> Just to keep the record straight, ILS doesn't use "nanosecond
> measurements".  When ILS was designed, it was difficult to build
> a device that could measure to the nanosecond and impractical to
> build it small enough to carry on board an aircraft.  Think
> 1950's tube (aka valve) based electronics.

> ILS is totally analog.  Localizer (horizontal guidance) uses a
> VHF carrier (from 108.1 to 111.9MHz) with overlapping signal lobes,
> one modulated at 90Hz and one at 150Hz.  The centerline is defined
> as the plane of points where the modulation levels are equal.
> Glideslope (vertical guidance) uses the same technique but on
> a different, higher ~330MHz carrier frequency.

That's 1930's to 1940s technology :-).
I've just been rereading parts of Professor Dr R V Jones' "Most Secret 
War" ("The Wizard War" if you are unfortunate enough to have the US 
named version) and that description nicely matches the German X beams 
system actually in use in the very early 1940's and on test before the 
war (1939) and similar British systems.


            Russell

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