A review about local ties and eccentricity vectors: strategies, results, potentials and open issues

Transcription

1 A review about local ties and eccentricity vectors: strategies, results, potentials and open issues P. Sarti L. Vittuari, C. Abbondanza, J. Dawson, G. Johnston, M. Negusini, S. Montaguti

2 Theoretical aspects: Reference point definition (physical vs. geometric): gravitational (Clark and Thomsen, NASA Tech. Memo , 1988) and thermal deformations (e.g. Wresnik et al. 2007, J Geodesy) Information contained in the tie Alignment of the eccentricity vector into ITRF (local to global) Operational aspects: What to talk about? Tie precision and accuracy (old vs. new/modern ties) Surveying strategies (approach, measurements, techniques) Reference point realization Inter-technique eccentricities (tracking point vs. instrument reference point) Time dependence: Monument stability (stability of the RPs) Local network stability and deformation (reproducibility of local frame)

3 What to measure? A vector between two or more tracking points: Space geodetic instrument reference point (S) VLBI and SLR: based on fixed and moving axis GPS and DORIS: Antenna Reference Point Reference marker (M) Possible inter-technique eccentricities (between S and M): they must be measured

4 How to do it? Three approaches may be adopted: Indirect methods (e.g. Dawson and Johnston 2005, IERS Tech. Note 33; Sarti et al J Geodesy) Mixed methods (e.g. Nothnagel et al. 2002, EU TMR Report) Direct method Different strategies: Purely terrestrial observations (trilateration, triangulation, spirit levelling) Sideshots on some targets Redundant forward intersection on some targets GPS based surveys mixed

5 What must the outcome be? Eccentricity vector Complete vcv SINEX file

6 Indirect approach: observations Terrestrial observations Targets positions are recovered with angles, distances and height differences Targets positions are related to the RP with geometric models (conditioning) GPS observations Markers are surveyed with GPS A rapid static survey is performed and GPS antenna positions are related to the RP with geometric models (conditioning)

7 Indirect approach: software Axis and Clem&nt are post-processing software for indirect methods They are capable to estimate an eccentricity between any space geodetic instrument Their performances have been compared on different data sets

8 Yaragadee: location of space geodetic instruments SLR DORIS GPS

9 VLBI GPS Medicina: location of space geodetic instruments

10 Indirect approach: software Axis and Clem&nt are post-processing software for indirect observations They are capable to estimate an eccentricity between any space geodetic instrument Their performances have been compared on different data sets The geometric conditioning can be varied

11 a b p3 p2 c p1 Additional ancillary conditions (SLR and VLBI) Realization of the axes (SLR and VLBI)

12 Indirect approach: software Axis and Clem&nt are post-processing software for indirect observations They are capable to estimate an eccentricity between any space geodetic instrument Their performances have been compared on different data sets The geometric conditioning can be varied The degree of conditioning is important (Dawson et al. 2007, J Geodesy)

13 Error Invariant Point w.r.t theoretical value East mm (1σ) North mm (1σ) Up mm (1σ) Model 0.8 (1.0) 0.2 (1.1) 3.4 (3.9) Axisa 0.2 (0.8) 0.2 (0.9) 1.2 (1.7) Axisb -0.1 (0.8) 0.2 (0.8) 0.1 (0.9) Axisc 0.0 (1.4) 0.1 (1.4) 2.1 (1.7) Clem&nta 0.0 (1.4) 0.1 (1.4) 2.1 (1.8) Clem&ntb 0.0 (1.4) 0.1 (1.4) 0.3 (1.4) Clem&ntc a = indipendent circles b = a + common normal vector c = a + b + co-linearity

14 Indirect approach: software Axis and Clem&nt are post-processing software for indirect observations They are capable to estimate an eccentricity between any space geodetic instrument Their performances have been compared on different data sets The geometric conditioning can be varied The degree of conditioning is important (Dawson et al. 2007) Their output is a SINEX file Both software are based on rigorous statistic methods and allow precise estimation of the eccentricity Axis and Clem&nt results are consistent!

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18 U IPF= ± (m) U IP0= ± (m) U IPE= ± (m) (U IPE -U IPF) 1 cm

19 Montaguti et al. this session

20 Reference points: geometry vs. physics A geometric realization is based on the RP definition: in this case 1 mm accuracy is achievable but The real, physical observing point is different and varies during the observations Geometric models are applied to space geodetic observations (e.g. PC variation files for GPS or offset/zenith angle model for VLBI) Most probably they are not enough

21 From local to global The terrestrial observations are performed in a topocentric frame The local to global transformation is based on common points surveyed with GPS Its parameters are stochastically determined It can affect the inner information contained in the tie

22 From local to global The terrestrial observations are performed into a topocentric frame The local to global transformation is based on common points surveyed with GPS The time repeatibility of the local frame is uncertain

23 Medicina LOCAL TIE SURVEYS TERR t.b. performed GPS BASED

24 From local to global The terrestrial observations are performed into a topocentric frame The local to global transformation is based on common points surveyed with GPS The time repeatibility of the local frame is uncertain the local stability of the ground control network has to be constantly investigated (Abbondanza et al. this session)

25 Tie s Modulus (m) according to the geometrical modeling applied (parenthesized are the formal error a 1-σ level in mm) TER 2001 Cum1 (loosely conditioned solution) (0.7) Cum2 (+intra-group parallelism) Cum3 (+ inter-axial orthogonality) Cum4 (Axis offset estimation) (0.7) (0.7) (0.7) TER (0.4) (0.4) (0.4) (0.4) TER (0.2) (0.3) (0.3) (0.3) ABS (1.5) (2.0) (2.0) (2.0) ABS (0.8) (0.8) (0.3) (0.8)

26 final remarks The indirect approaches can be considered useful, versatile, flexible and precise tools Should local ties be referred to the geometric definition of RP? Local to global transformation is very important Local frame stability and monument stability can be detected with regularly repeated ties Could the creation of a small and active group of curious people help answering some questions?

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