[time-nuts] Best Chance GPS module

Bob Camp kb8tq at n1k.org
Wed Nov 30 16:23:54 EST 2016


A few basics:

1) GPS receivers really can’t / don’t do a lot about multipath. The newer devices with a lot 
of correlators help a bit, but that’s about it. Simply put - newer is better. 
2) Because of near the omnidirectional nature of GPS, antennas don’t do a lot for multipath. They
can help a bit on low angle stuff, but that’s not going to be your problem. 
3) Unless you can reasonably expect 4 sat’s in view at all time, don’t bother with setting up a GPS
timing system. It will just make you angry with all the issues. A USB GPS on your PC will give you a 
pretty good idea of what you can or can’t pick up. 
4) If the sole reason to do this is for NTP, consider simply setting up a local server and doing sync
over your internet connection. Much less fuss ….


> On Nov 30, 2016, at 4:11 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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