Phase referencing permits imaging of target sources that are too weak to give detectable fringes in a coherent integration time. Many details of this process are given in VLBA Scientific Memo No. 24. Typically, at 128 Mbps, targets weaker than about 50 mJy at centimeter wavelengths, and 100-200 mJy at 22 and 43 GHz, require phase referencing.

When phase-referencing in the nodding style, a VLA-like sequence of calibrator-target-calibrator is employed. The calibrator should be within about 2°-3° of the target source, be dominated by a point source, and have a compact flux density of at least about 150 mJy. At 1.4, 22, and 43 GHz, smaller target/calibrator separations are desirable, and may be essential. At 86 GHz, smaller target/calibrator separations are essential. Users should consult the VLBA Calibrator Survey. Due to the extreme problems with the ionosphere, there have been no reports of successful phase-referencing in the nodding style at 0.3 or 0.6 GHz. Due to the extreme problems with the troposphere, there has been only one report of successful phase-referencing in the nodding syle at 86 GHz (Porcas & Rioja 2002).

Cycle times can be estimated from equations (17-9) and (17-10) in Beasley & Conway (1995). Tabulated and plotted estimates based on those equations, as a function of source elevation and weather, also are in VLBA Scientific Memo No. 20. The time estimates in Scientific Memo No. 20 only take the troposphere into account; modifications to account for the ionosphere may be found in VLBA Scientific Memos No. 18 and 22.

(Adapted from the Novice's Guide to Using the VLBA.)

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