"Gerry Caron" wrote in message ...

It works by using a Mode A interrogator and multiple receivers around the
periphery of the airport. By measuring the time difference of arrival
(TDOA) of your transponder replies, it calculates your position relative to
the airport and the approach path. It then broadcasts thru an omni antenna
an "ILS" signal modulating the 90 and 150 Hz tones to drive your CDI to
indicate the proper guidance to the approach path.

Fascinating the things people thought up back in the days before a
constellation of satellites could provide a 3D positional fix with an
accuracy measured in inches.

With WAAS underway, it seems predictable that non-precision approaches
will be joining colored airways in the dustbin of history. If the FAA
had the resources and motivation it could probably be accomplished in
5 years, though I suppose 20 is more likely...

-cwk.