Two relevant points on GPS/LORAN 1 - GPS has two positioning systems 1 - SPS Standard Positioning Service which is what all civillian uses of GPS utilize for positioning and timing uses and this can be degraded or disabled with no notice to the user community by the National Command Authority. 2 - PPS Precision Positioning Service this is the military GPS system which uses encrypted signals on a different frequency to provide location services accurate to 30 cm. SPS can be disabled with no effect on PPS. I have no knowledge of why there are two systems since the system was initially designed for military use only but as a guess the SPS system was designed as a test system so GPS system functionality could be checked without the need to disclose keys. 2 - GPS is more accurate than LORAN however the SPS is much less repeatable by design than LORAN. A LORAN may not give you as accurate a Fix as the GPS but the LORAN will always bring you back to the same spot +/- a few feet which is why Aviators and Sailors like LORAN better than GPS. 2.5 - Both systems use atomic clocks for their time reference systems. Scott C. McGrath On Thu, 2 Oct 2003, joe mcguckin wrote:
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 ?