I have a real scenario problem regarding GPS/NTP time synchronisation and since I am not such an expert I'd like to ask the GPS wizards here for help.
As discussed in this group before there are GPS based reference clocks which can be put into so called "fixed" or "stationary" mode. In simple terms this means that the GPS will calculate its precise position only during startup and not continuosly during operation. In return it will happily provide GPS based time even if only a single satellite is in view. Since the GPS receiver is stationary, it simply remembers its position! This all works really well as long the stored position information is accurate. I was wondering, how inaccurate the time provided by the GPS receiver will become if the position information is incorrect by, for example, 100km. Is it correct to assume that this is equivalent to the time the GPS radio signal takes to travel 100km? If so, the time would be wrong by approximately(!) 0.3ms. This issue is actually related to a real problem we have with a GPS synchronised simulcats radio paging network. For some unknown reason (this might be non-technical, e.g. operator error) we had a GPS reference clock in stationary mode with a position error of about 200km. If this would translate into a time error of at least 0.6ms this would be sufficiently bad to corrupt messages transmitted simultaneously by several paging transmitters at 512 bps (512 bps means that in the most trivial encoding scheme one bit takes about 2.5ms to transmit - 1000ms divided by 512). The essential question is whether we have to check about 200 GPS receivers for incorrect position data or whether this wouldn't really cause any problem. Thanks a lot Markus _______________________________________________ questions mailing list [email protected] https://lists.ntp.isc.org/mailman/listinfo/questions
