FRINGE FINDERS: which are strong sources near the source
of interest to estimate delay and rate errors due to correlator
model errors other than source position. Particularly handy
to correct large clock errors.





PHASE REFERENCING: where observations of a calibrator source
are interleaved with those of the target. Then fringe fit solutions
for the calibrator source can be used to calibrate delay, rate
and phase residuals of the target.

• 
  http://astrogeo.org/calib/search.html — An interactive VLBI
  calibrator search engine.
  





• 
  http://astrogeo.org/calib/rfc — The most complete catalogue
  of source position produced from dedicated VLBI absolute astrometry
  experiments (~5000 objects);
  





• 
  http://astrogeo.org/vlbi_images — The database of source
  brightness distributions in FITS format accompanied with images
  from prior VLBI observations for over 4000 objects;
  





• 
  http://astrogeo.org/lcs/cat/flux_density.html — The table
  of correlated flux densities at different baseline lengths for over
  500 sources with declinations below -40°.
  





• 
  http://www.vlba.nrao.edu/astro/calib/ — The
  VLBA calibrator search tool.
  

Note 2: Many sources are variable and exhibit changes in their structure.