= Radio interferometer calibration
Radio interferometer calibration estimates instrumental and propagation effects that alter a <complex visibility>. A simple narrow-field model is
$$
V_{12}^{\rm obs}=g_1g_2^*V_{12}^{\rm sky}+n_{12},
$$
where $g_1,g_2$ are complex receiver gains and $n_{12}$ represents additive <noise>. A <point-like astronomical source> of known position and stable strength constrains these gains: its expected <complex visibility> has a known <wave phase> and amplitude. A resolved or variable reference source requires a better source model. Errors in the reference model can be transferred into the inferred sky.
In a proposed extension, <Martin Ryle> described simultaneous observation of a nearby reference point source to correct atmospheric <wave phase> errors, using an additional dish at each element. He presented this as a possible extension, rather than the standard <One-Mile Telescope> arrangement.<Radio Telescopes of Large Resolving Power by Martin Ryle>[Ryle (1974), p. 202: proposed reference-source correction]{ref}
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