VLBI observation biases induced by antenna gravitational flexures

Transcription

1 VLBI observation biases induced by antenna gravitational flexures Pierguido Sarti Institute of Radio Astronomy Bologna National Institute of Astrophysics Italy AGU Fall Meeting Dec. 7, 2012

2 The early warnings

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4

5 Signal path variation (SPV) α (Clark & Thomsen, 1988) L( e) = R( e) + F( e) + V ( e) R α Originated by gravity-induced flexure of radio telescopes It is a linear combination of three contributing terms: Displacement of the receiver along the line of sight Deformation of the primary reflector Displacement of the reflecting system along the line of sight SPV depends on the pointing elevation! F α V

6 Clark & Thomsen (1988) Focused on the Fairbanks antenna (Alaska): 26 m dish X-Y mount S/X receiver in primary focus position Used a Finite Element Model of the antenna The coefficients were determined α i assuming a uniform grading on the primary reflector The resulting at all elevations! L ( e) 0

7 How do gravitational flexures impact VLBI observations?

8 Focal length Receiver Vertex CRP O D f e = D f + O cose

9 How does gravity deform the telescope? a b a) g F c b) R a = primary reflector g b = quadripod c = vertex c) g V

10 g

11 Focal length Receiver Vertex

12 ΔL(e)=α F ΔF(e)+α R ΔR(e)+α V ΔV(e)

13 g

14 g

15 g

16 Focal length ΔF Φ (m) Δ Fmax (mm) (Clark & Thomsen 1988) /24.4 (Sarti et al. 2009) (Holst et al. 2012) ΔL(ΔF(e), )

17 Laser beam GS200 Laser

18 Standpoint Target

19 Elevation 90 deg

20 Elevation 75 deg

21 Elevation 60 deg

22 Elevation 45 deg

23 Elevation 30 deg

24 Elevation 15 deg

25 ΔL(e)=α F ΔF(e)+α R ΔR(e)+α V ΔV(e)

26 g Displacement of the receiver

27 g Displacement of the receiver

28 g Displacement of the receiver

29 g Displacement of the receiver

30 Receiver ΔR ΔL(ΔR(e), )

31

32

33 18 15 R2 Displacement (mm) Topography FEM Elevation (deg)

34 18 15 R1 Displacement (mm) Topography FEM Elevation (deg)

35 18 15 R3 Displacement (mm) Topography FEM Elevation (deg)

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37

38 ΔL(e)=α F ΔF(e)+α R ΔR(e)+α V ΔV(e)

39 Vertex

40 Vertex ΔV ΔL(ΔV(e), )

41 g

42 g Displacement of the vertex

43 40 30 (mm) Focal length variation (Laser) Receiver displacement (Topography) Vertex displacement (FEM) Elevation (deg)

44 40 30 (mm) Focal length variation (Laser) Receiver displacement (Topography) Vertex displacement (FEM) Elevation (deg)

45 How were the Δs determined so far? Laser scanning for ΔF: Sarti et al. 2009; Holst et al. 2012; (J Surv Eng) Finite Element Model: all Δs in Clark and Thomsen 1988 (NASA Tech Rep); ΔF in Sarti el al (J Geodesy) Triangulation and trilateration: ΔR in Sarti el al (J Geodesy) ΔL(e)=α F ΔF(e)+α R ΔR(e)+α V ΔV(e)

46 z r u k u r I = ) ( 2 ) ( r g g g e k r I β = b R r k r I b b β = 2 1 ) ( 2R

47 = S i R ds r I r h ) ( ) ( α R V R F ' 1 ' ' 1 ' α α α α = = Abbondanza & Sarti (2010), J Geodesy R V R F " 2 1 " " 2 2 " α α α α = = Primary focus configuration: Cassegrainian configuration:

48 Abbondanza & Sarti, 2010 J Geodesy

49 L in VLBI data analysis (Sarti et al. 2011, J Geodesy) VTD/Solve allows computation of theoretical VLBI path delay (Petrov 2008, Ref Manual) 7.08 millions of VLBI observations processed with VTD/Solve 345 experiments for Medicina; 150 for Noto Ran two solutions: R1: reference solution without SPV model A1: as R1 but SPV models for Medicina and Noto applied

50 Station U (mm) E (mm) N (mm) # Sess DSS MATERA MEDICINA NOTO NYALES ONSALA WETTZELL Sarti et al. 2011, J Geodesy

51 Conclusions Gravitational deformations may deform the structure of the telescope to a non negligible extent modify the Up component of the telescopes Reference Point do not modify the estimate of other S-geodetic parameters may impact the scale of the VLBI network L models are strictly telescope-dependent and as such should be determined specifically for each VLBI telescope must be computed properly: with α i coefficients computed with realistic IF with R, V and F models deriving from ad hoc high precision terrestrial surveys and/or FEM Cannot be inferred from the telescope dimensions (NO scaling!) The height bias cannot be determined relying on VLBI data alone!