Study of systematic errors in VLBI caused by polarization impurity


Content:

Introduction

In according with specification of MARK-III and MARK-IV VLBI system receiving system should record signal of a specified circular polarization. Conventionally right circular polarized (RCP) signal is recorded in ordinary geodetic VLBI experiments. However, actual implementation has inevitable deviations from an ideal. It was shown that normally VLBI system records not 100% RCP signal but 1-10% of unwanted left circular polarized (LCP) signal. Signal from extragalactic source is transformed before recording: it reflecting antenna surface, contr-reflector, feed-horn; then signal comes through the receiver. All these component may change polarization of the input signal what results in recording some portion of LCP. Since feed-horn rotation has opposite sign for RCP and LCP, presence of LCP signal corrupts measurements of correlated phases. We can expect that instrumental polarization is a frequency dependent effect. In that case the presence of LCP will corrupt bandwidth synthesis and group delays will get additional contribution. This effect will be different for polarized and unpolarized sources. Author [1] states that polarization impurity is a dominating instrumental effect which affects on measurement of group delays.

Objectives

The general aim is to reduce the level of instrumental errors and improve precision and accuracy of estimation of the targeted parameters: station coordinates, source coordinates, Earth orientation parameters and others quantities.

The objectives of the study is to answer the questions:

Experiments

  1. Polarization test during Europe46 experiment. Usual 24 schedule was extended for this experiment. 4 scans of the strongest sources were added. All stations made 2 scans of these sources in usual mode then two antennas: MEDICINA and CRIMEA swapped polarization from RCP to LCP and all stations made 2 additional scans: 6 stations observed RCP and 2 stations observed LCP.

    Preliminary results of this experiment are available.


  2. Dedicated polarization experiment BRD01 (01-JUL-99)
    Results of polarization test inspired idea of making full 24 hours experiment for investigation effects of polarization impurity in geodetic VLBI.

    The experiment is scheduled on 01-JUL-99 10:00 UTC.
    Proposals for this experiment and a preliminary schedule are available.


Conclusions

I don't have yet...

References

1. T.A. Herring "Precision and accuracy of intercontinental distance determination using radio interferometry", Ph.D. Thesis, Hanscom AFB, Massachusetts 01731, 442 p. 1983
2. A.E.E. Rogers, "Instrumentation Improvements to Achieve Millimeter Accuracy", NOAA Technical Report NOS 137, NGS 49, American Geophysical Union, Washington, D.C., pp.1-7, 1991
3. D.H. Robertson and J.H.C. Wardle, "Linear polarization radio imaging at milliarcsecond resolution", The Astrophysical Journal, vol. 427, pp. 718-744, 1994
4. M. Massi, G. Comoretto, M. Rioja and G. Tofani "Baseline errors in European VLBI Network measurements II. Instrumental polarization", Astronomy and Astrophysics Supplement Series, vol. 116, pp. 167-176, 1996
5. M. Massi, M. Rioja, D. Gabuzda, K. Leppanen, H. Sanghera, K. Ruf and L. Moscadelli, "Baseline errors in European VLBI Network measurements III. The dominant effect of instrumental polarization", Astronomy and Astrophysics, vol. 318, pp. L32-L34, 1997
Back to "Development of advanced methods of VLBI data analysis"
Back to Leonid Petrov's Home Page
Back to home page of the Goddard VLBI group

This page was prepared by Leonid Petrov
Last update: 11-JUN-99 13:35:07