[time-nuts] GPS for ntp

Chris Albertson albertson.chris at gmail.com
Tue Oct 21 01:03:03 EDT 2014

NTP is not nearly good enough to use for measuring speed of light
delays.  It works at the microsecond level at best.  I think what you
want is each station to have a local oscillator that runs in phase
with the 1PPS signal that comes from GPS receivers.  Then you measure
the incoming signal relative to the phase of the local oscillator.

On Mon, Oct 20, 2014 at 6:39 PM, Joseph Gray <jgray at zianet.com> wrote:
> What does everyone think of this GPS module for ntp use? According to the
> specsheet, it uses a Ublox Neo-7N.
> http://www.ebay.com/itm/RY725AI-10Hz-UART-USB-interface-GPS-Glonass-QZSS-antenna-module-flash-memory-/181562403752
> I'm thinking about using it for a Beaglebone ntp server. I know there was
> some discussion about Beaglebones a while back. Has anyone gotten ntpd with
> PPS and time stamping running on a Beaglebone yet (like on the Soekris
> Net4501)?
> Does anyone want to take a wild guess as to the best/worst case UTC offset
> I might get with such a setup? I realize that I'd have to try it to really
> find out, but guesses and advice from those more knowledgeable is always
> appreciated.
> What I'm really after is to eventually have several Beaglebones scattered
> around the area as part of a TDOA DF network. I have spare radios to
> dedicate to the task. Each fixed node will timestamp incoming transmissions
> and then relay that information to a central location. The central system
> will use the data to calculate the location of the transmitter.
> Does anyone here have any experience with this sort of thing? Would you
> recommend a better way of accurately timing the transmissions vs using ntp
> on each Beaglebone?
> I know that TDOA is used by the cellular companies. Is anyone aware of some
> "user level" projects or source code for this? I've Googled a bit, but
> haven't turned up much. There was one thesis about an Amateur doing part of
> this, but he didn't have much detail on the satellite nodes and didn't
> describe anything about the centralized processing part.
> I have one Beaglebone White that I got cheap, so I have something to get
> started with. Later, as I figure out things, I'll buy a few Beaglebone
> Blacks.
> Joe Gray
> W5JG
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Chris Albertson
Redondo Beach, California

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