GPS / Global Positioning System - satellite navigation system / - Integrated electronic-technical system that consists of a combination of ground and space equipment is designed for accurate positioning of the various moving and static objects on the ground. The project was implemented and is owned by U.S. military, and he had initially conceived only for military purposes (also known as NAVSTAR - Navigation Satellite Time and Ranging). The main elements of this complex are: operating in an integrated network of satellites that emit signals to ground traffic control system orbiting satellite receiver client equipment - GPS-navigator in an additional element to be considered a system of beacons operating at improving the quality of the signal - a differential GPS (DGPS) satellites Satellite system consists of 29 satellites, for the purpose of positioning the 24 satellite, the other 5 - back. They are all on six different circular orbits at an angle of 60 ° to each other, so that from any point of the earth's surface visible from four to twelve of these satellites. On each orbit is located at the 4 satellites, the height of the orbit is approximately 20,200 km and the orbital period of each satellite around the earth 12 hours. Each satellite is measuring about 5 meters weighs about 800 kg, incorporates a high-precision atomic clock, computing device, a transmitter power of 50 W, emitting at a frequency of 1575.42 MHz. Satellite signal contains the personal code, the data on the status of the satellite and an almanac. Personal code is used to identify the transmitting satellite. All of them are numbered from 1 to 32 and this number is displayed on the screen of the GPS-receiver during his tenure. The greater the number of rooms (the satellites) makes GPS-service network: a new satellite can be launched, tested and commissioned before the old failure. Such a satellite will simply assigned a new number (from 1 to 32). Each satellite transmits information about its state (working or nonworking), the current date and time. Almanac data suggest that, where during the day should be in all GPS-satellites. Each of them transmits almanac containing the parameters of its orbit, as well as all the other satellites. Control systems are not fully autonomous, its performance is monitored stations in Earth observation. Territorial monitoring stations are located in Hawaii, Kwajalein Atoll, Ascension Island, Diego Garcia and in Colorado Springs, all information is recorded and transmitted to the main command station located at a military base in Colorado from Falcon, adjust the orbits and navigation information. GPS-Navigation GPS-receiver based on the received information from satellites determines the distance to each satellite, their relative position and calculates its coordinates according to the laws of geometry. If the distance from one satellite is known, and we can describe the scope of a given radius around it when it becomes known and the distance to the second satellite, determined the location will be located somewhere in the sector, given by the intersection of two spheres, and the third satellite defines two points on the circle remains to calculate which of them is the desired location. One of the points can always be discarded, since it has a high rate of displacement or under the Earth's surface. Thus, knowing the distance to three satellites, one can calculate the coordinates of the defined terms. In this case, to determine the 2-coordinates (latitude and longitude) is enough to get signals from three satellites, and to determine the height above sea level - with four. Speed ??of propagation of radio signals is constant and equal to the speed of light, distance to the satellites is determined by the delay time the message was received GPS-receiver relative to the time the message is sent from aboard the satellite. Having determined the location of the user, the receiver can calculate quantities such as speed, steering angle, trajectory, distance, distance to destination, sunrise and sunset times and much more. The GPS system works in any weather conditions, but there are factors that reduce accuracy: ionospheric and tropospheric delay As you progress through the atmosphere slows down the signal. The GPS system uses a built-in model which defines the average delay for the partial correction of errors of this type. This reflected signal occurs when the GPS signal is reflected from objects such as tall buildings or cliffs, and falls into the GPS-receiver. The increase in travel time of the reflected signal results in an error. Insufficient accuracy of the receiver clock in clock is GPS-receivers are inferior in accuracy of atomic clocks on board satellites. This may cause small errors in determining the time of the signal. A small number of visible satellites more satellites "see" GPS-receiver, the higher the accuracy. Buildings, elements of relief, and sometimes even dense foliage can interfere with reception of signals GPS, resulting in errors in positioning or its impossibility. The deliberate reduction in the accuracy of the GPS signal selective access program (Selective Availability, SA) U.S. Department of Defense included the deliberate introduction of errors in the signal GPS. The purpose of this program was to prevent the possible use of civilian GPS-receivers for military purposes. Poor geometry of visible satellites concept of satellite geometry "means that they are located relative to each other and the GPS-receiver. If, for example, the receiver receives information from four satellites and all four are located in the northern and western areas, the satellite geometry is rather poor, sometimes so much so that the receiver can not determine the location because all distances are measured to satellites, will lie in one global direction . This means that the triangulation would be bad, and that the region of intersection of a straight is quite large (ie, the region likely will take up a lot of space, and pinpoint the coordinates of the impossible). In this case, even if the receiver gives some values ??of the coordinates and their accuracy can not be good enough (probably 100 - 150 m). If these four satellites will be in different directions, the accuracy will increase significantly. In this case, even if we take into account the effect of SA, the accuracy can not be worse than 30 m. Thus, the ideal is a satellite geometry, the angles between the directions of them are large. Bad consider such a geometry, when the satellites are located on the same line or close to it. DGPS DGPS uses an additional, fixed at one point the GPS-receiver to determine the Correction of satellite signals. At the present time, there are several free and paid services of this kind. For example, the U.S. Coast Guard and U.S. Army Corps transmit GPS-correction through marine radio beacons. They operate in the range 283.5 - 325.0 kHz and can use them free of charge. You only need to be purchased DGPS-receiver. This receiver connects to a GPS-navigator with 3-wire cable on which the amendment is passed in the usual serial form in a format called RTCM SC-104. Paid DGPS-service work in the UHF range or broadcasts via satellite. In these cases also need a special DGPS-receiver for receiving the amendments and send them to the GPS-navigator. Price depends on the required accuracy. The former includes WAAS (Wide Area Augmentation System) in the Americas, European EGNOS and Japan's MSAS. Alternative satellite navigation system: Currently working or preparing to deploy the following satellite navigation system: GLONASS GLONASS (Global Navigation Satellite System) - is at the deployment phase constellation. Belongs to the Ministry of Defense of Russia. Beidou deployed in China currently subsystem GNSS, intended for use only in this country. Feature - a small number of satellites in geostationary orbit. The European Galileo system, located at the stage of a satellite constellation. It is expected that Galileo will be operational in 2008, when the orbit will withdraw all of the 30 planned satellites (27 operational and 3 reserve). Space segment will be complemented by ground-based infrastructure, which includes two control centers and a global network of transmitting and receiving stations. It is estimated that Galileo will significantly improve the accuracy of directly determining the location of the user, bringing it up to one meter without the use of special modes and additional subsystems. In contrast to the American GPS and Russian GLONASS, Galileo was created solely for civilian use and nominally controlled by any government or military institutions. In recent years, experts are increasingly talking about the possibility of creating a single global system that combines information from all three satellite navigation systems - GLONASS, Galileo and GPS. Roscosmos in the future, even going to create a single station, receiving signals from all three systems. In "Roskosmos" confirm that a tripartite agreement with NASA and the European Space Agency is already preparing.
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