ILS Localiser Beam Characteristics
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 3.1.3.7.4, 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?
Cheers.