# Why do localizer/glideslopes use 90/150Hz specifically?

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?

• Do you have any reason in mind why they would use other modulation frequencies besides 90 and 150 Hz? – Terran Swett Jun 13 '18 at 23:36
• Because if they used 75 Hz people would ask why they don't use 90. – Michael Hall Sep 3 '19 at 4:12

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.

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.

• Thnx Gerry that seems interesting and logical answer however i have different theory !!! Back in old days to generate electricity u need Alternator( turbines) that run at frequency of 60Hz. So to choose anything which is multiple of 60Hz would be easy but if we use e.g 60Hz or 120 Hz in ILS, will cause interference so chose frequencies of 90Hz and 150Hz which were multiple of 30Hz and remember 30Hz was fundamentsl LF frequency easily developed out of 60Hz. I hope it makes sense :) – Rumi Apr 14 '18 at 18:45
• @aadeez That is a valid assumption but it's not just 'back in the old days'. Any system that has to generate an AC signals has to consider how to most effective do so. Almost all use a single reference, whether an internal oscillator or an external signal. Using the ground power 60Hz as a reference would simplify making 90Hz and 150Hz signals as they are 1 1/2 times and 2 1/2 times the reference. But that's not the only reason, just one of many considerations. In the end, somebody ran through the trade-offs and picked these as the best choice. – Gerry Apr 14 '18 at 19:23
• Agreed Gerry :) i posted one more query of why localiser use VHF and glideslope, UHF? still to be answered by some one. Any thoughts? – Rumi Apr 15 '18 at 19:55
• I think he wants to know why they chose specifically 90/150 Hz TONES on the radio signal, not the radio signal freq itself. I'll answer below. – Bob Denny Sep 11 '18 at 14:18
• Never mind, Gerry's answer is technically correct (bandwidth considerations). Also they are not harmonically related. The envelopes cancel out via the antenna radiation pattern such that 90Hz cancels out to the right of the course and below the glide slope, and vice versa for the 150Hz. Very clever actually. It has certainly stood the test of time. Back in the 1970s the FAA attempted to replace ILS with Microwave Landing System. It failed. – Bob Denny Sep 11 '18 at 14:26

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.

• Javelin thanks but your source? – Rumi Jan 28 '19 at 6:10

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.

• Hi Geoff well thanx but its still unclear ! You mentioned about 75MHz but i am talking about Low frequency modulating signals of 90Hz and 150 Hz, not MHZ mate :) Also the Chart that u shared describes as low as 3Khz only.... my point is Why only 90Hz and 150Hz? we could have used 45Hz too :) there must be some logic i guess – Rumi Apr 13 '18 at 23:15