Department of Education

administration of Balashovsky municipal district

District scientific and practical conference of students

"Young leaders of education"

_______________________________________________________

Municipal educational institution

Humanitarian and pedagogical boarding school

balashov, Saratov region

"Satellite navigation systems"

section: "Informatics and modern computer technologies"

Completed: Yaroslav Esikov,

student of the 11th class

Head: Barsukova M.A.,

iT-teacher

Balashov 2008

Introduction

History of satellite navigation systems

Low-orbit satellite navigation systems (SNS)

Middle orbit satellite navigation systems SNS GPS

Satellite navigation systems.

How navigation systems work

• Global navigation system NAVSTAR

  Russian satellite navigation system GLONASS

  European system GALILEO

More about GLONASS and GPS

The attitude of the Russian leadership to the GLONASS system

Conclusion

Bibliographic list

Introduction

Relevance

For many years, everything related to high-precision determination of the location of moving objects remained the lot of "privileged" systems; these methods were used exclusively in navigation, aeronautics and mapping. The creation of GPS and GLONASS systems radically changed the situation. Today GPS / GLONASS receivers have become part of our life, and location determination has become a common mobile communication service.

Initially, the guaranteed positioning accuracy of both systems was about 100 m. However, after the main GPS service provider (US Department of Defense) abandoned the selective access mode in 2000, the positioning accuracy has increased by almost an order of magnitude. Note that the use of the differential correction mode increases the accuracy by several tens of times. It would seem that today all categories of consumers of navigation information are satisfied. However, work is actively continuing on the European project of the Global Navigation Satellite System (GNSS), created at the initiative of the EC and the European Space Agency.

Object of study

Satellite navigation systems

Subject of study

The principle of operation of satellite navigation systems

Structure

The work consists of an introduction, 4 parts, a conclusion and a bibliography.

History of satellite navigation systems Low orbit satellite navigation systems (ss)

The problem of using mobile landmarks placed in outer space for navigation purposes acquired a practical solution after the launch of the world's first Soviet artificial earth satellite (AES) on October 4, 1957.

SNS Transit ("Transit") It began to be developed in 1958 in the USA. In 1959, the first navigation artificial Earth satellite was launched into orbit, and in 1964 the system for navigation of American nuclear-powered missile submarines "Polaris" was put into operation. For commercial operation, the Transit SNS was provided in 1967, and the number of civilian users soon significantly exceeded the number of military users. By the end of 1975, six navigation satellites (SC) were in circular near-earth orbits (about 1000 km altitude), and the coordinates of the observer were calculated based on the reception and selection of the Doppler frequency shift of the transmitter of one of them. The weight of the satellite was 56 kg. The satellite emitted a signal at two frequencies - 150 and 400 MHz, the root mean square error (RMS) of determining the location of the object on the earth's surface was 100 m. In 2000, the system was taken out of service.

SNS "Cicada" - this Russian system dates back to 1967, when the first navigation satellite, Kosmos-192, was launched into orbit. The system was fully commissioned in 1979 as part of four spacecraft launched into circular orbits with an altitude of 1000 km, an inclination of 83 degrees and a uniform distribution of orbital planes along the equator. The system allowed the observer to determine the coordinates of his place every 1.5-2 hours with the duration of the navigation session up to 10 minutes. Over time, as a result of the modernization of the system, the UPC for determining the location of the object reached 80-100 m. Tsikada also used the Doppler frequency shift of the transmitter signal to determine the coordinates of the location. Later, the spacecraft of this system were equipped with equipment for detecting objects in distress, equipped with radio beacons emitting special signals. Currently, Cicada has limited use in navigation. To determine the coordinates of the ships of the USSR Navy, the low-orbit satellite navigation system "Tsikada-M" was used, which has characteristics close to the system "Tsikada".

