MTV wrote:Frankly, the FAA designed this system almost exclusively for jets, not for GA airplanes. Its utility for GA aircraft is severely limited in many ways. But, we're all paying for it.
I think this is a fair summary. There is a tremendous utility for everyone that flies to use these services. However, the requirements for cadence and accuracy for the GPS sources are really only useful for:
a) Fast aircraft, where slower cadence and position predictions/reporting and/or 28 ft accuracy vs 8 ft accuracy are more important, and
b) Precision approaches, where the position and reporting solution has to be equal to or better than currently available primary radar and ILS vertical guidance in the terminal phase of the approach.
Unfortunately, the same high precision and high cadence (rep rate) GPS required for small, slow, low, vfr bug smashers is useless since the potential positional errors are absolutely smaller than for faster aircraft, and the additional precision provided by the bug smasher is of no use to the other aircraft in the service area because the errors simply are not as large as with faster aircraft. In other words, the error for a slow aircraft with precision position information is tiny compared to the error for faster aircraft.
Allowing low precision GPS for VFR-only, slower (say <200 kts, but could perhaps even be some other larger number) aircraft would immediately decrease the costs of compliance by allowing older TSO position sources and even less expensive modern sources to be used (as was the original plan, actually). In each and every source document I've read discussing the origins of required cadence and precision for the ADS-B system, high speed errors typical for commercial airliners and precision approach errors were explicitly used to demonstrate lower limits for performance requirements. These calculations look very different for slower aircraft without a need for IFR navigation.
It seems pretty silly to me to require the same performance for position reporting for a C-140 and a Citation X. I don't think a pilot will change their course of action for a traffic conflict if the other target was known to within 28 feet instead of 8 feet of an antenna location.
However, I've heard more than one pilot express dismay (with a straight face) the FAA was not making everyone install GNSS carrier phase equipment to provide centimeter positioning "like surveyors use every day". Because more expensive is always better for some of them, and because they believe centimeters vs 28 feet somehow translate into countless saved lives. I believe that thinking is bunk. I believe having more installations flying through lower costs in more aircraft would have an impact on flight safety.
