Hi Hal, The almanac is helpful, but not required to track sats. Only the ephemeris info is needed, which is repeated every 30+ seconds or so if I remember correctly, which is why modern receivers can be locked in as little as 36 seconds or less from cold start without almanac or assistance from a network.
The gps to utc offset is for some crazy reason not transmitted in the ephemeris, but only in the almanac, so on average it will take about 6 minutes to receive this info and the nmea time may jump a second or more at that point. It may take a user until the next leap second happens to notice that their system has this error, and is causing problems.. Bye, Said Sent from my iPad On Mar 3, 2011, at 3:24, Hal Murray <[email protected]> wrote: > >> I suspected it might have something to do with the almanac. The Tripmate has >> been on now for 100 minutes, but has not been able to track four satellites >> during that entire time. Does the GPS need to track at least four satellites >> continuously during the 12.5 minutes it takes to download the almanac? If >> so, then I'll probably need to place this thing outside tomorrow. > > I'm pretty sure it only needs one satellite to get the almanac. The whole > idea is that the almanac contains all the details about where the satellites > are and you need that to compute the Doppler so you know what frequency to > listen to. It's sort of a chicken and egg problem. If you start from > scratch, you just hunt (in frequency) until you get lucky and find a > satellite, then you get the almanac from it, then you can find the others. > > >> I discovered two things about the Tripmate. First, the sensitivity sucks >> indoors. It is currently showing 13 satellites visible, but only tracking >> three. > > If you are indoors, I'm not surprised that an older unit is having troubles. > > >> Second, the time data from the GPRMC sentence is about two seconds behind >> UTC (using my calibrated eyeball for measuring). > > Some units are off-by-one. As long as it is consistent, you can fix it up in > software. I don't think I've seen any that are off by two, but it wouldn't > surprise me. > > Here is a hack I wrote when I was chasing that sort of glitch: > http://www.megapathdsl.net/~hmurray/ntp/ntpd/timing.c > The first column is HHMMSS.sss (UTC from your local system) so it should be > easy to see if your system time matches the time from a NMEA device. > > > -- > These are my opinions, not necessarily my employer's. I hate spam. > > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
