The realization of a 3D Reference System

Transcription

1 The realization of a 3D Reference System Standard techniques: topographic surveying and GNSS Observe angles and distances either between points on the Earth surface or to satellites and stars. Do not observe positions with respect to the geocenter, the rotation axis and the reference meridian.

2 RSs must be realized by networks of benchmarks or continuously monitored stations, whose coordinates are: estimated in the RS, published in catalogues. Then, local networks can be estimated by standard techniques wrt these reference networks. System: the geometric definition Frame: the realization, i.e. the catalogue of the coordinates of a set of points that provide the reference (frame) to estimate the positions of other points.

3 For historical reasons and applications different definitions: different Reference Systems. Given a unique System, for practical reasons: different observations campaigns: different Frames.

4 Reference Frames They can be classified in Global RFs Realized by global networks of fundamental points that cover the whole planet by satellite geodesy techniques (VLBI, SLR, GPS) to estimate positions, displacements and deformations (geodynamics) of the planet.

5 Local RFs Defined at the local scale (continental, national,, structure control) by: At the scale of the continents/nations, 1. GPS networks adjusted in the global network, 2. horizontal and vertical national networks (historical topographic/cartographic/cadastral). At the local scale GPS/topographic network for surveying and monitoring.

6 Continuously monitored RFs Realized by networks of continuously observing stations, whose coordinates are continuously estimated: permanent stations and networks. Static RFs Realized by benchmarks surveyed and estimated in a single campaign.

7 Note Continuously monitored RFs guarantee the maximum consistence, because at each epoch, the distributed coordinates are consistent with the accuracy of the measurement/estimation methods. Static RFs provide crystallized coordinates; even if they had been surveyed with the maximum accuracy, their errors grow in time because of local phenomena (breaks, maintenance works,...) and regional deformations (subsidence, geodynamics,...).

8 A permanent GNSS station A H24 operating GNSS receiver publishes raw data is continuously monitored, provides its coordinate estimates. In Figure: Como PS.

9 Permanent networks A permanent network consists of: 1. a set of permanent stations, with a homogeneous spatial distribution in the interested area. 2. one or more control centers that: manage the PSs, check their data quality, process the network data, estimate PSs coordinates, if needed, distribute network data and products to the users.

10 PNs aims The aims of a PN can be different: global reference frame (IGS) monitoring, local reference frame (EPN) monitoring, local control: alpine geodynamics, landslides, dams,... positioning services: GPS users supporting at different latency and accuracy scales.

11 International Terrestrial Reference System Geocentric Unit of length: meter, consistent with the TCG scale. Initial axis orientation: BIH 1984 orientation. Time evolution of the axes orientation: no net rotation with respect to the Earth tectonics

12 ITRS Permanent networks Four spatial geodesy techniques: VLBI: Very Long Baseline Interferometry SLR: Satellite Laser Ranging DORIS: Doppler Orbit determination and Radiopositioning Integrated on Satellite GNSS. Global Navigation Satellite Systems (GPS+GLONASS+GALILEO+...)

13 The global VLBI network (International VLBI Service) About 30 stations, implemented by space telescopes.

14 The global SLR network (International SLR Service) About 90 stations, by laser telescope.

15 The global GNSS network: International GNSS Service A IAG service, established in 1993, with the following goals: to contribute to the realization and distribution of ITRS; to distribute GNSS products (ephemerides, EOP, ); to define the standards for GNSS permanent networks; to support GNSS research. About 470 PSs. several Analysis centres, Working groups, Pilot projects, Services, a Central Bureau.

16 The IGS PN

17 ITRF (ITR Frame) by IERS From 1988 the results provided by VLBI, SLR, GPS are analyzed by the IERS (International Earth Rotation Service), with the following estimation goals: Earth Orientation Parameters (EOP), International Celestial Reference Frame, International Terrestrial Reference Frame, tectonics of the Earth crust.

18 ITRF The number of PS s (VLBI, SLR, GPS) increase; the estimation algorithms improve. ITRF (coordinates catalogues) evolves with one update about every 5 years: the main are ITRF89,, ITRF2005, ITRF2008.

19 A ITRF consists of a catalogue of coordinates and velocities of the PS s contributing to the solution. In the map: ITRF2008 PSs.

20 Estimation and distribution of ITRF VLBI: quasar observations link Earth rotation to celestial rerence frame provides earth orientation parameters estimates the Z axis of ITRF SLR: satellites orbits estimation allows to estimate the geocenter estimates the origin of ITRF IGS PN is linked to VLBI and SLR and distributes ITRF to the GNSS user community

21 Distributed estimates for each PS Initial geocentric position x ( t0) and related covariance C, 00 velocity!x and related covariance C vv for the reference epoch t 0. ITRF2008 estimates for some Italian GPS PS s Nome Xm ( ) Ym ( ) Zm ( )!X (m / y) Y(m! / y) Z(m! / y) Bolzano Matera Cagliari

22 The coordinates can be propagated at each epoch t t by the 0 x(t) = x(t 0 ) +!x (t t 0 ), C() t = C + C ( t t ) 00 vv 0 2 Characteristics of ITRF Accuracies of the last ITRFs are of some mm. a low update rate (each about 5 years), the linear hypothesis on the PSs velocities can be too much rigid.

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24 Cartographic applications and regional RFs Earth crust is moving: each geodetic point moves Given a region, global movements should be removed for local positioning, then 1. cartographic purposes: typically at the continental/national scale 2. local monitoring of inner deformations: local geodynamics, landslides,... Local reference frames are useful/defined in these cases.

25 The European Terrestrial Reference System , ETRS89 ETRS89 coincides with ITRS in , but moves and rotates with the stable (mean) part of Europe. Realized by the IAG European Reference Frame (EUREF) Commission, by EPN. EPN (European Permanent Network) About 275 GNSS PSs

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27 Present realization: ETRF2000-R08 ITRF coordinates and velocities of EPN stations are estimated, the transformation parameters between ITRF and ETRF are published and distributed, ETRF coordinates and velocities of the stations are computed by applying the transformations. ETRF2000 provides a RF with minimum motion wrt Europe: useful for all the cartographic applications.

28 EPN Webpage of COMO PS

29 COMO displacement in ETRF

30 Transformation of ITRF results to ETRF ITRF coordinates are transformed to ETRF at epoch t, by: x(t) P,E = t(t) + (1+ µ(t))x(t) P,I + δ R(t)x(t) P,I δ R(t) = r 0 + (t )!r, t(t) = t 0 + (t )!t, µ(t) = µ 0 + (t )!µ The mean motion of the Europe from to the current epoch is subtracted (the motion from to is embedded in constant parameters; the motion from 2000 to current epoch is given by annual rates).

31 Transformation parameters and their rates are given by official EUREF documents

32 All the transformations are now implemented in EPN webpage.

33 The Italian (static) realization of ETRF: IGM95 The national geodetic network, monumented and surveyed by Istituto Geografico Militare Italiano (IGM) is called IGM95. IGM95 is composed of about 1250 benchmarks.

34 Example of IGM95 monograph (Lentate)

35 First IGM95 adjustment In the 90, in ETRF89 at epoch Problems: deformations and errors due to survey instruments and techniques, data elaboration algorithms, adjustment constraints, less accurate in 90 than now. Errors: in horizontal up to 5 cm, in heights up to 10 cm, sparse larger blunders.

36 The new national network and the adjustment of IGM95 Dynamic National Network (RDN). A PN of 99 PSs to establish the new national zero order network.

37 In RDN (one month of data) has been adjusted in ITRF2005. Coordinates have been transformed to ETRF2000 IGM95 has been linked ( 45 baselines) to RDN and adjusted in ETRF2000. New IGM95 monographs report updated ETRF2000 coordinates.

38 RF fixing in GNSS positioning Two cases: point positioning & relative positioning / network adjustment Point positioning In point positioning, the RF is imposed by fixing the satellite coordinates Relative positioning Baselines estimation and network graph

39 Relative positioning A local geodetic 3D net surveyed by GNSS is a network of baselines A 3D translation must be fixed

40 Cheap approach to minimal constraints One (reference) point of known x P coordinates included in the network: its a priori coordinates are fixed in the adjustment. An error in the reference point completely propagates into the whole network.

41 Optimal approach to minimal constraints (adjustment of the network) Include several reference points in the network. The local network is adjusted in a network of higher order.

42 Optimal approach to minimal constraints The barycenter of the a priori coordinates of the reference stations x Ri is computed by the x = x 0, R 0 Ri, i the barycenter is kept fixed in the adjustment xˆ = x xˆ x = ξˆ = 0 Ri, 0 Ri, Ri, 0 Ri, Ri, i i i i

43 Guidelines to adjust a local network in IGS / EPN Some PSs from IGS/EPN are included in the local network, their raw data are downloaded by public EPN/IGS ftp sites and processed with the local network data, their IGS / EPN coordinates are computed at the campaign epoch t by the linear propagation of official coordinates and velocities and are (minimally) constrained in the adjustment of the local network. The network coordinates are 'aligned' to ITRF / ETRF at epoch t

44 The praxis to adjust a local network in IGM95 Some benchmarks of IGM95 are included in the local network, they must be occupied by receivers and processed with the local network data, their coordinates are (minimally) constrained to the monographs values in the adjustment of the local network. The network coordinates are estimated in IGM95. The praxis is less accurate than the ITRF/ETRF procedure: it should be adopted only for cadastral and topographic surveying.