LESSON H
PART I: HOW THE GPS WORKS

You're probably familiar with the term GPS which stands for Global Positioning System. You've heard it mentioned, and you know it has something to do with finding out your location, but most likely, you've never used one. Or if you have, you're not quite sure how the whole thing works. So let's take a look at what the GPS is and what lies behind it before we move on to learning how to use a GPS receiver.

It's hard to imagine that the first proposals for a GPS were presented as early as 1940 for flight and water navigation. The current system in use today in North America is NAVSTAR, developed by the US Department of Defense. It is a radio positioning system that provides your three-dimensional position and velocity by means of satellites and receivers.

The Global Positioning System can be broken down into three main parts:

1. the space segment
2. the control segment
3. the receiver

Basically, the space segment consists of 24 satellites split among six orbital planes. These orbit around the earth and send radio signals. The various control stations on earth monitor the performance of these satellites. The receiver is used to determine your exact position based on the radio signals it receives from different satellites.

Understanding the way that the system works as a whole is simple. And the best way to understand what the GPS does is through the following example which doesn't use a GPS at all:

You've gone out for a sail from New Harbor, Maine and have been diligently practicing all the different techniques in our on-the-water skills section. After hours of tacking, trimming the sails, and gybing, you are ready for a break and decide to set anchor near the shore of a small remote island. You look around but there is nothing telling you where you are. You look at your map:



Looking at your map, you realize that you've zigzagged through the waters so much, that it would be hopeless to try and guess which of the islands this one might be. But somehow, you have to figure out your route back to New Harbor. You reach for your trusted GPS only to discover you've left it at home! Luckily, there are three other boats that have dropped anchor for a rest too. You ask them if they might know where you are.

Unfortunately, they've been having a bad sailing day, and, assuming you're a rookie, give you a hard time by only providing you with the following information that they read from their GPS instruments:

• Old Sailor Bob sailed a straight course from Port Clyde. He's traveled 24.25 nautical miles.
  
  
• Sue left from Clark Island and is 18 nautical miles from her point of departure.
  
  
• John says he's 21 nautical miles from Vinalhaven, his home town.
  
  

In spite of their efforts to snub you, you know you have enough information to pinpoint your location on the map. You take out your compass and draw circles around each person's point of departure, each circle bearing a radius equal to the distance they've traveled until it looks something like this:



Now you know where you are! Right where the three circles intersect: Large Green Island. It'll be a longer haul home than you thought! But you've managed to impress the others, including Old Sailor Bob who is scratching his head and mumbling, "I guess you young kids are more clever than I thought!"

The GPS works in a similar fashion based on this geometry principle. The satellites are equipped with atomic clocks that keep precise time. Part of the signal sent out by GPS satellites reports the time and its position.

Because we know that for all practical purposes, radio signals travel at the speed of light (approx. 186,000 miles per second), the GPS receiver registers the delay (the amount of time passing) between the signal being sent by the satellite and the receiver picking up the signal. By using the forumla:

distance = time x velocity

it can calclulate its distance from the satellite.

Working with one satellite, the receiver can only determine its position along a circle, which would be distance x from the satellite. Using our example above, if you only had Sue's information, you could have been anchored near Large Green Island or Georges Islands or anywhere else along the circle. But with John and Bob's information, you were able to narrow it down to one point. Similarly, with information from three satellites, the receiver can pinpoint your location on the water as the intersection of the three radii.

But what if you're in the mountains? You've probably realized by now that more than just giving you your possible position along a circle, the GPS is giving you your coordinates along the surface of a sphere in three-dimensional space. Finding out your altitude requires knowing your possible spheres of distance from four satellites. And this is why the Global Positioning System is set up with 24 satellites split among six orbits. You need to be able to receive information from four satellites to determine your three-dimensional coordinates. Actually, you almost always are able to receive information from as many as eight at any given moment!



On to Lesson H, Part II: Using the GPS -->