GPS or Global Positioning System is a network of orbiting satellites that send precise details of their position in space back to earth. The signals are obtained by GPS receivers, such as navigation devices and are used to calculate the exact position, speed and time at the vehicles location.
GPS is well-known for its military uses and was first developed by the US to aid in its global intelligence efforts at the height of the Cold War. Ever since the early 1980s, however, the GPS has been freely available to anyone with a GPS receiver. Airlines, shipping companies, trucking firms, and drivers everywhere use the GPS system to track vehicles, follow the best route to get them from A to B in the shortest possible time.
GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user’s exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with 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 locked to signals 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 3D position ( latitude , longitude and altitude ) . Once the user’s position has been determined , the user or vehicle position information to be sent using the GSM inovatrack to the server via the nearby BTS . GPS unit can calculate other information , such as speed , bearing , track , trip distance , distance to destination , sunrise and sunset time and more.
How Accurate Is GPS ?
Accuracy or precision of a concern for determining the coordinates of a point / location. The coordinates of this position will always have ‘error factor’, better known as the ‘degree of accuracy’. For example, the tool suggests a point coordinates with an accuracy of 5 meters, meaning that the actual position can be anywhere within a radius of 5 meters from the point of coordinates (location) is. The smaller the number accuracy (meaning accuracy is higher), then the position will become increasingly precise tools.The accuracy of GPS Data depends on many factors. For example, the quality of the GPS receiver, the position of the GPS satellites at the time the the data was recorded, the characteristics of the surroundings (buildings, tree cover, valleys, etc) and even the weather.In the daily use, accuracy level is more often influenced by peripheral factors that reduce the strength of the satellite signal. Because the satellite signal can not penetrate solid objects properly, then when using the tool, it is important to consider the vast sky that can be seen.When the tool is in the blank spot (area where no signal for communication), for example, in the forest, or valley. Then the accuracy rate will be much lower, even GPS devices are not able to capture the signal so that the appliance can not deliver the vehicle position information to the server system Inovatrack. But with Inovatrack GPS, vehicle position information will still be delivered to the BTS (inovatrack system) station on exit from the blank spot, despite experiencing delays. Because navigation tool relies fully on the satellite, the satellite signal becomes very important. This satellite-based navigation tools can not work optimally when there is interference on satellite signals.
The GPS Satellite System
The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling at speeds of roughly 7,000 miles an hour.
GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there’s no solar power. Small rocket boosters on each satellite keep them flying in the correct path.
Here are some other interesting facts about the GPS satellites (also called NAVSTAR, the official U.S. Department of Defense name for GPS) :
The first GPS satellite was launched in 1978.
A full constellation of 24 satellites was achieved in 1994.
Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.
A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
Transmitter power is only 50 watts or less.
What’s The Signal?
GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go 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 I.D. code that identifies which satellite is transmitting information. You can view this number on your Garmin GPS unit’s satellite page, as it identifies which satellites it’s receiving.
Ephemeris data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time. This part of the signal is essential for determining a position.
The almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system.
Sources Of GPS Signal Errors
Factors that can degrade the GPS signal and thus affect accuracy include the following :
Ionosphere and troposphere delays – The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.
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 travel time of the signal, thereby causing errors.
Receiver clock errors – A receiver’s built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.
Orbital errors – Also known as ephemeris errors, these are inaccuracies of the satellite’s reported location.
Number of satellites visible – 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. GPS units typically will not work indoors, underwater or underground.
Satellite geometry/shading – This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.
Intentional degradation of the satellite signal – Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.