gistfile1.txt

http://www.satsignal.eu/ntp/Raspberry-Pi-quickstart.html
http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html
https://bart.motd.be/node/79
http://ava.upuaut.net/?p=726

!!! https://gist.github.com/LebedevRI/20019255686c7cb1b578

sudo raspi-config
1. Expand Filesystem 
2. Advanced Options -> Disable Serial Shell (optional) 
Reboot

-------------- /boot/config.txt --------------

dtoverlay=pps-gpio,gpiopin=18

--------- /etc/modules ---------

pps-gpio

--------- /etc/udev/rules.d/09-pps.rules ---------

KERNEL=="ttyAMA0", SYMLINK+="gps0"
KERNEL=="pps0", OWNER="root", GROUP="dialout", MODE="0777", SYMLINK+="gpspps0"

----------------- /etc/ntp.conf -----------------

# NMEA /dev/gps0, /dev/gpspps0. Mode 17: 9600 baud (16) and process $GPMRC
server 127.127.20.0 mode 17 minpoll 0 maxpoll 0 iburst prefer
fudge 127.127.20.0 flag1 1 # enable PPS signals processing
fudge 127.127.20.0 flag2 0 # capture PPS pulse on rising edge
fudge 127.127.20.0 flag3 1 # use kernel clock dicipline
fudge 127.127.20.0 refid GPS
fudge 127.127.20.0 stratum 15

# Kernel-mode PPS reference-clock for the precise seconds
server 127.127.22.0 minpoll 0 maxpoll 0 prefer
fudge 127.127.22.0 flag2 0 # capture PPS pulse on rising edge
fudge 127.127.22.0 flag3 1 # enable kernel PPS discipline
fudge 127.127.22.0 refid kPPS
fudge 127.127.22.0 stratum 0

# add prefer to pool's/server's !

-------- recompile ntp with pps-tools added to build-depends -----------