Definition
One of the two audio modulation frequencies used by the ILS localizer and glide slope to define course and path guidance. The 150 Hz signal is radiated to one side of the on-course (localizer) or on-path (glide slope) line; the 90 Hz signal is radiated to the other side. The aircraft's receiver compares the strength of the two tones, and the difference drives the course deviation indicator. When both tones are received equally, the aircraft is on course or on glide path.
Plain English
A specific tone built into the ILS signal. The localizer and glide slope each transmit two tones, 90 Hz and 150 Hz, on opposite sides of the correct path. The cockpit instrument shows you which side you're on by comparing the strength of the two tones.
Context Anchor
Seen in ILS discussions, especially localizer, glide slope, and ILS error descriptions.
Derivation
Hz is short for hertz, the unit for cycles per second, named after the German physicist Heinrich Hertz. So 150 Hz means a tone vibrating 150 times per second. The number itself was chosen by ILS system designers to pair cleanly with the 90 Hz tone and be easy for a receiver to separate.
Why Pilots Care
Correct interpretation of the 150 Hz versus 90 Hz balance lets the pilot know which way to correct to stay on the localizer during an instrument approach.
Grounding Statement
For ILS use, think of 150 Hz as one of the two repeated tones the equipment listens to in order to show aircraft position relative to the course or glidepath.
Intuition Check
150 Hz is not the ILS radio frequency you tune in the cockpit. It is a tone carried inside the ILS signal after the navigation receiver is tuned.
Example Sentence 1
If the 150 Hz signal is stronger than the 90 Hz signal at the receiver, the localizer needle will deflect, telling the pilot to correct back toward the on-course line.
Example Sentence 2
Terrain reflection can unbalance the 150 Hz and 90 Hz tones and produce false course indications.