AC-DC if the aircraft in question is operating under a CofA as my volts free Auster is until the next annual then you are looking at a time consuming and expensive Major Mod to be approved by the CAA. Off the top of my head I would say it would take a year and cost around 2,000 pounds. Add on the cost of the Mode S Transponder and you are looking at around 4-5,000 quid at today's prices. The current value of my aircraft is about 14,000. Do you see why I am reluctant to fit Mode S? This all changes when the Austers go onto a Permit to Fly later this year then the modification costs drop significantly but given that my flying is VFR only what benefit is Mode S going to give me? Will most of my fellow bimblers have fitted TCAS? I don't think so. I think I'll be fitting a Mode C transponder after I've fitted the electrical system then upgrade to Mode S in 2012 when (hopefully) there will be a low electrical demand, low cost unit available for us bimblers.