> It’s unclear to me why there’s so much delay and jitter in the PPS timestamping.<p>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.<p>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: <a href="https://www.developerdan.com/ntp/" rel="nofollow">https://www.developerdan.com/ntp/</a>
Set up my own GPS-referenced NTP server using an RPi3, very easy. Needed to use a powered antenna though, that little PCB thing I got with the module just didn't cut it.<p>Been working augmenting it with a 10MHz ovenized crystal oscillator (OXCO) for kicks, in case GPS is offline. You can second-hand units cheap on AliExpress and Ebay, which is nice as they're pre-aged so shouldn't drift as much.<p>Quality can vary a bit between them, some have lived a hard life perhaps, so I suggest getting 5 and checking with a scope.<p>Chrony somewhat recently gained support for multiple PPS sources, so should be able to just hook it up. This means I shouldn't need to worry about the absolute frequency of the OXCO as long as it is stable, which is nice.
fyi r5s is supported by openwrt upstream so you don't need to deal with vendor firmware (friendlywrt) anymore<p><a href="https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=c06a71f0b3fae00dbd402840ea6e3b532c35b0da" rel="nofollow">https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=c0...</a>