Patent ID: 7773661

Claim:
A method of monitoring a redundant transmitter of a radio communications system having an active transmitter for transmitting data signals to at least one receiver, the method comprising the steps of: a) during normal operation, transmitting over the redundant transmitter a spread-spectrum signal within a frequency band of the data signals, said spread-spectrum signal being of low spectral power in comparison with said data signals; and b) at said at least one receiver, detecting a presence of the spread-spectrum signal, an absence of the spread-spectrum signal being taken to indicate non-integrity of the redundant transmitter; c) wherein the detecting step is performed by a process of cross-correlation, the spread-spectrum signal being provided by feeding the redundant transmitter with a first pseudo-noise signal, and the cross-correlation being performed between a received signal and a second pseudo-noise signal, the second pseudo-noise signal having the same characteristics as the first pseudo-noise signal, the first pseudo-noise signal passing through the same components in the redundant transmitter as would the data signals, were the redundant transmitter called upon to take over from the active transmitter.