Here are some more thought about the landing system, and! on-topic.

 I don't know how today's ILS works, but I do know how the Lorentz low
visibility radio guided landing helper worked during WWII.
 Generate 2 adiacent radio beams that overlap a little. Beams could be
several km wide, but the overlap is quite narrow.

 The beam on the left is modulated with audio pulses with let's say 25%
fill ratio and the beam on the right is modulated with audio pulses
(same freq) with fill ratio 75%, in sync with the other beam.
 When you are in the overlap area, you hear a continuos sound. If you
hear short pulses and long pauses, you are too far to the left, so go
right. And the reverse.

 Now assume the particularities of models and an airport for them.

 Assume that your model has some navigation built into it and it can
position itself mapwise in an pretty good corridor to come in for a
landing. (assume GPS here, also see below for more).

 Have an ultrasonic altimeter onboard that CAN measure heights down to
let's say 10cm and up to several tens of meters. Now you have a good
height indication (GPS is totally useless for this due to unacceptable
tolerances 10m error, 100m error, accuracy of the map you reference
to).

 Build a Lorentz like beam system, but instead of radio waves, use
ultrasound. Have the pair of transmitters at the far side of the
airfield and a receiver onboard (you can definitely build it with a
PIC!!!) that can control the plane's direction left/rigth, after it
gets adequately aligned to the airfield by other means, so that it
stays on course guided by the Lorentz beam.

 Navigation :

 Assume that you are flying at an airfield usually used by you and
other model aircraft. You could build on the ground the necessary
support systems for a navigation system that will work around that
airport.

 Although I have absolutely no technical knowledge about how GPS
actually works, I guess it could be an extension of the Oboe system
developed and used for aircraft navigation by the british, also during
WWII. This system was considered to be the most accurate radio
navigation system developed during the conflict.

 The Oboe system : you have several stations on the ground at different
location. Each transmits a pulse modulated radio wave, and all are in
sync. Each station is identified uniquely.
 A receiver will pick up one station and use it as a reference, then it
picks up another station and computes the time difference between the 2
signals. By some math (which I do not know), one can compute a
hyperbola which is the geometric locus where the receiver can be, with
one of the stations (i think the  reference one) in it's focal point.
 Now pick up a third station and compute the time difference between
the signals and you get a second hyperbola. Where the two hyperbolas
intersect, there you are. If you are in doubt and you may have 2
intersections, measure the difference between signals 2 and 3 and get a
third one. all three will have a common intersection point.

 (I assume GPS and similar systems use a similar method, using atomic
clocks to have the signals in sync and expanding the concept to 3D, to
include height and to consider the fact that you cannot simplify to 2D
when your source stations are orbiting above...)

 According to the accuracy of the measurements and the floating point
precision used, one can get a pretty accurate navigation system.

 Now take this concept and try to apply it to sound instead of radio.
This is because sound travels much slower and on such short distances,
computing picosecond differences between signals is not a practical
thing to do.

 I leave it to you to extrapolate.

 One more thing: please, if anyone develops such a system, let us know
about it, and hopefully, will be under GPL.

P.S.
 And let's not forget that WE ARE TALKING ABOUTMODEL AIRCRAFT, not a
military jet, not a huge airliner. I think, in general, a model
aircraft is not flown in hurricane winds, and it doesn't go up in a
thunderstorm so it get's lost in the clouds, etc,etc,etc.



=====
ing. Andrei Boros
Centrul pt. Tehnologia Informatiei
Societatea Romana de Radiodifuziune

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