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Re: [gpsd-dev] PPS over USB

From: Jim Thompson
Subject: Re: [gpsd-dev] PPS over USB
Date: Tue, 1 May 2012 22:07:20 -0500

Hi Dave,

There is a third option in-play, but I've not spoken about it in public.  

I have an ARM (xscale) board with OpenWRT (amd gpsd) running that has 
provisions for a Trimble Copernicus-based GPS module with the PPS output tied 
to a GPIO pin on the Xscale, and the UART on the GPS module connected to the 
Xscale at TTL signal levels. 

The module is in-transit, so I've not completed the integration or started 
testing.  I should be able to use the fast-path IRQ stuff on the xscale to 
significantly reduce the latency & jitter over a USB-based solution. I know 
it's fast enough to be able to implement a distributed CSMA scheme among 
multiple 802.11 radios, with timing sufficient to occur in the same timespan as 
two short OFDM symbols (800ns each, so 1.6 microsec.) 

Outdoor enclosures, POE, etc. are all sourced, so the whole ball of wax could 
be put outside, with either Ethernet or Wi-Fi connectivity.  Alternatively, an 
antenna with DC power could be used, (it's available).

When they're ready, of course you can have one to play with.  I'm doing it to 
support your efforts wrt bufferbloat. 


On May 1, 2012, at 8:40 PM, Dave Taht <address@hidden> wrote:

> I'm a little confused.
> There are two threads going by today. One is about a sirfIII and the
> other a ublox 6
> I have been assuming they are one and the same, but perhaps I'm confused.
> Secondly an item that has not been looked into much is the quality of
> the antennas, or the quality of the clock source when only a single
> sat is available....
> Thirdly, wow.... when can I get one of these puppies to play with?
> :me looks out his bay window wistfully:
> On Tue, May 1, 2012 at 3:14 PM, Gary E. Miller <address@hidden> wrote:
>> Yo All!
>> As some of you may know, esr has been helping the bufferbloat project
>> with some gpsd issues.  Their goal is to get good time from a USB
>> connected GPS.
>> Esr negotiated with Navisys to special build three units of a ublox 6
>> and a pl2303 with PPS conencted to USB.  They call them a GR-601 and I
>> just received the samples.  The preliminary results are pretty good if a
>> clock stable to about 1 milliSec is your goal.
>> A preliminary result from a dual core laptop.
>> # ntpq -p
>>     remote           refid      st t when poll reach   delay   offset
>> jitter
>> ==============================================================================
>> -backup          .SOC1.           1 u   45   64  377    0.233 -0.006   0.067
>> +fuzzy           .GPS1.           1 u   47   64  377    0.241 -0.047   0.071
>> +SHM(0)          .GPS.            0 l   10   16  377    0.000 -2.995   1.656
>> *SHM(1)          .GPS1.           0 l    9   16  377    0.000  0.317   0.428
>> backup and fuzzy each have a PPS clock directly connected over serial.
>> The jitter on backup is about 2 microSec and fuzzy about 0.5 microSec.
>> And they tend to agree over ntp to about 20 microSec or better.
>> All the NTP servers are adjacent to each other and connected over GigE.
>> The jitter over the GigE seems to be about 100 microSec or better.
>> SHM(0) is NMEA time over USB.  SHM(1) is PPS time over USB.
>> As you can see the NMEA/USB short term jitter is about 2 milliSec and
>> the PPS/USB time is about 0.5 milliSec jitter.  Long term NMEA/USB drift
>> is quite a bit larger, maybe 100 milliSec or more.
>> So the jitter of PPS over USB is about 50x worse than PPS over GigE, but
>> I think for the purposes at hand pretty good.  Of course I expect the
>> results to be worse when run on a single core router.
>> The important part of the ntp.conf:
>>    # for gpsd
>>    server minpoll 4 maxpoll 4
>>    fudge time1 0.142  refid GPS
>>    # for PPS and gpsd
>>    server prefer  minpoll 4 maxpoll 4
>>    fudge time1 0.001500 refid GPS1
>> I start the daemons this way:
>>    ntpd -p /var/run/ -N -u ntp:ntp -g
>>    gpsd -n /dev/ttyUSB0
>> Any questions or suggestions?
>> ---------------------------------------------------------------------------
>> Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
>>        address@hidden  Tel:+1(541)382-8588
> -- 
> Dave Täht
> SKYPE: davetaht
> US Tel: 1-239-829-5608

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