RPI + Adafruit GPS sanity check

[Patrick]

Hi all,

I used the following docs to setup an RPI 2B + Adafruit GPS HAT.
Primarily following:
	https://www.ntpsec.org/white-papers/stratum-1-microserver-howto/
while using the following for sanity checks:
	https://www.morcant.com/blog/stratum-1-ntp-server/
	http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html

And it all appears to be working. I have some questions though:

Is the caveat against NMEA refclock still valid?
https://www.ntpsec.org/white-papers/stratum-1-microserver-howto/#_why_gpsd
I tried both gpsd and refclock NMEA configs, and they both threw
falsetickers, which I chalked up to my HAT's limited skyview.
Since the PPS is /dev/pps0, gpsd can be dropped.

Is there anything that should be done to prevent mishaps like the USNO
has had over time? It seems like there just needs to be a nagios check
that runs against local time servers to make sure they have a large
enough odd number of peers?

Does refclock NMEA time2 or refclock shm time1 calibration matter?
Depending on skyview, the NMEA refclock may bounce around too much and be
declared a falseticker at times due to too large an offset -- so with a limited
skyview, it's better to think of the NMEA clock as a backup time source.

It seems that the more important source is the PPS which will discipline
when the consensus second starts. PPS should always be flagged with a
leading 'o' and its jitter should be less than 5 microseconds.

ntpq -pn
     remote           refid      st t when poll reach   delay   offset   jitter
===============================================================================
oPPS(0)          .PPS.            0 l   10   16  377   0.0000  -0.0008   0.0002
*NMEA(0)         .GPS.            0 l    9   16  377   0.0000  26.6331  44.7632
+10.8.0.10       218.186.3.36     2 u    6   16  377   0.3630  -0.1715   0.1588
+10.8.1.7        10.8.0.10        3 u    5   16  377   0.3528  -0.1748   0.0125
-209.58.172.142  85.199.214.98    2 u   58   64  377   3.2658  18.1020   0.0932
-133.243.238.164 .NICT.           1 u   31   64  377  75.7035   2.9800  38.0955
-203.217.204.135 133.243.238.163  2 u   62   64  377  71.4136  -4.2721   0.1610



Thanks!

P.S. Below are notes that others might find useful regarding HAT
configuration and timeN calibration, along with the configs that
were used.




# GPS HARDWARE INITIALIZATION

Configure the GPS to output NMEA MRC (has date + time) once per second
https://www.gpsinformation.org/dale/nmea.htm
https://cdn-shop.adafruit.com/datasheets/PMTK_A08.pdf

N.B. GPS commands are check-summed as per the last page of the PMTK datasheet.

python3
def checksum(cmd):
  ck, *cmd = [ ord(x) for x in cmd ]
  for c in cmd:
    ck ^= c
  return hex(ck)[2:].upper()

assert checksum("PMTK605") == '31'
assert checksum("PMTK226,3,30") == '4'
assert checksum("PMTK251,115200") == '1F'


# GPS CALIBRATION

GPS offsets can vary depending on skyview ( what satellites are seen when ) so
calculate the offsets online-average over at least 12 hours ( Medium Earth
Orbit period ).

Run the following to generate the NMEA refclock's time1 offset

while sleep 1; do
  ntpq -pn
done | awk '
  BEGIN { n = 0 ; mean = 0; }
  /^.NMEA/ {
    n += 1;
    mean += ( $9 - mean ) / n;
    printf("%f\t\r", mean);
	system("");
  }
'




####### /etc/rc.local #######

#!/bin/bash
GPS=/dev/ttyAMA0

gps_config() {
	printf '$%s\r\n' "$1" > $GPS
	sleep 0.2
}

stty -F $GPS raw 9600 cs8 clocal -cstopb
gps_config 'PMTK251,9600*17'

# only output RMC
#          $PMTK314,1,1,1,1,1,5,0,0,0,0,0,0,0,0,0,0,0,0,0*2C<CR><LF>
gps_config 'PMTK314,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0*29'

# output once per second
gps_config 'PMTK220,1000*1F'


# ntp talks directly to the adafruit
ps -C ntpd &>/dev/null && {
	echo killing ntpd
	killall -TERM ntpd
}
nice -19 /usr/local/sbin/ntpd -g -c /etc/ntp.conf


####### /etc/ntp.conf #######

refclock pps         refid PPS ppspath /dev/pps0 minpoll 4 maxpoll 4 prefer
refclock nmea        refid GPS path /dev/ttyAMA0 baud 9600 minpoll 4 maxpoll 4 mode 1 time2 0.480
# mode 1 cos only RMC has date + time

# local servers
server 10.8.0.10     iburst minpoll 4 maxpoll 4
server 10.8.1.7      iburst minpoll 4 maxpoll 4

server sg.pool.ntp.org iburst
server jp.pool.ntp.org iburst
server kr.pool.ntp.org iburst

restrict default kod limited nomodify nopeer noquery
restrict -6 default kod limited nomodify nopeer noquery

restrict 10.8.0.10
restrict 10.8.1.7
restrict 127.0.0.1
restrict -6 ::1

driftfile /var/lib/ntp/ntp.drift