GEETEE: The Gauribidanur $T$ Array

GEETEE is a low frequency radiotelescope operating at $34.5$ MHz. It is situated near Gauribidanur, $\sim80$ km from Bangalore, India. The antenna system is a $T$ shaped array with $1000$ dipoles, $640$ in the $1.4$ km long $E-W$ array and 360 in the $0.45$ km long $S$ array. It's collecting area in the $EW \times S$ correlation mode is 18,000 Sq m and has a resolution of $26'\times 42'$ $Sec(\delta-14^0.1)$. The $EW$ array consists of four rows of dipoles in the $NS$ direction, with $160$ dipoles in each row. The $S$ array consists of $90$ rows in the $NS$ direction with four dipoles each placed in the $EW$ direction.

A multibeam-forming receiver has been built for $GEETEE$ to obtain long periods of interference free observation over as large a patch of sky as possible in one day. A short observing time for a wide field survey at low frequencies minimises the effects of the ionosphere. For multibeam operation a single row of $EW$ is used in the meridian transit mode. Single row was chosen to maximise the coverage in declination. A single beam in the $EW$ direction was considered sufficient, as the images are confusion limited. $90$ outputs of the $S$ array are transmitted to the observatory in 23 open-wire transmission lines using time division multiplexing. In the observatory building, the signals from the EW and $S$ arrays are down-converted to an intermediate frequency of $4$ MHz. Then each of the $S$ array output is correlated with the $EW$ array output using one-bit correlators. This gives $90$ visibilities sampled at $5$ m intervals along the NS direction. The Fourier transform of these visibilities gives $90$ multiple beams in the $NS$ direction covering a span of $\pm47^0$ of Zenith angle along the meridian. A two dimensional image of the sky is obtained by stacking successive scans across the meridian.