The most important measurement for radio astrometry is that of the
actual fringe phase of a connected-element interferometer (or similarly the group
delay in VLBI). Let be the angle between the reference direction and the
meridian plane of a given interferometer baseline. The phase is then defined by
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(17.4) |
![]() |
(17.5) |
Accounting for uncertainties in the baseline and source position vectors the
actual phase is
![]() |
(17.6) |
![]() |
(17.7) |
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(17.8) |
The two limiting cases
, and
correspond to those where we either calibrate the baseline or determine the exact
source position.
In the first case the source coordinates are perfectly known and by comparing
the observed phase with the reference phase
one determines
and hence the true baseline
. The
reference sources observed for baseline calibration are bright quasars or galactic
nuclei whose absolute coordinates are accurately known. The most highly accurate
source coordinates are those of the radio sources used to realize by VLBI the
International Celestial Reference Frame (ICRF); distribution of coordinate errors
are below one milliarcsecond. However, the ICRF catalogue is insufficient for phase
and baseline calibrations of millimeter-wave arrays because most sources are not
bright enough in the millimeter-wave domain. The IRAM calibration source list is
thus a combination of several catalogues of compact radio sources.