Allow for use of cheap USB GPS modules (eg VK-172)

[geerlingguy]

I've been playing around with a cheap USB GPS module, the $9.99 WWZMDiB VK-172 USB GPS Dongle Receiver.

It may be nice to find a way to hack it's PPS output into a reference point on the Pi or on a NIC, for a low-cost entry-level option (to get someone's feet wet with a ~30-60ns accurate Pi Time server for much less than $100 all-in).

I wrote a blog post about the module, testing it on my Mac at home (I was pleasantly surprised): Trying out a cheap USB VK-172 GPS dongle on a Mac.

[geerlingguy]

I've also tried this $16 Ublox 8 module now, and it works a little nicer, has a U.fl connector for an external antenna, and has a PPS signal exposed on a bare trace, which you could solder a little GPIO jumper to...

It worked on my Mac mini, but I'd also like to test getting PPS working on it with the Pi's GPIO:

[geerlingguy]

I have one running just with Chrony + PPS to GPIO pin 18 (physical pin 12) on a MirkoPC V2 board, and it's nice and accurate:

pi@mirkopc:~ $ chronyc tracking
Reference ID    : 50505300 (PPS)
Stratum         : 1
Ref time (UTC)  : Fri May 23 22:56:04 2025
System time     : 0.000000015 seconds fast of NTP time
Last offset     : +0.000000549 seconds
RMS offset      : 0.000000376 seconds
Frequency       : 2.195 ppm slow
Residual freq   : +0.008 ppm
Skew            : 0.040 ppm
Root delay      : 0.000000001 seconds
Root dispersion : 0.000013334 seconds
Update interval : 8.0 seconds
Leap status     : Normal

That's using my external GPS antenna through a distribution amp.

It'd be nice for this project to support that mode of operation in addition to the TimeHAT (with PPS from GPS wired directly to the NIC's PHC). Or at least have an example config so you can set up Chrony but not discipline the PHC directly from GPS PPS? Basically make this playbook a little more flexible for various price ranges / use cases.

I followed the guide in Austin's Nerdy Things - Revisiting Microsecond Accurate NTP for Raspberry Pi with GPS PPS in 2025, with some slight modifications.

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