Hardware

Adafruit Ultimate GPS, MAX3232 breakout, FT232R breakout, DB9 breakout, YwRobot 3.3V power supply.
PPS and TX go to both MAX3232 and FT232R, RX only comes from FT232R.
DB9 serial "null modem" cable is used, so PPS goes on DTR to show up on the DCD pin
MAX3232/DB9 goes to sandfish
FT232R/USB goes to rpi. rpi's pps-gpio comes from a navspark-mini (GPS+Beidou reciever)

Timeline of changes (all times in UTC):
May 23 20:48 - restarted ntpd with "flag2 1" on USB-PPS
May 23 22:06 - restarted ntpd with "time1 0.000550" on USB-PPS
May 25 19:57 - restarted ntpd with noselect on USB-PPS

Graph: Comparison of both local PPSes

Graph: local clock stability (or: ability for ntpd to measure the local clock's frequency accurately)

Graph: comparison with local clocks (50th percentile offset expected to be <75us)

Graph: PPS-USB offsets