[time] GPS Time Server Survey

Kipton Moravec kip
Thu Apr 24 21:19:22 UTC 2008

On Thu, 2008-04-24 at 13:45 -0700, Chris Kuethe wrote:
> On Thu, Apr 24, 2008 at 9:14 AM, Kipton Moravec <kip at kdream.com> wrote:
> > I am designing a system with a GPS module, and started looking at all of
> >  the things it could do, including time.
> >
> >  Since you folks are all interested in time, I was wondering which of two
> >  products would be of interest to you.
> >
> >  1. A GPS module with the ability to expand the serial port to more than
> >  10 to allow you to use the NTP NEMA driver. The time pulse would be
> >  accurate to about 30 nS RMS. Retail cost about $100.
> yes. although i'm wondering where you're going to get such a
> non-crappy chip. Also, will it output raw measurements (CP/PR/D)?
> While you're at it, could you mount this on a (mini)?pci card with a
> dual-port UART - plug it in, it looks like 2 more com ports, but then
> you can use one port as your time source and the other port for
> "control" purposes.

The problem with putting it in the PC is that the PC is most often not
near the sky. GPS needs some view of the sky.

You loose a lot of signal with a long antenna lead, that is why you
rarely see more than a 15 foot antenna lead.

> >  2. A time server with 2 Ethernet ports, with GPS time reference, to
> >  directly support between 50 and 100 users using GPS. Retail Cost about
> >  $200
> could be fun - depends on what's in it.
Just a 32-bit microcontroller with a good crystal and a GPS. All running
linux. No keyboard, no monitor, just ssh to it over the Ethernet. Not
that exciting.

> >  Both would be Stratum 1 by using the GPS (and later Galileo also)
> >  Satellites.
> >
> >  Any additional specifications? How much more would you pay for the
> >  additional specs?
> >
> >  Or is this not interesting enough for you, that I would be wasting my
> >  efforts.
> my home time server is a soekris net4801 driven by a garmin gps18;
> pretty low-end, but internal diagnostics say that it's tracking within
> 1 usec of the garmin's PPS output. that's tolerable, considering the
> garmin is only specified to 1 usec accuracy, and the 4801 isn't really
> a marvel of computing precision. And the fact that it's
> ethernet-connected to the ass end of an ADSL line - there goes all my
> precision anyway.
 This is 30 times more accurate than the Garmin, but the PC is no where
near that accurate, so it does not matter.

> also, the soekris can support more than 100 users; i've seen 182
> unique ip's today, and the day is barely half over.
> as i'm also a bit of a gps nut, i like the fact that i can run gpsd on
> my time server to get a view on how the GPS is performing, as well as
> being able to generate a sky view (http://gpsd.mainframe.cx/). For
> that matter, I like the fact that I can put perl/php/python on this
> machine and whip up little scripts as it need them. So a cute little
> low-power SBC would be neat, but I hope it's not too low-power...

All that will be possible, you could have access to the NEMA data.

Is it important?

Kipton Moravec KE5NGX
"Always do right; this will gratify some people and astonish the rest."
--Mark Twain

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