# Why are the modulation depths of the localiser and glideslope different?

Why do we use a modulation depth of 20% (for 90Hz and 150Hz each) in the case of the localiser and 40% (for 90Hz and 150Hz each) in the case of the glideslope? Why not use 20% for both?

• Where did you find the 20% and 40% numbers? Apr 16, 2018 at 12:13
– Rumi
Apr 16, 2018 at 13:42
– Rumi
Apr 16, 2018 at 13:42

From the book "Principles of Avionics" By Albert Helfrick:

The amount of modulation is twice that found in the localizer. Because the glide slope does not have ident or speech audio, the percentage modulation for 90 and 150 Hz tones is increased.

• Fiddlesticks i would humbly disagree to this !!! I dnt think this is the reason. Mod depth is doubled in glideslope's case than localiser and also note down glide slope is using UHF band not VHF unlike localiser. There must be some other reason. Cole on folks come up with proper reason m dying to hear :(
– Rumi
Apr 16, 2018 at 16:10
• @aadeez "Mod depth is doubled in glideslope's case..". Yes, that is precisely what the book I linked to says! (Click on the link if you haven't already). Apr 16, 2018 at 16:26
• Fiddlesticks This book that you referred has been bothering me for few days now. Would u mind explaning in detail the the topic 3.1.2 (Page 80 to 85) ? I would appreciate that
– Rumi
Apr 18, 2018 at 9:31
• @Rumi, you mean pages 88 to 93? That explains, in somewhat complicated way, how a directional signal is transmitted using two or several simple dipole antennas and suitable phase shifts. I think the Wikipedia article on Phased Array does better job of explaining the general principle. Sep 13, 2018 at 18:51
• @Rumi, why would there have to be another reason? This one sounds pretty reasonable. Sep 13, 2018 at 19:00

Ok folks in my view the reason of using different mod depths for localiser and glideslope is this : As we all know that increasing the mod depth, increases the Total power or Peak Envelope Power of the signal. Now Glideslope uses UHF band and for 90Hz and 150Hz it needs increased mod depth to be demodulated by the ils receiver in aircraft, whereas localiser uses VHF band hence less mod depth of only 20% is needed for each tone of 90Hz and 150Hz in this case. I hope you all will now excuse of all the brain storming that you have done so far. CrossRoads my friend was it me?

Appendix B of this document https://www.faa.gov/documentlibrary/media/order/6750.24e.pdf appears to refer to ICAO Annex 10, http://cockpitdata.com/Software/ICAO%20Annex%2010%20Volume%201 . See 3.1.3.5.3.3 and 3.1.5.3.3.4 for the Localizer, and 3.1.5.5.3 for the Glideslope. That's why they use what is used currently; the International Standard says to.

As to the why of the particular modulations, I don't have the history of how the particulars were developed over time. A google search of "ils approach history" yields quite a few hits tho. This one seems quite detailed.
http://instrument.landingsystem.com/

• Crossroads my friend the query of mine is still unanswered !!! There must have been a reason to use mod depth of 20% and 40% . Nothing in aviation is without logic. Someone can easily question why in localiser they didnot use 40% mod depth of 90hz and 150hz each? So my question still remains unanswered sadly which is quite disappointing to see :(
– Rumi
Apr 16, 2018 at 14:52
• You need to go back to the 1940s for that I fear: ethw.org/… I'm still reading. Apr 16, 2018 at 14:55
• From Pages 5-2 and 5-4 here, casa.gov.au/sites/g/files/net351/f/_assets/main/pilots/download/… The localizer beam is centerline +/- 2.5 degrees, while the glide slope beam is centerline +/- 0.5 degrees. Perhaps that is why the modulation is different. But I haven't seen any words that say modulation degrees corresponds to transmitted beam width. Apr 16, 2018 at 15:03
• CrossRoads i am still waiting for your indepth explanation my friend :)
– Rumi
Apr 20, 2018 at 3:35
• Sorry, I haven't seen anything more as to how they came up with the depths used. Apr 20, 2018 at 12:08