| The Global Positioning System (GPS) is a | | | | GPS satellites (also called NAVSTAR, the official |
| satellite-based navigation system made up of a | | | | U.S. Department of Defense name for GPS):o |
| network of about 18-24 satellites placed into orbit. | | | | The first GPS satellite was launched in early |
| GPS was originally intended for military | | | | 1978.o A full constellation of 24 satellites was |
| applications, but in the late 1970s, the government | | | | achieved in late 1994.o Each satellite is built to last |
| made a system available for civilian use. GPS | | | | about 10-15 years. Replacements are constantly |
| works in any weather conditions, anywhere in the | | | | being built and launched into orbit.o A GPS satellite |
| world, 24/7. There are no cost for the use. | | | | weighs approximately 1,500 pounds and is about |
| How it works | | | | 16 feet across with the solar panels extended.o |
| GPS satellites circle the earth twice a day in the | | | | Transmitter power is only 50 watts or less. |
| same orbit and transmit signal information to | | | | What's the signal? |
| down to mother earth. GPS take this information | | | | GPS satellites transmit two low power radio |
| and use triangulation to calculate the user's exact | | | | signals, designated L1 and L2. Civilian GPS uses the |
| location. The GPS receiver compares the time a | | | | L1 frequency of 1575.42 MHz in the UHF band. |
| signal was transmitted by a satellite with the time | | | | The signals travel by line of sight, meaning they |
| it was received. The time difference tells the GPS | | | | will pass through clouds, glass and plastic but will |
| receiver how far away from the satellite it is. | | | | not go through most solid objects such as |
| Now, with this distance measurements from a | | | | buildings and mountains. |
| few more satellites, the receiver can determine | | | | A GPS signal contains three different bits of |
| the user's position and display it on the unit's | | | | information -- a pseudorandom code, ephemeris |
| electronic map. | | | | data and almanac data. The pseudorandom code |
| A GPS receiver must be locked on to the signal | | | | is simply an I.D. code that identifies which satellite |
| of at least three satellites to calculate a 2d | | | | is transmitting information. You can view this |
| position (latitude and longitude) and track | | | | number on your Garmin GPS unit's satellite page, |
| movement. With four or more satellites in view, | | | | as it identifies which satellites it's receiving. |
| the receiver can determine the user's 3D position | | | | Almanac data, which is constantly transmitted by |
| (latitude, longitude and also altitude). Once the | | | | each satellite, contains important information about |
| user's position has been determined, the GPS unit | | | | the status of the satellite (healthy or unhealthy), |
| can calculate other information, such as speed, | | | | current date and time. This part of the signal is |
| track, trip distance, distance to destination, sunrise | | | | essential for a good position view. |
| and sunset time and a lot more. | | | | Sources of GPS signal errors |
| How accurate is GPS? | | | | Factors that can degrade the GPS signal and thus |
| Today's GPS receivers are extremely accurate, | | | | affect accuracy include the following:o Ionosphere |
| thanks to parallel multi-channel design. Garmin's 12 | | | | and troposphere delays -- The satellite signal |
| parallel channel receivers are quick to lock onto | | | | slows as it passes through the atmosphere. The |
| satellites when first turned on and they maintain | | | | GPS system uses a built-in model that calculates |
| strong locks, even in dense foliage or urban | | | | an average amount of delay to partially correct |
| settings with tall houses. Certain atmospheric | | | | for this type of error.o Signal multipath -- This |
| factors and other sources of error can affect the | | | | occurs when the GPS signal is reflected off |
| accuracy of GPS receivers. Garmin® GPS | | | | objects such as tall buildings or large rock |
| receivers are accurate to 15 meters on average. | | | | surfaces before it reaches the receiver. This |
| Newer Garmin GPS receivers with WAAS (Wide | | | | increases the travel time of the signal, thereby |
| Area Augmentation System) capability can | | | | causing errors.o Receiver clock errors -- A |
| improve the accuracy to less than three meters | | | | receiver's built-in clock is not as accurate as the |
| on average. No additional equipment or fees are | | | | atomic clocks onboard the GPS satellites. |
| required to take advantage of WAAS. Users can | | | | Therefore, it may have very slight timing errors.o |
| also get even better accuracy with Differential | | | | Orbital errors -- Also known as ephemeris errors, |
| GPS (DGPS), which corrects GPS signals to within | | | | these are inaccuracies of the satellite's reported |
| an average of three to five meters. The U.S. | | | | location.o Number of satellites visible -- The more |
| Coast Guard operates the most common DGPS | | | | satellites a GPS receiver can "see," the better the |
| correction service. This system consists of a | | | | accuracy. Buildings, terrain, electronic interference, |
| network of towers that receive GPS signals and | | | | or sometimes even dense foliage can block signal |
| transmit a corrected signal by beacon | | | | reception, causing position errors or possibly no |
| transmitters. In order to get the corrected signal, | | | | position reading at all.o Satellite geometry/shading |
| users must have a differential beacon receiver | | | | -- This refers to the relative position of the |
| and beacon antenna in addition to their GPS. | | | | satellites at any given time. Ideal satellite |
| The GPS satellite system | | | | geometry exists when the satellites are located |
| The 18-24 satellites that make up the GPS space | | | | at wide angles relative to each other. Poor |
| segment are orbiting the earth about 12,000 miles | | | | geometry results when the satellites are located |
| above us. They are constantly moving, making | | | | in a line or in a tight grouping.o Degradation of the |
| two complete orbits in less than 24 hours. These | | | | satellite signal -- Selective Availability (SA) is an |
| satellites are travelling at speeds of roughly 7,500 | | | | intentional degradation of the signal once imposed |
| miles an hour. | | | | by the U.S. Department of Defense. SA was |
| GPS satellites are powered by solar energy only. | | | | intended to prevent military adversaries from |
| They have backup batteries onboard to keep | | | | using the highly accurate GPS signals. The |
| them running in the event of a solar eclipse, when | | | | government turned off SA in May 2000, which |
| there's no solar power. Small rocket boosters on | | | | significantly improved the accuracy of civilian GPS |
| each satellite keep them flying in the correct path. | | | | receivers. |
| Here are some other interesting facts about the | | | | |