[ntp:questions] Synchronize distributed PCs with GPS 1 PPS and NTPD for OWD measurements

Charles Elliott elliott.ch at comcast.net
Tue Sep 22 15:15:36 UTC 2015


The USGlobalSAT MR350P or MR350PS4 is a pretty good COTS 
GPS unit with PPS.  You can find the data sheet here: 
http://usglobalsat.com/s-18-serial.aspx.  You must look 
on the datasheet to find the PPS because it is not advertized.  
I have a MR350P, so I know there are two problems with it.

1.  The PPS signal is not brought out to a connector, so you 
have to cut the cable to find it; the data sheet says it is 
in yellow.  But cutting the cable w/o damaging the wires 
presents a barrier.

2. My Samsung Galaxy S4 consistently finds 22 or more satellites 
in a sky that the MR350P finds at most 6-8 satellites, and that 
is with the MR350P puck outside taped to the top of a fence post.  
But the phone cost $650 and the MR350P was about $65, so that 
result may be logical.  When the MR350P is stationary on the fence 
post, the average of 4 days of GPS location readings is 39.99321667N, 
-75.12598333W.  The Samsung S4, located right next to the MR350P, 
says the location is 39.993218N, -75.125984W (after 10 minutes), and 
that position locates on Google Maps only about 5-6 feet south of its 
actual location, whereas the MR350P's coordinates place it about 2 
feet east of the fence and maybe 2 feet south of its actual location.  
So, over a period of time the MR350P is pretty accurate, position-wise.

W/O PPS the time signal on the MR350P is unusable.  It wanders around 
about +- 60ms about the correct time with a frequency of about 36 hours.

I never could make myself cut the cable on the MR350P to find the PPS, 
since I am about as handy with electrical stuff as Gordon was untying 
knots.  But if you could find a MR350P at a decent price, it is a COTS 
GPS with PPS.

Charles Elliott

> -----Original Message-----
> From: questions [mailto:questions-
> bounces+elliott.ch=comcast.net at lists.ntp.org] On Behalf Of sandip
> gangakhedkar
> Sent: Monday, September 21, 2015 12:24 PM
> To: Joachim Fabini
> Cc: questions at lists.ntp.org
> Subject: Re: [ntp:questions] Synchronize distributed PCs with GPS 1 PPS
> and NTPD for OWD measurements
> 
> Joachim,
> 
> Thanks for your detailed solution, which was along the lines of my
> thinking..
> 
> Do you know of any off-the-shelf GPS receiver with RS232 and PPS
> capability?
> 
> Hal,
> 
> >> What sort of distance accuracy are you expecting?
> 
> Not very stringent..up to 10m will do.
> 
> 
> >> What sort of distances will you be operating over?
> 
> 50m - 1000m
> 
> >> How unreliable is your link?
> 
> Very. The packet error rate can vary between 0 - 100%.
> 
> >> I would ignore NTP and do everything yourself.
> >>
> >> Do something like ping.  That takes 2 packets, but you don't need to
> know the
> >> time on the other end.  If both ends need to know the distance, you
> can
> make
> >> a measurement with 3 packets.
> >>
> >> Or you can send a dozen packets and use the minimum time assuming
> the
> others
> >> had delays in the interrupt handler.  (and use the spread in the
> times
> as an
> >> indication of quality)
> >>
> >> You will probably need to calibrate the response times of the CPUs
> and
> the
> >> delays through the radios so you can subtract it out.  The radio
> delays
> may
> >> may depend on signal strength which varies with distance, but will
> also
> >> change if you go behind mountains or trees or buildings.
> 
> Interesting technique, but for One way delay measurements, it will
> introduce a lot of measurement error, particularly as the links are
> unreliable and asymmetric (uneven delays).
> 
> 
> Best,
> Sandip
> 
> 
> On Mon, Sep 14, 2015 at 1:40 PM, Joachim Fabini
> <Joachim.Fabini at tuwien.ac.at
> > wrote:
> 
> > On 14.09.2015 12:43, Hal Murray wrote:
> >
> > > Joachim Fabini said:
> > >> - Re-compile your kernel for LinuxPPS support, following the
> > instructions on
> > >> http://linuxpps.org/wiki/index.php/LinuxPPS_installation .
> > >
> > > That hasn't been necessary for a long long time.
> >
> > Most recent distributions have PPS line support enabled by default.
> > Still, kernel build is necessary if you have special driver
> requirements
> > or change options -  I recompile measurement kernels for increasing
> the
> > Hz rate. The page contains some outdated information but some of it
> is
> > valuable (in particular the reference to the ppstools repository and
> > timepps.h copy that is essential for building ntp).
> >
> > > What version of the kernel are you using?
> >
> > 3.8 - 3.13
> >
> > > Recent kernels need something like:
> > >   ldattach 18 /dev/ttyS0
> > > which creates /dev/pps0
> >
> > Yes, that's what I do. Ideally you can automate it and trigger some
> > additional ntp prerequisites (udev-related) as recommended on the ntp
> > pps support page
> (http://linuxpps.org/wiki/index.php/LinuxPPS_NTPD_support
> > )
> >
> >
> >
> > _______________________________________________
> > questions mailing list
> > questions at lists.ntp.org
> > http://lists.ntp.org/listinfo/questions
> >
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