On Jun 15, 2009, at 9:07 AM, Dave Baxter wrote:
Is this good value, and a trusted seller?
I know nothing about the seller, but the price (US $159) looks
reasonable, considering the inclusion of the antenna and power supply.
My intended main application, is to drive a local NTP server (Network
Time Protocol) as I'm getting more and more P'd off with my ISP's
ineptitude in maintaining a network where NTP (or anything else not
related to web crawling or email) works with any reliability. (Huge
and
variable latency, ping to ping, at different times of day, confirmed
by
other users of the same ISP.)
I run (just because I can!) a HF beacon monitor station, running the
Faros software from Alex VE3NEA. That uses NTP and only NTP to
synchronise it's software clock. (http://www.dxatlas.com/Faros/) The
results of that can be seen at http://g8kbv.homeip.net:8008/ The
white
delay spots illustrate where the NTP source is messed up.
The 10MHz output would perhaps be useful, once I have modified a radio
or two to use that as a reference for their synthesizers, but that is
not necessary as yet. I had a GPS with a 1pps output, but due to a
PSU
malfunction, it doesnt work any more sadly. :-(
Mind you, I'm, still having difficulty configuring a FreeBSD box for
NTP
server use. The instructions to do so I have are good it seems
(http://www.satsignal.eu/ntp/FreeBSD-GPS-PPS.htm) just that the system
does not seem to co-operate, and I'm not that familiar with
Linux/FreeBSD etc at the basic user level, let alone re-compiling the
kernel. As evidenced by a non booting system, the two times I
eventually got it to compile (that takes hours to complete) and that
was
not even with modified sources, just what came on the CD
unmolested. As
each reload takes an age, I sort of lost the plot with that method!
You should be able to get an operating NTP configuration off of the
stock FreeBSD install. Please note that the web page you cited is
somewhat outdated, the current production FreeBSD release is 7.2. The
first thing I would suggest is to get a default FreeBSD install
running with NTP using servers from the internet (if you're in the UK
I would start with the UK NTP pool servers 0.uk.pool.ntp.org,
1.uk.pool.ntp.org, etc.). You should be able to do this without any
recompiling.
Once you have the FreeBSD system operating with NTP I would then work
on the GPS synchronization. Note that the Thunderbolt PPS width is 10
microseconds, and for PPS detection to work on a FreeBSD serial port
the pulse should be on the order of a couple of milliseconds wide.
You can use a pulse stretcher, such as the TAPR FatPPS (http://www.tapr.org/kits_fatpps.html
) to generate a long enough pulse. Using PC hardware, with FreeBSD
you should be able to get the NTP server synchronized within a few
tens of microseconds of UTC, using PPS into the serial port.
You may also consider a device such as the Garmin GPS18 if all you
want is an economical stratum 1 time server.
The local Linux User Group is not that much help either, no one has
any
experience with this sort of "engineering" thing.
Ultimately, I'd like to run something like this on a non-PC hardware
platform.
Now you get into the fun stuff. A device such as the Soekris Net4501
(124 Euro, US $148 new) <http://www.soekris.com/> makes a wonderful
NTP server. Unmodified it works quite well. One simple hardware
modification allows much more precise timestamping of the PPS,
allowing synchronization within a couple microseconds. Going further,
replacing the stock crystal with a precision frequency source, which
you can derive from the Thunderbolt's 10 MHz, will allow the NTP
server to synchronize itself within 150 ns. John Ackerman describes
this at <http://www.febo.com/pages/soekris/>. I have built one based
on John's description, with a few alterations, and it works extremely
well.
First things first, though. For a beginner I would: 1) NTP
synchronized to internet servers using stock FreeBSD install, 2) then
get NTP working with PPS, 3) Then look at embedded-type device such as
Net4501.
I, and I'm sure there are others here, would be happy to help with any
questions, going off-list if need be to avoid cluttering the list.
Ralph
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