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Re: [gpsd-dev] PPS over USB

From: Ed W
Subject: Re: [gpsd-dev] PPS over USB
Date: Sun, 06 May 2012 14:34:25 +0100
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:12.0) Gecko/20120428 Thunderbird/12.0.1

On 05/05/2012 00:04, Gary E. Miller wrote:
That said, I would personnally love to find a USB 2.0 equivent to the
USB1.1 PL2303.  Doing so would increase the time resolution from about 1
milliSec to 125 microSec.  An 8x improvement.  Right now the GR-601W is
three orders of magnitude worse than local PPS over serial and 16 times
worse than being on GigE adjacent to an ntpd server with serial PPS.
Getting to 125 microSec would make the USB/GPS only 2x worse than being
adjacent to a serial PPS.

Incidentally I have a load of these (very inexpensive):

They are detected as a "cp210x: v0.09:Silicon Labs CP210x RS232 serial adaptor driver". Some googling suggests this is a USB2 chipset?

The actual GPS is a Venus6, which is a -160dB tracking chipset (so not far off your uBlox 6).

According to someone on your thumbgps list these have very predictable latency to the first $

Simply using gpsd to feed chrony gives only millisec kind of accuracy, presumably too much userland latency and error in reading (does gpsd set low_latency when opening the port?)

$ chronyc_auth_cmd sourcestats
200 OK
210 Number of sources = 4
Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev
GPS0 7 5 98 -8.099 72.469 -69us 734us GPS1 0 0 0 0.000 2000.000 +0ns 4000ms 21 11 39m 75.348 0.370 -4872us 218us 19 10 37m 75.493 0.294 -5350us 162us

Ed W

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