Selecting a GPS device

The Global Positioning System (GPS) is currently the only fully functional Global Navigation Satellite System (GNSS). More than two dozen GPS satellites are in medium Earth orbit, transmitting signals allowing GPS receivers to determine the receiver's location, speed and direction.

Before you choosing a suitable GPS for your hiking needs, it is important to have a reasonable understanding of what is to offer along with how they work.

There are several components that need to be looked at. There is the GPS receiver (or antenna) itself, which plays an important part. Then there is the portion of the system which uses the received information to estimate where the device is located on an internal map.

Then there are the various capabilities for route planning and recording, which vary from unit to unit. Finally, the display portion of each solution type can vary considerably, from limited location information to full color three dimensional maps of the immediate surroundings and advised route.

The actual principal of GPS is very easy to appreciate, since it produces similar results to traditional "triangulation" although GPS does not use angles. If one imagines an orienteer needing to locate themselves on a map, they first need to be able to find at least three points that they recognize in the real world, which allows them to pinpoint their location on the map.

They can then measure, using a compass, the azimuth (This is the direction of a celestial object, measured clockwise around the observer's horizon from north) that would be needed to take them from the point on the map to their current position. A line is then drawn from each of the three points, and where the three lines meet is where they are on the map.

Translating this into the GPS world, we can replace the known points with satellites, and the azimuth with time taken for a signal to travel from each of the known points to the GPS receiver. This enables the system to work out roughly where it is located, it is where the circles representing the distance from the satellite, calculated on the basis of the travel time of the signal, intersect.

Of course, this requires that the GPS locator has the same coordinated time as the satellites, which have atomic clocks on board. To do this, it cross checks the intersection of the three circles with a fourth circle, which it acquires from another satellite.