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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING GPS - is everything so simple and reliable?. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications GPS in translation from English is a global positioning system. The American satellite radio navigation system GPS is intended for high-precision navigation support on a global scale for all branches of the US armed forces and civilian users. The US government has spent more than $10 billion to create this system and continues to spend money on its further development and support. The development of the system began in the 70s. In 1978 the first satellites were launched. In 1983, the system was opened for civilian use, and in 1991 restrictions on the sale of receiving equipment in Russia were lifted. Russian ships and vessels began to be actively equipped with GPS receivers. In the first decade of the new millennium, satellite navigation systems will become the main means of positioning for land, air and sea objects. With modern integrated circuit technology, GPS and GLONASS receivers are small, reliable and cheap, so that in time tourists, car enthusiasts and even mushroom pickers will be able to purchase them. The receiver connected to the pager can tell you where your child is currently walking or where, for example, your car is. And it will be done with high precision. With the help of the receiver, not only the location of the moving object is determined, but also the speed of its movement, the distance traveled, the distance and direction to the intended point, the time of arrival and deviations from the set course are calculated. Apparently, it is useful to recall the principles of operation of a satellite navigation system. To accurately determine their location on the ground, they traditionally use geodetic signs or geodetic landmarks or astronomical objects (the Sun, stars). In radio navigation systems, such geodetic markers are radio beacons, the location of which is well known. A satellite navigation system works similarly, where instead of geodesic signs and radio beacons, satellites emitting special signals are used. The current location of the satellites in orbit is well known. Unlike geodetic signs, they are mobile, their period of revolution around the Earth is 12 hours. Satellites themselves transmit information about their location. Their distance is determined by measuring the amount of time it takes for a radio signal to travel from a satellite to a radio receiver and multiplying that by the speed of the electromagnetic wave. Synchronization of the clocks of the satellites (which use atomic reference frequency generators) and the receivers provides an accurate measurement of the distances to the satellites. To calculate the coordinates of a place on Earth, you need to know the distances to the satellites and the location of each of them in outer space. The GPS satellites are in high orbits (20 km) and their coordinates are . can be predicted with great accuracy. The US Department of Defense tracking stations regularly detect even the smallest changes in orbits, and this data is transmitted to satellites. The measured distances to the satellites are called pseudoranges because there is some uncertainty in their determination. The thing is. that the ionosphere and troposphere of the Earth cause delays in satellite signals, introducing an error in the calculation of the distance. There are other sources of errors - in particular, the computational errors of the on-board computers, the electrical noise of the receivers, and the multipath propagation of radio waves. An unfortunate relative position of the satellites in the sky can also lead to a corresponding increase in the total positioning error. To determine distances, satellites and receivers generate complex binary code sequences called pseudo-random codes. The determination of the signal propagation time is carried out by comparing the delay of the pseudo-random code of the satellite with respect to the same code of the receiver. Each satellite has its own two pseudo-random codes. In order to distinguish between range-measuring codes and information messages of different satellites, the receiver selects the corresponding codes. Pseudo-random ranging codes and satellite information messages allow the transmission of messages from all satellites simultaneously, on the same frequency, without mutual interference. The radiation power of the satellites is low, and the mutual influence of the signals from the satellites is negligible. Measurement accuracy can be improved by using differential measurements A ground reference station with precisely known geodetic coordinates calculates the difference between its receiver's position and its actual position. The difference in the form of a correction is transmitted to consumers via radio channels to correct the readings of the receivers. These corrections eliminate a significant part of the errors in distance and location measurements. Calculation of coordinates in the receivers is performed automatically and is provided to the user in a convenient cartographic form. The global GPS system includes three segments. The first, space, includes 24 IC3. rotating in six orbits, four satellites each, at an altitude of 20 km. The second, ground-based, consists of a complex of ground-based control stations, control and data input for correcting satellite navigation information. The lead station is located at the Joint Military Space Systems Control Center in Colorado Springs. Monitoring stations constantly measure the parameters of satellite ephemeris and transmit corrective information to satellites through transmitting stations for transmission to consumers. The third segment includes user equipment: satellite signal receivers, which determine and present all the required navigation data. The main consumer of GPS information is the US Department of Defense. GPS receivers have been introduced on all combat and transport aircraft and ships, as well as in the guidance systems of high-precision cruise missiles and in the guidance systems of the new US guided bombs. This means that the US military can plan to deliver precision-guided missile strikes from a distance of 1000 km, not only against buildings and structures, but even with the accuracy of hitting a certain window. Moreover, these strikes can be delivered from submarines and from the air. The main advantages of GPS receivers. listed above. - Reliability, ease of maintenance, round-the-clock location capability, and compliance with international standards. However, not everything is as simple and accessible as it seems at first glance. The main parameter characterizing the navigation system is positioning accuracy. Since 1983, as already mentioned, the GPS system has been open for civilian use, but we will clarify, not completely. For professional civilian GPS receivers, typically under $5000, only C/A mode is available, the so-called free access signal emitted by satellites at a frequency of 1575,42 MHz (L1), which provides positioning accuracy within 100 m. in fact, the magnitude of errors can reach 300 m. The