I re-confgured the GT-87 to just use GPS satellites (the posted data had GPS, SBAS, and GLONASS enabled). The 1PPS span was mostly +/- 2.5 ns, with a few excursions to the +/- 3.5 ns range. I'll try GLONASS only next. I looks like Furuno is not going to add Galileo support to the GT-87 firmware.