Updated EVN station positions
This message concerns improved station positions for EVN antennas which do not participate regularly in geodetic experiments. A special 6-cm experiment using geodetic techniques was performed to measure these. The analysis has now reached the point where accuracies of the station position estimates are estimated to be better than 10cm. These results have appeared in Charlot et al., 2001, in Proceedings of the 15th Working Meeting on European VLBI for Geodesy and Astrometry, ed. D. Behrand & A. Ruis (Barcelona: IEEC/CSIC), p.194. Further presentations of this work will appear in the near future. But we feel obliged to inform EVN users at this point and to begin to use the updated station positions for correlation. We have combined the positions with locally determined position offsets to update the coordinates of some other stations (i.e., Jb1 and Cm from Jb2, Wb_arr from Wb7 - note that the calculation of the Wb array position was improved in February 2002). Independently, we also received an update for the position of the Metsahovi mm-antenna and Arecibo from Dave Graham.
These new station positions were used to reanalyze some phase-reference test observations, as well as more recent Network Monitoring Experiments that had a phase-referencing schedule. Noticeable improvements in phase residuals and the resulting phase-reference maps themselves were evident. We are thus confident that these new coordinates will improve users' results when using EVN stations for phase-referencing via the standard way in AIPS (i.e., using correlator-residual phases).
There is a recipe for incorporating these updated positions into the analysis of previously correlated experiments in AIPS (via the task CLCOR), so that users who already have their FITS files can also benefit from these improvements. There is also a summary of the reanalysis of the phase-reference test observations.
These improved EVN station positions will be used at the VLBI correlators that regularly process data from EVN telescopes (Bonn, Dwingeloo, Socorro). The SCHED databases will be updated in the next distribution. Further, experiments correlated at JIVE prior to mid-April 2002 did not include the effects of station velocity. PIs of phase-reference experiments correlated at JIVE will have received individual e-mails detailing the particulars of their experiment(s) with regard to these station-position considerations in May 2002.
Contact Bob Campbell (mail icon below) with further questions.
A list of the updated station coordinates (ITRF2000, epoch 1997.0; a right-handed frame) that have resulted from this effort. The table also lists the correct (non-zero) axis offsets for the stations.
You can judge the overall effect of the updated station position with the figure below. It shows the (first-order) effects on difference phase, spanning a 1m station position change in each Cartesian coordinate and a reference-target source separation of 1o in each direction on the sky. Each panel shows the difference-phase correction in degrees, per GHz of observing frequency. You use these plots by following along a horizontal line corresponding to the declination of your source; this yields a full sidereal-day sinusoid (the horizontal axis is GHA =Greenwich Apparent Sidereal Time - Right Ascension of your source). To first order, the total effect on difference phase for your situation (actual 3-D station position correction, 2-D target-reference separation, and observing frequency) can be estimated by linearly "combining" the panels in the appropriate ratios. Click on the image to download a full-size gzipped postscript file.
EVN webmaster (jive{at}jive.eu)