On Friday, April 3, 2015 at 7:23:35 AM UTC-7, John Carlyle wrote:
BillT, Darryl, thanks for your replies. I really got confused with 14 CFR 91.215 and 91.225! I reread them again, and now I see where I went wrong.

It's nice that I won't have to spring for an expensive C145a GPS source. My glider is experimental so I do have the option to feed it with the PowerFlarm GPS. I won't ever be going into Class B or C airspace, though, so I'm not sure what I'd gain by implementing ADS-B Out via this option.

A question regarding TCAS - it calculates target range using reply timing rather than received power level, doesn't it? In other words, TCAS doesn't work like the PowerFlarm or Zaon PCAS, so I don't have to worry about getting too close to airliners because I have a 130W instead of a 250W transponder.

-John, Q3

On Thursday, April 2, 2015 at 9:36:07 PM UTC-4, Darryl Ramm wrote:
On Thursday, April 2, 2015 at 5:42:12 PM UTC-7, Bill T wrote:
John, I'm sure Darryl will be along shortly.
Gliders are not required to have a transponder or ADS-B Out.
You are not required to add Out GPS data to the Trig unless you chose to. But if you do it must meet the standards.
FAA has recently reworded the language that allows "Experimental" kit built aircraft to install non TSO'd ADSB out systems as long as they meet the same standards. I'm not sure if that applies to experimental gliders.

We have Trig21 on some of our gliders, inside the Mode C Veil, we are not planning to add GPS to the Trig.
BillT

Bill is right. You won't need to do this, not unless the regulations are changed, and that would be a huge surprise.

You need to read 14 CFR 91.225.

Because of the the glider/electrical system exemption you can fly within a Mode C Veil after 2020 without ADS-B Out. Obviously when that close to lots of traffic I'd hope glider owners do equip their aircraft with a transponder, as you have--thanks! And that transponder provides lots of protection, importantly it makes you visible to the TCAS systems in just about all airliners and fast jets.

If you chose to install ADS-B Out and your glider is certified then you must meet all the requirements of the 2020 ADS-B Out Mandate, that unfortunately is not something that a Trig TT-21 transponder can do, you need to have a TT-22 because of FAA's silly power requirements for ADS-B Out. And yes you would also likely need a TSO-C145a (or later) GPS source. If your glider is experimental then you have cheaper options (but you need to be careful that say if you connect a commodity GPS to the Trig it won't give you any post 2020 mandate privilege e.g. not allow your glider to enter class B or C airspace... probably not an issue :-)).

TCAS is it's own interrogator (so are the GA oriented "TCAD" systems, some of which evolved to be TCAS I certified), it measures other aircraft's response time to it's interrogations to measure slant distance, and it reads the transponder Mode C replies (or the Mode S equivalent) to get the other aircraft altitudes.

And those TCAS interrogators are one of the reasons why PCAS works at all in areas where you don't have line of sight to a ground based SSR interrogator to cause those transponders to reply... if you are anywhere near popular flight paths. Those TCAS systems are interrogating aircraft out to several tens of miles.

Bonus reading:

TCAS only ever issues Mode C interrogations (or Mode S equivalent), not Mode A for several reasons, one is there is just not much point--the aircrew don't need to know the other aircraft squawk code, and to just cut down on wasting bandwidth, and to avoid confusion about whether a reply from old Mode A/C transponders is a Mode C altitude or a Mode A squawk code.

TCAS II uses a crude predicted time to impact to prioritize threats and when to issue TA and RAs. As the threats get close the TCAS system gets "pretty interested" in the treat and will start hammering away with rapid transponder interrogations. TCAS II works intelligently using selective interrogation of Mode S transponders to cut down on wasting bandwidth. Enough Mode A/C equipped traffic near TCAS based interrogators wastes bandwidth with all the stupid Mode C interrogation replies. That is one reasons that I wish the FAA had said way back in the 1990s that Mode A/C would eventually go away and be replaced with Mode S only...give the industry a clear 1-2 decade roadmap. And that could have lead to a simpler, more affordable and more usable 1090ES only ADS-B system in the USA. Organizations like AOPA lobbying for keeping Mode A/C helped encourage the complex dual-link mess we ended up with.