[ntp:questions] ESR looking for good GPS clocks

Michael Deutschmann michael at talamasca.ocis.net
Sat Mar 3 22:08:35 UTC 2012


On Sat, 3 Mar 2012, Chris Albertson wrote:
> On Fri, Mar 2, 2012 at 4:03 PM, Michael Deutschmann
> > (Not my dream though.  If I was to go to the trouble of installing a box
> > outside my home and routing a cable through the wall to my computers,
> > I'd like some weather sensors in the deal as well as just time...)
>
> No point in that because the weather data can be sent wirelessly from
> outdoors.   I tried to route wires and very quickly found it was
> easier to place the NTP server near the GPS and use a two foot cable.
>  The whole setup is on a top shelf of an interior closet

In my fantasy device, the weather is logged by a daemon running on the
same computer as NTPD.  So the extra features would have no effect on the
cable length.

I don't like consuming spectrum if I don't need to.  And even with
wireless, sensors need power.

Although that brings up one other idea -- a wireless GPS clock.  That
would be basically a relay that listens for long-range-line-of-sight
signals, and then synthesizes a short-range-building-piercing signal for
its owner to use.  Only a simple pulse modulated signal would be needed --
the seconds could be labeled with NTP over 802-whatever wireless.

*

Back to ESR's plans, reading the comments I think he may actually not be
adventurous enough.  He estimates the budget of the volunteers who will
run his sensors at under $200 and absolutely zero hardware work -- not
even building an enclosure for a bare circuit board.  Of that budget, $100
is taken by a linux router that will run his surveying software.  So he
figures a $75 maximum price for his GPS fob.  (He could easily make this
if not for the PPS problem.)

It might make more sense to ask the hardware designers to invent a linux
router and timing-grade GPS *in one box*, making the entire budget
available to the custom part.  Also, the USB conversion overhead would be
gone -- you could even weld the PPS signal to a dedicated IRQ.

Such a box would be just as popular with NTP hobbyists outside ESR's
project as a USB fob.  The sort of NTP hobbyist who rejects "router"
boxes in favor of a customized old desktop will be holding out for RS-232
anyway.

---- Michael Deutschmann <michael at talamasca.ocis.net>



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