Monitoring a GPS NTP Server

Beyond monitoring Linux OS and basic NTP statistics of your stratum 1 GPS NTP server, you can get some more values from the GPS receiver itself, namely the number of satellites (active & in view) as well as the GPS fix and dilution of precision aka DOP. This brings a few more graphs and details. Nice. Let’s go:

NMEA 0183

The GPS-based NTP server consists of two inputs: The NMEA sentences to get the date and time (along with much other information as the position, which is the primary usage of GPS) and the PPS pulse per second ticks. In order to get some live stats about the satellites and the precision of the GPS signal, we leverage some of the NMEA sentences. Since the gpsd daemon is already running on the Raspberry Pi, we can easily use another tool called “gas pipe” which simply lists all incoming NMEA data. Sample usage of

gpspipe -r

:

pi@ntp2-gps:~ $ gpspipe -r
{"class":"VERSION","release":"3.16","rev":"3.16-4","proto_major":3,"proto_minor":11}
{"class":"DEVICES","devices":[{"class":"DEVICE","path":"/dev/ttyAMA0","driver":"u-blox","subtype":"Unknown","activated":"2019-03-21T13:34:24.189Z","flags":1,"native":1,"bps":9600,"parity":"N","stopbits":1,"cycle":1.00,"mincycle":0.25}]}
{"class":"WATCH","enable":true,"json":false,"nmea":true,"raw":0,"scaled":false,"timing":false,"split24":false,"pps":false}
$GPZDA,133425.00,21,03,2019,00,00*6E
$GPGGA,133425,5132.2081,N,01356.7869,E,2,08,1.00,197.16,M,47.711,M,,*4F
$GPRMC,133425,A,5132.2081,N,01356.7869,E,0.0471,136.251,210319,,*2E
$GPGSA,A,3,2,6,12,19,24,25,32,123,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,2.0,1.0,1.7*08
$GPGBS,133425,1.95,M,2.98,M,9.89,M*3A
$GPZDA,133426.00,21,03,2019,00,00*6D
$GPGGA,133426,5132.2082,N,01356.7868,E,2,08,1.00,196.74,M,47.711,M,,*4B
$GPRMC,133426,A,5132.2082,N,01356.7868,E,0.0557,219.635,210319,,*22
$GPGSA,A,3,2,6,12,19,24,25,32,123,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,2.0,1.0,1.7*08
$GPGSV,4,1,13,02,23,113,40,06,24,077,38,12,83,015,38,14,27,311,18*7C
$GPGSV,4,2,13,17,04,037,21,19,24,044,19,24,50,138,46,25,53,266,32*77
$GPGSV,4,3,13,29,21,198,17,32,36,295,13,123,28,151,41,127,18,126,30*7B
$GPGSV,4,4,13,128,01,102,31*70
$GPZDA,133427.00,21,03,2019,00,00*6C
$GPGGA,133427,5132.2084,N,01356.7867,E,2,08,1.00,196.66,M,47.711,M,,*40
$GPRMC,133427,A,5132.2084,N,01356.7867,E,0.1544,279.026,210319,,*2B
$GPGSA,A,3,2,6,12,19,24,25,29,32,123,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,2.0,1.0,1.7*03
$GPGBS,133429,1.95,M,2.98,M,9.89,M*36
$GPZDA,133430.00,21,03,2019,00,00*6A
$GPGGA,133430,5132.2086,N,01356.7865,E,2,08,1.00,195.86,M,47.711,M,,*4B
$GPRMC,133430,A,5132.2086,N,01356.7865,E,0.0151,297.432,210319,,*2D
$GPGSA,A,3,2,6,12,19,24,25,29,32,123,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,2.0,1.0,1.7*03
$GPZDA,133431.00,21,03,2019,00,00*6B
$GPGGA,133431,5132.2088,N,01356.7864,E,2,08,1.00,195.57,M,47.711,M,,*49
$GPRMC,133431,A,5132.2088,N,01356.7864,E,0.2654,279.194,210319,,*2A
$GPGSA,A,3,2,6,12,19,24,25,29,32,123,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,2.0,1.0,1.7*03
$GPGBS,133431,1.95,M,2.98,M,9.89,M*3F

Piping this output into

grep XY -m 1

to stop after the first output, and piping it again into

awk -F ',' '{print $X}'

to have the comma as separator as well as printing only the X-th position, we can use the following NMEA messages to gather this information:

• satellites in view: GPGSV at the fourth position
• satellites being active: GPGGA after the seventh comma = position 8
• GPS fix (0 = no fix, 1 = 2D fix, 3= 3D fix which is the best): GPGSA at position 3
• positional dilution of precision aka PDOP (should be under 10 for a good position, but irrelevant for our NTP timing): GPGSA again, normally at position 15, but for whatever reason on my output it is at position 54.

A sample run with these four values is:

pi@ntp2-gps:~ $ gpspipe -r | grep GPGSV -m 1 | awk -F ',' '{print $4}'
13
pi@ntp2-gps:~ $ gpspipe -r | grep GPGGA -m 1 | awk -F ',' '{print $8}'
08
pi@ntp2-gps:~ $ gpspipe -r | grep GPGSA -m 1 | awk -F ',' '{print $3}'
3
pi@ntp2-gps:~ $ gpspipe -r | grep GPGSA -m 1 | awk -F ',' '{print $54}'
2.4

That’s it from the NTP server’s side of view. Now we have to bring those values into the monitoring server:

SNMP into MRTG

I am still using my fairly outdated MRTG with Routers2 and RRD installation. Please consider using some other tools such as Zabbix, Icinga 2, or PRTG for monitoring purposes. However, the following procedures in getting the raw values from the NTP server into some kind of SNMP monitoring system are the same.

