David Taylor <[email protected]> wrote: > On 12/02/2013 09:54, Rob wrote: > [] >> You are describing implementation details in some particular receiver. >> Why not read the actual specs of the protocol instead? > [] > > Do you have the URL of the official $GPZDA protocol, please? Is it any > more than the receiver's best guess, given that there is no "locked" > indicator? > > Oh, and a URL which the mere mortal can read: > http://www.nmea.org/pub/0183/ => no permission.
I have studied the NMEA 0183 protocol some years ago and it was very apparent that the standard did not intend to provide accurate time to the outside world. All those positioning sentences provide the "time of fix", the time at which the other information in the sentence was calculated. There was no clear specification of the time that may have passed since the time of fix. E.g. an early receiver may have been able to calculate a fix only once every 5 seconds, and the sentence sent would include the time in 5 second steps. For the GPZDA sentence, the time is supposed to be the current time, but there is no specification how much that sentence is delayed from the moment that this was the current time. Also not very usable. Normally the receivers also provide a manufacturer-specific binary protocol, and often (but not always) there is tighter specification of the time packet. E.g. the time is valid at the moment the leading edge of the first startbit of the packet was sent. However, that is not very useful information either, as the receiving computer often does not timestamp that exact moment so you do not really know when it occurred (there can be varying delays e.g. because of FIFO buffers and interrupt coalescing). Without PPS, there really isn't much you can do with a GPS receiver to derive time information. And even with PPS, it is not as easy as it seems, because the uncertainty of the serial time information is of the same order of magnitude as the "pinning" provided by the PPS. I.e. when you are not careful, you still can be off by one second. _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
