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Comment by lightswitch05 | Hacker News Reader

I had this happen before. I used it as an excuse to learn how to setup NTP with a GPS receiver. I made a little blog on it if anyone is interested in the results. Be sure to click the sandwich menu for some real-time data: https://www.developerdan.com/ntp/

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Quick question, because I've followed a number of how tos before that don't, if you disconnected all network connectivity, would it sync to the PPS source? Second, have you tested this, I only ask because 1# I found a lot of time sync was really slow, and #2 some of them reported that they had sub-microsecond time sync but when tested they were several milliseconds out.

Anyway, I solved these problems with sbts-aru, but maybe it's interesting for you to test if you haven't already done this.

Good question, it absolutely syncs without any network connectivity. For better or for worse, I have it setup NOT to sync to anything else- being a stratum 1 source is enough. Now… I will admit I’m just a hobbyist with this stuff. Anyways, the way I got around it trying to sync to another source when using the PPS was to define the GPS as two separate sources. One for the PPS signal and another for the NMEA sentences:

    refclock pps ppspath /dev/gpspps0 prefer time1 0.004 minpoll 3 maxpoll 4 iburst 
    refclock nmea baud 57600 time2 0.068 minpoll 3 maxpoll 4 prefer iburst

To get around issues with sync being slow, I have NTPD configured with some extra flags to allow large time jumps at startup:

    NTPD_OPTS="-ggg -N"

I have two raspberry pies setup this way- which I don’t use for much else. My primary computers have these two servers configured as time sources- but also have external sources- so as long as network connections are available, they can determine if they are providing a sane time or are false tickers. As for accuracy, you can view the ntpviz output for each setup in the hamburger menu. For catwoman, its regularly within 4 microseconds.

Ah okay. Because I found that the default start order of gpsmon and chrony was incorrect meaning that the shared memory communications wasn't working but there were no error messages stating this. So sometimes, it would sync (slowly) and then chrony sources would say that it was having microsecond sync time but when I connected a switch (On/Off type) to the same gpio on two separate Pi's with linked earth connections and had a python program output the system time in nano seconds I found that despite telling you it was keeping time accurate to microseconds it was in fact many milliseconds out. The only thing I can conclude is that despite the setup indicating that it should use the PPS source that use the interrupt, this was in fact not happening and it was somehow getting it's time from the serial line only. Without indicating that this was happening.

When I switched the startup of gpsmon and the chrony daemon to the other way around time sync was achieved within 1/2 minute to a minute (Real fast) and the reported time on two separate Pi's with the joined up gpio's (So you can get very close to triggering a time check at the same time) was then sometimes as low as 30 ns's from each other, which is damn good considering that most of the difference would be jitter.

And none of the how-to's that I read talked about changing the start order of chronyd and gpsmon so I can only assume they all might be suffering from the same problem. It's important to note that the diagnostic commands from chrony will not indicate this and the only way you will notice if this is happening if you setup a rig like I mentioned above. I noticed because my sound localization calculations were showing errors that didn't make sense.

I must admit I've never used chrony, so I'm unfamiliar with how to configure it. I've read a lot say saying gpsmon's shared-memory-segment is a really great interface and to go with it instead of NTP's own GPS drivers, but I cannot say that lines up with my experience. I do have one computer setup this way, and it works, but I found on the PPS configurations, it seemed to me to have better accuracy and less jitter using NTP's own GPS drivers and taking gpsmon out of the equation. Its quite possible I just don't know the right configuration combination, but ntpsec's own GPS drivers work great for me.

Ah okay. Conversely I've never tried that setup :) Nice to hear about it though.

This is off-topic, but I was looking at your sbts-aru project and remembered having read the hackaday post on it about fireworks. I'm curious if you've ever seen the RaspberryShake BOOM sensor? If so, any thoughts on how your project and it differ?

https://shop.raspberryshake.org/product/turnkey-iot-atmosphe...

I hadn’t seen that. It uses a pi so in principle it could sync the time the same way. How well related to localizable recordings would also depend on how it establishes the actual time arrival with the clock and how it deals with jitter.

Currently I only support USB mics at 16 bits that can talk to jackd. This was a conscious decision because it was all that was needed but also because jacks makes it essentially have multiple source sinks. Such as as well as recording a real time audio processing pipeline that potentially leads to real time gunshot and other sound event localization.

But it would be cool if the infrasound sensor had a USB sound interface. As then it could be useful in localizing elephants.

If it syncs time accurately by any method maybe you are able to localize some very interesting events! The localization code I provide with my project should fine with times obtained from this project. I seriously doubt whether that project uses a memory overlayFS but it’s to add as an improvement.

Interesting! What is the accuracy like when lacking "PPS precision"/Clayface? I'm wondering about the magnitude by which it is off (seconds, minutes, or other).

PPS turned up quickly on Wikipedia, so that one is readily answered

> PPS signals have an accuracy ranging from 12 picoseconds to a few microseconds per second, or 2.0 nanoseconds to a few milliseconds per day based on the resolution and accuracy of the device generating the signal.

You can see for yourself the level of accuracy in the ntpviz output. Notice the units of measurement on the graphs:

* Clayface: https://www.developerdan.com/ntp/#./clayface/7-days/

* Catwoman: https://www.developerdan.com/ntp/#./catwoman/7-days/

That Wikipedia quote should mention temperature! Temperature variations have a big impact at this level of accuracy. These really cheap GPS receivers do not have temperature adjusted clocks. Unfortunately my server closet (this is just a hobby) does not have well regulated temperature, so you can see the impact of temperature on the clock accuracy. Also, I found if I start running a bunch of stuff on these computers - that makes the CPU heat up, which also affects the jitter. If you really want high-precision, you'll have to shell out some extra cash then I did: https://www.sparkfun.com/products/18774

rtk gnsses really rock. I would to save up for one for next year :)

As for the GT-U7, this software can be used to avoid Windows, it seems:

https://github.com/semuconsulting/PyGPSClient

> While not intended to be a direct replacement, the application supports most of the UBX configuration functionality in u-blox's Windows-only u-center © tool (only public-domain features are supported).

I'm going to have to check this out! Thank you for sharing!

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