Expert: Colin Woehrle - 2/8/2006

Question
Hi there, I am a third year student studying Aviation Technology at the University of Leeds, England.

I am currently writing my dissertation which is mainly focused on Instrument Landing Systems.

I know that when glideslope antenna arrays emit their radio signals they produce a 150 Hz lobe and a 90 Hz lobe and where these lobes overlap is essentially the "glideslope".

However, I seem to recall from somewhere that due to the way the antenna produce the 150 Hz lobe, a secondary 150 Hz lobe is also produced above. This results in the production of a secondary "false" glideslope.

I would just like to confirm that this is true and if possible find out why the antenna produces a secondary 150 Hz lobe. I know that this is probably more the work of a radio engineer but I am hoping you may be able to help me first.

Feel free to include any additional information on ILS in general. Anything would be very much appreciated.

Thanks a lot.

Answer
Hi Chris,

Thanks for the question. It's a good one. I can't seem to find reference to this anywhere but I recall from my studies at the University of North Dakota that the false glide slope of the 150 Hz lobe is produced when it is reflected either off the ground or some other obstruction on the ground, allowing it to be projected well above the primary and correct 150 Hz lobe. It can produce a 12 to 12.5 degree descent path = 2000 ft/min descent rate - very dangerous. It makes sense why airports have ILS Critical Areas that you are suppose to stay out of at the beginning of a runway when ILS approaches are in use because your aircraft may interfere with the signals being emitted to other aircraft on approach.

I hope this helps!  If I run into our avionics tech at work I'll try to get more information, but for now, this is what I remember. You'll hear from me again if I find out more.

Good luck to you on your dissertation!

Sincerely,

Colin D. Woehrle