Matt Barrow wrote:
"Tauno Voipio" wrote in message
...

Jose wrote:
That's right - due to inevitable ground reflections, there
are always false glideslopes above the correct one.


Would you be so kind as to explain that one? False glidescopes?

An ILS signal is finally created at the receiver antenna
as a combination of three signals which are sent separately.

(For the perfectionists: This is a simplified description).

A localizer antenna group is built of three sections: left,
center and right. There are several antennas per section,
often 4 to 8. The center section radiates a signal called CSB
(carrier and sidebands) which alone will create a centerline
display at the receiver. The side sections radiate a signal
called SBO (sidebands only) which contains a kind of difference
signal between the centerline signal and a side indication.
The left and right side signals are radiated in opposite phase,
so the right signal is positive when the left signal is negative.
When the side antennas are at equal distances from the aircraft
antenna, the side signals cancel at the receiver. When there is
a distance difference (as it is when the aircraft is off-centerline)
the side signals combine producing the corresponding side signal.

The glideslope is produced in the same way, but there is a problem:
We cannot install a proper bottom antenna, as it should be some
tens of feet below ground. Here, we have luck: If an antenna is
put above a conductive flat surface, the field pattern is similar
as the pattern with an opposite-phase antenna at the same distance
at the other side of the surface, and that is just what is needed
here. We'll use ground surface as the reflector to create the illusion
of a bottom antenna.

The pattern must then be tilted upward by the glideslope angle.
This creates an asymmetry in the pattern, and we get false
glideslopes above the correct one. For a 3 degree glideslope,
the first false glideslope is usually around 10 degrees.

HTH (hope this helps)

--

Tauno Voipio
tauno voipio (at) iki fi