Re: [gpsd-dev] PPS over USB

[Ed W]

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):
    
http://www.bluenext.co.uk/bluetooth-products/gps/92-bn-903s-gps-dongle.html

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 $
    
https://lists.bufferbloat.net/pipermail/thumbgps-devel/2012-March/000006.html

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
91.220.24.240              21  11   39m     75.348      0.370  -4872us   
218us
91.220.24.241              19  10   37m     75.493      0.294  -5350us   
162us


Ed W