The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of around 18-24 satellites placed in orbit. GPS was originally intended for military applications, but in the late 1970s the government made a system available for civilian use. GPS works in any weather condition, anywhere in the world, 24/7. There are no costs for use.
How does it work
GPS satellites circle the earth twice a day in the same orbit and transmit signal information to mother earth. GPS takes this information and uses triangulation to calculate the user’s exact location. The GPS receiver compares the time a satellite transmitted a signal with the time it was received. The time difference tells the GPS receiver how far away it is from the satellite. Now, with these distance measurements from a few more satellites, the receiver can determine the user’s position and display it on the unit’s electronic map.
A GPS receiver must be connected to the signal from at least three satellites to calculate a 2d position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user’s 3D position (latitude, longitude, and also altitude). Once the user’s position has been determined, the GPS unit can calculate other information such as speed, route, travel distance, distance to destination, sunrise and sunset times, and much more.
How accurate is GPS?
Today’s GPS receivers are extremely accurate, thanks to the multiple parallel channel design. Garmin’s 12 parallel channel receivers connect quickly to satellites when first turned on and maintain solid lock-ons, even in areas with dense vegetation or urban settings with tall houses. Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. Garmin® GPS receivers are accurate to within 15 meters on average.
Newer Garmin GPS receivers with WAAS (Wide Area Augmentation System) capability can improve accuracy to less than three meters on average. No additional equipment or fees are required to take advantage of WAAS. Users can also get even greater accuracy with Differential GPS (DGPS), which corrects GPS signals by an average of three to five meters. The US Coast Guard operates the most common DGPS correction service. This system consists of a network of towers that receive GPS signals and transmit a corrected signal through beacon emitters. To get the correct signal, users must have a differential beacon receiver and beacon antenna in addition to their GPS.
GPS satellite system
The 18 to 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are in constant motion, making two complete orbits in less than 24 hours. These satellites travel at speeds of approximately 7,500 miles per hour.
GPS satellites work solely with solar energy. They have backup batteries on board to keep them running in the event of a solar eclipse, when there is no solar power. Tiny rocket boosters on each satellite keep them flying on the correct path.
Here are some other interesting facts about GPS satellites (also called NAVSTAR, the official US Department of Defense name for GPS):
o The first GPS satellite was launched in early 1978.
o By the end of 1994, a complete constellation of 24 satellites was achieved.
o Each satellite is built to last between 10 and 15 years. Replacements are constantly being built and launched into orbit.
o A GPS satellite weighs about 1500 pounds and is about 16 feet wide with the solar panels extended.
o Transmitter power is only 50 watts or less.
What is the signal?
GPS satellites transmit two low-power radio signals, called L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band. The signals travel by line of sight, which means they will pass through clouds, glass, and plastic, but will not pass through most solid objects, such as buildings and mountains.
A GPS signal contains three different bits of information: a pseudorandom code, ephemeris data, and almanac data. The pseudorandom code is simply an identification code that identifies which satellite is transmitting information. You can see this number on the satellite page of your Garmin GPS unit, as it identifies which satellites it is receiving.
Almanac data, which is constantly transmitted by each satellite, contains important information about the satellite’s status (good or not), current date and time. This part of the signal is essential for a good display of the position.
Sources of GPS signal errors
Factors that can degrade the GPS signal and therefore affect accuracy include the following:
o Ionospheric and tropospheric delays: The satellite signal slows down as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of bearing to partially correct for this type of error.
o Signal Multipath – This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the signal’s travel time, which causes errors.
o Receiver clock errors: A receiver’s built-in clock is not as accurate as the atomic clocks aboard GPS satellites. Therefore, you may have very minor synchronization errors.
o Orbital errors: also known as ephemeris errors, these are inaccuracies in the reported location of the satellite.
o Number of visible satellites: The more satellites a GPS receiver can “see”, the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all.
o Geometry/shading of the satellites: this refers to the relative position of the satellites at a given moment. Ideal satellite geometry exists when the satellites are located at wide angles to each other. Poor geometry results when the satellites are located in a line or in a close group.
o Satellite Signal Degradation — Selective Availability (SA) is an intentional signal degradation once imposed by the US Department of Defense. SA was intended to prevent military adversaries from using satellite GPS signals. high accuracy. The government shut down SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.