How Does A Handheld GPS Work?

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Originally sent into orbit by the United States Department of Defense, these satellites were originally intended to be used for military purposes. In the mid 1980's, the government allowed the public to use these satellites and there are no setup fees or subscription fees to use the GPS systems.

How does it work?

In a very precise orbit, 24 satellites circle the earth, twice a day and transmit a signal back to earth. This information is taken in by GPS navigational receivers and uses a triangulation system to calculate the user or GPS unit's position. What happens is the GPS receiver compares the time a transmitting signal was given and the time a transmitting signal was received. This difference in time tells the GPS navigational receiver unit just how far the distance is between it and the satellite is. And then, with several more time measurement differences, the GPS receiver will know be able to determine the position of the GPS user and display that on the screen. It's all in the timing.

In order for a 2D GPS receiver to work properly, it must be locked into at least two satellites in order to tack movements and calculate latitude and longitude. Using four or more satellites, the GPS receiver can establish the position of altitude, latitude and longitude. From the user's position, it can then calculate other things such as bearing, trip distance, and distance to destination, speed, sunrise and sunset times, locations of points of interest and much more.

What is the signal?

Using line of sight to travel, each GPS satellite transmits two low powered radio signals. This means the signals can travel through clouds, plastic and glass but not through buildings or mountains which are solid objects.

There are three different bits of information contained in each GPS signal - almanac data, pseudorandom data and ephemeris data. The almanac data is contains important information about the satellite, whether it is healthy or not and the current date and time. This portion of the satellite data is vital for determining the positions. Pseudorandom data is an identification code or ID that identifies which satellite is transmitting data at what time. The ephemeris data tells the GPS receivers where each satellite should be throughout any given day. This is also the orbital information each satellite.

Accuracy

Thanks in part to their parallel multi-channel design, GPS are more reliable and accurate then they were just a few years ago. Most GPS receivers are quick to lock on to a satellite and remain with that satellite throughout your travels; however, there are a few atmospheric interferences that cannot be helped. The 24 satellites are orbiting approximately 12,000 miles above the earth and are constantly moving, making two complete rotations around the earth every 24 hours. There speed is nearly 7,000 miles per hour. These satellites are motorized by solar energy and carry backup batteries in case of a solar eclipse of the sun.

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