Global Positioning System (GPS) Receivers use an interesting technique to keep people from getting lost. Not all GPS Receivers are equal, however. Some of the less expensive, “pocket” size GPS Receivers may have more difficulty telling you where you are when used in densely wooded areas for instance. However, most GPS Receivers can quite accurately keep one from getting lost, or help one find a way out if one is lost.
GPS Receivers are patched into a system of 27 satellites orbiting Earth. They each have an electronic almanac which tells them where each satellite should be at any given time. They receive data from the satellites about where the receiver is located. The way this works is through a technique called three-dimensional trilateration.
When a person attempts to find his or her location, the GPS receiver picks up signals from three or more of the closest satellites. Each satellite measures GPS Receivers as a distance from the satellite. With more measurements comes greater accuracy, since each new measurement helps to precisely identify the exact location of the person on earth.
It helps to understand this by evaluating two-dimensional trilateration. A lost person asks three people for directions. He is told by the first person “You’re 80 miles from Sacramento.” The second person tells him “You’re 40 miles from San Jose.” The third person remarks, “You’re 60 miles from Santa Rosa.” By comparing these values, the person could conclude he’s in San Francisco. More information would make these comparisons even easier.
The satellites and the GPS Receivers use three-dimensional trilateration since they are not measuring flat distances, but distances from satellites to earth. Most people are within the receiving range of at least three GPS Receivers at any given time, and this helps specifically identify where they are. Greatest accuracy is achieved when GPS Receivers can reach at least four satellites.
GPS Receivers measure distance is by measuring the time it takes for a signal to reach a given satellite. Normally, to get the best accuracy, both the GPS receiver and the satellite would require atomic clocks for extreme accuracy. However, atomic clocks are extremely expensive. So only the satellite is equipped with an atomic clock. GPS Receivers have a quartz clock that resets according to satellite readings from an atomic clock.
Some inaccuracies occur in GPS Receivers because they rely on the assumption that radio waves will all travel at the same speed. This is not the case. In cities, radio waves can bounce off skyscrapers and skew results. One of the fixes for this is to have stationary GPS stations on earth that can look at GPS results and fix known problems.
Continuing modifications are made to newer GPS Receivers to pick up the signals of more satellites and account for small changes that could affect measurements. However, with GPS Receivers we are on the way to putting ourselves in the position to never get lost again.