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?

  • $\begingroup$ Where did you find the 20% and 40% numbers? $\endgroup$
    – CrossRoads
    Apr 16, 2018 at 12:13
  • $\begingroup$ Crossroads its v.basic thing that u are asking however for your reference see the link below $\endgroup$
    – Rumi
    Apr 16, 2018 at 13:42
  • $\begingroup$ google.com/url?sa=t&source=web&rct=j&url=https://… $\endgroup$
    – Rumi
    Apr 16, 2018 at 13:42

4 Answers 4


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.

  • $\begingroup$ 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 :( $\endgroup$
    – Rumi
    Apr 16, 2018 at 16:10
  • $\begingroup$ @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). $\endgroup$ Apr 16, 2018 at 16:26
  • $\begingroup$ @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. $\endgroup$
    – Jan Hudec
    Sep 13, 2018 at 18:51

In short

In addition of the two navigation tones 90 and 150 Hz, the LLZ signal can contain two other components:

  • a telegraphic identification tone of 1020 Hz,
  • a voice channel, between 1 kHz and 3 kHz (3dB). This channel can be used by ATCO to talk to aircraft crews when their regular communication receiver has failed.

All components have a modulation depth specified by ICAO. The total depth is the sum of the individual depths.

There is no need to duplicate the identification and voice elements on the UHF carrier, the GP signal carries only the navigation tones.

To have LLZ and GP carriers modulated with a similar (total) depth, the individual depth allocated to the two navigation tones is smaller for the LLZ than for the GP.

The resulting total depth is between 68% and 95% for the LLZ, and between 75% and 85% for the GP.

If you are interested, relatively complex details follow.

Modulation index for a compound signal

What ICAO calls the depth of modulation is better known as the modulation index, this is the wording I use here. In DSB-AM the index is the ratio of the total amplitude of the modulation to the carrier amplitude.

Strictly speaking, the total amplitude of the modulation is itself the sum of the phasors of individual modulations. Since the allowed zero-crossing phase difference for tones is only 10° or 20° and there are restrictions for other elements, phasors are nearly aligned. In such a case the total peak amplitude can be approximated by the sum of individual peak magnitudes, and the total index can be approximated by the sum of individual indexes. Let's use this approximation and work with indexes only.

Total indexes for LLZ and GP

For the LLZ, with identification keying and voice capability:

  • The minimum index is 68%: 18% for each navigation tone, 5% for the identification, and a voice channel 9 times the identification tone (45%).

  • The maximum index is 109%: 22% for each navigation tone, 15% for the identification and 50% for the voice channel. But this would over-modulate the carrier, which is not desirable as this increases the VHF channel spacing and prevents simple demodulation by envelope detection. ICAO limits the total index to 95%.

On the other hand the GP signal contains only the two navigation tones which individually modulate the UHF carrier with an index between 37.5% and 42.5 %, this means the GP carrier is modulated between 75% and 85%.

Both the LLZ and the GP carriers are modulated with about the same total index. The part allocated to the two navigation tones must be different for LLZ and GP to get this result.

Of course, without a voice capability, the LLZ index is lower, between 41% and 59%, hence the navigation tones index could be increased in this case. However the receiver expects fixed indexes.

The two next sections summarizes Annex 10 specifications for reference.

LLZ specifications

  • § 90/150 Hz nominal index of 20%.
  • § Tolerance: 18% to 22%.
  • § The two tones are phased-locked.
  • § Voice index 50%, ratio 9:1.
  • § 1020 Hz tone index between 5% and 15%.
  • § Total index limited to 95%

The annex also provides a limit on the sum 90 Hz + 150 Hz:

  • § Between 30% and 60% (after January 2000).

This is related to how the LLZ signal is created. While it appears to be a single signal to the receiver, it is actually the space modulation result of three physical signals sent on the same frequency by the antenna array:

  • The Carrier with Sidebands (CSB), a DSB-AM signal sent along the course axis with no phase shift.

  • The Sidebands Only (SBO), a carrier-suppressed AM signal, sent on one side of the course axis with a +90° phase shift, and with a -90° phase shift on the other side.

    enter image description here

CSB and SBO are modulated by the two navigation tones with the same index. The DDM is created by the space modulation after they are radiated. Navigation tones from CSB and SBO add on one side of the course axis, and subtract on the other side, in different a proportion according to the azimuth. For details on the ILS principle, see this question or have a look at this video.

GP specifications

  • § 90/150 Hz nominal index of 40%, tolerance: 37.5% to 42.5%.
  • § The two tones are phased-locked.

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?

  • $\begingroup$ Disagreed. I dnt think its the case, the DVOR also is in VHF band with extremely low mod depth. Why the 40% mod depth was chosen for GP is still a mystery. $\endgroup$
    – Rumi
    May 7 at 13:20

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 and for the Localizer, and 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.

  • 1
    $\begingroup$ 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 :( $\endgroup$
    – Rumi
    Apr 16, 2018 at 14:52
  • $\begingroup$ You need to go back to the 1940s for that I fear: ethw.org/… I'm still reading. $\endgroup$
    – CrossRoads
    Apr 16, 2018 at 14:55
  • 1
    $\begingroup$ 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. $\endgroup$
    – CrossRoads
    Apr 16, 2018 at 15:03
  • $\begingroup$ CrossRoads i am still waiting for your indepth explanation my friend :) $\endgroup$
    – Rumi
    Apr 20, 2018 at 3:35
  • $\begingroup$ Sorry, I haven't seen anything more as to how they came up with the depths used. $\endgroup$
    – CrossRoads
    Apr 20, 2018 at 12:08

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