reference systems and frames 5. ITRS/ITRF ETRS89/ETRF 6. The role of EUREF 7. Summary

48 5- ITRS/ITRF ETRS89/ETRF ITRS - INTERNATIONAL TERRESTRIAL REFERENCE SYSTEM ADOPTED IN 1991 (VIENNA) BY THE IUGG DEFINITION, REALIZATION AND PROMOTION BY THE IERS (successor of the BIH) (International Earth Rotation and Reference Systems Service)

49 5- ITRS/ITRF ETRS89/ETRF ITRS Origin at the Earth s centre of mass (considering the mass of the solid part, liquid part and atmosphere) Unit of lenght: meter (SI) consistent with TCG (Geocentric Coordinate Time) Orientation of axes consistent with the BIH definition at epoch Non-rotation condition wrt horizontal tectonic motions

50 5- ITRS/ITRF ETRS89/ETRF ITRF - INTERNATIONAL TERRESTRIAL REFERENCE FRAME Set of geodetic references Coordinates estimation based on space geodetic techniques VLBI (Very Long Baseline Interferometry) SLR (Satellite Laser Ranging) GPS (Global Positioning System) DORIS (Doppler Orbitography Radiopositioning Integrated by Satellite)

51 5- ITRS/ITRF ETRS89/ETRF ITRF AND SCIENCE REQUIREMENT Long-term stable ITRF: 0.1 mm/yr Stable: linear behaviour of the TRF parameters, i.e. with no discontinuity : Origin Components: 0.1 mm/yr Scale 0.01 ppb/yr (0.06 mm/yr) Current situation: probably not better than 1 mm/yr

52 5- ITRS/ITRF ETRS89/ETRF 579 locations (920 stations) ITRF CO-LOCATIONS (Zuheir Altamimi, 2010) 461 North 118 South 584 discontinuities

53 5- ITRS/ITRF ETRS89/ETRF ITRF DATUM SPECIFICATIONS (Zuheir Altamimi, 2010) Origin: SLR Scale : average of SLR & VLBI Orientation: aligned with ITRF2005 using 179 stations at 131 locations: 104 N, 27 S

54 5- ITRS/ITRF ETRS89/ETRF TRANSFORMATION PARAMETERS ITRF ITRF2005 T1 (mm) T2 (mm) T3 (mm) D (10-9 ) R1 (mas) R2 (mas) R3 (mas) Epoch Parameters / Rates /

55 5- ITRS/ITRF ETRS89/ETRF (Zuheir Altamimi, 2008)

56 5- ITRS/ITRF ETRS89/ETRF ITRF2005 ITRF2005 STATION POSITIONS AT EPOCH AND VELOCITIES GPS STATIONS DOMES NB. SITE NAME TECH. ID. X/Vx Y/Vy Z/Vz Sigmas m/m/y M006 GRASSE GPS GRAS M S001 CASCAIS GPS CASC S (

57 5- ITRS/ITRF ETRS89/ETRF (Zuheir Altamimi, 2010)

58 5- ITRS/ITRF ETRS89/ETRF ITRF2005 ITRF2005 STATION POSITIONS AT EPOCH AND VELOCITIES GPS STATIONS DOMES NB. SITE NAME TECH. ID. X/Vx Y/Vy Z/Vz Sigmas m/m/y M006 GRASSE GPS GRAS M S001 CASCAIS GPS CASC S ITRF2008 ITRF2005 STATION POSITIONS AT EPOCH AND VELOCITIES IGS STATIONS DOMES NB. SITE NAME TECH. ID. X/Vx Y/Vy Z/Vz Sigmas m/m/y M006 GRASSE GPS GRAS M S001 CASCAIS GPS CASC S (

59 5- ITRS/ITRF ETRS89/ETRF ITRF2008 Vertical Velocity Field (Zuheir Altamimi, 2010)

