Accurate time without a network connection?
Arthur W. Neilson III
art at pilikia.net
Wed Apr 23 12:30:48 PDT 2003
--On Tuesday, April 22, 2003 1:45 PM -0500 David Kelly said:
| Looking for options to keep system time moderately accurate at a site
| without network access. So the normal application of ntpd over IP is
| not viable. Dialup modem not allowed either.
| Definition of "moderately accurate": doesn't have to be any better than
| I can set my watch, travel to the site, and set the system time. It just
| has to eliminate "travel to the site."
| A GPS is one possible solution using the appropriate ntpd driver. Am not
| sure I can place the GPS antenna in a suitable location.
| A WWV receiver would be another viable option but I am not having any
| great success in finding a prebuilt receiver with decoder suitable for
| direct connection to serial port and ntpd.
Hey David. I built a gps clock for ~200 bucks, it's really good.
It's based on the Motorola UT+ board which is 5" x 3" all SMT and
prebuilt, this is interfaced to a smaller daughtercard from TAPR
(Tuscon Amateur Packet Radio club) I am a ham myself and also
a freebsd guy since v2.1.2 ... the daughtercard converts the TTL
PPS (pulse per sec) signal from the gps to rs232 with a MAX232
chip, you'll need some soldering skills to build the daughtercard,
connect things together and mount it in the enclosure of your
choice. Here check it out at my website:
The info is all on the site. Freebsd has support for kernel PLL
via an external PPS signal, just add option PPS_SYNC to your
kernel config also ntpd has builtin support for the Motorola
Oncore UT+ the second page of my refclock stuff explains it.
The antenna is very small since the GPS frequency is in the
1 Ghz band (1575.42Mhz to be precise) it's the size of a mouse
and can be placed anywhere you can see some sky. The ntp distro
oncore driver inits the GPS in mode 0 (time only) if you run mode
3 that's 3D you need to see 4 birds at all times to get a 3D postion
in space (latitude, longitude, height above ground) if you don't
care about your position (the gps clock just sits there in the
same spot all the time) then run it in mode 0 so you only need
to sync to a single bird for the time.
I live in a condo and the antenna sits at the edge of my balcony
I put a small metal plate under it for a ground plane and my building
faces south. I use BNC type connectors and 25 feet of RG-174 coax
between the GPS and the antenna, if you make the cable run too long
you'll get too much signal loss especially with RG-174 which isn't
exactly low loss cable. Accuracy to the clock is in the 50 ns range,
+ or - ~30 ns.
Arthur W. Neilson III, WH7N - FISTS #7448
Bank of Hawaii Network Services
art at pilikia.net, aneilson at boh.com, wh7n at arrl.net
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