PHOTO & GPS - [About GPS]
GPS stands for Global Positioning System. It is a satellite navigation system that can help you to find out your exact geographic position on earth. First it was used for military purposes only but in the last decade more and more civilian applications became available. GPS applications exist in many forms.
Everyone knows the car navigation systems, where the GPS is combined with a digital map of a country. By using voice navigation the GPS can guide you to a certain destination. But GPS also know many applications that aren't using digital maps. Like watches which are capable to tell your exact position in a numeric format. A third kind of devices are the logging modules. They contain a data logger that stores your exact position every few seconds. Later you can download the track to a computer and view it on a map.
GPS is possible because there are over 24 GPS satellite orbiting earth. Each satellite broadcasts a signal, including:
A GPS device is looking for some of those signals from at least three different satellites. Once a good signal was found, the GPS receiver can calculate the distance between receiver and satellite. Once the receiver knows the distance between itself and at least three satellites, it can calculate its own exact location by using trilateration.
In many electronic devices the GPS is integrated on a small external module. This module usually communicates with the device through a serial communication, which is connected to the (micro)processor of the GPS device.
The GPS module takes care of all the necessary processing to calculate the exact position, and sends the results to the microprocessor in the form of NMEA sentences (ASCII Strings). NMEA is a protocol that was invented to communicate between marine electronic devices. Below you can find a sample of a NMEA communication. The microprocessor will have to analzse these sentences, and extract the coordinates from these lines. The coordinates are stored in the lines beginning with $GPGLL. Other lines only contains information about the strength of the GPS signals, ...
GPS devices can help you to navigate through a certain area, but many devices also have many logging features. This means your entire trip is stored into the memory of the GPS.
After a trip or ride you can download the GPS track to your computer (it all depends on the GPS device you are using). The downloaded GPS tracklog has often the .GPX extension. Actually this .GPX file is a regular XML file which can be opened with a plain text editor such as notepad.
You can also import the GPX track into Google Earth. By doing so you will for example see your tours on a real aerial map. Below you can find an example walk in the streets of Paris (Google Earth). I logged this route using the Garmin eTrex Vista Hcx GPS device. This is a portable walking GPS with an autonomy of over 20 hours (running on 2 AA batteries). It can log thousands of hours.
