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

Hal Murray hmurray at megapathdsl.net
Mon May 13 15:17:42 EDT 2013

jimlux at earthlink.net said:
> 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.

No.  Sigh.

I've been looking for a low cost, no soldering, GPS unit.  I haven't found 
one.  USB seems like the obvious choice.  The problem is that they all have 
crappy software, at least if you are interested in timing.  The problem is 
100 ms of long term wander.  It's a first cousin to a hanging bridge.  You 
can't filter it out.

Here is a graph of the SiRF chip set:
The Garmin USB-18x is similar.  I don't have a graph handy.  I'll put one 
together if anybody is curious.

The USB-18 (no x) was pretty good.  Then they updated to the USB-18x.  It's 
as crappy (for timing) as all the SiRF units.

USB timing may not get to 1 ms, but it will be close.


Linux has 2 types of PPS support.  NTP uses the one in the kernel: the kernel 
saves a time stamp.  NTP polls for it.  gpsd uses a user-wakeup ioctl: user 
code gets a wakeup (interrupt?) and grabs the time.  That's obviously slower 
and noisier, but it's good enough if you aren't on a busy system or after 
microsecond accuracy.

Eric Raymond (gpsd) got one of the GPS over USB vendors to add the wire to 
connect the PPS signal to the DCD pin on the USB-serial chip.  He picked a 
unit that used a friendly USB-serial chip, meaning documentation was 
available.  So there was no problem getting the software working.

That was several months ago.  I don't know if they ever made it to retail.


> I have.an ARM board handy with serial and usb, but never set up NTS
> with a GPS. Is it just NEMA commands?

The Garmin USB drives don't speak NMEA.  Most of the low cost GPS over USB 
units use SiRF chips that do speak NMEA.  (They also have a binary mode.)

Anybody know if uBlox over USB units are available?  I just took a quick look 
and didn't find anything.

These are my opinions.  I hate spam.

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