NTP, possible solutions, and best implementation

Thu Oct 2 21:42:24 UTC 2003

> 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 ?