5 Interfacing Options

P6 Serial → Login Shell (no) → Enable Hardware (yes)

Add the following to the bottom

enable_uart=1
dtoverlay=pi3-disable-wifi
dtoverlay=pi3-disable-bt #same with rpi4
#dtoverlay=pi3-miniuart-bt #if you want software BT
dtoverlay=pps-gpio,gpiopin=18
init_uart_baud=9600

Look for the following

[ 2.443494] pps_core: LinuxPPS API ver. 1 registered
[ 2.446699] pps_core: Software ver. 5.3.6 - Copyright 2005-2007 Rodolfo Giometti <giometti@linux.it>
[ 2.471796] pps pps0: new PPS source pps.-1
[ 2.471886] pps pps0: Registered IRQ 498 as PPS source
[ 6.965166] pps_ldisc: PPS line discipline registered
[ 6.966569] pps pps1: new PPS source ttyAMA0
[ 6.966664] pps pps1: source "/dev/ttyAMA0" added

ppstest /dev/pps0 #pps on gpio

cat /dev/ttyAMA0 #serial from gps module

Change servers as appropriate, stuff at the bottom is the gpsd and pps link

Can have no servers and only use gps/pps as source, or have more than 1 other. Two sources (yourself and some other server) is bad.

# Enable this if you want statistics to be logged.
#statsdir /var/log/ntpstats/

statistics loopstats peerstats clockstats
filegen loopstats file loopstats type day enable
filegen peerstats file peerstats type day enable
filegen clockstats file clockstats type day enable


# You do need to talk to an NTP server or two (or three).
#server ntp.your-provider.example

#Can have no servers and only use gps/pps as source, or have more than 1 other. Two sources (yourself and some other server) is bad.
#server tock.usask.ca
#server ntp2.torix.ca
#server ntp.zaf.ca

# Access control configuration; see /usr/share/doc/ntp-doc/html/accopt.html for
# details.  The web page <http://support.ntp.org/bin/view/Support/AccessRestrictions>
# might also be helpful.
#
# Note that "restrict" applies to both servers and clients, so a configuration
# that might be intended to block requests from certain clients could also end
# up blocking replies from your own upstream servers.

# By default, exchange time with everybody, but don't allow configuration.
restrict -4 default kod notrap nomodify nopeer noquery
restrict -6 default kod notrap nomodify nopeer noquery

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

# Needed for adding pool entries
restrict source notrap nomodify noquery

# Clients from this (example!) subnet have unlimited access, but only if
# cryptographically authenticated.
#restrict 192.168.123.0 mask 255.255.255.0 notrust


# If you want to provide time to your local subnet, change the next line.
# (Again, the address is an example only.)
#broadcast 192.168.123.255

# If you want to listen to time broadcasts on your local subnet, de-comment the
# next lines.  Please do this only if you trust everybody on the network!
#disable auth
#broadcastclient
#
server 127.127.22.0 minpoll 4 maxpoll 4
fudge 127.127.22.0 flag3 1 refid PPS

server 127.127.28.0 minpoll 4 maxpoll 4 iburst prefer
fudge 127.127.28.0 time1 +0.105 flag1 1 refid GPSD stratum 1
tos mindist 0.015 #something about accepting a certain amount of jitter from serial sources

Edit devices and options as shown below

# Default settings for the gpsd init script and the hotplug wrapper.

# Start the gpsd daemon automatically at boot time
START_DAEMON="true"

# Use USB hotplugging to add new USB devices automatically to the daemon
USBAUTO="true"

# Devices gpsd should collect to at boot time.
# They need to be read/writeable, either by user gpsd or the group dialout.
DEVICES="/dev/ttyAMA0 /dev/pps0"

# Other options you want to pass to gpsd
GPSD_OPTIONS="-n"

tar -xvf ntpsetup.tar

cd ntpsetup

./install.sh

This copies the gpscontrol files and creates/enables its service. Also makes a gpsd requirement for ntp

echo "" > /etc/dhcp/dhclient-exit-hooks.d/ntp
echo "" > /var/lib/ntp/ntp.conf.dhcp
chattr +i /etc/dhcp/dhclient-exit-hooks.d/ntp
chattr +i /var/lib/ntp/ntp.conf.dhcp

http://www.unixwiz.net/techtips/raspberry-pi3-gps-time.html

https://ava.upuaut.net/?p=951

apt -y install cpufrequtils
echo 'GOVERNOR="performance"' > /etc/default/cpufrequtils
systemctl restart cpufrequtils

apt -y purge bluez triggerhappy wpasupplicant alsa-utils
systemctl stop avahi-daemon
systemctl disable avahi-daemon
apt -y autoremove
apt clean

crontab -e
@reboot setserial /dev/ttyAMA0 low_latency