On Tue, Aug 12, 2008 at 4:29 PM, Hal Murray <[EMAIL PROTECTED]> wrote: >> Yep, that's to be expected, given the iterative solver they're using. >> Try turning on the ZDA message, apparently the $ sign is aligned to >> the start of the second ... if its implemented in your version of >> firmware. Since you're running a BU-353 you could have all kinds of >> broken things in your firmware (not impressed by Globalsat). > > Do you expect the ZDA message to be better synchronized than any of the > others?
Yes. In SiRF receivers ZDA is specifically aligned to the second. I just checked my copy of the manual I got with my BU-353, and Table 1-1 NMEA Output Messages says "ZDA PPS timing message (synchronized to PPS)." RMC is produced after the navigation solution is computed and that does not happen in constant time. I found another little gem in my old email: the XTrac (High Sensitivity) version of the software does not support PPS. XTrac is often designated by "ES" in the version string. I also found that the PPS line is just a GPIO. Perhaps in the XTrac build they cut out the GPIO control code, but left in the scheduling of ZDA... *shrug* > Is the BU-353 any better or worse than any of the other units using the SiRF > chips? I've tried several different brands. I can't tell them apart unless > I look at the physical package. The hardware is about the same, but I don't trust Globalsat to not load some customized broken firmware... >> I've used the LVC w/ PPS as the reference clock... not sure that I >> trust the delay/jitter characteristics of USB enough to give me better >> time than a wrist-watch. > > USB isn't fundamentally evil. It's polled, so you won't get great response > to something like a PPS interrupt. But the polling is handled automagically > with modern hardware so It's not much worse (maybe better) than the typically > interrupt batching that RS-232 chips do. I know it's quick enough... usb ethernet, webcams, sound devices all work. But I don't think the usb implementation in most GPS receivers was designed to support timing applications. > I think USB will be good enough if your target is a few milliseconds rather > than a few microseconds. Yes, I've found that with a BU-303 (sirf2), using just NMEA timestamps, I can sync to about 5ms. Good enough for kerberos and nfs :) CK -- GDB has a 'break' feature; why doesn't it have 'fix' too? _______________________________________________ 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.