Thus, since the time of medieval sailors, the method of determining the coordinates of an object on the Earth's surface has not fundamentally changed, but has only become much easier thanks to the widespread use of computing devices and sensitive receiving equipment. To solve the problem of determining the coordinates from the magnitude of the Doppler frequency shift of the signal emitted by the satellite, the receiving equipment calculated the speed of the spacecraft located at an altitude of 1000 km. In addition, it was necessary to know the position of the spacecraft in orbit (the spacecraft “dumped” this so-called "ephemeris information" to the consumer) and to have a highly stable frequency generator on the spacecraft and in the receiving equipment.

In principle, the distance could be measured simultaneously to two satellites or sequentially in time to the same satellite. In practice, the difference in distances to the same satellite was measured at 20-second time intervals. Therefore, the satellite navigation system included a ground control complex (with means of measuring and transmitting data on its position in orbit to the spacecraft - "ephemeris information").

Satellite navigation GPS has long been a standard for creating positioning systems and is actively used in various trackers and navigators. In Arduino projects, GPS is integrated using various modules that do not require knowledge of theoretical foundations. But a real engineer should be interested in understanding the principle and scheme of GPS operation in order to better understand the possibilities and limitations of this technology.

GPS work scheme

GPS is a satellite-based navigation system developed by the US Department of Defense that provides precise coordinates and time. Works anywhere in the world in all weather conditions. GPS consists of three parts - satellites, stations on Earth, and signal receivers.

The idea of \u200b\u200bcreating a satellite navigation system originated in the 50s of the last century. An American team of scientists observing the launch of Soviet satellites noticed that as the satellite approaches, the frequency of the signal increases and decreases with its distance. This made it possible to understand that it is possible to measure the position and speed of a satellite, knowing its coordinates on Earth, and vice versa. The launch of satellites into low-earth orbit played a huge role in the development of the navigation system. And in 1973, the DNSS (NavStar) program was created, under this program, satellites were launched into medium-earth orbit. The program received the name GPS in the same 1973.

The GPS system is currently used not only in the military field, but also for civilian purposes. There are many areas of GPS application:

• Mobile connection;
• Plate tectonics - tracking plate vibrations;
• Determination of seismic activity;
• Satellite tracking of transport - you can monitor the position, speed of transport and control their movement;
• Geodesy - determination of the exact boundaries of land plots;
• Cartography;
• Navigation;
• Games, geotagging and other entertainment areas.

The most important disadvantage of the system can be considered the impossibility of receiving a signal under certain conditions. GPS operating frequencies are in the decimeter wavelength range. This leads to the fact that the signal level may decrease due to high clouds, dense foliage of trees. Radio sources, jammers, and in rare cases even magnetic storms can also interfere with normal signal transmission. The accuracy of data determination will deteriorate in the circumpolar regions, since the satellites are not high above the Earth.

Navigation without GPS

The main competitor to GPS is the Russian GLONASS system (Global Navigation Satellite System). The system began its full-fledged work in 2010; attempts to actively use it have been made since 1995. There are several differences between the two systems:

• Different encodings - the Americans use CDMA, the Russian system uses FDMA;
• Different sizes of devices - GLONASS uses a more complex model, therefore, power consumption and device sizes increase;
• The placement and movement of satellites in orbit - the Russian system provides a wider coverage of the territory and a more accurate determination of coordinates and time.
• Satellite Life - American satellites are getting better quality so they last longer.

In addition to GLONASS and GPS, there are other less popular navigation systems - the European Galileo and the Chinese Beidou.

Description of GPS

How GPS works

The GPS system works as follows - the signal receiver measures the signal propagation delay from the satellite to the receiver. From the received signal, the receiver obtains data on the location of the satellite. The signal delay is multiplied by the speed of light to determine the distance from the satellite to the receiver.

From the point of view of geometry, the operation of the navigation system can be illustrated as follows: several spheres, in the middle of which there are satellites, intersect and the user is in them. The radius of each of the spheres is correspondingly equal to the distance to this visible satellite. Signals from three satellites provide information on latitude and longitude, the fourth satellite provides information on the height of an object above the surface. The obtained values \u200b\u200bcan be summarized into a system of equations, from which you can find the user's coordinate. Thus, to obtain an accurate position, it is necessary to take 4 measurements of the distances to the satellite (if we exclude implausible results, three measurements are sufficient).

