[time-nuts] Can I get 1 millisecond accuracy with a USB GPS-18

Jim Lux jimlux at earthlink.net
Mon May 13 22:20:53 EDT 2013

On 5/13/13 8:24 AM, Chris Albertson wrote:
> Why not run an experiment?  Connect a serial and a USB GPS-18 to the
> same NTP server. and run BOTH ref clocks at the same time.   Let it
> run for a day or so them please, post the results for NTPQ here.

I will, but, before I launch into a science project, I figured I'd ask 
if anyone else has done it.
> Windows is a poor platform for timing.  You are best off with Linux or
> BSD UNIX and a real hardware serial port.  The combination of Windows
> and USB makes 1 millisecond about the limit.  But you can get  2 - 5
> uSec. with Linux or BSD and a serial port.   The world record (as far
> as I know) broke 1 uSec but required external hardware and motherboard
> mods.

Yes, I'm aware of the issues with Windows, but at this stage I want to 
keep my options open.  That's why I was asking if anyone had tried it, 
and what their results were..

> An experiment would answer this.  But you'd need to run both GPSes on
> the same NTP server.
> If you don't have the port, it is worth the effort to do a Motherboard
> swap.  Intel makes a few Atom boards with real serial rs-232 ports and
> the Atom is so low powered that it can use a passive heat sync.   If
> you run this 24x7 the power savings can pay for the new $90 board.
> $90 includes the cpu chip, mother broad and cooling and the serial
> port.   What is your cost per KWH?   With a hardware serial port and
> Linux (or BSD Unitx) single digit uSec is to be expected.

I'm running an Atom board and it has RS232.   The system is battery 
powered, so power cost isn't really an issue, per se... although bigger 
batteries are more expensive.

> A common mid tower PC can burn 100W.  THat works out to 876 KWH per
> year.  I pay 27 cents for each KWH so the cost is $234 per year.  The
> new board is only about 10W.  You can see a payback on only 6 months
> and you make $100 in the next 6 months.  The power savings can also
> pay for replacing the GPS receivers with serial units.
> On Mon, May 13, 2013 at 7:43 AM, Jim Lux <jimlux at earthlink.net> wrote:
>> rummaged through archives and couldn't find anything..
>> I've got some GPS-18's with the RS232 and 1pps output.  BUT, I'm wondering
>> if anyone has tried to get timing with the USB version (Linux or Windows..),
>> and if so, is getting 1 millisecond absolute accuracy feasible.
>> The underlying USB thing has 8kHz frame rate, but I suspect that the serial
>> port emulation (which is probably what they use) might not hold it that
>> well.  I can see there being some sort of fixed offset (going through hubs
>> and such), but once the configuration is known, is that stable?
>> Any practical advice?
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