Neo Freerunner GPS as OpenBSD timedelta sensor

I recently discovered the nmea(4) serial line discipline in OpenBSD which was written by Marc Balmer in 2008. This is a driver that can be attached to a serial device and which interprets NMEA 0183 data typically produced by GPS devices. It extracts time data (not position) from the GPS stream and makes it available through the OpenBSD sensors framework. This timedelta sensor can then be used by OpenNTPD to keep your clock in sync. Now I have a Neo Freerunner smartphone which comes with a GPS device that produces NMEA data and I wanted to test it out.

First of all on the phone side you have to power on the GPS device, for example like this:

echo 1 > /sys/class/i2c-adapter/i2c-0/0-0073/pcf50633-regltr.7/neo1973-pm-gps.0/power_on

Then you need to transfer the GPS data from the phone to your OpenBSD machine and emulate a serial device that provides the data.

socat (net/socat) does a great job at this. On the phone side make sure GPSD is running and do something like the this:

socat EXEC:"gpspipe -r" TCP-LISTEN:31415

alternatively you can talk to the raw device without GPSD like that:

socat /dev/ttySAC1,raw,echo=0,crnl TCP-LISTEN:31415

Now on the computer side you need to attach this to a pseudo terminal:

socat TCP:192.168.0.14:31415 /dev/ptypa,raw,echo=0,crnl

You should now be able to query the GPS device on your computer. Attach the nmea(4) discipline to the serial device using:

ldattach nmea /dev/ttypa

If everything worked fine you should see an nmea sensor come up which you can query using sysctl or systat for example.

Here’s my relevant output from systat:

SENSOR                                          VALUE  STATUS  DESCRIPTION
nmea0.percent0                                100.00%    OK    Signal
nmea0.timedelta0                              2.237 s    OK    GPS autonomous

To use this sensor with OpenNTPD you simply need to add the following to /etc/ntpd.conf:

sensor nmea0

ntpd is opportunistic about the timedelta sensors, meaning it will use them when they are available but will run just as smooth when they are not.

Here’s some ntpd output:

sensor nmea0 added (weight 1, correction 0.000000, refstr HARD)
sensor nmea0: offset -2.236736

I advise you to read the relevant man pages to get a better understanding of the process.