lightswitch05

в прошлом годуdiscuss

I can +/-5ms jitter using a cheap GPS receiver with serial over USB.

Using the same receiver, but with a PPS wire to a GPIO pin on a raspberry pi, I get +/- 5µs jitter.

If you are interested, I have a write up with live graphs in the hamburger menu here: https://www.developerdan.com/ntp

lightswitch05

2 года назадdiscuss

I’m not familiar with Chrony. With NTPsec, the PPS driver docs say [0]:

> While this driver can discipline the time and frequency relative to the PPS source, it cannot number the seconds. For this purpose an auxiliary source is required;

And so (with NTPsec), you need to define two sources even though it’s coming from the same device. One for the PPS signal for clock discipline, the other for the clock value.

> refclock pps ppspath /dev/gpspps0 prefer

> refclock nmea baud 57600 prefer

0: https://docs.ntpsec.org/latest/driver_pps.html

lightswitch05

2 года назадdiscuss

> It’s unclear to me why there’s so much delay and jitter in the PPS timestamping.

I’ve messed around with this on a couple different GPS chips. I’ve found improvements can be made by increasing the baud rate to the maximum supported. 9600 tends to be the default, but 57600 works a lot better. Also, disable all NMEA sentences except the one you are using. Finally ramp up the update interval to be much more often. The default tends to be every 1000 milliseconds, but 100 milliseconds works better for less jitter. I’ve been using NTPsec, not Chrony, so maybe there are more nuances.

Im just a hobbyist, but I have a bit more details written up here, checkout the poorly designed hamburger menu for some charts and graphs: https://www.developerdan.com/ntp/

lightswitch05

2 года назадdiscuss

I get weird looks when I speak with people about my GPS PPS NTP servers. This is website amazing. Most of it is way beyond my comprehension, but it’s given me a lot to read up on.

lightswitch05

2 года назадdiscuss

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...

lightswitch05

2 года назадdiscuss

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.

lightswitch05

2 года назадdiscuss

> 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!

lightswitch05

2 года назадdiscuss

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

lightswitch05

2 года назадdiscuss

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.

lightswitch05

2 года назадdiscuss

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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