PPS over USB

[Dan Drown]

Quoting Hal Murray <hmurray at megapathdsl.net>:
> PPS over USB will be off on average of 1/2 the polling interval.  It's easy
> to see if you have a decent place to stand.  In some sense, it's not a big
> deal.  On the other, it's a systematic error so it's silly not to document
> the fix, especially in a document that is mostly cut-n-paste.
>
> Eric: Please plug your 701W into your Pi with HAT and collect some data.

Here's 7 days worth of data:

https://dan.drown.org/rpi/usb-vs-gpio-pps.html

The Rapsberry Pi 2's GPIO-PPS based local clock offsets at 0.1  
percentile and 99.9 percentile were -3.61 microseconds and 4.13  
microseconds.

The Raspberry Pi's USB-PPS based local clock offsets at 1 percentile  
and 99 percentile were -582.97 microseconds and 539.08 microseconds.