Thank you Gary for the purchasing recommendation — it sounds like a fun project, and I will look into it if I don’t find an easy fix for my current hardware. Amazing what can be done with a Raspberry pi.
Thank you David and Greg for the further info and links about PPS. David, in the first link, this is a great detailed procedure, but my situation is not exactly the same, because the author is using pps_gpio in a Raspberry Pi, while my signal is coming through a serial port. I imagine I somehow need to choose and locate a different driver instead of pps_gpio? I’ve never done anything with kernel modules before. I did try the commands lsmod|grep pps and dmesg|grep pps, and I find that I do have pps_core installed and running, but it is “used by” ptp, which seems to be associated with the ethernet adapter rather than the serial port.
The driver on the NTP.org page sounds great, but again I have no idea how to get it on my system. Do I have to install ntpd in place of chrony, and will the driver come with the package?