[time-nuts] Best Chance GPS module

Chris Albertson albertson.chris at gmail.com
Thu Dec 1 01:51:14 EST 2016


First question:   How accurate does your local NTP server need to be?   If
the answer is "a few tens of milliseconds" then you don't need GPS.  All yu
need is a decent Internet connection.

Second.  NTP is a VERY light load and certainly does not need to run on a
dedicated computer.   Any mail server, web server or any computer that runs
24x7 can run NTP and you will not notice the extra load on the system.
And even in the case where you need GPS, one need just ONE computer.
 Almost any computer is powerful enough and you only need 100 Bit Ethernet,
gigabit is not better for timing.

About the GPS receiver.   Even the (within reason) worst GPS receiver with
a partial view of the sky and some multiparty will by ODERS of MAGNITUDE
more accurate then needed for running NTP.  The reason is that the BESTR
one can hope for with a near perfect NTP setup is a few micro seconds error
and even a quite poor GPS will do better than 100 nanoseconds.     I'd say
if yu can get the GPS to run at all it will be good enough for NTP.

One other thing:  Your GPS ABSOLUTELY MUST produce a one pulse per second
output.

If on the other hand your were trying to build a frequency standard or a
GPS controlled oscillator then you'd be worried about tons and tons of
details like you listed.   You literally just can't be good enough for
running a GPSDO.  But for NTP it is easy because even a run of the mill GPS
receiver is dramatically better than needed for NTP.

But take a serious look at your requirements.   I got into running NTP long
before therefore affordable GPS receivers.  In fact long before we all have
"always on" Internet connections and we all had dial-up phone modem.   So I
ran NTP over a phone modem and used that time standard to run an
astronomical telescope and was able to aim it at stars.   Many times your
requirements are not so hard and something simple works well.

Don't struggle getting the last nanosecond of accuracy out of a GPS when
NTP is only about to transfer millisecond level time over Ethernet.

On the other hand, if yu time stamping needs to be at the microsecond level
then the GPS must be physically connected to the computer doing the
stamping, yu can't transfer microseconds over an Ethernet and if you need
nanosecond level time stamps you can't get that in software




On Wed, Nov 30, 2016 at 1:11 PM, MLewis <mlewis000 at rogers.com> wrote:

> (resending as I tried posting in html...)
>
> Hello,
>
> I'm a novice at time issues. I've been gathering info and I think I'm
> ready to start asking questions.
>
> I'm after a time solution for my personal computer. It looks like I need:
>
> *    a standalone box to be my own local personal NTP server (so the
> variable processing load on my main box can no longer affect host polling
> software timestamps...)
> *    run standard NTP Client & Host software on that box
> *    discipline it with PPS from GPS, using an active antenna
> *    use NTP to obtain the NTP time from this local personal NTP server as
> required
> *    I don't know if I need two additional boxes: one for the NTP host and
> another for receiving the GPS PPS signal, like a Pi in kernal mode.
>
> GPS issues.
>
> *    I'm in a semi-basement apartment.
> *    Building faces due South.
> *    Building is 1960's brick & concrete, steel trusses & sheets in place
> from the concrete floor pours.
> *    Windows from 14" to 38" from the ground. My antenna has to be between
> those.
> *    Window frame is plastic retrofit over wood Pearson-sliders.
> *    There is a bank of buildings due south of me that leaves me with
> clearance elevations between 6 and 12 degrees. I'm at 45.42 North.
> *    Due South within what open sky I have, are two heights of electrical
> wires, around 35 feet out from the building at the edge of the parking lot.
>
> I'm expecting multipath from: those buildings, my building and I assume
> the wires and possibly the cars?
>
> Between the multipath and a less than full view of the sky, I believe I'm
> needing a GPS module:
>
> *    that is sensitive,
> *    good multipath handling,
> *    GPS & GLONASS, to get as many sats possible out of the sky-view I
> have.
>
> Due to my location difficulties, considering newer more sensitive GPS
> modules seems to make sense, even if they're not timing modules. I was
> tempted by a Qualtec L86 with -148|-165 dBm or a SkyTraq Venus838LPx-T.
> Then a ublox Max-M8Q with -165|-167 dBm & its stationary mode. But then I
> discovered I can get a ublox NEO-M8T with -165|-167 dBm sensitivity, its
> multipath handling and concurrent reception of GPS/QZSS, GLONASS, BeiDou &
> Galileo.
>
> A ublox NEO-M8T with an active timing antenna (GPS | GLONASS ?) seems to
> be my best chance of being successful at my location.
>
> Questions:
>
> *    Is there a more suitable GPS module than the ublox NEO-M8T for my
> location issues?
> *    Can anyone recommend an affordable suitable antenna for GPS & GLONASS?
> *    One box or two, for running a local NTP host and receiving the PPS
> from a GPS module?
> *    Is there a better solution for a micro board than a Rasp Pi with its
> USB controlled Ethernet, or the BBB with it's RF issues?
> *    What am I not considering that could end up biting me?
>
> Thanks,
>
> Michael
>
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-- 

Chris Albertson
Redondo Beach, California


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