View Full Version : ILS Localiser Beam Characteristics

12th Jul 2011, 10:42
Can anybody shed some light on what typical ILS localiser DDM characteristics are like as an aircraft traverses the beam? The reason I ask is because in ICAO Annex 10, section, it suggests that the DDM vs. angular displacement is "substantially linear" up to DDM values of +/-0.180. Beyond these angles, the DDM shall not fall below an absolute value of 0.155. What this says to me is that if an autoflight system was to reject DDMs outside of the +/- 0.155 limits, the signal will be robust enough for guidance. This is the ideal. Now for reality...

What we have seen is that on the angular fringes of localiser coverage, the DDM can be within +/-0.155 and very noisy. As we fly towards the runway extended centreline, the DDM rises outside this range, reaches a peak (receiver limits) of 0.4, then starts to fall linearly to 0 on the centreline. So, within the DDM range of +/-0.155, we have multiple angular displacements from the runway for a given DDM, i.e. the aeroplane can be either on the fringes of coverage, or in the nice linear region that is fit for guidance. This makes it difficult to design autoflight control laws and moding because what we thought was a safe DDM range to use is in fact not.

Are these DDM characteristics typical?


12th Jul 2011, 10:59
Hi FCSoverride,

I don't know what a DDM is - but I've experienced the autopilot attempting to lock onto a "false" LOC signal several times. (Stupidly large heading change commanded)
We simply re-select heading again, and re-arm the LOC when closer to the runway centre line and airport.

12th Jul 2011, 11:39
DDM = Difference in Depth of Modulation. The localiser is formed from two radio beams of different frequency. They are each pointed in slightly different directions, with some overlap. The aircraft can deduce its angular position relative to the runway by measuring the DDM between the two beams. I am sure google can offer a far better explanation than I can!


12th Jul 2011, 12:39
Typical is as per the Annex 10 specs. If not, the facility will be failed during flight inspection. Sometimes they'll drop right down to the threshold on the edge of LOC coverage, and you are likely to have false nulls just beyond the coverage volume.

Going from outside of the coverage volume to centreline, what you are likely to see is:

1) False null (fly left/right inverted) just outside of the coverage volume. Very steep (high sensitivity), and with lots of noise.

2) Steep rise to specified levels around the edge of the coverage volume, exceeding the required value before entering. Sometimes this can be close, but usually the margin is good.

3) A constant high value as you are approaching the course sector, usually far exceeding the specifications.

4) A drop just prior to entering the course sector, within specs but sometimes getting close, again rising before you enter the course sector.

5) A linear decrease/change of sign through the course sector. For most facilities, this really is linear but in some places it can be slightly S-shaped.

The most critical points are on the edge of the coverage sector and the drop prior to entering the course sector. At no point should you see anything but full deflection when inside the coverage volume and outside of the course sector.

Sir George Cayley
12th Jul 2011, 20:58
So what causes 1) to 5)? Are these single or multiple events and what is the cause?

Not my area (as you can no doubt tell) but very interested.


13th Jul 2011, 01:36

The beam spreads out over distance and is affected by a number of issues, mostly bending due to prox to heavy iron/steel concentrations such as bridges and buildings, hence "For most facilities, this really is linear but in some places it can be slightly S-shaped."

The glass in a building will also have an effect, all glass buildings will tend to reflect the signal that has reflected off of an aircraft, hence the aircraft may receive multiple bounces of the signal. hence "but I've experienced the autopilot attempting to lock onto a "false" LOC signal several times."

Following a large aircraft mass will upset the signal as well, perhaps giving multiple signals reflecting of various strengths, hence "Very steep (high sensitivity), and with lots of noise."

Construction density around airport facilities is a major concern, and the reason we are looking at shutting down the ILS/NDB and other beam type signals, and replacing the system with GBAS.

13th Jul 2011, 06:30

There are a few things I would like to clarify. When you refer to the "coverage volume", how are the boundaries of this volume defined for a specific airport/ runway, e.g. are they defined as an angle/ range on the approach charts? I was thinking of the coverage boundary as when the localiser RF signal exceeds a certain threshold.

So to confirm my understanding, basically, the localiser behaves as per ICAO Annex 10 when within the coverage volume. When outside of this volume, at the fringes of where the receiver on the aircraft flags that it has detected a strong enough signal to think it is in coverage, the localiser can be "out of spec", i.e. within +/- full scale and very noisy.

Many thanks for the info everybody.