Now that you can access the GPS device, let's work on accessing the data programmatically. Your project should now have the GPS device connected and have access to query the data via the serial port. In this section, you will create a program to use this data to discover where you are, then you can determine what to do with that information.

If you've completed the previous section, you should be able to receive the raw data from the GPS unit. Now you want to be able to do something with this data; for example, find your current location and altitude and then decide whether your target location is to the West, East, North, or South.

First, get the information from the raw data. As noted previously, the position and speed is in the $GPMRC output of the GPS device. Write a program to simply parse out a couple of pieces of information from that data. So, open a new file (you can name it location.py) and edit it, as shown in the following screenshot:

#!/usr/bin/python 
import serial 
ser = serial.Serial('/dev/ttyUSB0', 4800, timeout = 1) 
x = ser.read(1200) 
pos1 = x.find("$GPRMC") 
pos2 = x.find("
", pos1) 
loc = x[pos1:pos2] 
data = loc.split(',') 
if data(2) == 'V': 
   print 'No location found' 
else: 
   print "Latitude = " + data[3] + data[4] 
   print "Longitude = " + data[5] + data[6] 
   print "Speed = " + data[7] 
   print "Course = " + data[8] 

The code lines are explained as follows:

The following screenshot is an example showing the result that appeared when my device was able to find its location:

Once you have the data, you can do some interesting things with it. For example, you may want to figure out the distance from and the direction to another waypoint. There is a piece of code at http://code.activestate.com/recipes/577594-GPS-distance-and-bearing-between-two-GPS-points/ that you can use to find the distances from and bearings to other waypoints, based on your current location. You can easily add this code to your location.py program to update your robot on the distances and bearings to other waypoints.

Now your quadcopter knows where it is and the direction it needs to go toward to get to other locations! There is another way to configure your GPS device that may make it a bit easier to access the data from other programs; it is using a set of functionality held in the gpsd library. To install this capability, type sudo apt-get install gpsd gpsd-clients, and this will install the gpsd software. For a tutorial on this software, go to http://wiki.ros.org/gpsd_client/Tutorials/Getting%20Started%20with%20gpsd_client. This software works by starting a background program (called a daemon) that communicates with your GPS device. You can then just query this program to get the GPS data. To start the process, type sudo gpsd /dev/ttyUSB0 -F /var/run/gpsd.sock. You can run the program by typing cgps.

The following screenshot shows a sample result:

The preceding screenshot displays both the formatted and some of the raw data that is being received from the GPS sensor. If you get a timeout error when attempting to run this program, type sudo killall gpsd to kill all the running instances of the daemon and then type sudo gpsd /dev/ttyUSB0 -F /var/run/gpsd.sock again. You can also access this information from a program. To do this, edit a new file called gpstry1.py. The code will look like this:

#!/usr/bin/python 
import gps 
session = gps.gps("localhost", "2947") 
session.stream(gps.WATCH_ENABLE | gps.WATCH_NEWSTYLE) 
while True: 
   report = session.next() 
   if report['class'] == 'TPV': 
      if hasattr(report, 'time): 
         print report.time 

The following are the details of your code:

Once you have created the program, you can run it by typing python gpstry1.py. The following screenshot shows how the output should look after running the program:

Now you have access to your position; you can use it for a number of different capabilities. Perhaps the most useful is planning your flight path. This information and the path-planning procedures are discussed at http://code.activestate.com/recipes/577594-GPS-distance-and-bearing-between-two-GPS-points/ in more detail. As a brief summary, if you know your current position and your desired future location, you can plan a path between the two. This will include an angle and a direction.