On 2/19/15 9:11 AM, Matt wrote:

An inexpensive GPS receiver with a 1pps output will easily get you to
much better than 1 millisecond.  The Garmin 18 is but one choice.  It
has the advantage that it's already packaged, as opposed to, say, one of
the little modules designed to be attached to a Arduino.

Configuring ntp to use it is just a matter of setting up the file
properly. NTP will use the 1pps coming in on one of the modem control
signals (DTR, DCD, RI, etc.)

(I use a USB cable to get 5V to run my GPS-18x-LVC, and wire 1pps to
DCD, pin 1 on the 9 pin connector)

So really, it's a matter of finding a place to put your Garmin receiver
and string a cable that's not too long to your *nix box running ntp.

People have done it with a Rpi, if you want to go that route.

That's exactly what you want to do.

http://www.satsignal.eu/ntp/FreeBSD-GPS-PPS.htm

http://www.rjsystems.nl/en/2100-ntpd-garmin-gps-18-lvc-gpsd.php

(everything there should work fine with the current GPS-18x-LVC, but I'm
sure someone on the list has actually done it and can confirm.)

http://www.catb.org/gpsd/gpsd-time-service-howto.html

Yeah, the Garmin is cheap ($85 US), so you're not going to get
nanosecond timing, just microsecond level.  Since you need milliseconds,
it's plenty good enough.

I'll second that.  sub-millisend timing using NTP is very easy and not
expensive.  An old Motorola timing GPS receiver can be bought for about $20
and then all you need in some kind of computer.  NTP can run on any
existing computer while it does it's normal functions.

Getting below a microsecond is MUCH harder but getting 100X better than
your millisecond level goal is cheap and easy.

I like the old Motorola Oncore series because of their price and
performance. They are very inexpensive an 50 nanosecond 1 sigma error is
typical.  But the Garmin will work too.  Use what you have.  NTP really
can't make use a nanosecond level clocks.  NTP works in microseconds.

On Thu, Feb 19, 2015 at 2:34 PM, Jim Lux jimlux@earthlink.net wrote:

--

Chris Albertson
Redondo Beach, California

Hi,

My university would like to have a <1ms precise source of time to do
some networking experiments (measure one way propagation delays
etc...). So I wandered on the internet to find the best choice with a
budget of ~1000€ (~1100 American dollars).
I've been overwhelmed by the number of possibilities (atomic
clocks/GPS signal etc...) and as no price appear on the seller
websites, it's difficult to rule out options.

I hope it's within the scope of the mailing list but I would like some
advice on good hardware with the previous constraint (budget ~ 1100 $,
precision < 1ms).
[]
I wonder if we should buy a specific box or if we could not plug the
antenna to a linux box with gpsd/NTPd on it ?

Any advice ?

Best regards
Matt

Matt,

As others have said, adding a GPS/PPS device to an existing Linux box should
be fine (if you can find one with a COM port these days!).  Here are some
performance measurements - you can see that even with the low-power and
low-cost Raspberry Pi you can get sub ten microsecond results easily:

http://www.satsignal.eu/mrtg/performance_ntp.php

source would meet your needs.  There is a quick-start guide for the
Raspberry Pi here:

http://www.satsignal.eu/ntp/Raspberry-Pi-quickstart.html

Cheers,
David

SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor@blueyonder.co.uk

On Thu, 19 Feb 2015 18:11:08 +0100
Matt mattator@gmail.com wrote:

Can you say a little bit more about your setup?
There are many choices about how to get time accurately and precisely
to a computer, but which one is the "best" depends highly on your setup
and location (and what your requirements are, of course).

		Attila Kinali

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson

Not sure how small your University is, Matt. But most telco/networking
departments will have an NTP infrastructure already, that may include local
GPS clocks. If you look around at the ntp servers on the university LAN and
find one or more stratum-1's with millisecond or less delay, you probably
already have the source you want.

Tim N3QE

On Thu, Feb 19, 2015 at 12:11 PM, Matt mattator@gmail.com wrote: