The PPS is just something you get when the GPS chipset has a lock, though (and sometimes when it doesn't, depending on how you have it configured). Most non-timekeeping GPS chipsets give you a PPS output for free, and 100% of timekeeping GPS chipsets give you one.

Pretty sure about 50ns. Datasheet for the Trimble Thunderbolt E: http://trl.trimble.com/docushare/dsweb/Get/Document-383329/0... ...second page, "PPS accuracy: 15ns (one sigma)" (I picked a larger number than the spec sheet simply so that hopefully nobody would go "oh, that's the ONE SIGMA value, lets argue".)

My Thunderbolt, in a completely temperature-uncontrolled environment with ~20 degree daily swings, generally has a PPS accuracy of ±45ns, in what is pretty crap conditions (first rule of precision timekeeping is "temperature stability")