The obtained equations are corrected by the discrepancy between the calculated and actual position of the satellite. The error that arises as a result of this is called ephemeris and ranges from 1 to 5 meters. Interference, atmospheric pressure, humidity, temperature, ionospheric and atmospheric influences also contribute. In total, the totality of all errors can bring the error up to 100 meters. Some errors can be resolved mathematically.

To reduce all errors, GPS differential mode is used. In it, the receiver receives by radio channel all the necessary corrections to coordinates from the base station. The final measurement accuracy reaches 1-5 meters. In the differential mode, there are 2 methods of correcting the received data - this is the correction of the coordinates themselves and the correction of the navigation parameters. The first method is inconvenient to use, since all users have to work on the same satellites. In the second case, the complexity of the positioning equipment itself increases significantly.

There is a new class of systems that increases the measurement accuracy to 1 cm. The angle between the directions to the satellites has a huge impact on the accuracy. With a large angle, the location will be determined more accurately.

Measurement accuracy can be artificially reduced by the US Department of Defense. For this, a special S / A mode is set on navigation devices - limited access. The mode is designed for military purposes, so as not to give the enemy an advantage in determining the exact coordinates. Since May 2000, the restricted access regime has been canceled.

All error sources can be divided into several groups:

• The error in calculating the orbits;
• Receiver errors;
• Errors associated with multiple signal reflections from obstacles;
• Ionosphere, tropospheric signal delays;
• Satellite geometry.

Main characteristics

The GPS system includes 24 artificial earth satellites, a network of ground tracking stations and navigation receivers. Observation stations are required for determining and monitoring orbital parameters, calculating ballistic characteristics, adjusting deviations from motion trajectories, and monitoring equipment on board spacecraft.

Characteristics of GPS navigation systems:

• Number of satellites - 26, 21 main, 5 spare;
• Number of orbital planes - 6;
• Orbit height - 20,000 km;
• The service life of satellites is 7.5 years;
• Operating frequencies - L1 \u003d 1575.42 MHz; L2 \u003d 12275.6 MHz, power 50 W and 8 W, respectively;
• Navigational determination reliability - 95%.

There are several types of navigation receivers - portable, stationary and aviation. Receivers are also characterized by a number of parameters:

• Number of channels - modern receivers use 12 to 20 channels;
• Antenna type;
• Availability of cartographic support;
• Display type;
• Additional functions;
• Various technical characteristics - materials, strength, moisture protection, sensitivity, memory capacity and others.

The operating principle of the navigator itself - first of all, the device tries to communicate with the navigation satellite. As soon as the connection is established, the almanac is transmitted, that is, information about the orbits of satellites located within the same navigation system. Communication with a satellite alone is not enough to obtain an accurate position, so the remaining satellites transmit their ephemeris to the navigator, which is necessary to determine the deviations, perturbation coefficients and other parameters.

Cold, warm and hot start of the GPS navigator

After turning on the navigator for the first time or after a long break, a long wait begins for receiving data. The long waiting time is due to the fact that the almanac and ephemeris are missing or outdated in the navigator's memory, so the device must perform a number of actions to receive or update data. The waiting time, or the so-called cold start time, depends on various indicators - the quality of the receiver, the state of the atmosphere, noise, the number of satellites in the field of view.

To get started, the navigator must:

• Find a satellite and establish communication with it;
• Receive an almanac and keep it in memory;
• Receive ephemeris from the satellite and save them;
• Find three more satellites and establish contact with them, receive ephemeris from them;
• Calculate coordinates using ephemeris and satellite positions.

Only after going through this entire cycle, the device will start working. This launch is called cold start.

A hot start is significantly different from a cold start. The navigator's memory already contains the current almanac and ephemeris. Almanac data is valid for 30 days, ephemeris data - for 30 minutes. It follows from this that the device was turned off for a short time. With a hot start, the algorithm will be simpler - the device establishes a connection with the satellite, updates the ephemeris if necessary and calculates the location.

There is a warm start - in this case the almanac is up to date and the ephemeris needs to be updated. This takes a little more time than a hot start, but much less than a cold one.

