Abstract: An MLS system comprising ground based equipment which includes a microwave signal transmitter arranged to feed microwave signals to radiator elements of an array antenna via phase shifter modules one of which is provided for each of the said elements, whereby the production of a scanned microwave signal beam is facilitated, signal monitor/detector means effective to provide a detected sample signal derived from the scanned beam, filter means responsive to the detected sample signals for providing a filtered signal derived in dependence upon effective sidelobe signals in the said beam, and comparator means responsive to the filtered signal for providing a transmitter shutdown signal when the filtered signal exceeds a predetermined threshold level for more than a predetermined time during a predetermined number of successive scans.