Square Kilometer Array (SKA) Concept

In one way or another, all of the various research directions in radioastronomy are limited by our current instrumental sensitivities. Only by ensuring the continued access to order-of-magnitude improvements in our capabilities, can we ensure a continued high rate of discovery! The sensitivity of radio telescopes, in the time between $1940$ and $1980$, have shown an exponential improvement, over at least $6$ orders of magnitude $(10^0$ mJy to $0.1$ mJy for $1$ minute integration time). The radio astronomers are toying with the idea of building a telescope with an improvement in sensitivity by a factor of 100 and are hoping that it will lead to fundamental scientific advances (Braun, 1996)

Consideration of the many varied scientific drivers suggests the following basic technical specifications for the instrument:

1. A frequency range of 200 to 2000 MHz.
2. A total collecting area of $1$ km$^2$
3. Distribution over at least 32 elements.

The NFRA in their study of the SKA concept suggest that a broad-band, highly integrated phased array antennas should be adopted for such an array. Some of the advantages are:

1. Phased arrays give ``complete" control of beam. The main application considered being the adaptive suppression of RFI environment.
2. Multiple independent beams possible resulting in multiple programs and rapid surveys.

They are planning development work in this direction in several steps: Adaptive array demo, one sq. meter array and a thousand element array and proof of principal arrays. Discussion of all these aspects is beyond the scope of this chapter. Instead we end with the principle of an adaptive array.