[time-nuts] Low cost GPS for NTP use (was: RE: time-nuts Digest, Vol 60, Issue 44)

[Dave Baxter]

Bob (Fluke.1) has some GPS16HVS's at a decent price.   Very short (cut
off) leads though.  I have two of them (one just in case) and am about
to graft on cable extensions to test them.

These are units that do have real RS232 and 1PPS output as standard, &
work from an 8 to 40V supply, for a fraction of their new cost!   Garmin
still hosts the data sheets, user and tech manuals, and configuration
software, freely downloadable

I believe from what I hear on other lists, that there are probably 100's
of these units being "decommissioned" all over the world.  They are not
that old in real terms.

I was also experimenting with a SiRF start based "mouse" type GPS.  The
ones with the long tail and 6 pin RJ connector for the serial IO.  And a
red flashing light on the mouse, that flashes (changes once a second!)
when it has acquired the signal.   I was trying to track down the PPS
signal that triggers the flip-flop that drives the LED.  You never know
(there are unused pins on it's IO connector inside the case....)    But
the circuitry is just too small for me to see, let alone probe with a
'scope!   So, I'll keep them for use with trackers.


Best Regards..

Dave Baxter.
G0WBX.


---- snip ----

Message: 7
Date: Thu, 09 Jul 2009 12:09:32 -0700
From: Hal Murray <hmurray at megapathdsl.net>
Subject: Re: [time-nuts] Time servers on a well known web site.
To: Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Message-ID: <20090709190933.409E0BCF1 at ip-64-139-1-69.sjc.megapath.net>
Content-Type: text/plain; charset=us-ascii


> Wonder too just how accurate it is, using a USB based (non PPS) GPS?

The picture looked like a GlobalSat BU-353.  I guess you could build a
NTP 
server out one, but I wouldn't expect time-nuts quality.


I've been looking for low cost GPS units that work well with NTP.  I
haven't 
found much.

USB has a bad reputation for timing because it's polled, but that
polling is 
done in hardware on the order of a ms.  You can sanity check things by 
feeding a GPS with a serial connection to both a normal serial port and
a 
RS-232 to USB gizmo.
  http://www.megapathdsl.net/~hmurray/ntp/GPS18LVC-usb-off.gif
I think this says that USB works reasonably well and my kernel/whatever
isn't 
getting the low-latency stuff right.

The old non-x Garmin GPS-18-USB was pretty good.
  http://www.megapathdsl.net/~hmurray/ntp/GPS18USB-off.gif
The 18-x is pretty bad.
  http://www.megapathdsl.net/~hmurray/ntp/GPS18LVCx-off.gif

Most of the low cost USB units seem to use the SiRF chip sets.  They are

horrible for timing.
  http://www.megapathdsl.net/~hmurray/ntp/GPSSiRF-off.gif

If anybody figures out how to get reasonable timing out of one of these 
things, please clue me in.


-- 
These are my opinions, not necessarily my employer's.  I hate spam.