"Mike Rapoport" writes:
"Gerry Caron" wrote
"Trent D. Sanders" wrote

Have you ever heard of, or have any experience with a thing called the
"Transponder Landing System", a form of IFR approach using a
transponder? What is it, how does it work, does it work, do you have
to have a special "endorsement" to fly it, etc?

TLS is a substitute for an ILS where an ILS can't be installed due to
terrain or other factors. It's also a whole lot cheaper than an ILS so
it's useful for smaller airports. That's good, because it's capacity
limited (more later) so it's not a good solution for busy airports.

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.

To fly it, you have to set your transponder to the squawk code on the
approach plate (so it knows who is flying the approach) and tune your ILS
receiver to the "ILS" freq. The you fly it just like an ILS approach. No
special endorsements or training.

The capacity limitation I mentioned above comes about because only one a/c
can have the approach squawk code set at any one time or the system will
get confused and flag.

I haven't flown one. Most are in the pacific northwest at small airports
in the mountains.

Try the following link: http://anpc.com/

It isn't cheaper when you consider that it requires an operator.

The descriptions given above and other followups don't make it
clear: What's the function of the operator?