[time-nuts] GPS 18 behavior

[Tom Van Baak]

> when I'm in a GPS denied environment, it's not just because we're 
> indoors, it's because we're somewhere that GPS isn't available, so what 
> I'm really doing is providing a sort of flywheel to keep my little 
> modules synced with each other.  I don't need super accuracy in an 
> absolute sense: I just need to tag the data all with the same time tag 
> within a few seconds.

Jim,

If the requirement is just a couple of seconds, did you consider one of the high-accuracy Dallas RTC chips? That might be a simpler solution than a GPS receiver (knowing when and when not to trust its 1PPS, decoding NMEA, trusting NMEA, needing antenna with sky view).

If the requirement were sub-second, the solution is GPS; if the requirement were minutes, it's TCXO. Sounds like you're in the gray area where GPS is not necessarily the simplest, cheapest, or robust solution.

The other thing about multiple, portable or remote sensors is that in some cases they don't actually need to agree on time or rate internally as long as you can post-process the data and retroactively apply a time and frequency correction to the data they have already collected or are collecting. For example, if you know one is 2.3 seconds ahead and slow by 45 ppm you can still obtain highly accurate time-stamps from it -- the unit itself does not need to be on-time, or on-frequency.

/tvb