In case your Raspberry Pi board is not connected to the internet, the date and time of your Raspberry Pi will not be updated and may drift over time. To enable automatic updating of a Raspberry Pi “isolated” in any place , without network, you can set up an external synchronization by GPS. This solution clearly brings great accuracy!

Once GPS is connected to your Raspberry Pi, the following command should allow you to verify that the data are available.

cat /dev/ttyACM0
$GPRMC,191745.80,V,,,,,,,160314,,,N*7B
$GPVTG,,,,,,,,,N*30
...

The GPRMC frame format is

$GPRMC,hhmmss.ss,A,llll.ll,a,yyyyy.yy,a,x.x,x.x,ddmmyy,x.x,a*hh

1 = UTC of position fix
2 = Data status (V=navigation receiver warning)
3 = Latitude of fix
4 = N or S
5 = Longitude of fix
6 = E or W
7 = Speed over ground in knots
8 = Track made good in degrees True
9 = UT date
10 = Magnetic variation degrees (Easterly var. subtracts from true course)
11 = E or W
12 = Checksum

Download GPSDATE the program using git.

if you have not already git on your Raspberry Pi, install it with the command

sudo apt-get install git

Install GPSDATE

git clone https://github.com/adamheinrich/gpsdate

Install the program from the doanload directory and typing the following commands:

cd gpsdate
make all

Now you can use gpsdate to update your system time. Simply with the command:

sudo ./gpsdate /dev/ttyACM0
Local datetime was: 2014-03-16 20:19:06 (CET)
GPS   datetime  is: 2014-03-16 19:19:07 (UTC)
Succesfully updated local time.

For the system to be updated daily, we’ll add a command in the CRON file

sudo crontab -e

Updating the system clock every day at 1:10 am, add the following line:

10 01 * * * /home/pi/gpsdate/gpsdate /dev/ttyACM0

This article is an example of the many resources available for the Raspberry Pi and the repeater controlboard SvxLink Card (svxcard) on the site svxcard.f5uii.net

 

Sources:

• Parabole satellite de James Halliday (CC BY-SA 2.0)