Introduction

next up previous
Next: Correcting Data - The Up: VLBA TEST MEMO 69 Previous: VLBA TEST MEMO 69

Introduction

A significant issue has been found with the Earth Orientation Parameters (EOP: UT1-UTC and pole position) used in VLBA correlator job scripts since May 2003. The correlator is supposed to use rapid service values of EOP, that are based on measurements, when they are available at the time jobs are generated. Instead, because of a software bug, it was using predictions from about 2 weeks before the observe date. The Earth's rotation rate and pole position are sufficiently unpredictable that this can make a difference of tens of milliarcseconds in the observation geometry.

The problem adversely affects projects that depend on an accurate correlator model. Such projects include those that use phase referencing, especially at high frequencies or with large separations between calibrator and target or with very stringent position determination requirements. Projects that use sources scattered around the sky to solve for the atmosphere (AIPS task DELZN (Mioduszewski & Kogan 2004)) can have the atmospheric solutions degraded. Also spectral line observations that use a line feature for phase calibration but depend on a calibrator to take out phase slopes (delay) will be impacted at a low, but perhaps significant, level. Projects that are not affected include those that depend primarily on self-calibration, such as imaging of strong sources. Also unaffected are geodetic and astrometric projects that use the total delays from the correlator.

While investigating this bug, it was realized that some projects get processed with predicted rather than measured EOP even when the system is working normally. This usually happens when correlator jobs are prepared before the rapid service EOP values are available in Bulletin A. For this reason, users with projects outside the time period in which the bug was in effect might want to check the EOP used for their project and, in affected cases, make corrections.

This memo starts with a quick recipe on how to correct data. Users who are not interested in the details, but who want to make corrections can read that section only, or even just the CLCOR help file in AIPS. Later sections describe the problem in detail and present test results that show the possible effects and show that the AIPS correction works.

next up previous
Next: Correcting Data - The Up: VLBA TEST MEMO 69 Previous: VLBA TEST MEMO 69
Craig Walker 2005-10-06

Connect with NRAO

The National Radio Astronomy Observatory and Green Bank Observatory are facilities of the U.S. National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Founded in 1956, the NRAO provides state-of-the-art radio telescope facilities for use by the international scientific community. NRAO telescopes are open to all astronomers regardless of institutional or national affiliation. Observing time on NRAO telescopes is available on a competitive basis to qualified scientists after evaluation of research proposals on the basis of scientific merit, the capability of the instruments to do the work, and the availability of the telescope during the requested time. NRAO also provides both formal and informal programs in education and public outreach for teachers, students, the general public, and the media.

The National Science Foundation (NSF) is an independent federal agency created by Congress in 1950 "to promote the progress of science; to advance the national health, prosperity, and welfare; to secure the national defense..."

AUI collaborates with the scientific community and research sponsors to plan, build, and operate cutting-edge facilities. We cultivate excellence, deliver value, enhance education, and engage the public.