US government reserves the right to reduce the accuracy of the available C / A signals at any time by turning on the so-called selective access mode. In other words, a significant offset of the satellite clock is deliberately created and the parameters of the pseudo-random C / A code are changed, which deliberately introduces distortions into the information about the current location of the satellite. All this leads to significant errors in positioning, and in fact - to the impossibility of accurate navigation using the GPS system, which creates the prerequisites for accidents and even disasters. The thing is that such a deliberate distortion of data concerns all consumers of the civil GPS signal. At the same time, the introduction of selective access provides military users with GPS receiver indicators for normal operation and the necessary high accuracy. To do this, satellites on a second frequency - 1227,6 MHz (L2) - emit a military PY code that provides high accuracy, but it is not available to civil receivers. The cost of receiver indicators with access to the military code is on average $50. But here, too, the US military has foreseen the case when receiver indicators with access to the PY military code can get to users hostile to the United States. To prevent this, a RY code encryption mode is introduced, which will make it impossible for unauthorized users to navigate. The accuracy of location when accessing the military code is also increased due to the comparison of the arrival times of signals at frequencies L1 and L2. Reception of signals with the code C / A on only one frequency does not give such an opportunity. The peculiarity of satellite navigation systems is that. that when individual satellites fail, all its characteristics gradually deteriorate. Then periodically there are zones and large areas where the measurement accuracy becomes lower than acceptable, which can lead to serious incidents. Many malfunctions of the GPS system are detected only by the ground complex of stations. Users are notified about this with a delay of 15 minutes to 4 hours. One such event occurs approximately once every four months. Russian GPS users, as a rule, are deprived of the opportunity to receive information about such failures in a timely manner. A more detailed study of the features of the GPS system turned out that the WGS-84 geocentric coordinate system used by it for calculations is oriented primarily to Western consumers. In Russia, for accurate mapping, its own coordinate system PZ-90 has been created. which is not the same as WGS-84. since they are based on different models of the Earth ellipsoid. Consequently, the same geodetic latitudes and longitudes of one point on the ground may differ. In other words, when determining a place on a Russian map using GPS receivers, additional errors are inevitable due to differences in coordinate systems. Unfortunately, in the modern world, economic and information confrontation has become a normal phenomenon. At the same time, in the new economic conditions, when many American goods and services flooded into the market, our consumers also had the opportunity to use the services of the GPS system. The Americans hurried to "stake out" their product in all possible cases of its use Whether you like it or not, but when sailing, say, in North American waters and regions of US allies, please use the GPS system. Owners of ship receivers of this system can become hostages of the US military anywhere in the world and at any time. In addition, even when sailing in the coastal seas of Russia with the help of GPS, you can run aground due to a mismatch in geocentric coordinate systems. A detailed study of the situation has shown that the accuracy of the GPS system, in particular in North America, is based on a huge number of corrections for various cities and locations in the United States. which, as a rule, are previously entered into the electronic memory of GPS receivers. For the territory of Russia, there are no such corrections in these receivers. In view of the foregoing, the use of only the GPS system in Russia leads to a violation of the interests of its national security. Since 1982, work has begun in our country on the creation of a domestic global navigation satellite system - GLONASS, which operates on approximately the same principles as GPS, and provides the possibility of civilian use. Already in September 1993, this system was officially put into operation, and in 1995 it was deployed with a full complement of satellites. The main distinguishing feature of the Russian GLONASS system is that in the civilian application mode, it achieves positioning accuracy. close to the accuracy provided by GPS receivers using the military PY code. In addition, GLONASS receivers operate in both the GPS WGS-84 coordinate system and the Russian PZ-90 coordinate system. In 1996, the government of the Russian Federation provided the GLONASS system as one of the components of the world navigation system. Russian factories have produced a number of GLONASS navigation devices: "Breeze", "Reper". "Skipper", "Gnome-M", "Leader", "Goliath". Moreover, these receiver indicators are a combined version of GLOHACC / GPS. The market for such equipment in Russia is just beginning to take shape. Taking into account the noted features of the GPS system, domestic equipment can seriously compete with almost sixty foreign companies supplying a wide range of GPS receiver indicators to the Russian market. Moreover, the International Maritime Organization (IMO) in the same 1996 approved GLONASS and GPS only as components of the worldwide radio navigation system and recommended using them in a combined mode. Domestic receivers. as a rule, they are combined in these two systems and have a differential mode, due to which they have an advantage in comparison with non-aligned foreign-made GPS receivers. Russian users should seriously weigh the pros and cons before purchasing imported equipment. For example, we present the typical characteristics of the combined navigation receivers GLOHACC/GPS of the Moscow Design Bureau "Korund". The receiver indicators are designed to receive signals from GLONASS and GPS systems (C/A code) simultaneously from 14 satellites. The root-mean-square positioning error is 10 m, height - 15 m with a favorable location of GLONASS satellites (for GPS receivers - 30 m and 60 m, respectively). Accuracy of determination of coordinates in differential mode - 1...3 m, height - 1,5...4 m. Speed measurement error - 0,1 m/s. Coordinate systems PZ-90, SK-95, SK-42, WGS-84 are used. There is an RS-232 interface for communication with control systems and information processing. Dimensions - 180x195x70 mm, weight - from one to two kilograms. Russian manufacturers of combined receiver-indicators of global satellite systems entered into a difficult market struggle with foreign companies producing equipment similar in purpose. There is every reason to hope that the products created by domestic specialists will be quite competitive with foreign receiver indicators. Author: V. Kuryshev, Severomorsk, Murmansk region.
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