Author: D. José Luis Delpón Ramos. Aeronautical Advisor.

European Engineer, 12884ES registration number. (European Engineer recognition in Paris, 1992).

Lead Auditor for maintenance quality management System – ISO 9000 / 2000. Lead Auditor for, Certified number SGS/ICS/LAC/550194/P.3081 (Madrid)

Counsellor of the Academic Council del European Institute of Aviation Training and Accreditation de la Universidad Rey Juan Carlos de Madrid.

ICAO expert for air navigation in programs of PNUD (Plan de Naciones Unidas para el Desarrollo)


Influence of the state of the reflection plane of the GP (ILS), on the approach trajectory.


The glide path of the ILS uses the theory of the antenna image for the formation of the glide path, therefore, it will be necessary to study deeply the influence of the plane of reflection, which is in front of the GP antennas, on the formed path. The bad or good condition of this part of the terrain is very important, since it affects the antenna assembly to form a projected glide path trajectory.


The plane of reflection can be affected by variations in the conductivity of the ground when changing the water table or increasing the height of the grass, or by accumulation of snow. Even by dragging earth by torrential rain, it breaks the ground and rocks that, being numerous and together, can reach roughness to the projected trajectory


Any of these variations on the ideal plane of reflection will change the antenna heights virtually, which will cause changes in the angle of the glide path. These changes will be different depending on whether the measurements are made in the far field, the position of the aircraft, or in the near field, the position of the monitor, due to the different relative phase changes that occur in the signals transmitted by the antennas.


Changes in the water table will produce, as the ground dries up, decreases in the plane of reflection and, therefore, increase in antenna heights. The increase in the height of the grass will produce refractions in the signals before arriving at the plane of reflection, with which this one will behave as if they had lowered of the same form the antennas.


The accumulation of snow on the plane of reflection when it has a high degree of conductivity, that is, at temperatures not too low, produces the same effect as if the reflection plane were raised and, therefore, the heighs of the antennas. Everything indicated until now can be applied to any type of glide path.






By means of a simulation program, the necessary data for the study have been obtained and, when compared with the effects detected in practice at the airport, its accuracy has been verified. With these data you can draw the following conclusions for far field:


  • When the reflection plane rises above the ideal, the angle of the glide path also increases and the half-sector of course narrows


  • When the plane of reflection falls below the ideal, the angle of the glide path decreases and the half-sector of course narrows with respect to the nominal.


  • When the reflection plane goes down, the value of the DDM decreases at low angles, which can affect the coverage


When studying the values corresponding to near field, we see that the DDM indication of the monitor will be inverted with respect to what actually appears in the far field, due to the inversion of phases between the antennas A1 and A3 that exists at that distance and that the values of the DDM are much higher than those detected by the airplane


If we consider the tolerance of the angle of the glide path established by ICAO in Annex 10, we see that it is +/- 0.075q for CAT II, which gives us +/- 0.055 DDM, therefore, it can be allowed an elevation of the reflection plane up to 0.50 m. without the angle of the glide path in the far field being out of tolerances. The narrowing produced in the course half-sector will also be within tolerances.


However, it can be observed that in order to obtain the same value in the monitor, it is only necessary that the plane of reflection is raised 0.30 m. Therefore, the monitor position alarm will jump before the far field signal actually goes out of tolerances. Due to this, it will be necessary to ensure that in the case of snowfall, the layer of snow in the plane of reflection of the GP does not exceed a height of 0.30 m, otherwise the monitor leaves an alarm.


Another way to allow a greater operability of the equipment is to allow a layer of snow of 0.50 m. in the entire plane of reflection, which, as we have seen, keeps the far field signals within tolerances and have a counter-antenna between the GP antenna and the monitor detector, from which the snow can be removed when it exceeds the 0.30 m. Tall. In addition, the installation of this counter-antenna will make the measurements of the monitor more stable in all conditions


As can be seen, it is very important to maintain the plane of reflection of the GP in perfect condition in order to obtain a stable glide path. One way to maintain the plane of reflection is to plant it with grass and keep it at a maximum height of 0.20 m., because in this way the soil moisture and, therefore, its conductivity will not very much. Of course, the plane of reflection must be free of any obstacle that may cause reflection in the SBO signals transmitted by the GP antennas.