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Re: [gpsd-dev] PPS over USB

From: Hal Murray
Subject: Re: [gpsd-dev] PPS over USB
Date: Mon, 07 May 2012 10:58:19 -0700

address@hidden said:
> That's news to us.  What GPS chip is in your devices?  If it's  Venus 6
> that's interesting - suggests they don't have the timing  wobble that SiRF
> chips do. 

The timing wobble on data from the serial port is just a firmware 
quirk/bug/feature.  Take your pick.

The old Garmin GPS 18 (non x) was pretty good.  The GPS 18x is as bad as the 
SIRF chip sets but the pattern is different.


address@hidden said:
> 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? 

There are two issues:  accuracy and stability.

Your suggestion might improve accuracy.  It won't help stability.

In the context of bufferbloat, accuracy is not a big deal.  We could go 
through a calibration dance if that helps.

> 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? 

That's reasonable.  Thanks for the data point.  It just means that the 
firmware on the Venus chip isn't as brain damaged (from a timing perspective) 
as the firmware on most other chips.

These are my opinions, not necessarily my employer's.  I hate spam.

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