Facilities > VLBA > News for VLBA/HSA/VLBI Proposers and Observers

News for VLBA/HSA/VLBI Proposers and Observers

by Amy Mioduszewski last modified Jul 09, 2019 by Walter Brisken
Contributors: jromney

July 9, 2019 W. F. Brisken

Saint Croix antenna downtime for major maintence (update)

Metalwork repair on the Saint Croix antenna is complete and the first coat of new paint has been applied. Remaining work includes completing the second coat of paint, reinstalling external cables, and recommissioning the antenna. VLBA staff are expecting return to service by approximately August 9, 2019.

April 5, 2019 W. F. Brisken

Saint Croix antenna downtime for major maintence (update)

The Saint Croix VLBA antenna will be going offline on April 22 for the work noted below. Exact date of return-to-service is not yet known but the duration of the downtime is expected to be approximately 3.5 months.

December 19, 2018 W. F. Brisken

Saint Croix antenna downtime for major maintence

The VLBA antenna on the island of Saint Croix was hit by a pair of hurricanes in September, 2017. After 6 months, primarily in restoring electrical power and internet services to the island, the antenna was returned to service. The VLBA received disaster relief funding to restore the VLBA antenna and site to pre-hurricane conditions. Some of this work has begun and has not impacted observing. The major upcoming effort will be corrosion mitigation and painting of the 25m antenna. This will require significant non-observing down-time. Exact dates are not yet known, but this work could begin by mid March, 2019 and will likely last 4.5 months. Please revisit this page for updates.

September 28, 2018 —  W. F. Brisken

Unstable phase on the GBT

For a period of at least three years the GBT was operating with a local oscillator that had its phase reset at the start of each observing scan, even if the tuning did not change, with potentially serious effect on VLBI
data.  There is no evidence or reason to believe that delay jumps happen in conjunction with the phase jumps.  It is believed this problem began when GBT's LO synthesizers were replaced in June 2014 and continued through approximately Sep 20, 2017.  This problem would have affected most phase-referenced VLBI observations made with the GBT in this period.  Usually this is done for weak sources where self calibration is not possible, or in the case of astrometry where relative offset between a calibrator and a target source is the desired measurement.  In these cases the GBT data would have produced data of no value to the experiments.  In cases where in-beam calibration was employed (where a suitably bright source within the primary beam of the GBT is simultaneously observed, but separately correlated, is used to calibrate the weaker target), the GBT data are likely still usable, however users of such data will need to be careful not to determine calibration across scan boundaries.  In cases where the GBT was affected, baselines not including the GBT would still be valid.  In cases where self calibration was used on the target or where group delays are the primary observable, the GBT data would be usable. If the observations were made with pulse-cal enabled, the pulse-cal data distributed within the FITS file (PC table) might be usable to correct for the phase jumps.  Please see documentation for the PCCOR task in AIPS.

September 28, 2018

RDBE Delay and Phase Jumps: Update

A pair of VLBA memos has been released providing more explanation of the delay jumps and some additional advise to users.  The following two memos are available:

September 16, 2016 —  J. D. Romney

RDBE Delay and Phase Jumps

Delay and phase jumps have been detected in observations made using both the DDC and PFB personalities of the RDBE. These occur only quite infrequently (as far as we are aware, at least), and characteristics of the jumps vary substantially, so it has not been possible to describe all the effects in detail, nor to recommend countermeasures beyond careful analysis of observational delays. General descriptions of the most common effects are sketched below.

1> Delay jumps upon resumption of an observation following interruption for short USNO operations at PT, MK, and possibly other stations:  Any restart of an observation may require a reload of the appropriate FPGA firmware, and a resynchronization of the internal timekeeping may occur. A general rule for these cases is that return to delay and phase cannot be guaranteed on a schedule resumption, and comparison of delays at the affected stations before and after the interruption is necessary. Notification of the interruption has been included in the VLBA operators' logs for a long time, and was augmented recently to advise such a check.

2> DDC delay jumps: Such jumps have been seen sporadically for the last ~2 years, and have been the focus of a major investigation, and of numerous attempted corrections in the DDC firmware, all without complete success to this point. In some but not all cases, such jumps occur (if at all) on the second use of any given setup in the observation schedule, and do not recur subsequently within the same schedule. This effect, and the overall low rate of occurrence, both limit efficient analysis.  Some test schemes have been developed to gather more data than available previously (at the cost of substantial observing time). Tests suggest a basic jump quantum that is inversely proportional to the channel bandwidth: 256ns / BW[MHz] for 2<=BW<=16;  jumps of this type are not known to occur for BW=1 nor BW>=32.  Some cases have been seen with small-integer multiples of that jump quantum.  A major rewrite of the DDC FPGA firmware is currently under way, aimed both at preventing such jumps, and at facilitating tests to verify error-free performance.

