Abstract
For some time now, several ATS Units (ATSUs) have been using an alert function that is based on data sent by the Enhanced Mode S transponder. This transponder version provides a down-link of various airborne parameters that includes the flight level or altitude selected by the pilots in the altitude window of the associated auto-flight system. If the pilot selected flight level or altitude does not correspond with the cleared level/altitude stored by the controller in the ATM (Air Traffic Management) system, an alert is triggered at the controller station. This warning is used by the controller to verify the pilot selected level/altitude and provides a valuable safety net.