Garmin 120 GPS

From Schaffter

The Garmin GPS unit

The GPS (Global Positioning System) is invaluable in the Sahara desert. I don't have the knowledge of how to orient by the stars. So, I trust the GPS very much. Normally, we are about three to eight cars when going out there. For security reasons, we always bring more than one GPS unit, mounted in different cars. The major risk is that the US Coast Guard (USCG) would suddenly receive an order from the US Army to turn the system off, while we are in the middle of the sand. Horrible thought.

There is a lot of detailed information available about how the GPS works, and I will not duplicate it here. But, a few words may still be of interest. The unit receives signals from GPS satellites, always in orbit above our heads. This means that there must be an antenna in the unit or attached to it. For vehicle mounted units, this normally means having an external antenna.

On my Toyota 4Runner, the antenna is placed on the hood, and kept in place by it's magnetic foot. The main unit is placed inside the truck, on top of the dashboard. When orientating with help from the GPS only, it's nice to have it rather close to your driving view.

The practical precision of the system is about a hundred meters horizontally, which is absolutely enough for our needs. (What you do not find within one hundred meters by car, must be a berried treasure. Unfortunately, we have no co-ordinates to those treasures. :-) Sometimes, it may be as good as fifty or even thirty meters, but one cannot always count on this precision. Given enough strong satellite signals, it will also calculate the altitude. But this information is even less precise. Do not expect more than two hundred meters in vertical precision. And being close to the sea can give some strange results. For instance, I have been told that I was actually sixty meters below sea level. :-\ All within the limit of the precision, of course. Even if I do have a snorkel mounted on my truck, it seemed a little bit too dry to believe. For sailors, the horizontal precision will definitely not be enough. By adding an optional beacon receiver, the precision may be as good as a few meters. This requires the existence of land based additional transmitters, though. And those are primarily found close to the coast lines. Not in the middle of the deserts. The US Coast Gard Navigation Center provides the latest GPS information on-line.