3 ms·

I wonder what kind of accuracy could be had with a cheapie USB GPS receiver. I wouldn't use that anywhere I had Internet connectivity, but in a disconnected loc

I wonder what kind of accuracy could be had with a cheapie USB GPS receiver. I wouldn't use that anywhere I had Internet connectivity, but in a disconnected location I wonder if it would get within a few ms of GPS time. I have an old USB GPS that shows up as a serial device (FTDI, or more likely a counterfeit) that I used back in my wardriving days, and later for moving map navigation on a laptop on road trips (as a dedicated navigator/passenger). It might be fun to dig it up and try it out. Since I know Jeff frequents HN I'll go meta for a second and say that I noticed the Neatpatch cable management in the rack. I've been a fan of Neatpatch for a good long time. I don't use them in racks there there will be much churn, but they're mighty nice in locations that get wired once and only touched again when switches get replaced. It really makes a rack look-- erm-- nest.

michaelt 2y ago
People have taken various measurements, such as https://digitalnigel.com/wordpress/?p=3449 https://digitalnigel.com/wordpress/?p=3449 https://kloppenborg.net/blog/gps-pps-over-usb/ https://kloppenborg.net/blog/gps-pps-over-usb/ https://blog.dan.drown.org/pps-over-usb/ https://blog.dan.drown.org/pps-over-usb/ Of course, if you only need 'good enough' accuracy, a battery-backed RTC and NTP when you're online will generally be good enough.
deleted 2y ago
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lightswitch05 2y ago
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 https://www.developerdan.com/ntp
geerlingguy 2y ago
> Since I know Jeff frequents HN I'll go meta for a second and say that I noticed the Neatpatch cable management in the rack. I love Neat Patch units just to make it so when I'm popping things in and out of my racks, I can drop a few loops of slack into the box and not have it hanging out the back. As long as you keep it relatively organized, it's a tidy, if a bit expensive, solution. For long-term use on a lab rack, I don't like the 1U Neat Patch though. It's too short for frequent use, I often get a little slice from the rigid plastic fingers. The taller 2U units have enough space to work inside—and larger punchouts on the back, for easier cable passthrough.
fsagx 2y ago
have a look at this write up: https://austinsnerdythings.com/2021/04/19/microsecond-accurate-ntp-with-a-raspberry-pi-and-pps-gps/ https://austinsnerdythings.com/2021/04/19/microsecond-accura... (not usb though -- uses pps)
- auspiv 2y ago
  Austin of austinsnerdythings.com here - that article does use PPS but there is a different writeup that does use USB that I refer to as my "millisecond accurate" guide - https://austinsnerdythings.com/2021/09/29/millisecond-accurate-chrony-ntp-with-a-usb-gps-for-12-usd/ https://austinsnerdythings.com/2021/09/29/millisecond-accura...
  - fsagx 2y ago
    Well done. Thanks for those.

michaelt 2y ago

People have taken various measurements, such as https://digitalnigel.com/wordpress/?p=3449 https://digitalnigel.com/wordpress/?p=3449 https://kloppenborg.net/blog/gps-pps-over-usb/ https://kloppenborg.net/blog/gps-pps-over-usb/ https://blog.dan.drown.org/pps-over-usb/ https://blog.dan.drown.org/pps-over-usb/ Of course, if you only need 'good enough' accuracy, a battery-backed RTC and NTP when you're online will generally be good enough.

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lightswitch05 2y ago

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 https://www.developerdan.com/ntp

geerlingguy 2y ago

> Since I know Jeff frequents HN I'll go meta for a second and say that I noticed the Neatpatch cable management in the rack. I love Neat Patch units just to make it so when I'm popping things in and out of my racks, I can drop a few loops of slack into the box and not have it hanging out the back. As long as you keep it relatively organized, it's a tidy, if a bit expensive, solution. For long-term use on a lab rack, I don't like the 1U Neat Patch though. It's too short for frequent use, I often get a little slice from the rigid plastic fingers. The taller 2U units have enough space to work inside—and larger punchouts on the back, for easier cable passthrough.

fsagx 2y ago

have a look at this write up: https://austinsnerdythings.com/2021/04/19/microsecond-accurate-ntp-with-a-raspberry-pi-and-pps-gps/ https://austinsnerdythings.com/2021/04/19/microsecond-accura... (not usb though -- uses pps)

auspiv 2y ago
Austin of austinsnerdythings.com here - that article does use PPS but there is a different writeup that does use USB that I refer to as my "millisecond accurate" guide - https://austinsnerdythings.com/2021/09/29/millisecond-accurate-chrony-ntp-with-a-usb-gps-for-12-usd/ https://austinsnerdythings.com/2021/09/29/millisecond-accura...
- fsagx 2y ago
  Well done. Thanks for those.