LBA Calibrator survey: 1st experiment correlation is finished results are available

[Leonid Petrov <Leonid.Petrov@xxxxxxxxxxx>]

Dear colleagues,

  The correlation of v254b is finished. I made a preliminary analysis
and derived positions of 121 sources. The catalogue of source positions
and the correlator output can be found at the updated project web page
http://vlbi.gsfc.nasa.gov/lcs . We are still waiting for system temperature
data from Parkes. Except, this the data are ready for imaging. Later, 
I will compute and apply the ionosphere contribution based on TEC estimates 
from GPS and then the final catalogue will be produced. I do not anticipate 
applying the ionosphere contribution will shift sources positions more 
than 2 sigmas i.e. 4-6 mas.

  I have already commented the experiment. Group delay ambiguities were
successfully resolved. I scheduled each source in three observations instead
of t scans per source in VLBA Calibrator Surveys. Analysis confirmed that 
it was right strategy. For three source with less than 10 observations 
(i.e. a source was detected in two scans, but at some baselines it was 
missing) group delay ambiguity resolution in the preliminary solution was 
wrong. According to our previous experience with VLBA, 7 detections were 
sufficient to rule out an error in ambiguity resolution. For LBA data with 
7 times smaller ambiguity spacing and 5 times smaller network, this limit 
should be raised to 10-13. Changes in scheduling algorithm that require the 
interval between scans be no less than 4 hours will reduce the probability
that the error in ambiguity resolution will be unnoticed. Anyway, 
30 detections (3 scans at 5 stations) are sufficient. 

  The second experiment v271a was observed last Sunday, 2008.08.10. Hope, 
we will see results this year.

Sincerely,
Leonid Petrov
13-AUG-2008 18:51:39