"C Kingsbury" wrote in message =
om...
"Gerry Caron" wrote in message =
...
=20
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.
=20
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.
=20
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...
=20
-cwk.

Five years? Twenty years?
Probably about a year ago, the FAA mentioned the rate at which they were
converting (IIRC) GPS approaches to LNAV/VNAV approaches.
The number sounded fairly good, until doing the math showed it would =
take
them about thirty years to finish the current number of runways at that =
rate.
---JRC---