60 5- ITRS/ITRF ETRS89/ETRF TIME SERIES - ITRS

61 5- ITRS/ITRF ETRS89/ETRF TIME SERIES - ITRS

62 5- ITRS/ITRF ETRS89/ETRF MOTIVATION FOR CREATION OF ETRS89 ITRS coordinates : ~2,5 cm/y in Europe unusable for day-to-day geo-referencing activities European Terrestrial Reference System 1989 (ETRS89) coincident with ITRS at the epoch fixed to the stable part of the Eurasian Plate Coordinates expressed in ETRS89 have minimal time- dependency and are consequently useable for geo-referencing in Europe. Transformations : ITRFyy ETRFyy published in Memo (Boucher & Altamimi, 2008)

63 5- ITRS/ITRF ETRS89/ETRF ETRFyy REALIZATION OF ETRS89 ITRF2008 ETRF2008 IERS(ITRF) transformation ITRF2005 ITRF2000 ITRF97 ETRF2005 ETRF2000 ETRF97 ITRF93 Memo Boucher& Altamimi (2008) ETRF93

64 5- ITRS/ITRF ETRS89/ETRF TIME SERIES ETRS89

65 5- ITRS/ITRF ETRS89/ETRF TIME SERIES ETRS89

66 CONTENT 1. Review of the INSPIRE IR on CRS 2. A brief look into the past 3. Space geodetic observing system 4. Geodetic reference systems and frames 5. ITRS/ITRF ETRS89/ETRF 6. The role of EUREF 7. Summary

67 6- The role of EUREF COMMISSION 1 REFERENCE FRAMES SUBCOMMISSIONS SC1.1: Coordination of Space Techniques SC1.2: Global Reference Frames SC1.3: Regional Reference Frames SC1.4: Interaction of Celestial and Terrestrial Reference Frames INTER-COMMISSION STUDY GROUPS / WORKING GROUPS IC-SG 1: Theory, implementation and quality assessment of geodetic reference frames IC-SG 2: Precise orbit determination and reference frame definition IC-SG 3: Time series analysis IC-WG 1: Environment loading: modelling for reference frame and positioning applications

68 6- The role of EUREF SUBCOMMISSION 1.3 GENERAL PURPOSE SC1.3 Regional Reference Frames deals with the definitions and realizations of regional reference frames and their connection to the global International Terrestrial Reference Frame (ITRF) Moreover, it offers a home for service-like activities addressing theoretical and technical key common issues of interest to regional organisations

69 6- The role of EUREF Sub-commission 1.3 Regional Reference Frames J. Torres Sub-commission 1.3c North America (NAREF) M. Craymer R. Snay/J. Griffiths Sub-commission 1.3b South America (SIRGAS) C. Brunini L. Sánchez Working Group Regional Dense Velocity Fields C. Bruyninx Sub-commission 1.3a Europe (EUREF) J. Ihde Sub-commission 1.3d Africa (AFREF) R. Wonnacott Sub-commissions 1.3e Asia-Pacific (APREF) J. Dawson S. Matsuzaka Sub-commissions 1.3f Antarctica (SCAR) R. Dietrich

70 6- The role of EUREF ORGANIZATION 8 Chairman and Secretary (general coordination and management) 8 Technical Working Group (coordenation of the scientific and technical work) 8 Members from (almost) all European countries 8Universities and Research Institutions 8National Mapping Agencies (NMA) 8...

71 EUREF definition, realization and maintenance of the European Reference Systems cooperation with the pertinent IAG components and EuroGeographics use the most accurate and reliable terrestrial and space-borne techniques available develop the necessary scientific background and methodology

72 6- The role of EUREF EUROPEAN TERRESTRIAL REFERENCE SYSTEM 89 (ETRS89) Definition The IAG Subcommision for the European Reference Frame, following its Resolution 1 adopted in Firenze in 1990, recommends that the terrestrial reference system to be adopted by EUREF will be coincident with the ITRS at the epoch and fixed to the stable part of the Eurasian Plate. It will be named European Terrestrial Reference System 89 (ETRS89).

