[ntp:questions] Inexpensive home reference clock - WWVB/CDMA?

Jason Rabel jason at extremeoverclocking.com
Sun Jul 1 23:46:46 UTC 2007

>All I have is a handheld GPS navigation system (Garmin eTrex). The 
>window in my apartment faces another building about 15 feet away, and 
>there's an overhanging roof. There's a sliver of sky coverage, and some 
>signal makes it through the roof, but it's really not much. Sometimes 
>there's moderate coverage from 3 satellites, while other times there's 
>none at all.

>Apartment management doesn't take kindly to stuff like that, and the 
>balcony is outside their front door (relatively small studios), so 
>there's not really any sky coverage either (see the overhanging roof 
>part above).

Okay, that's pretty understandable situation. I don't know how sensitive
those devices are, but you could try a high-gain timing antenna (I think
they are rated at like 38dB gain - usually about $40 on eBay) with a timing
receiver (I've seen the Motorola VP for $8 on eBay). Once your 3D position
is determined then all you would need is one satellite for timing.

If you still have long outages then you could get a GPSDO package which will
'flywheel' via an ovenized oscillator when there are no satellites.

That's my last thoughts on GPS. I won't keep trying to push the issue. ;)

>My CDMA phone gets full or almost full signal strength here, so there's 
>no issues with that here. Still, the WWVB clock is significantly 
>cheaper, and I'm perpetually tight on money.

I was looking at their CDMA product after I replied before, and it looks
like it uses a pretty cheap Multitech modem (which I've seen sell on eBay
for $10). The problem with that modem (and many other cheap ones) is they
only give time down to the second, and it is a polling command. So while
more expensive, it seems to provide a lesser quality of time (you could do
better with NTP over the Internet or ACTS). Those people say .1 second
accuracy, which is probably just from continuously polling the modem and
finding when the time changes. Really it's not going to provide that great
of a time, I've seen better CDMA products though they are more expensive
(and you wanted cheap).

Again, your OS is going to be somewhat of a factor here. If you are running
windows, I don't know which builds support which reflocks (if any). If you
are running BSD, Linux, or Solaris then you should be pretty safe.

You can see what their return policy is, so if the WWVB unit doesn't work
out you can return it without a fuss (hopefully). Or you could get one of
those WWVB wall / desk clocks and see if it gets a signal and syncs up (plus
give you a clock that's always on-time around the house).

If you have a half-decent broadband connection you should be able to get
~100ms or better time via NTP over the Internet.

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