ILS-glide path TCH and RDH 1

25.10.2004

Submission:

The "reference datum height (RDH) or "threshold crossing height" (TCH) of the ILS is determined physically by several factors

Position of the glide-slope mast

Geometry of the radiators (height, lateral and forward offset)

Characteristics of the terrain in front of the glide-slope mast.

The terrain in front of the glide-slope mast is three-dimensional in the general case and may be limited /truncated laterally and/or longitudinally. The terrain is called also "reflection terrain". "Three-dimensional terrain" means that the reflection terrain cannot be described sufficiently by two-dimensional parameters:

The forward or longitudinal slope

The lateral slope and in case by additional

Step heights in certain cases; e.g. if the runway is elevated related to the reflection terrain.

If the approximate two-dimensional description of the reflection terrain is justified, the terrain is in fact a non-horizontal skewed plane.

Of course in the design process the position of the mast and the geometry should be determined according to the terrain characteristics. Generally speaking the design process must be adapted to the terrain characteristics. The general three-dimensional design method covers in principle all types of terrain. These 3D-methods constitute advanced numerical methods integrated into system simulation tools.

If the RDH/TCH is re-determined later after the design and installation process by numerical or measurement means, again, only the adequate methods can be used for reliable and accurate results. If the terrain has only a forward slope and a small negligible lateral slope, the formula in Annex 10 (§2.4 green pages) may be used. Large errors occur if this assumption is not met.

Other more advanced methods or formulas cover the two-dimensional case. The sets of formulas given in FAA order 8260.3B chg18 (appendix 2) do not really constitute a two-dimensional treatment. In any case it is not a general three-dimensional treatment. The resultant errors for RDH/TCH can be unacceptably large despite the specifications for the RDH/TCH given in the SARPS of Annex 10.

Because the so-called ICAO-formula as well as the FAA-formula are approximations and applicable in different cases, it is worthwhile to discuss the meaning and effects of neglecting the "Y"-quantity given in the Annex-10-formula. However, the Y is negligible in the trivial case when there is no forward and no lateral slopes.

It should be pointed out that right now the FAA-orders are in a change process in relation to the RDH/TCH. The FAA-order 8240.47 for flight inspection is in a relatively advanced status defining a general so-called "BFSL"-method to determine RDH/TCH which does not use the simplifying formulas of Annex 10 (Att. C) nor the formulas in 8260.3B.

Generally speaking it can be stated that the results of approximate formulas are the better, the more the terrain is tailored to its range of applicability. However, the tailoring process can be very costly in the appropriate cases, i.e. realising in case a huge earth movement and grading process. This is if a classical image type glide-slope is requested. A novel alternative end-fire glide-slope system is often the better alternative in these cases.

More information can be found on the Internet web site "http://www.navcom.de". Search for the "news"-section and then for the "crossing"-section.

Dr.-Ing. G. Greving

NAVCOM Consult

Germany