> It depends upon how low a probability failure you're willing to consider > and how paranoid you are. For one thing, the U.S. National Command Authority > could decide that GPS represents a threat to national security and disable > or derate GPS temporarily or indefinitely over a limited or unlimited area. >
Derating GPS wouldn't affect the time reference functionality. Turning off GPS entirely would seriously affect military aviation operations. > It is well known that GPS is vulnerable to deliberate attacks in limited > areas, perhaps even over large areas (see Presidential Decision Directive > 63). Backup systems are officially recommended for "safety-critical > applications" and the US government is actively intersted in developing > low-cost backup systems (presumably because they're concerned about GPS as a > SPOF too). > > The US government, and other entities, do perform "GPS interference > testing". This basically means they interfere with GPS. The government is > also actively investigating "phase-over to private operation", which could > mean changes to operation, fee system, or reliability of the GPS system. > > One could also imagine conditions that would result in concurrent failures > of large numbers of satellites. Remember what happened to Anik E-1 and E-2 > (space weather caused them to spin out of control). > > If you do develop a system with GPS as a SPOF, you should certainly be > aware of these risks and monitor any changes to the political and technical > climate surrounding GPS. I do believe that it is currently reasonable to > have GPS as a SPOF for a timing application that is not life critical (that > is, where people won't die if it fails). > > Aviators try very, very hard not to trust their lives to GPS. > As opposed to LORAN ?