| Originally sent into orbit by the United States | | | | Using line of sight to travel, each GPS satellite |
| Department of Defense, these satellites were | | | | transmits two low powered radio signals. This |
| originally intended to be used for military purposes. | | | | means the signals can travel through clouds, |
| In the mid 1980's, the government allowed the | | | | plastic and glass but not through buildings or |
| public to use these satellites and there are no | | | | mountains which are solid objects. |
| setup fees or subscription fees to use the GPS | | | | There are three different bits of information |
| systems. | | | | contained in each GPS signal - almanac data, |
| How does it work? | | | | pseudorandom data and ephemeris data. The |
| In a very precise orbit, 24 satellites circle the | | | | almanac data is contains important information |
| earth, twice a day and transmit a signal back to | | | | about the satellite, whether it is healthy or not |
| earth. This information is taken in by GPS | | | | and the current date and time. This portion of the |
| navigational receivers and uses a triangulation | | | | satellite data is vital for determining the positions. |
| system to calculate the user or GPS unit's | | | | Pseudorandom data is an identification code or ID |
| position. What happens is the GPS receiver | | | | that identifies which satellite is transmitting data at |
| compares the time a transmitting signal was given | | | | what time. The ephemeris data tells the GPS |
| and the time a transmitting signal was received. | | | | receivers where each satellite should be |
| This difference in time tells the GPS navigational | | | | throughout any given day. This is also the orbital |
| receiver unit just how far the distance is between | | | | information each satellite. |
| it and the satellite is. And then, with several more | | | | Accuracy |
| time measurement differences, the GPS receiver | | | | Thanks in part to their parallel multi-channel design, |
| will know be able to determine the position of the | | | | GPS are more reliable and accurate then they |
| GPS user and display that on the screen. It's all in | | | | were just a few years ago. Most GPS receivers |
| the timing. | | | | are quick to lock on to a satellite and remain with |
| In order for a 2D GPS receiver to work properly, | | | | that satellite throughout your travels; however, |
| it must be locked into at least two satellites in | | | | there are a few atmospheric interferences that |
| order to tack movements and calculate latitude | | | | cannot be helped. The 24 satellites are orbiting |
| and longitude. Using four or more satellites, the | | | | approximately 12,000 miles above the earth and |
| GPS receiver can establish the position of altitude, | | | | are constantly moving, making two complete |
| latitude and longitude. From the user's position, it | | | | rotations around the earth every 24 hours. There |
| can then calculate other things such as bearing, | | | | speed is nearly 7,000 miles per hour. These |
| trip distance, and distance to destination, speed, | | | | satellites are motorized by solar energy and carry |
| sunrise and sunset times, locations of points of | | | | backup batteries in case of a solar eclipse of the |
| interest and much more. | | | | sun. |
| What is the signal? | | | | |