Re: [gpsd-users] [gpsd-dev] RFC - drop "tag" fields from the public interface

[Eric S. Raymond]

Hal Murray <address@hidden>:
> How does Watch work today?  How does/will it work for the timing case?  There 
> are several NMEA sentences that provide timing.  If the transmitter is 
> stable, it would help to get the time from the same sentence type each time.

Time is collected at the same point in the cycle every time. What
sentence that is varies by GPS type but is always the same sentence
type for any one GPS.

There's a cycle detector in the code that watches timestamped
sentences (ignoring others) looking for where the stamp changes from T
to T+1.  It then marks the sentence type just *before* the T+1
sentence as the end-of-cycle type.  Thereafter, reports (including time)
are issued on end-of-cycle.

The uBlox 4 LEA is a representative example.  Each cycle it ships
$GPZDA, $GPRMC, $GPVTG, $GPGGA, $GPGSA, $GPGSV, $GPGLL. Timestamps
occur in ZDA RMC GGA GLL; the timestamp is incremented on ZDA.

In this case, gspd notices that the timestamp is incremented on ZDA,
which comes after GLL.  GLL is marked as end of cycle, reports are
issued on every GLL using info accumulated from  ZDA RMC GGA GLL,
and GLL fills the tag field of the resulting TPV.
-- 
                <a href="http://www.catb.org/~esr/";>Eric S. Raymond</a>