On Mon, Aug 20, 2012 at 9:13 AM, unruh <un...@invalid.ca> wrote: > On 2012-08-20, Hahn, Ron <ron.h...@fmr.com> wrote: > > Colleagues, > > > > Chris Albertson, made the following statement: > > > > --< begin >-- > > I'm using the t-bolt. It seems to work. I guess there might be more > > functions the driver could implement. Perhaps better support for > > self-survey or logging or whatever. I'd like to see internal temperature > > logged. But for normal NTP use it works fine. > > > > If you only have one GPS the t-bolt is the one to have. But if you can > > afford two there are lower priced GPSes that use less power that work > well > > with NTP. I have the t-bolt and two UT+ units. > > > > Whatever you get make sure it is a TIMING gps. The Garmin units and the > > Sure Electronic board at not timing GPS although they do have PPS > outputs. > > They work but they are "uSec" level devices. I'd look for 100ns or > better. > > No The sure is 10s of ns device. Unfortunately this is useless as you > cannot get the time into your computer to better than usec. The > interrupts are not serviced fast enough on any PC to give better than a > few us and the interrupt routines get delayed on a working system by > more than that at times (eg due to disk priority, or things switching > off interrupt processing). > > You would have to build a special board for your computer to better than > usec. > > The errors add together. If the GPS and the computer both had 1 uSec of uncertainty the total is 2uSec. So there is an advantage to having a GPS that is in effect "zero". I think this is the same ia when you build a circut with a bunch of 5% tolerance resisters. You can get much larger than 5% error because of the ways resisters add.
I think I would some day like to try and build an external PPS interface that works at nanosecond resolution. It would not be hard. A fast counter is PPL'd to a 10Mhz lab reference and then the PPS captures the counter and then interrupts the computer. the computer then reads my external box rather then the CPU counter. Only a few lines of C code need to change inside the Linux driver. Chris Albertson Redondo Beach, California _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
