vlbi observations to satellites of the gnss · 2017. 12. 11. · estimated zenith wet delays (zwd)...

VLBI observations to satellites

of the GNSS A first time series

6th International Colloquium on Scientific and Fundamental Aspects of the Galileo, October 23-25 2017, Valencia, Spain

Andreas Hellerschmied • Johannes BöhmTechnische Universität Wien, Austria

Lucia McCallum • Jamie McCallumUniversity of Tasmania, Australia

Tim NatuschAuckland University of Technology, New Zealand

6th International Colloquium on Scientific and Fundamental Aspects of the Galileo,October 23-25 2017, Valencia, Spain

Very Long Baseline Interferometry (VLBI)

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Group delay

baseline vector b

position of quasar s0

Earth orientation

X/S Band

NASA GSFC

bs0


Satellite observations with VLBI

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• Motivation

– Establish inter-technique ties in space

„Co-Location in space “ (Plank L, 2014)



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• Simulations by L. McCallum (Plank) et al. 2016

– 16 fictitious stations, 6 GPS satellites, obs. every minute

– Monte Carlos simulation including turbulence and 30 ps white noise



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– 16 fictitious stations, 6 GPS satellites, obs. every minute

– Monte Carlos simulation including turbulence and 30 ps white noise

One-day skyplot at Svetloe with fast switching (left) and continuous observations (right)



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– weekly station position repeatabilities



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• Can these observations be carried out successfully?

• Earlier work on the observational side with GNSS

– Tornatore, Haas, et al. 2014

– Haas et al. 2014 report on successful correlation for GLONASS experiment (Wz-On)

– ..


Our tests in 2015/2016

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• L1 + L2 signals

• Tracking of GPS and GLONASS

Aim:

„enable and streamline the process

from scheduling to analysis“

Exper. code Date Duration Target Stations Comments

--- June 2015 --- GPS, Glonass Ho, Cd Initial tracking tests

179a 28.6.2015 2 h GPS, Glonass Ho, Cd Change frequ. For each satellite

236a 24.8.2015 4 h GPS Ho, Cd Fixed frequ., dual polarization

238a 26.8.2015 4 h GPS, Glonass Ho, Cd Fixed frequ., dual polarization

126b 5.5.2016 6 h GPS Ho, Cd DBBC in Ho, no Mk4

131a 10.5.2016 6 h GPS Ho, Cd DBBC+Mk4 in Ho (redundant recording)

g336 1.12.2016 3 h GPS Ho, Cd, Wa First 3 station experiment

Warkworth 30m


Overview

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• Starting and finishing with the Vienna VLBI and Satellite Software (VieVS), we have developed a complete process chain

• Whenever possible, we used standard procedures

schedulingTLE

DBBCMk5C

30m

Ceduna - Cd1.2-1.7 GHz

Mk4/DBBCMk5A/Mk5B

26m

Hobart - Ho1.2-1.7 GHz

Observations

DBBCMk5B

30m

Warkworth- Wa

+

correlationDiFX

fourfit

analysisSP3


Scheduling

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• Observation planning with VieVS Satellite Scheduling Module

– Time sequence of VLBI experiment

Control files for stations (antennas & recorders) and correlator

• Experiment design

– 4 to 5 GNSS satellites repeatedly observed in 5 min tracks in each session

– Strong quasars observed for calibration (start + end)

Sky plots of experiment 131a


Observations

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• Mode: 8 IF channels, 16 MHz

bandwidth, 2 bit sampling,

dual-polarization (linear)

2 channels for L1 and L2 each

4 channels for quasar group delays

• „Stepwise“ satellite tracking

– New source position every 10 sec

– Continuous data recording

~ 1 TB data per station over 6 hour session

Live L1 GPS signal (spectrum

analyser) during 179a.


Correlation & Fringe Fitting

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• Application of standard tools

• Correlation with DiFX software

– The standard a priori delay model was replaced by „near field delay model“ calculated in VieVS

– Clock model adjusted using quasar observations

• Fringe fitting with fourfit to derive delay observables

– Per observation four polarization products (XX, YY, XY, YX) in two bands (L1 and L2) 8 delays per observation epoch

– Combination of polarizations and bands still pending


Resudial Delays (L1, XX-polarization)

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• For 126b and 131a we find residuals (observed minus computed) within 8 ns or ~2.5 m for all observed satellites over the entire session of 6 hours



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– Applying ionosphere correction (from TEC maps; Tierno Ros et al. 2011) these residuals drop to 4 ns or ~1.2 m

Ionosphere corrections Corrected residuals



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– Applying ionosphere correction (from TEC maps) these residuals drop to 4 ns or ~1.2 m

Corrected residuals

• Rapid changes in residuals probably caused by unresolved issues with polarization and gain


Resudial Delays: Ho-Cd-Wa (g336)

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• AGC in DBBC deactivated during scans at Cd & Ho Constant gain

Cd-Ho: ~ 3 ns

Cd-Wa: ~ 10 ns

Ho-Wa: ~ 10 ns


Analysis

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• In VieVS

• Standard geodetic estimation

– Clocks (offset + rate)

– ZWD (two-hourly)

– Stations fixed for single-baselines

Post-fit residuals at the level of 10-20 cm

estimatesReference values

from GNSS

Estimated zenith wet delays (zwd) of session 131a.


Summary & Outlook

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We have developed a closed process chain for VLBI satellite observations, from scheduling to analysis

Plank et al. (2017), VLBI observations to satellites of the GNSS: from scheduling to analysis, J Geod, Vol 91

First three station experiment in December 2016 (Ho-Cd-Wa)

• Next steps:

– Improve recording (8-bit mode for higher dynamic)

– Combination of X-Y polarization products is still an issue

– More tests to further improve processes


Summary & Outlook

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• Is there utility in putting special quasar-like senders to the Galileo satellites?

– Antennas on satellites are not point-like

– Special point-like senders will be necessary to betterconnect GNSS to VLBI

– Setting up international observing sessions on a regularbasis might be a challenge

– Apart from political issues, would the VLBI community beprepared to sacrifice parts of its accuracy to thoseobservations?


Thank you for your attention!

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Erwin Schrödinger Fellowship J 3699-N29

Project SORTS – I 2204

References:Böhm et al. (2012), The New Vienna VLBI Software, Proceedings of the 2009 IAG Symposium, Buenos Aires, Argentina, 31 August 2009 - 4

September 2009, Series: International Association of Geodesy Symposia, Vol. 136, Kenyon S, Pacino MC, and Marti U (eds.), ISBN 978-3-642-20337-4, pp 1007-1012.

Hellerschmied et al. (2015), Scheduling of VLBI Observations to Satellites with VieVS, Proceedings of the IAG Symp., REFAG 2014. Luxembourg, Series: International Association of Geodesy Symposia, DOI 10.1007/1345_2015_183, available online.

Plank et al. (2014), Precise station positions from VLBI observations to satellites: a simulation study, J Geod, Vol 88, pp 659–673.

Plank et al. (2016), Simulated VLBI satellite tracking of the GNSS constellation - Observing strategies, IAG Symposia series.

Plank et al. (2017), VLBI observations to satellites of the GNSS: from scheduling to analysis, J Geod, Vol 91, pp 867-880.

Tierno Ros et al. (2011), Use of GNSS-derived TEC maps for VLBI observations, EVGA working meeting proceedings, Bonn, 2011, pp 114-117.

Contact:[email protected]

vlbi observations to satellites of the gnss · 2017. 12. 11. · estimated zenith wet delays (zwd)...

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