I am using the “EXTENDING THE AGENT” section within the snmpd.conf in order to be able to poll these values through SNMP. That is: 

sudo nano /etc/snmp/snmpd.conf

adding the following lines at the EXTENDING THE AGENT section:

extend-sh gpsfix        gpspipe -r | grep GPGSA -m 1 | awk -F ',' '{print $3}'
extend-sh gpspdop       gpspipe -r | grep GPGSA -m 1 | awk -F ',' '{print $54}'
extend-sh gpssatact     gpspipe -r | grep GPGGA -m 1 | awk -F ',' '{print $8}'
extend-sh gpssatview    gpspipe -r | grep GPGSV -m 1 | awk -F ',' '{print $4}'

Followed by a

sudo service snmpd restart

.

Using snmpwalk on the MRTG server to find the relevant OIDs, such as:

weberjoh@jw-vm01-mrtg:~$ snmpwalk -v 2c -c THISISTHEKEY udp6:ntp2.weberlab.de .1.3.6.1.4.1.8072.1.3.2.3.1.1
iso.3.6.1.4.1.8072.1.3.2.3.1.1.5.116.101.115.116.49 = STRING: "Hello, world!"
iso.3.6.1.4.1.8072.1.3.2.3.1.1.5.116.101.115.116.50 = STRING: "Hello, world!"
iso.3.6.1.4.1.8072.1.3.2.3.1.1.6.103.112.115.102.105.120 = STRING: "2"
iso.3.6.1.4.1.8072.1.3.2.3.1.1.7.103.112.115.112.100.111.112 = STRING: "2.7"
iso.3.6.1.4.1.8072.1.3.2.3.1.1.9.103.112.115.115.97.116.97.99.116 = STRING: "04"
iso.3.6.1.4.1.8072.1.3.2.3.1.1.10.103.112.115.115.97.116.118.105.101.119 = STRING: "13"

In the end my two MRTG targets for all four values (that is: 2x graphs) are as follows:

Target[ntp2-gps-satellites]: 1.3.6.1.4.1.8072.1.3.2.3.1.1.9.103.112.115.115.97.116.97.99.116&1.3.6.1.4.1.8072.1.3.2.3.1.1.10.103.112.115.115.97.116.118.105.101.119:THISISTHEKEY@ntp2.weberlab.de::11:5::2
MaxBytes[ntp2-gps-satellites]: 24
Title[ntp2-gps-satellites]: GPS Satellites -- ntp2-gps
Colours[ntp2-gps-satellites]: DARKGREEN#006600, PINK#FF00FF, GREEN#00CC00, BLUE#0000FF
Options[ntp2-gps-satellites]: gauge integer
YLegend[ntp2-gps-satellites]: Satellites
Legend1[ntp2-gps-satellites]: Satellites active
Legend2[ntp2-gps-satellites]: Satellites in view
Legend3[ntp2-gps-satellites]: Peak satellites active
Legend4[ntp2-gps-satellites]: Peak satellites in view
LegendI[ntp2-gps-satellites]: Satellites active:
LegendO[ntp2-gps-satellites]: Satellites in view:
ShortLegend[ntp2-gps-satellites]:  
routers.cgi*Options[ntp2-gps-satellites]: fixunit nomax nototal
routers.cgi*ShortDesc[ntp2-gps-satellites]: GPS Satellites ntp2-gps
routers.cgi*Icon[ntp2-gps-satellites]: globe-sm.gif

Target[ntp2-gps-fixdop]: 1.3.6.1.4.1.8072.1.3.2.3.1.1.6.103.112.115.102.105.120&1.3.6.1.4.1.8072.1.3.2.3.1.1.7.103.112.115.112.100.111.112:JESUSISTHEKEY@ntp2.weberlab.de::11:5::2
MaxBytes[ntp2-gps-fixdop]: 100
Title[ntp2-gps-fixdop]: GPS Fix 'n DOP -- ntp2-gps
Colours[ntp2-gps-fixdop]: DARKGREEN#006600, PINK#FF00FF, GREEN#00CC00, BLUE#0000FF
Options[ntp2-gps-fixdop]: gauge
YLegend[ntp2-gps-fixdop]: Fix 'n DOP
Legend1[ntp2-gps-fixdop]: Fix
Legend2[ntp2-gps-fixdop]: Positional DOP
Legend3[ntp2-gps-fixdop]: Peak Fix
Legend4[ntp2-gps-fixdop]: Peak Positional DOP
LegendI[ntp2-gps-fixdop]: Fix:
LegendO[ntp2-gps-fixdop]: PDOP:
ShortLegend[ntp2-gps-fixdop]:  
routers.cgi*Options[ntp2-gps-fixdop]: fixunit nomax nototal
routers.cgi*ShortDesc[ntp2-gps-fixdop]: GPS Fix 'n DOP ntp2-gps
routers.cgi*HRule[ntp2-gps-fixdop]: 10 "PDOP should be under 10 for a good position (not relevant for timing)"
routers.cgi*Icon[ntp2-gps-fixdop]: globe-sm.gif

This gives the following graphs. Note the daily period for the satellites, depending on the position of the GPS antenna. Satellites graph (in the weekly view):

And the GPS fix and PDOP graph (in the weekly view as well):

Two more graphs, both in the yearly view, that show my change from a small GPS antenna to a bigger one at the beginning of February 2018. While the average number of active satellites increased from about 5 to almost 8, the daily peak for the PDOP decreased from about 33 to under 10. Very good!

That’s it. Have a nice day. :D

Featured image “Cockpit” by Roger Schultz is licensed under CC BY 2.0.