Roach412 wrote:
...simply put, the operator is completely detached from the thing - it gives rise to issue that pilot responsibility is compromised when you're no longer physically "at-risk" of a failure, or stupid pilot error...or intentional destructive desires....
You make a good point. There's an old joke in aviation that the reason they put the cockpit in the nose of the plane is so the pilots will be the first to arrive at the scene of the crash. It generally is believed that it's MUCH more stressful to be an ATC controller than to be a pilot, because when a pilot REALLY screws up, his troubles are over. If a controller really flubs it, his nightmare has only begun.
Karlsweldt, the requirement to have a transponder doesn't come from a/c size, it comes from the type of operation and the type or airspace being flown in. The more restrictive of the two applies. Scheduled passenger air carriers operate under Part 121 or 135 of the FARs, both of which require a minimum of a Mode C transponder, which must be in operation any time the a/c is in motion, regardless of airspace. But if it's a training flight, and there are no ticketed passengers on board, the same a/c operates under Part 91 of the FARs, which governs General Aviation, and which has no overarching requirement to have a transponder. You can fly an 800,000# Boeing 747 under Part 91 without a transponder, but you can't fly an ultralight into JFK (Class B airspace) without a Mode C transponder (and a flight plan).
And I think you misunderstand the way a "plain jane" (Mode A/C) transponder operates. They do not communicate amongst themselves, and they do not transmit preemptively. They wait for an ATC radar to interrogate them, then they reply with the 4-digit code. All Mode A does is give ATC the ability to assign a unique identifier to a blip on the radar screen. It does nothing to aid in traffic separation or collision avoidance (excepting that ATC can provide it, based off radar & transponder information). Unless it's an AWACS or an electronic warfare platform, no other a/c has the capability to detect whether any particular a/c has a transponder in operation.
A Mode C transponder is a Mode A that also has altitude encoding. The only thing it does differently from a Mode A is it includes the pressure altitude reading from the a/c's altimeter along with the 4-digit code. ATC's surveillance radars are completely unable to judge an a/c's altitude and must rely on Mode C. But a Mode C-equipped transponder does nothing to improve on Mode A's failure to provide traffic separation or collision avoidance.
Some a/c (like the Piper Cub) have no transponder because their engine has no alternator. Older GA a/c generally have a Mode A transponder. Commercial and business a/c, and more recent GA a/c (pretty much the overwhelming majority of everything that flies) will have a Mode C transponder. It's only when you get to the tip of the iceberg that you'll find any a/c with an active collision avoidance system, like the TCAS (which I made reference to in an earlier post in this thread).
There is a thing called a Mode S transponder, which does provide traffic separation and collision avoidance, but that isn't a transponder in the traditional sense, it's a hybrid device, a Mode C transponder merged with TCAS-like equipment. Mode S isn't in widespread use, and probably won't ever be, because of what's coming down the pike.
There is a GPS-based system in the wings, called Automatic Dependent Surveillance-Broadcast, which sounds tailor-made for UAVs, especially autonomous ones, especially since its coverage isn't so degraded as radar's when flying close to the ground. Basically anywhere cell phone signal reaches, ADS-B signal also can reach. It's just a matter of infrastructure. And $$$$$$$$$. Should be online around 2020.
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