On Mon, May 7, 2012 at 1:48 PM, Gary E. Miller
<address@hidden> wrote:
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?
> 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.
His is one of the special case GPS that has a solid PPS synced start bit.
> 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.
On ONE of the more jittery chipsets
> 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.
Not if the USB latency in the CP210x is constant