The GMRT Array

The GMRT, consists of 30 fully steerable parabolic dishes, each of 45m. diameter spread over 25km in an approximate 'Y' configuration. Twelve of the 30 antennas are in a central compact random array of 1km x 1km in size and the remaining 18 antennas are stretched out along the arms of 'Y', each 14 km long. GMRT currently operates in the frequency bands 151, 235, 325, 610 and 1000$-$1400 MHz. GMRT is a very sensitive aperture synthesis array at low radio frequencies. The central array and the long arm antennas provide good sensitivity for detecting large diffuse objects and high angular resolution of a few arc-second, needed for small objects respectively. GMRT is currently being used for a variety of astrophysical studies, e.g., detection of red-shifted 21cm line radiation, pulsars, HII regions and supernova remnants in our Galaxy, diffuse emission from nearby galaxies, giant radio galaxies, distant quasars, damped Lyman-alpha systems as well as the nearest star, the Sun.

The figure 2.2 shows the location of GMRT antennas.

Figure 2.2: Location of GMRT antennas.

The advantage in building an array (GMRT) in India is because at low radio frequencies, $i)$ The surface accuracy requirements are not so stringent; hence construction could be done using manual labor, resulting in lower cost. $ii)$ Low radio frequency electronics is relatively inexpensive and can be built in-house. $iii)$ Very high angular resolution is not required, since the interesting science at low radio frequencies is in diffused extended regions.

The disadvantages are $i)$ The sky background is higher. $ii)$ Ionospheric distortion increases towards lower radio frequencies. $iii)$ Man made noise is higher. (Ref. Swarup etal, 1991, Ananthkrishnan & Rao, 2002).