We review millimeter interferometric phase variations caused by variations
in the precipitable water vapor content of the troposphere, and we discuss
techniques proposed to correct for these variations. We present observation
s with the Very Large Array (VLA) at 22 and 43 GHz designed to test these t
echniques. We find that both the fast switching and paired array calibratio
n techniques are effective at reducing tropospheric phase noise for radio i
nterferometers. In both cases, the residual rms phase fluctuations after co
rrection are independent of baseline length for b > b(eff). These technique
s allow for diffraction-limited imaging of faint sources on arbitrarily lon
g baselines at millimeter wavelengths. We consider the technique of troposp
heric phase correction using a measurement of the precipitable water vapor
content of the troposphere via a radiometric measurement of the brightness
temperature of the atmosphere. Required sensitivities range from 20 mK at 9
0 GHz to 1 K at 185 GHz for the millimeter array (MMA) and to 120 mK for th
e VLA at 22 GHz. The minimum gain stability requirement is 200 at 185 GHz a
t the MMA, assuming that the astronomical receivers are used for radiometry
. This increases to 2000 for an uncooled system. The stability requirement
is 450 for the cooled system at the VLA at 22 GHz. To perform absolute radi
ometric phase corrections also requires knowledge of the tropospheric param
eters and models to an accuracy of a few percent. It may be possible to per
form an "empirically calibrated" radiometric phase correction, in which the
relationship between fluctuations in brightness temperature differences an
d fluctuations in interferometric phases is calibrated by observing a stron
g celestial calibrator at regular intervals. A number of questions remain c
oncerning this technique, including the following: (1) Over what timescale
and distance will this technique allow for radiometric phase corrections wh
en switching between the source and the calibrator? (2) How often will cali
bration of the T-B(rms) - phi(rms) relationship be required?