In VLBA polarimetric observations, BB channels are assigned in pairs to opposite hands of circular polarization at each frequency. Such observation can be recorded in VLBA or in Mark III formats.
Although straight-forward conceptually, calibration of continuum polarimetery has traditionally been very difficult (Cotton 1993, 1995b). Steps that must be followed include normal amplitude calibration; fringe-fitting; self-calibration and Stokes I image formation; instrumental polarization calibration; setting the absolute position angle of electric vectors on the sky; and correction for ionospheric Faraday rotation, if necessary. The polarization calibration path in AIPS is currently being actively developed, tested, and documented. This path includes powerful global fringe-fitting techniques to locate weak cross-polarized signals. A polarization self-calibration technique is described by Leppänen, Zensus, & Diamond 1995).
To permit calibration of the instrumental polarization at centimeter wavelengths, VLBA users should include observations of sources either with simple linearly polarized structure (e.g., J1407+2827, see Table 4) or no linearly polarized emission (e.g., J0319+4130, see Table 5). To set the absolute position angle of electric vectors on the sky, VLBA users will want to observe a source whose linear polarization is known at the epoch of their project. BL Lacertae objects can be used for this purpose (Cotton 1993; Gabuzda 1994), with the caveat that their linearly polarized emission typically varies rapidly in time.