On Saturday, March 21, 2015 at 5:53:06 AM UTC-7, Bill T wrote:
Thanks Andy, will do. We have an ancient KT Mode C, we'll need to add approved GPS receiver, 1090ES and compatible altitude encoder, plus an ADS-B out box to tie together.

BillT

Hi Bill,

It wasn't totally clear from your post, but to clarify...Modern Mode S transponders should transmit 1090ES Out with the addition for a properly configured GPS. The 'ES' stands for 'Extended Squitter' with appends the GPS information to the Mode S broadcast. Since it operates on the same frequency as Mode C, you should be able to upgrade transponders without changing antenna installation. It seems likely that if you wait, there will be available on the market an integrated Mode S/1090ES Out box with built in GPS, in which case all you'd be able to reuse the Mode C antenna, antenna cable run and power and simply find a spot for the GPS antenna. I'm not sure about the altitude encoder - I think generally access to a static line is the only necessity.

Keeping the old Mode C and adding ADS-B UAT Out entails not only the GPS, but running additional power and (probably) a circuit breaker, one or maybe two additional ADS-B antennas and coax runs, double the inspection and maintenance and in all likelihood more debugging for interoperability. To the extent that the UAT Out has some sort of control head you'll need find a place to mount that too. Lastly you'll have to face the inevitability of being stuck with two systems forever (even after your Mode C finally gives up the ghost). Since Mode S and UAT operate on different frequencies they cannot be combined and will always require separate antennae. If you make the investment in UAT, switching back over to 1090ES Out once your Mode C dies entails also throwing out the relatively newer UAT gear, pulling the antennae, etc. Would be a bit messy and expensive.

Andy, 9B