Re: [gpsd-dev] NMEA time calculation question

[Gary E. Miller]

Yo Ed!

On Mon, 07 May 2012 16:44:36 +0100
Ed W <address@hidden> wrote:

> Hi, Could Eric or similar give me a confirmation on the current gpsd 
> nmea time calculation?

I'll try.
 
> My understanding is that you take the final GPZDA sentence, then work 
> *back* the timestamp of the initial $ by counting the number of 
> characters divided by baud rate?

Nope, nothing anywhere near so fancy.  And since the start of gps 
sentences is so jittery there is no point to trying except for a 
few special case GPS.

> Granted NMEA time is not traditionally considered incredibly
> accurate, but *if* the calculation is as I describe above, then we
> could potentially increase measurement accuracy by 0.5-1ms?

Right now the NMEA uncertainly is 70 milliSec or much worse.  1 milliSec
is noise.

> The reason for asking is that my Venus6 + CP210x has NMEA time that 
> appears to measure almost as accurately as a theoretical PPS over
> USB.

Until the CP210x kernel driver is fixed there is no point thinking about
any usage of CP210x for timing.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        address@hidden  Tel:+1(541)382-8588

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