Shouldn't a GPS unit be able to determine the present time much more quickly than a position fix? Sure, getting super-accurate (<<1sec) time will require multiple satellites, but I would think that a single satellite is all that is needed for a basic timestamp. Sean On Mon, Dec 31, 2012 at 9:12 PM, RussellMc wrote: >> The field units are mains powered and draw hundreds of Watts each, so >> the added power consumption is not an issue. >> >> As long as the GPS locks onto the time within 5 minutes, I think the >> client will be happy. They are also fixed position. Would this help? >> >> Maybe using the GPS to correct an on-board RTC would be a better option? > > Off the cuff low practical experience comments: > > GPS sounds like a good fit. > A typical GPS (all modern?) maintain ephemeris (short term) and > almanac (long term) satellite constellation descriptions to allow > location of the satellites. If the system is always on the time signal > is essentially continous. If you turn the system on regularly and > frequently (say hourly to maybe a few hours) the ephemeris information > is usually relevant enough to allow reacquisition in under about a > minute and sometimes much less. If you turn the system on daily or > weekly it can take minutes to acquire enough satellites to start > producing information. (1 =3D somewhere apparently :-), 2 =3D a torus, 3 > =3D where you are or out closer to lunar orbit, 4 =3D where you are, 5+ = =3D > better.). > > Away from cities you are unlikely to seldom experience catastrophic > loss of enough satellite signals to lose the time signal. > > So, if you have a RTC that has enough stability for at least 10 > minutes (not exactly hard even on the far side of the black stump on a > hot Australian summer day) then a GPS fix "when required' will keep it > stable enough. > An RTC less stable than that or no RTC would benefit from permanent > GPS operation. > > > http://en.wikipedia.org/wiki/GPS_signals > > > R > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .