De GPS 12XL (one of the types GPS-receivers) calculates its position and movement with the radiosignals the GPS satellites send. Thereīs a total of 24 satellites and every single one rotates round the world twice a day. The satellite sends singals continuously which can be received by the GPS-navigator. These signals will be used to calculate your position. For this purpose, there have to be at least 3 satellites in view by the GPS.

Your GPS receiver will without satellite signals not be able to provide you with navigation information and for the best result there has to be a clear view of the sky. In short this means: the better the view of the sky, the better the information will be. The picture on the right shows a screenshot of my GPS12XL. The bargraph shows the quality of the received signals and the amount of visible satellites (9 of them in this case).

The GPS will automatically use another satellite to maintain the position when one or more satellites get out of sight. The minimum amount of visible satellites is 3 (for 2-Dimensional information). With 4 satellites in view it gives a 3D position (with altitude).

GPS signals are pretty weak and donīt go through rock, buildings or other obstructions.  The GPS signals will go through glass and plastics. But a (very) dense forest may weaken the signals. Thatīs why connecting an external antenna to your GPS receiver may be necessary. In my case there has never been the need to use one yet.

Because there are a lot of technical terms in this business, you may want to visit this link for an explanation and a brief summary in the list of frequently used technical terms.

There are a lot of different types of GPS-receivers. I chose for the Garmin GPS 12XL receiver, because of the following reasons:

• Robust, compact en rainwaterproof, no moving (mechanical) parts.
• Backlight screen.
• You can connect it to a PC or handheld, which makes you capable to program it. This makes you independent from the manufacturer.
• Communication protocol NMEA 0183 V2.0 or the Garmin protocol.
• Possibility to connect an external antenna (not necessary for me yet).
• Relatively cheap, but very expensive accessories (so make your own cable).
• External power supply: 4 to 40VDC. This makes it direct connectable to your car, motorbike or the power supply of the handheld pc (in my case a Psion 5MX).
• Huge group of users, thus well supported.

To take a look at the specifications of the 12XL, take a look here.

To let the system (as meant on this site) work, you have to make the following preparations in the menu (for the GPS 12XL):

• Choose main menu -> setup menu ->interface ->NMEA/NMEA, 0183 2.0, 4800baud.for the programs Streetplanner and Routeplanner, or
• Choose main menu -> setup menu ->interface ->Garmin/Garmin, Host for programs like PsiNMEA or Realmaps.

For an explanation of the programs mentioned, just keep your head up and click on to the next page......;)