In interferometry the geometric delay suffered by a signal (see Chapter 4) has to be compensated before correlation is done. In an analog system this can be achieved by adding or removing cables from the signal path. An equivalent method in digital processing is to take sampled data that are offset in time. Mathematically, is the sample delayed by with respect to (where is the sampling frequency). In such an implementation of delay it is obvious that the delay can be corrected only to the nearest integral multiple of .

A delay less than (called *fractional delay*) can
also be achieved digitally. A delay introduced in the path of
a narrow band signal with angular frequency produces a
phase
. Thus, for a broad band signal, the delay
introduces a phase gradient across the spectrum. The slope of the
phase gradient is equal to the delay or
.
This means that introducing a phase gradient in the FT of
is equivalent to introducing a delay is . Small enough
phase gradients can be applied to realize a delay . In the GMRT
correlator, residual delays is compensated using
this method. This correction is called the Fractional Sampling
Time Correction or FSTC.