Raspberry Pi -- ntp server config. FROM: https://frillip.com/raspberry-pi-stratum-1-ntp-server/

ntp.conf

# /etc/ntp.conf, configuration for ntpd; see ntp.conf(5) for help

# Local
server 127.127.1.0  
fudge 127.127.1.0 stratum 10

# GPS with PPS enabled
server 127.127.20.0 mode 17 minpoll 4 maxpoll 4 iburst true prefer  

# GPS time
#fudge 127.127.20.0 flag1 1 refid GPS

# Internet time servers for sanity
# server 0.pool.ntp.org iburst prefer  
# server 1.pool.ntp.org iburst  
# server 2.pool.ntp.org iburst  
# server 3.pool.ntp.org iburst

# added by me
server 0.north-america.pool.ntp.org
server 1.north-america.pool.ntp.org
server 2.north-america.pool.ntp.org
server 3.north-america.pool.ntp.org
server ntp.ubuntu.com
server 0.ubuntu.pool.ntp.org

# By default, exchange time with everybody, but don't allow configuration.
restrict default kod nomodify notrap nopeer noquery  
restrict -6 default kod nomodify notrap nopeer noquery

# Local users may interrogate the ntp server more closely.
restrict 127.0.0.1  
restrict -6 ::1

# Drift file etc.
driftfile /var/lib/ntp/ntp.drift