3> PFB phase jumps: These have been rare enough to be thought not to occur any more, but at least one event was seen recently. The jump occurred only in the phase-cal results at one station, and was not seen in the actual interferometer phases.


September 10, 2015

RDBE_based VLBA Gains

VLBA pointing and gain measurements in the past in the past have been made based on power levels detected by the legacy Baseband Converters (BBCs). Since February 2014, power levels have also been measured in the new RDBE broadband system. The analysis which produces the gain values given in the vlba_gains.key has been run in parallel with both data sets.  Until late August 2015, the vlba_gains.key derived from the BBCs was the one placed on the ftp site and used as the sources of the values distributed with exported data sets in the gain table.  On September 2, 2015 we switched to using the gains derived from the RDBE data in the vlba_gains.key file. More information is presented in the VLBA Scientific Memo #37.

December 31, 2011

VLBA flag problems (Nov 10 to Dec 22)

During the second week of November, upgrades were made at Pie Town in support of the upgraded C-band receiver. These changes have transferred monitor and control of some antenna electronics from the "legacy" station computer to the modern system which is based on EVLA software. This new system is working well, however the legacy system, which is still responsible for generating flags for the station, was perpetually flagging data based on monitor points that no longer exist. The Hancock antenna C-band system was upgraded in December with the same issue beginning with observations made on Dec 10.

Data observed after Nov 10 and correlated before Dec 7 will have a single erroneous flag for Pie Town encompassing the entire observation. Data observed between Dec 7 and Dec 22 won't have any flags for Pie Town or Hancock. Data observed after Dec 22 should be back to normal.

VLBA operations staff have regenerated flag data for the Nov 10 through Dec 22 period. Users wishing to restore proper flags can follow the instructions to download cal.vlba.gz and then follow instructions in the AIPS Cookbook. The section titled Automatic formatting of VLBA and VLBA-like log files will explain how to use VLOG and UVFLG to load flags from the downloaded file. You may want to make a backup (with TASAV) of the original FL table before deleting it (with EXTD) just in case you want to go back to the original flags.

Questions regarding the flagging problem and its corrections should be directed at the NRAO helpdesk.

August 29, 2011

Hancock VLBA station temporarily shut down by hurricane Irene

The Hancock VLBA station was put into shutdown mode in the morning of August 27 in anticipation of high winds, heavy rains and power loss due to hurricane Irene.  Commercial power was lost at approximately 3 am Sunday morning and has not yet been restored.  The site techs were able to reach the station on Monday, August 29 and determined that there is no obvious damage to the antenna or structures.  Numerous trees are down around the site and on the roads and power lines have been downed by fallen trees.  It may take several days before power is restored, however, backup generators for critical systems (maser, cryogenics, etc.) are working so we anticipate a smooth return to operations once power is restored.

August 8, 2011

Error found in EOP corrections in AIPS

Versions of the AIPS task CLCOR released between Sept. 21, 2009 and Aug. 4, 2011 used the wrong sign on the station Y coordinate when calculating Earth Orientation Parameter (EOP) corrections. The main adverse impact will be on phase referencing projects for which sufficiently large EOP corrections were made that the difference in the relative correction between calibrator and target is significant. Check the Details of the CLCOR/EOP Error page for more details. 

July 6, 2011

Los Alamos antenna back in operation

In the evening of July 6, 2011 the danger posed by the Las Conchas fire had subsided sufficiently that the Los Alamos VLBA antenna was put back in operation.  It has been taking part in VLBA observations since.

July 2, 2011

Las Conchas Fire near Los Alamos

The Las Conchas Fire that started on June 26, 2011, came within a few miles of the LA VLBA antenna. The LA station is out of danger, but the fire continues to burn and is not yet contained. The Los Alamos National Laboratory remains closed. NRAO therefore has no access to the antenna at this time, and LA cannot currently be included in VLBA observations.

June 21, 2010

VLBA Pie Town azimuth track fixed

The Pie Town VLBA antenna was restored to full service on June 17, 2010, after antenna mechanics were able to effect a repair to the split in the rail. The damaged rail section was regrouted and the split rail was welded. It is believed that the split in the rail was caused by a poor quality weld of the rail to the mounting plate during the construction of PT. It appears that this weld cracked during cooling and then slowly propagated up the rail until the failure occurred.

June 1, 2010

VLBA Pie Town antenna down for repair of azimuth track

During a routine inspection of the Pie Town VLBA azimuth track on May 25, 2010, a split in the rail and grouting near bolt 20 was discovered. Until a repair is completed, the PT antenna will be unavailable for observing. Several options for the repair are under consideration, but PT will be out of service until at least July 1, 2010.