Very Long Baseline Interferometry for Geodesy and Astrometry

Transcription

1 VieVS User-Workshop 2017 Very Long Baseline Interferometry for Geodesy and Astrometry Johannes Böhm

2 VLBI How does it work?

3 VLBI a flowchart SINEX skd vex2 drudge drudge snp/prc snp/prc NGS Mark4 Mark4 db-v4 DiFX Calc/Solve fourfit fits

4 Scheduling SKED (Vandenberg 1999) or VieVS (Sun et al. 2014) At any instant, different subsets of antennas will be observing different sources All observations to one source at a time form a scan Different integration and slewing times need to be considered

5 Scheduling Various optimisation criteria sky coverage, covariance matrix,... sky coverage important for tropospheric delays Jing Sun

6 Data analysis

7 Data analysis VieVS uses NGS files and vgosdb files

8 Theoretical delays Before calculating the observed computed (o c) value, various models need to be applied IERS Conventions (Petit and Luzum 2010) IVS Conventions, e.g. thermal deformation

9 Station coordinates At first, coordinates (at a certain epoch, e.g. J2000.0) and velocities are taken from a specific realization of the ITRS, e.g. ITRF2014 TT (!) and tide free

10 Station coordinates Periodic corrections to get closer to true coordinates solid Earth tides ocean tide loading (ocean) pole tide loading tidal atmosphere loading (S1 and S2)

11 mm Station coordinates Aperiodic corrections atmosphere loading ocean non-tidal loading hydrology loading Radial displacement at August 1, 2008, 00 UT Atmosphere l. Dudy Wijaya

12 Earth orientation From the International Terrestrial Reference System (ITRS) to the Geocentric Celestial Reference System (GCRS) at the epoch of the observation t Q motion of celestial intermediate pole (CIP) in CRF R rotation of the Earth around the CIP W polar motion of CIP w.r.t. TRF

13 Earth orientation Remaining unmodelled parts of the celestial motion can be observed with VLBI and are called celestial pole offsets; partly due to unmodelled FCN Sigrid Böhm

14 Earth orientation Celestial pole offsets, UT1 UTC, polar motion need to be observed Daily values are used as a priori information IERS 14 C04, Finals Models for diurnal and sub-diurnal ocean tides Libration (forced polar motion)

15 Earth orientation Sigrid Böhm

16 Relativistic model for the VLBI time delay Following IAU Resolutions IERS Conventions Consensus model by Eubanks (1991) Arrival time at station 1 is reference Observations refer to TT (SI second on geoid), theory is geocentric thus, a scale factor needs to be applied

17 Relativistic model for the VLBI delay Lucia Plank

18 Troposphere delay modeling Troposphere delays Zenith delay times mapping function to determine a priori slant delays and to estimate residual zenith delays

19 Troposphere delay modeling Separation into hydrostatic and wet part Continued fraction form for mapping functions e.g., Vienna Mapping Functions, Global Mapping Functions, Global Pressure and Temperature

20 Troposphere delay modeling Tropospheric gradients Raytracing

21 Antenna deformation Snow and ice loading Gravitational deformation Thermal deformation Wresnik

22 Axis offsets.. are not absorbed by clock estimates when axes do not intersect VLBI reference point Nothnagel, 2009

23 Source structure Source structure effects SI 1 SI 4

24 Least-squares adjustment in VLBI Classical Gauß-Markov model Kalman Filter (Herring et al. 1990) Collocation (Titov and Schuh 2000) minimize the squared sum of weighted residuals

25 Least-squares adjustment in VLBI Observations equations (real and pseudo observations = constraints) Weight matrix

26 Least-squares adjustment in VLBI Auxiliary parameters: clocks (quadratic functions plus piecewise linear offsets; reference clock), zenith wet delays and gradients Clock breaks First and main solution Piecewise linear offsets at e.g. integer hours.. allows combination with other space geodetic techniques at normal equation level

27 Least-squares adjustment in VLBI Many other geodetic/astrometric parameters to be estimated E.g., Earth orientation parameters can be estimated daily or with higher time resolution

28 Least-squares adjustment in VLBI Global VLBI solutions from a (large) number of single solutions E.g., for station and source coordinates Often, auxiliary parameters are removed (= implicitly estimated) Results in SINEX file

29 Least-squares adjustment Global VLBI solutions

30 Least-squares adjustment in VLBI Conditions to prevent N matrix from being singular free networks need a datum rank deficiency is six (scale is defined by observations) NNR/NNT usually applied in case of longer time spans also NNR-rate/NNT-rate Episodic changes need to be considered (instrumental changes, Earthquakes)

31 Results from Geodetic VLBI and IVS Status 2010 of IVS main products (Schlüter and Behrend 2007)