Updating apt get
Shown below are the offset results with the Raspberry Pi in three configurations: with WAN-only connections syncing to the Internet (as you might find it a typical home situation), with LAN connections to a local stratum-1 server, and acting as a stratum-1 server itself with two different small GPS/PPS receivers as the reference clock.
Any glitches in the live data are likely to be the result of me rebooting, making configuration changes, or the GPS signal being less than normal. As expected, syncing from the LAN produces better results than from the Internet (WAN), and making the device into a stratum-1 server results in even lower offsets.
With older GPS receivers this required an outdoor antenna, but more modern units such as those mentioned here may well work indoors providing that some sky is visible, perhaps on the top floor of the building (as I am).
If you have an "RF-proof" roof (lead-lined, perhaps?!
Not shown on the graph, but the offset due to a CPU-heavy task (recompiling NTP from source, taking about 25 minutes) resulted in a 20 s positive excursion, followed by a 10 s negative excursion as temperatures cooled and NTP recovered.
When using a u-blox MEO-6M GPS module, with its PPS pin connected to the GPIO 24 pin for a kernel-mode "ATOM" ref-clock, similar results are obtained.
The Raspberry Pi is a credit-card size computer, available from distributors across the world.There are some helpful Linux commands scattered throughout this page.These notes are almost as much for my own records for the next time I need to visit this project, but I hope they may be helpful to others.Note that the Adafruit GPS Hat uses GPIO 4, physical pin 7, so you would need to change the commands given in this document.Later additions have included remote monitoring of the NTP server performance, and more general monitoring of the Raspberry Pi using the standard SNMP functions, with an additional CPU temperature monitoring add-on. Note that good performance is dependant on the GPS unit having a clear view of the sky, particularly the southern part of the sky if you are in the northern hemisphere.