The Raspberry Pi, when isn't connected to the Internet can not synchronize its clock (date and time). So for a Raspberry Pi isolated synchronize the date and time can be achieved by **GPS satellite synchronization**. In this example we use a GPS receiver with a USB connection. 

====== GPS receiver ======
Any GPS should head, insofar as it sending data to the **NMEA** standard. The time and date are provided in the frame ''GPRMC'', we use a model VK-162, available from [[http://www.banggood.com/fr/VK-162-Notebook-Usb-GPS-Navigation-Module-Support-Google-Earth-p-975418.html
|Banggood]] 

{{ :fr:documents:gps_usb.png?direct&150 |}}
====== Receive GPS data ======

Then check if it sends back NMEA sentences, i.e. using cat: (CTRL-C to quit)

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

For your information, the ''GPRMC'' frame is decoded by this way.
<code>
 $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
</code>
====== Install software to update time ======

Download the ZIP archive or fetch source code using git: 
<code bash>
git clone https://github.com/adamheinrich/gpsdate
</code>

To build the project, simply navigate to project's directory and call make:
<code bash>
cd gpsdate
make all
</code>
Now you can use the gpsdate tool to change your local time. Simply call

<code bash>
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.
</code>
====== Autoupdate date and time ======
==Auto update daily==

For autoupdated daily you need addd line in CRON files [[https://en.wikipedia.org/wiki/Cron#crontab|CRON]]
<code bash>
sudo crontab -e
</code>

Clock updated every day at 11h30pm, add folowed lines:
<code bash>
30 23 * * * /home/pi/gpsdate/gpsdate /dev/ttyACM0
</code>

You can save and quit using CTRL-X, Y and ENTER.