Friday, June 19, 2009

GPS: It Knows Where You Are

The advancements in consumer technologies never cease to amaze me. Take the GPS devices for example. These little units have become commonplace over the past few years as their prices have become more affordable to the average American. But what they do, and how they do it, is nothing short of amazing.

Let’s take a quick look at how these little gadgets know exactly where you are and where you’re going.

GPS stands for Global Positioning System. The system relies on a series of satellites. There are 24 of them in operation at all times. They orbit the earth twice per day, circling 12,000 miles above Earth’s surface, each of them in a different orbital path.

Each satellite has on board an atomic clock, accurate down to billionths of a second, and all of the clocks have to be perfectly synchronized. Each GPS device has a quartz clock, which is far less accurate than an atomic clock, but the clock in the GPS unit is synchronized using signals from the orbiting satellites once every second. So you can say that the GPS device has a virtual atomic clock.

Your position is pinpointed using a process known as trilateration. It’s similar to how they determine the epicenter of an earthquake. By measuring the distance to the epicenter from three different locations, you can tell where the earthquake occurred. That’s because if you draw a circle around each seismograph station with the radius of the circle representing the distance to the quake, the circles will intersect at only one place. That’s the epicenter.

So a GPS needs to get signals from at least three different satellites. The distance to each is calculated based on the speed of light and the time it takes for the signal to travel between the satellite and the GPS. A sphere around each satellite with a radius of the distance between it and the GPS will intersect with each other at the exact location of the GPS unit.

The distances are updated once per second, so any movement of the GPS can be calculated. The position and motion of the GPS is then placed as an overlay on a road map. The computer chip inside the GPS makes all these calculations almost instantly.

So, to determine your exact location, the computer chip and software in the GPS has to take into consideration that the GPS is moving and every one of the satellites it is tracking is moving at high speed as they circle the earth. It has to synchronize its internal clock every second using calculations from three satellites. And then it has to place this computed location on a map so that the map moves to keep up with the motion of the GPS. It also then has to select the best routes, calculate your speed, and estimate your time of arrival at your destination.

In addition to all that, the clocks in the satellites have to be updated and synchronized with an earthbound master station to make sure that they are always synchronized with each other. This all has to be done taking into consideration the effects of the atmosphere on radio signals.

It all results in the computation of your location within about five feet of accuracy. How cool is that?

No comments: