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Re: [gpsd-dev] NMEA time calculation question

From: Gary E. Miller
Subject: Re: [gpsd-dev] NMEA time calculation question
Date: Mon, 7 May 2012 10:48:42 -0700

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.

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

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