The On-Final phase-of-flight, as determined by the system, must be valid in order for the system to issue Landing Alerts. The system will issue Landing Alerts when the aircraft is below 250 feet AGL and within one nautical mile of the runway threshold. Nominally, the aircraft will be in the SurfaceWatch 'On-Final Phase-of- Flight' when the aircraft is on approach and nearing the airport to land (generally within a couple of nautical miles of the runway threshold).