VLA Baseline Corrections

Lists of baseline corrections

The basics

Format and use of lists of baseline corrections

Lists of Baseline Corrections

To access the Karl G. Jansky Very Large Array (VLA) baseline corrections for a given year, select the year below and press "View".

To access the pre-upgrade VLA baseline corrections for a given year, select the year below and press "View".

The Basics

NRAO monitors the positions of the VLA antennas on a regular basis -- roughly monthly, and more often during reconfigurations. This involves observing a number of calibrators spread around the sky, solving for the antenna-based gains, and from their behavior deriving the three-dimensional positions of those antennas (see e.g. chapter 12 of Thompson, Moran, and Swenson 1991, Interferometry and Synthesis in Radio Astronomy, for a fuller discussion). These corrections (generally less than a centimeter) are then inserted into the correlator model in the on-line system.

For observers the main point is that the positions used during correlation may not be correct, leading to phase errors and incorrect gridding of the uv-data (see Section 4.4.4 of the AIPS Cookbook). More importantly, better positions may have become available between the time your data was taken, and the time you reduce them. This is particularly likely if your observations took place during or shortly after array reconfigurations. The files given here list the NRAO determinations of the corrections to the antenna positions, the date on which those corrections were found, and the date on which they were entered into the system.

Applying corrections in CASA

Applying corrections in AIPS

Format and Use of Baseline Corrections Lists

A small section of the baseline corrections file might look like this:

                 1997 BASELINE  CORRECTIONS  (IN METER)

ANT
MOVED  OBSDATE   Put_In_ MC(IAT)  ANT  PAD    Bx      By      Bz   
 MAY28  JUN10      JUN12  19:50    7   W16  0.0000  0.0000 -0.0144
 MAY28  JUN10      JUN12  19:50   10   W10  0.0036  0.0000  0.0000
 MAY27  JUN10      JUN12  19:50   21   E2   0.0030  0.0000  0.0000
        JUN10      JUN12  19:50   25   W14  0.0000  0.0025  0.0000
        JUN10      JUN12  19:50   26   W12  0.0032  0.0000  0.0000
 JUN23  JUN24      JUN25  14:54    2   N18  0.0033  0.0152 -0.0060
 MAY27  JUN24      JUN25  14:54    6   E14  0.0049  0.0000  0.0000
        JUN24      JUN25  14:54    7   W16  0.0061  0.0000  0.0000
        JUN24      JUN25  14:54   11   N16  0.0041  0.0000  0.0000
 JUN23  JUN24      JUN25  14:54   15   N14 -0.0117 -0.0150 -0.0096
        JUN24      JUN25  14:54   25   W14  0.0040  0.0000  0.0000
        JUN24      JUN25  14:54   28   E16  0.0048  0.0000  0.0000
        JUN27      JUN28  00:46    1   N12  0.0030  0.0000 -0.0019
 JUN24  JUN27      JUN28  00:46    3   N10 -0.0051 -0.0110  0.0000
        JUN27      JUN28  00:46    7   W16  0.0000  0.0022  0.0000
 JUN24  JUN27      JUN28  00:46   14   E18  0.0072 -0.0082  0.0059

The meaning of these columns is as follows:

ANT MOVED

the date on which the antenna was moved to the indicated pad. A blank in this field does NOT mean "ditto" -- it means "not moved". Corrections in these entries have been re-determined relative to the previous entry for that antenna, i.e. these are corrections to the previous position.

OBSDATE

the date on which the observations used to determine these corrections were taken.

Put_In_

the date on which these corrections were added to the positions in the ModComps (the computers which run the VLA).

MC(IAT)

the IAT at which these corrections were entered into the ModComps.

ANT

the antenna number, as given e.g. in the AN table in AIPS.

PAD

the pad on which that antenna was sitting when the observations were made (as listed e.g. by the AIPS task PRTAN).

Bx, By, Bz

XYZ are in a right-handed Cartesian system. X and Y lie in a plane parallel to the equator. X points toward the intersection of the local meridian and the celestial equator, Y points East, Z points to the North pole. (reference: Thomson, Moran & Swenson, 2nd edition 2001, p86, chap. 4.1)

These corrections are ADDITIVE. To determine the appropriate correction for your observations, you should add up all corrections measured between the MOVE date and the next move after your observing run, excluding any corrections entered into the ModComps before your observing began.

EXAMPLE: Based on the above listing, antenna 7 was moved to pad W16 on 28 May 1997. The 10 June 1997 baseline run gave a -0.0144m correction to its z-coordinate, which was entered into the on-line system on 12 June 1998 at 19:50 IAT. Further corrections were determined on the 24th and the 27th. Let's assume this is the whole story, i.e. no further measurements were made before the telescope was moved to a new pad. If you observed between 28 May and 12 June, you would have to enter the sum of all three corrections; between 12 and 25 June, the sum of the 24 and 27 June corrections; beteen 25 and 28 June, only the 27 June correction; thereafter, no correction is needed.

In general, one should use the sum of all available corrections. However, antennas may indeed shift slightly over time, even when staying on the same pad. The test for an individual data set is either to solve for the antenna positions yourself, using those data; or to see what happens to your phases before and after these corrections, and judge for yourself whether the corrections have improved matters.

The procedure for applying these corrections inside AIPS is discussed in section 4.4.4 of the AIPS Cookbook).

If you have any additional questions regarding baseline corrections, please contact us through the NRAO help desk.