Definition
Additional signal lobes radiated by an ILS localizer or glide slope antenna, outside the protected approach area, that can produce on-course or on-glidepath indications when the aircraft is not actually aligned with the published approach path. They appear at angles other than the intended course or glide slope and can mislead pilots who intercept the ILS outside its protected service volume.
Plain English
An ILS antenna doesn't only send its guidance signal straight down the runway. It also sends weaker signals off to the side or at the wrong vertical angle. If a pilot picks one of these up by mistake, the cockpit needles can show 'on course' or 'on glidepath' even though the airplane is in the wrong place.
Context Anchor
Encountered in ILS approach discussions, especially when intercepting guidance far from the runway or outside the normal area where the signal is protected for use.
Derivation
False' here means 'not the real one' — a signal that looks valid but isn't. The localizer and glide slope each have one true course, and any other signal lobe that mimics it is therefore 'false.'
Why Pilots Care
Following a false course can produce large lateral or vertical deviations, especially on final approach, leading to unsafe flight paths or missed approaches.
Intuition Check
False does not just mean slightly inaccurate here, and courses does not mean training classes. In this context, false courses are misleading instrument-guidance paths that can appear real but are not the intended ILS approach path.
Example Sentence 1
The instructor explained that intercepting the localizer well outside the published fix risked picking up false courses instead of the real final approach path.
Example Sentence 2
Terrain near the airport created false courses that required extra vigilance on the inbound course.