But the GA aircraft I learned to fly in all had 4 dots on the CDI at full scale. I would agree this is common on air transport aircraft. You identified a generic convention of 2 dots at full scale.
The requirement is that the pilot can discern the center, the limits, and that the indication is linear within a few percent. And after installation they have to be tested to show they work together within the spec.īut what about the dots? The MOPS don't specify what the display looks like. The display - even if it's part of a glass PFD - has to meet the MOPS. In this case (which is common today), the radio is tested to comply at the "output to the display" and must have an interface spec to allow a display to be interfaced. Even if the display isn't part of the ILS receiver it's bound by the MOPS. The MOPS has a standard for display of deviation in terms of centering accuracy (0.0 DDM) and linearity of the display out to "full-scale".
There are minimum and maximum angular widths, so very short runways (<~3000 ft) will all have the same maximum width and very long runways will have the minimum width.īack to the ILS MOPS (DO-192 and DO-195). This standard means that shorter runways will have a wider LOC beam than a longer runway. With the LOC antenna at the other end of the runway, this sets the angular deviation guidance for the approach. The significance of 0.175 DDM is it defines the value at the limits of the course deviation display (full scale).Īnnex 10 requires the installation of an ILS requires (for most runways) the radiation pattern of the localizer to place full scale at the threshold of the runway at +/- 350 feet. It's also in DO-195 for localizer.īut what is 0.175 DDM and why is it referenced as "full-scale"? DDM is the Difference in Depth of Modulation of the 90 and 150 Hz audio tones that comprise the LOC and GS signals. There are various standards that come into play and I'll try to explain how they fit together.Īs you noted +/- 0.175 DDM is specified as "full-scale deflection." This is in Annex 10 and, as mins noted, DO-192 for glideslope. The problem you're having is that there is no standard definition of dots.
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