Measurement of the antenna locations to an accuracy of a fraction of
the wavelength (baseline calibration) as well as the measurement of
the fixed delays suffered by the signals from various antennas is
necessary for imaging with an aperture synthesis telescope like the
GMRT. These measurements were made during the course of this work.
The surveyed positions of the antennas, which were in error by 
m,
were used to bootstrap the baseline calibration process. Visibility
phase variation at 327 MHz as a function of Hour angle was used to
improve upon these values to an accuracy of 
cm. Later, using
1420 MHz observations, in combination with the GPS measurements,
antenna positions to an inferred accuracy of 
cm were
measured. Antenna fixed delays were measured from the phase gradient
across the band. Using the measured antenna positions and antenna
fixed delays, software was written for off-line fringe stopping. This
software was extensively used with the prototype 8-antenna correlator,
which did not have the facility for fringe stopping, as well as during
the early stages of the full 30-antenna correlator. Measurement of
flux densities tied to standard flux scales also requires measurement
of other system parameters like the primary beam attenuation, system
temperature, etc. All these measurements were also made.
The digital FX type correlator of the GMRT was used for these observations for system calibration and later for scientific observations. In the early stages of the commissioning of the telescope and the correlator, data was corrupted due to problems in the hardware (servo, telemetry, antenna tracking, Local oscillators systems, etc.) as well as the software (on-line control, data acquisition system, fringe stopping and correlator control, data acquisition systems, etc.). Various tests/simulations were done to gain understanding of the working of the telescope/correlator. Several test observations were also done to identify problems and potential sources of data corruption - particularly in the correlator hardware (which often resulted in high closure errors in the data) and fringe stopping software.
It was clear that in these early stages of this new instrument,
monitoring the data quality and the health of the system during
observations was of paramount importance. Since there were many
sources of data corruption and the manner in which they affect the
data were varied, it was useful to develop techniques and software
which can automatically identify as many of these problems, as early
on in the data analysis process as possible. Extensive software was
developed for this purpose in the form of general purpose object
oriented libraries, as well as in the form of programs for on-line and
off-line data processing and display. In order to make application
programs usable by others, a system of user interface and
documentation was also developed. This software system now runs into
lines of C, C++ and FORTRAN code.
The complex visibility function, 
, depends on a number of telescope
parameters like the system temperature, sensitivity, antenna fixed
delays, antenna positions, etc. Since the complex visibilities are a
function of a multitude of parameters, and different debugging
purposes require viewing with respect to various quantities, a
compact macro language was developed using which, data can be
extracted from the GMRT visibility database. This was implemented in
the form of a stand alone library as well as used in the application
program named xtract. The content and the output format can be
easily specified via this macro language, allowing easy access to
astronomical/engineering data. This also allows easy interface to
existing data display software for visualization. This was used
extensively during the course of this work and is now being widely
used for a variety of measurements like the system temperature,
antenna sensitivity, antenna beam shapes, etc.
Equations for solving for antenna based complex gains using the complex visibility measurements for an unresolved source were derived using complex calculus. This led to a simple interpretation of the algorithm to solve for the antenna gains, which in turn led to the development of an algorithm which is robust in the presence of often large amounts of severely corrupted/bad data. This was done by (1) doing two passes to eliminate dead/bad antennas, (2) automatically eliminating out-lying points using robust averaging (where the mean value is computed by iteratively eliminating points which deviate from the mean by greater than a threshold value). This algorithm was also used to identify and flag corrupted data in a semi-automatic fashion. This algorithm, implemented in the program rantsol, was also extensively used and is now used by others for various measurements and for the phased array mode of operation of the GMRT (Sirothia2000). It was also used for on-line data monitoring and for the semi-automatic identification of bad data. The output of this program was examined by another program (badbase) which identified consistently bad antennas and baselines in the data. This information was later used to flag data before using it for mapping. Data with high closure errors was often present on a few baselines out of a total of 435 baselines. The calibration program of AIPS is very sensitive to such bad data and the presence of about 10% bad baselines could result into poor solutions or sometimes no convergence at all. Automatic identification of such bad data was therefore important and a great time saver.
Signals from the two orthogonally polarized feeds can leak into each other at various point in the signal path. The planned Walsh switching of the signals from the two orthogonal feeds to minimized cross-talk has not yet been implemented for the GMRT. Besides, Walsh switching will not eliminate any leakage introduced before the switching point. Such leakage can result into closure errors even in the co-polar visibilities. This has been pointed out earlier by Rogers (1983) in the context of VLBA. A more detailed study of this was done by Massi et al. (1997) for European VLBI Network (EVN). Polarization leakage at a few percent level at most bands and at a much higher level at 150 MHz have been measured for the GMRT. The current GMRT correlator measures only the co-polar visibilities. A method was therefore developed to solve for the polarization leakage using only the co-polar visibilites for an unpolarized calibrator. The solutions, though have 3-parameter ambiguity (in the same sense as the phase ambiguity in Self-cal solutions), can be used to study the polarization properties of the individual antennas as well as to correct for the closure errors arising due to polarization leakage.
Other software developed during the course of this work includes software for time and bandpass calibration, data display, checking the consistency of the visibility database, fixing/editing data affected by problems in the system hardware/software, off-line correction of visibility phase due to antenna position offset/frequency offset, etc.