How GPS navigation works and how, GPS system signals, how the GPS receiver determines its position.

GPS Navigation The NAVSTAR GPS (NAVigation Satellite Timing And Ranging Global Positioning System) system consists of three parts or segments: space, ground and user equipment. 

How GPS navigation works and how, GPS system signals, how the GPS receiver determines its position.

Space part – These are 32 satellites orbiting in 6 orbits. The inclination of the orbits to the earth’s equator is 55 degrees, the angle between the planes of the orbits is 60 degrees. The height of the orbits is 20180 kilometers, the period of revolution is 12 hours. Such a period of revolution means that each satellite at the same time of the day flies over the same place. The power of the satellite transmitter is 50 W, the weight of the satellites is 1055 kg, the life of the satellites is up to 7.5 years. GPS satellites are capable of moving, filling in gaps in the system if one of them is out of order. An important element of the satellite is the atomic clock, rubidium and cesium, four on each. Satellites are identified by the PRN (Pseudo Random Number), which is displayed by the GPS receiver.

Ground part GPS consists of tracking stations. They track visible satellites and transmit data to the Command and Control Main Station (MCS) at an air base in Colorado, USA, to process satellite trajectories on complex computer program models. With this processing, the determination of ephemeris information is achieved, consisting of the parameters of the satellite orbits, their coordinates and derived coordinates. Through ground stations, the calculated ephemeris data is transmitted to the satellites, and then the satellite transmits them to the GPS receivers, where they are used to calculate the location coordinates..

GPS navigation, the NAVSTAR GPS system, is subordinate to the US Department of Defense, and its civilian use is supervised by the Department of Transportation. The system provides two types of services: SPS (Standard Position Service) – standard accuracy for civilians, and PPS (Precise Position Service) – high accuracy for the military. When developing the system, an SPS accuracy of 100 meters was considered sufficient for civilian purposes. As testing progressed, it turned out that the SPS subsystem was more accurate than anticipated. To preserve the advantages of high accuracy for the military since March 1990, the regime of access restriction SA (Selective Availability) was introduced, as a result of which civilian GPS navigation artificially reduces its accuracy.

User segment also underwent significant changes. The first single-channel receivers sequentially tuned to satellites. The initial location of the “cold start” location required an approximate position and took a lot of time, the “warm start” during subsequent starts also required several minutes. The signal often disappeared, lacked sensitivity. Then two-channel, and later fast switching, “multiplex” receivers appeared. Their sensitivity and speed seemed revolutionary. Since the end of the 90s, parallel 12-channel receivers tracking all visible satellites simultaneously became the consumer GPS standard. Improved anti-jamming and error correction algorithms, user interface.

GPS Signals.

All frequencies in the GPS system are multiples of the main clock frequency of the satellite clock – 10.23 MHz. The satellite transmits encoded signals in the ranges L1 = 1575.42 MHz and L2 = 1227.6 MHz. Phase modulation, the so-called pseudo-random. Signals contain two types of information: navigation messages and rangefinding pseudo-random code. The code is a sequence of ones and zeros, at first glance random, but changing according to a complex law. The pseudo-random code contains the satellite number (PRN). There are two kinds of code. Civilian GPS uses C / A (Coarse Acquisition) – a code now transmitted only on the L1 frequency. This code is also used for initial (rough) position determination in PPS. One code transmission cycle consists of 1023 bits and is repeated 1000 times / second.

Military GPS with high accuracy uses the P-code (Precise), which is transmitted at both frequencies, L1 and L2. In case of military danger, the P-code can be replaced by an encrypted Y-code. In the future, in addition to the abolition of the SA regime, for highly responsible civilian applications related to life safety, for example, aviation, a dual-frequency civil service will be introduced: from 2008 the C / A code on the frequency L2, in 2012-2014 the third civil frequency L5 = 1176.45 MHz. For the military, an M-code will appear that is better protected from interference.

Navigation messages GPS navigation transmits at a speed of 50 bps and also encodes with a pseudo-random code. Each message consists of 25 frames (pages) of 1500 bits. The full cycle of transmitting the entire message takes 12.5 minutes. The navigation message includes ephemeris data and almanac data, time data in the GPS system and coefficients for its conversion to universal time, keywords for the P-code and special messages, about the health of the equipment and others.

Ephemeris Are the satellite’s orbit parameters and some coefficients with which the receiver calculates the current and future position of the satellite using the Keplerian mathematical model. In addition, navigation messages contain corrections to the satellite clock and to the propagation delay in the ionosphere for C / A code users. Almanac This data on the ephemeris and the status of all other satellites in the system are stored in the GPS receiver. Thanks to these data, the receiver always knows where all the satellites of the system are located, even when it does not see them, and which satellites are best used to determine the coordinates.

How the GPS receiver determines its position.

GPS navigation uses a method for determining location coordinates from measurements of distances to satellite landmarks, determined using a pseudo-random code. To do this, the GPS receiver synchronously with the satellite generates a similar code of its own. By measuring the difference of the shift between the same sections of these codes in time, the receiver determines the so-called pseudorange. Using three pseudo-ranges, he can determine his exact position..

Determining location coordinates using one or two GPS satellites.

How GPS navigation works and how, GPS system signals, how the GPS receiver determines its position.

Why pseudo? The problem is that if the pseudo-random codes of the receiver and satellite were generated simultaneously, then the pseudorange would be ranges. However, the satellite clock is very accurate and corrected by signals from the Earth, and the receiver clock is less accurate, in addition, there are delays in the propagation of the signal in the ionosphere, troposphere, and so on, which creates a total error.

How GPS navigation works and how, GPS system signals, how the GPS receiver determines its position.

It is impossible to determine unambiguously two-dimensional coordinates from two circles of equal distances (position lines). For example, if the clock of the GPS receiver is lagging behind, the true position will be closer, but in each case proportionally closer to each of the satellites. Entering the position line from the 3rd satellite, we can get an unambiguous result.

The program of the receiver of the GPS navigator, using three measured pseudoranges, three position lines, by solving three equations together, calculates three unknown quantities: longitude, latitude, and clock error. That is why at least three satellites are needed for determining two-dimensional coordinates, and four for three-dimensional coordinates. The speed and direction of movement, the vector of the user, in non-professional models of GPS-navigator receivers, as a rule, are determined by the increment of coordinates.

Based on materials from the book All About GPS Navigators.
Naiman V.S., Samoilov A.E., Ilyin N.R., Sheinis A.I..

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