What is, precisely, the function of far field monitor in ILS?

For what I have understood, far-field monitor (FFM) is composed by a set of antennas that receive the signal from the localizer, and check its integrity. The checking is made by extracting the DDM from the signal and comparing its value with some tolerance established by the ICAO standards. What I don't understand is: since DDM changes with the azimuth (i.e, the position in which the aircraft is collocated with respect of the runway), how can the monitor know the proper DDM value to check? Is there a set of antennas for every azimuth?

Let me explain with this picture**:

what are the meaning of those three black dots forming the FFM? In my opinion, they could be,for example: one FFM for DDM at -10°, one FFM for DDM at 0°, and one FFM for DDM at +10°. But I am not sure, because I can't find any source that confirm this.

Is there anyone that could give me an exhaustive explanation of what is a FFM, how is it phisically implemented and how it works exactly?

** Qu, C.. (2015). Research on Signal of Field Monitor of 7220A Localizer Beacon Subsystem of ILS. Open Journal of Antennas and Propagation. 03. 37-50. 10.4236/ojapr.2015.34005.

Per FAA Order 6750.16, the far field monitors are placed on the final approach course centerline, usually near the inner and/or middle marker beacons. Since this is on the centerline, the expected DDM is 0 (or very close to 0 if the siting is not exact). The azimuth of the aircraft may change, but the FFM itself is not moving and so its azimuth is constant.

Far field monitors are only required for category II and III approaches. There's not much point monitoring the localizer signal 10 degrees off course- any aircraft that far off course has long since gone missed!

The FFM is essentially a localizer antenna like would exist on an aircraft. It knows where it is an compares where it is to where the localizer signal says it is. If this disagrees by too much for too long, it sends a signal to the control tower and to the localizer antenna. Depending on the level of disagreement, this may disallow category II or III approaches or may even cause the localizer antenna to be shut down.

You can read the gory details in this report by Texas Instruments of their development of some early category III ILS approaches back in the 70s.

• -10°/10° is the DDM linear sector for the localizer beam (DDM increases linearly from 0 at the center to 0.180 at 10°)
– mins
Commented Apr 25 at 15:35
• @mins Full-scale deflection is .155 DDM. The actual angle this corresponds to depends on runway length- the angle is set to whatever makes the full course width at the threshold 700 feet, or 6°, whichever is smaller. That means full-scale deflection (again at .155 DDM) is at most 3°, so I don't see how 10° could be .180 DDM and still be linear. Commented Apr 26 at 7:25
• You're right, I misinterpreted this diagram from Annex 10. There is a range for linear DDM increase within the 10° sector, up to 0.180, the rest is required to be >= 0.180, but not linear, up to the 10°. So my comment should have been: This range, defined by ICAO, is where the DDM increases from 0 to at least 0.180.
– mins
Commented Apr 26 at 9:42