Run command on GPS fix

Question

I have GPSD running on a Linux system (specifically SkyTraq Venus 6 on a Raspberry Pi 3, but that shouldn't matter). Is there a way to trigger a command when the GPS first acquires or loses the 3D fix, almost like the scripts in /etc/network/if-up.d and /etc/network/if-down.d?

Answer 1

I found a solution:

Step 1: With GPSD running, gpspipe -w outputs JSON data, documented here. The TPV class has a mode value, which can take one of these values:

• 0=unknown mode
• 1=no fix
• 2=2D fix
• 3=3D fix

Step 2: Write a little program called gpsfix.py:

#!/usr/bin/env python

import sys
import errno
import json

modes = {
  0: 'unknown',
  1: 'nofix',
  2: '2D',
  3: '3D',
}

try:
  while True:
    line = sys.stdin.readline()
    if not line: break # EOF

    sentence = json.loads(line)
    if sentence['class'] == 'TPV':
      sys.stdout.write(modes[sentence['mode']] + '\n')
      sys.stdout.flush()
except IOError as e:
  if e.errno == errno.EPIPE:
    pass
  else:
    raise e

For every TPV sentence, gpspipe -w | ./gpsfix.py will print the mode.

Step 3: Use grep 3D -m 1 to wait for the first fix, and then quit (which sends SIGPIPE to all other processes in the pipe).

gpspipe -w | ./gpsfix.py | grep 3D -m 1 will print 3D on the first fix.

Step 4: Put in in a bash script:

#!/usr/bin/env bash

# Wait for first 3D fix
gpspipe -w | ./gpsfix.py | grep 3D -m 1

# Do something nice
cowsay "TARGET LOCATED"

And run it:

$ ./act_on_gps_fix.sh

3D
 ________________
< TARGET LOCATED >
 ----------------
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||