Adequate baseline sensitivity is necessary for VLBI fringe
fitting, discussed in Section 15.3. The following
formula can be used in conjunction with the typical zenith 
s
for VLBA antennas given in Table 2 to calculate the RMS
thermal noise (
) in the visibility amplitude of a
single-polarization baseline between two identical antennas (Walker
1995a):
In Equation 2, 
accounts for the VLBI
system inefficiency (e.g., quantization in the data recording and
correlator approximations). Assume 
for data from a Mark III correlator; values are soon to be determined
for the VLBA correlator. 
is the bandwidth [Hz]; use the
full recorded bandwidth for a continuum target and use a spectral
channel for a line target. 
is the fringe-fit interval
[s], which should be less than or about equal to the coherence time
. Equation 2 holds in the weak source
limit and assumes 1-bit (2-level) quantization. About the same noise
can be obtained with 2-bit (4-level) quantization and half the
bandwidth, which gives the same bit rate. Moran & Dhawan (1995)
discuss expected coherence times. The actual coherence time
appropriate for a given VLBA project can be estimated using observed
fringe amplitude data on an appropriately strong and compact source.