GPS buyers guide

When you need to know where you’re going in life, you can let the spirits guide you or put your faith in GPS. Alex Kidman takes the electronic path, and explains the features worth having in a portable navigation device.

So, you’ve decided to throw away the folding maps, chuck out the street directories and apply some technology to the age old problem of getting to point A from point B as quickly and safely as possible. Or, in layman’s terms, you’re interested in a GPS. But which one should you buy? What’s the technology behind GPS? Can your GPS be used to track you? What are those black vans that keep parking outside my home?

How GPS works

GPS satelliteGPS (Global Positioning System) is a technology originally developed by the US Military for (not unsurprisingly) military purposes – keeping track of soldiers, firing missiles directly where you want them to be, and that sort of thing. It’s not a cheap system to run – the 24 satellites that make up the GPS system currently used cost around $US750 million a year to maintain.

A simple explanation of GPS is that signals are broadcast from each satellite down to earth. With a suitable GPS receiver, it’s possible to then triangulate from the received signals and get a very accurate reading of where on the planet one is. As the satellites are in constant orbit, more than 20 are needed to ensure planet-wide coverage, but typically it’s possible to get a good GPS lock with only four or five satellites. Understandably, it’s not possible to ‘see’ every GPS satellite from a single receiver down on earth.

While GPS had its roots in the military, decisions in the early 1980s to allow civilian usage, followed by the switching off of selective availability (which randomly scrambled signals momentarily to confuse ‘enemy’ missiles that might have been using signal) has led to it becoming a great tool for non-military uses. The most popular in the consumer sphere is without a doubt using GPS for vehicle navigation.

In-car GPS (sometimes referred to as Satellite Navigation, or SatNav) uses the basic GPS data (which is essentially just longitude and latitude readings) and overlays it on existing roadmap data. Marry the two, and you’ve got a way of identifying where you are that doesn’t involve pulling into a petrol station or craning your neck desperately trying to read passing road signs. Moreover, with a little bit of computing grunt and the right map data, you can use this tracking and mapping ability to plot out optimal courses to your destination, even if all you know is the final address.