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?
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.
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 18.104.22.168.3.3 and 22.214.171.124.3.4 for the Localizer, and 126.96.36.199.3 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.