Why is it that localizer and glideslope system are using 90 Hz and 150 Hz frequencies only, instead of some other LF such as 75 Hz?
Part two of my question: How does the modulation depth differ for aircraft using localizer and GS receiver?
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Sign up to join this communityWhy is it that localizer and glideslope system are using 90 Hz and 150 Hz frequencies only, instead of some other LF such as 75 Hz?
Part two of my question: How does the modulation depth differ for aircraft using localizer and GS receiver?
The frequencies are chosen to allow the system to produce a localizer and glideslope 'beam' that provides proportional guidance that can adequately guide an aircraft on the approach.
Read through this Question and Answer.
As you can see in the description of the beam-forming, the width of the antenna array and the choice of the modulation signals (90 and 150 Hz) drive the ddm proportionally to the angle of approach to the antenna array.
There are also considerations tied to the broadcast signal. Higher frequencies would require more bandwidth which would impact channel spacing.
Ultimately, the choice of the two frequencies is an optimization of a design for the approach guidance. For any given runway the current system can be set to provide consistent guidance (+/- 350 feet == full scale deflection) at the runway threshold. This is done by adjusting the distance between elements of the antenna array.
An AC motor in the USA will turn at 60 RPM and 90 is 3/2 and 150 is 5/2 of 60. Both can be generated using a simple gear drive which would reduce the costs during development. Similarly there are circuits to multiply or divide a driving frequency by whole numbers, with 60 hertz readily obtained. Simple gear ratios could convert 50 hertz used elsewhere.
90 is two digits and 150 is three digits. Left is four characters and Right is five characters which makes for a handy memory aid, as the greater number of digits matches the longer word. Similarly 180 hertz is commonly used to spin up X-ray tubes as a 60 rpm motor turns a generator via a 3:1 gear ratio. Analog clocks are kept accurate by exact 60 hz power and is better regulated than that of a common occilator, especially during development. Also the developers probably didn’t want to use 60 hz or a Harmonic of it to reduce noise from the power lines.
When ILS was developed, they actually used rotating capacitive plates with lobes. The 90 had 3 lobes and the 150 had 5. Easy way to modulate the RF with the phases locked. The depth of modulation was adjusted with a fixed plate with a variable gap between it and the rotating plate.
Answer to part 1: Radio frequencies are split up for different uses by the government (NTIA). The aviation world makes use of many of these ranges, including the range around 75 MHz, but most usable frequencies are already allocated for other purposes. This chart provides a great breakdown of the US allocation of radio frequencies.