73 6- The role of EUREF ACCESS TO ETRS89 Classic method: 1) GNSS data + ITRS pos/vel. 2) Add densification sites 3) Perform network analysis 4) Express network in ITRFyy using selected set of ITRF reference stations 5) Convert ITRFyy to ETRF2000

74 6- The role of EUREF EUREF PERMANENT NETWORK - EPN 244 GNSS stations of EPN (European Permanent Network) operating by the end of 2010 (59% GLONASS) 49% of the stations broadcast real time data, with 3 broadcasters : BKG, ASI e ROB

75 6- The role of EUREF EPN CUMULATIVE SOLUTION 15-weekly updated EPN cumulative position/velocity solution - Expressed in ITRFyyyy - Using ITRFyyyy discontinuities + discontinuities for non-igs stations + discontinuties since last ITRFyy Up to date ITRFyy positions/velocities with validity epochs ( - Possibility to use ITRF as well as non-itrf stations as fiducial stations - Need for classification of EPN stations

76 6- The role of EUREF EPN SITE CLASSIFICATION Class A Position: 1 cm-accuracy coordinates for any epoch of the station's lifetime Velocity repeatability < 0.5 mm/year Determined by comparing the estimated velocities between successive cumulative solutions Class B (mostly young sites) Position: 1 cm-accuracy for epoch of minimum variance; Velocity repeatability >0.5 mm/year - values not published. (Kenyeres, 2010, BGG)

77 6- The role of EUREF Class A sites: 199 EPN sites: 244 EPN CLASS A STATIONS Class A EPN EUREF symposium, Florence, 2009 EUREF guidelines for Densifications of the ETRS89: Fiducials: Class A stations and pos/vel. from cumulative EPN solution tied to ITRFyy

78 6- The role of EUREF ETRS89 NATIONAL CRS Each country has each own National Coordinate Reference System (CRS) Link between CRS and ETRS89 necessary in order to change national reference system to ETRS89 Information System (EUREF and EuroGeographics) Description of all national CRS Transformation parameters between CRS and ETRS89, provided and validated by NMCAs On-line transformation services 1-2 m accuracy (GIS applications)

79 6- The role of EUREF (

80 6- The role of EUREF EVRS - EUROPEAN VERTICAL REFERENCE SYSTEM Related European Vertical Datum (Normaal Amsterdams Peil) Gravity potential differences with respect to NAP or equivalent normal heights Realized by the Unified European Levelling Network (UELN) connecting the first order levelling networks of European countries in a common network

81 6- The role of EUREF EVRF2007 ADJUSTMENT PARAMETERS Datum realization by 13 datum points Reduction to the zero tidal system Reduction of the measurements to the epoch 2000 using the whole NKG2005LU model Number of adjusted levelling points 7900 A-posteriori standard deviation referred to 1 km levelling distance 1.1 mm 81

82 6- The role of EUREF LATEST EVRS REALIZATION: EVRF2007 UELN2007 and datum points of EVRF June Umeå 82

83 Memorandum of Understanding Memorandum of Understanding Memorandum ExGG (Expert of Group Undestanding on Geodesy) Project Euroboundaries Project Georail

84 SUMMARY The present geo-referencing systems are realized through an observing system based on geodetic space techniques

85 SUMMARY There is a strong international effort and cooperation for the maintenance of the global and regional geodetic reference frames The present geo-referencing systems are realized through an observing system based on geodetic space techniques

86 SUMMARY The close cooperation between research and governmental institutions is a key for the success of the projects (user-driven) There is a strong international effort and cooperation for the maintenance of the global and regional geodetic reference frames The present geo-referencing systems are realized through an observing system based on geodetic space techniques

87 SUMMARY The activities of EUREF are a key element for the establishment, maintenance and development of a geodetic infrastructure in Europe The close cooperation between research and governmental institutions is a key for the success of the projects (user-driven) There is a strong international effort and cooperation for the maintenance of the global and regional geodetic reference frames The present geo-referencing systems are realized through an observing system based on geodetic space techniques

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