[gpsd-dev] NMEA time calculation question

[Ed W]

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?

So my question would be: if using USB with it's 1ms polling interval, 
then it's possible to read the first part of the ZDA sentence, then it 
will be a known 1ms later that we get the last few characters and 
presumably then timestamp the sentence.  This would appear to give an 
approx 1ms additional delay over recording the timestamp of the first $ 
and using that if we later discover that it is the ZDA sentence?

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?

Actually, the thought occurs that given USB is around 1024Hz and serial 
around 600-5000Hz, but the sentences are 10+ characters long, we could 
use a custom timing mode for USB reads to increase timestamp accuracy.  
The logic would be that the usb timestamps have jitter, but the supply 
rate of the serial port is presumed of lower jitter - by examining the 
number of characters on each read we could get a far higher estimate of 
the timestamp that a character is actually emitted (and hence work back 
to figure out the timestamp of the $).


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.  
However, there is slightly higher than expected deviation and my initial 
results suggest that this has a positive skewed distribution, perhaps 
fitting the effect described above.

I'm going to collect a couple of hours results and plot them.  My 
observations over the last few months (without having thought it 
special) are that the Venus6+CP210x never visibly deviates more than 
around 1ms on the NMEA time.  I never see any of these few hundred ms 
drifts that others talk about?

The Venus6 is a -160dB tracking chipset. The ublox 6 is a -162dB 
chipset.  So not quite as good, but real use is likely to be more 
important than specs.  I don't own a ublox6 to compare

Regards

Ed W