I have three questions regarding ADS-B and ADS-C. (Hopefully you don't feel my questions are not related.)
(1) What's the scope of availability of ADS-B? Let's say an airliner equipped with all the necessary equipment for ADS-B in the US flies over to Tokyo. When or where does ADS-B stop working?
(2) I read from this post that ADS-C is used in some oceanic environments including the Pacific between certain flight levels. Roughly what percentage of transoceanic flights currently take advantage of the ADS-C system?
(3) I assume ADS-C is not a common transoceanic surveillance system, if it's true ADS-C is only used above certain flight levels. If so, what are alternatives to ADS-C? What other (surveillance) systems are being used to follow transoceanic flights? Self-reporting? GNSS? Or what?