Definition
One of the two audio modulation frequencies used by an Instrument Landing System (ILS) to define the position of the aircraft relative to the localizer centerline and the glide slope. The 90 Hz signal predominates on the left side of the localizer course and above the glide slope; the 150 Hz signal predominates on the right side of the localizer and below the glide slope. The aircraft's ILS receiver compares the strength of the 90 Hz and 150 Hz signals, and the difference drives the course deviation indicator (CDI) and glide slope needle.
Plain English
A specific tone the ILS transmits as part of its guidance signal. The receiver compares this tone with a second tone (150 Hz) to figure out whether the aircraft is left or right of the runway centerline, and above or below the proper descent path.
Context Anchor
Seen in ILS error discussions, especially when explaining how localizer and glideslope signals create course guidance.
Derivation
Hz stands for hertz, the unit of frequency meaning cycles per second, named after physicist Heinrich Hertz. So 90 Hz simply means a signal that oscillates 90 times per second -- an audio-range tone the ILS receiver can isolate and measure.
Why Pilots Care
The balance between the 90 Hz and 150 Hz tones drives the course-deviation indicator, directly affecting whether the pilot stays on the localizer during an ILS approach.
Analogy
Think of two steady beeps playing at different speeds. The receiver is not just hearing one beep; it is comparing which beep is stronger so it can tell which way the airplane needs to move.
Intuition Check
90 Hz is not the ILS radio channel frequency. It is a low-frequency tone carried on the ILS signal and used for guidance comparison.
Example Sentence 1
If the aircraft drifts left of the localizer centerline, the 90 Hz signal becomes stronger than the 150 Hz signal, and the CDI deflects to the right to command a correction.
Example Sentence 2
Stronger 90 Hz reception causes the CDI needle to deflect right, prompting a correction back toward the course.