As discussed in Chapter 21, all the GMRT receivers have the facility for noise injection. By injecting noise of known power the system temperature can be measured. The noise at any antenna can be switched on and off (on sub second time scales) according to a pre-determined pattern, which is encoded in PROMs in the Antenna Based Receiver (ABR). By synchronously measuring the total power, it is possible to calibrate the system temperature. The synchronous total power measurment however has not yet been implemented.
Signals from one antenna could leak into another antenna at various points along the signal flow chain. This is normally referred to as cross-talk. This would cause a spurious corrrelation between the base-band signals from these two antennas. This leakage can be minimized by switching the phase of the RF signal of each antenna by a pattern that is ortho-normal to the pattern used for all other antennas. At the correlator the exact reverse phase switching is done for each antenna so that the original phase is recovered just before the cross-correlation is done. Such a scheme would greatly reduce the cross-talk at all points between the RF amplifier and the base-band. Typically the ortho-normal functions used are Walsh functions, and this scheme is called Walsh Switching. The required Walsh patterns for each antenna are also encoded in PROMs situated at the ABR. However the Walsh demodulation at the correlator is yet to be implemented.