Accurate Source Position Service
NRAO offers to provide astrometric quality source positions in response to requests from users. The positions are measured during the joint NASA/USNO/NRAO 20 station geodetic/astrometric observations that occur about every 2 months and that mainly serve other purposes. A few sources can be measured during each of these observations. Formal position errors of between 0.1 and 3 mas have been obtained for sources measured under this program in the past.
Requests for positions should be received well ahead of the time when they are needed, for example, for use on a VLBI correlator. It takes a minimum of 2 to 4 months to obtain a position because of the wait between observing runs and the amount of time it takes to schedule, observe, fringe search, correlate, release, and reduce these large runs. If a position must be available for processing of another observation, the request should be made no later than when the other observations are proposed. Also, don't request too many sources. Only a small number can be added to each run.
If you have a source for which you need a position, first check that it really qualifies. We have received a number of requests for sources that were too weak, or for which good positions were already available. A source should have at least 0.1 Jy at 2.3 and 8.4 GHz and there should be some indication, either from past observations or from a flat spectrum, that it can be seen with VLBI. Some good resources for known source positions are the USNO Celestial Reference Frame Department, the International Celestial Reference Frame (ICRF), the Radio Fundamental Catalog (RFC), and the VLBA Calibrator Search Tool. Also, VLA positions for a large number of sources from the JVAS survey, good to 12-55 mas, are available by anonymous ftp at ftp.aoc.nrao.edu in file pub/sources.jvas.
Requests for positions should be made via the helpdesk, selecting the VLBA Scheduling Support Department. Please include the following information:
1. A short scientific justification.
2. Source names.
3. Source coordinates good to at least 0.5 arcseconds.
4. The expected flux densities at 2.3 and 8.4 GHz.
5. Evidence that there will be compact structure.
Requests for positions of more than 10 sources will not be considered. Accepted sources will be observed as soon as possible and positions will be returned by email. The results will be public domain immediately and source positions, and in many cases, even images will appear on the geodetic web sites.
If source positions obtained through this service are used in a publication, or are instrumental in obtaining the results in a publication, please include an acknowledgement that the source positions were provided by observations from the joint NASA/USNO/NRAO geodetic/astrometric observing program and that the positions are in the frame of the ICRF3 catalog. The ICRF3 catalog is the official IAU reference frame. A detailed description of the ICRF3 can be found in the Charlot et al. (2020). An acknowledgement will be appreciated by the NASA and USNO groups that actually do most of the work to derive the positions.
