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

A C agcarver+ntp at acarver.net
Tue Sep 22 15:39:40 UTC 2015

Most GPS units like that MR350P require a ground plane under them to
actually receive properly and have better performance (they are meant to
mount on a vehicle with a metal roof).  Try putting a very large square
of aluminum foil (or get an inexpensive aluminum foil baking pan or
pizza pan from the grocery store) under it.  Run the wire through a hole
in the foil so the mushroom top of the MR350 sits flush on top of the foil.

This applies to all GPS units that are meant for mobile operation if you
are intending to use them in a fixed location (especially any that have
magnetic antennas).  The only type of GPS that works for a fixed
application are those specifically designed for pole mounting at fixed
locations because they have internal ground planes or the antenna.  Some
marine GPS antennas fit this requirement since many boats roofs are made
of fiberglass and not aluminum or steel like the roof of a car.

On 2015-09-22 08:15, Charles Elliott wrote:
> 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
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