Restrictions on the purchase and use of homemade GPS modules

Russian legislation requires manufacturers to reduce the accuracy of detecting receivers. Working with uncoarse precision can be performed only if the user has a specialized license.

Under a ban in the Russian Federation there are special technical means intended for secretly obtaining information (STS NPI). These include GPS trackers, which are used to secretly control the movement of vehicles and other objects. The main feature of an illegal technical device is its secrecy. Therefore, before purchasing a device, you need to carefully study its characteristics, appearance, for hidden functions, and also view the necessary certificates of conformity.

It is also important in what form the device is sold. Disassembled device may not belong to STS NPI. But when assembled, the finished device may already be classified as prohibited.

Satellite navigation systems GPS and GLONASS were created based on certain requirements corresponding to their intended purpose. Their globality was implied; independence from meteorological conditions, terrain, degree of object mobility; business continuity and 24/7 availability; noise immunity; compactness of consumer equipment, etc.

Civil applications of the SNS, which developed after the development of the concept of GLONASS and GPS systems, especially such as civil air traffic control, navigation of ships, rescue operations, impose increased requirements on the SNS in terms of availability, integrity and continuity of service. Let's define these important terms:

Availability (readiness) - the degree of likelihood of the performance of the SNS before its application and in the process of application.

Integrity is the likelihood of detecting a system failure within a specified time or faster.

Continuity of service is the degree of probability that the system will remain operational for a given period of time.

A predetermined time period usually refers to the time period most important from a practical point of view, for example, the approach time of an airliner to land. At present, among civilian applications, the most critical to the performance of the SNS is air traffic control, including navigation support for aircraft. Availability requirements depend on the phases of flight and traffic intensity. Accessibility during a route flight should be no worse than 0.999… 0.99999; when flying in the airfield zone and uncategorized landing approach not worse than 0.99999. The integrity requirements reach, according to ICAO requirements, the value of 0.999999995 with an admissible warning time of no more than 1 s. The given data show how great are the requirements for the reliability of the SNS by consumers.

In SNS GLONASS and GPS, high performance characteristics at the structural level are achieved through the joint functioning of three main segments:

Space segment;

Control segment;

Consumer segment.

In addition to the main segments, there is such a functional addition as a differential subsystem (DGPS) and a number of auxiliary elements: special terrestrial and space communication channels, means for putting satellites into orbit, etc.

The basis of the concept of SNS GLONASS and GPS was the independence and demandlessness of navigation definitions. Independence implies the determination of the required navigational rules, but at the current level of development of electronics, such complication no longer matters. The system's lack of demand means that all calculations in the consumer's equipment are calculated only on the basis of passively received signals from the satellite with precisely known orbital coordinates in advance. In turn, the absence of the need to transfer the request from the consumer to the NSA makes it possible to make the consumer's equipment very compact and economical.

Space segment.

The positioning accuracy and stability of the SNS operation to a large extent depends on the relative orbital position of the satellites and the parameters of their signals. As a rule, it is required that at least 3 - 5 satellite vehicles are in the consumer's visibility zone. In practice, the orbital structure is constructed in such a way that for most consumers more than 6 NSs are constantly visible and the consumer has the opportunity to choose the optimal constellation according to a specific algorithm embedded in the receiver's computer. In addition to the existing NSA, the completed SNS has several backup satellites, which can be promptly introduced to replace the failed ones or to increase the coverage of a certain region. Operating satellite can be regrouped (within limited limits) on command from the ground control station. The currently operating medium-altitude orbits with an altitude of about 20,000 km allow receiving signals from each satellite on almost half of the Earth's surface, which ensures the continuity of the radio navigation field and sufficient redundancy when choosing the optimal constellation of the satellite. GPS and GLONASS systems are often called network SNS, since the mutual synchronization of the satellite in orbital coordinates and parameters of the emitted signals is of fundamental importance for their operation, i.e. unification of the NSA group into a network.

The main value of the NSA is the formation and emission of signals necessary for the consumer to solve the problem of positioning and monitoring the health of the NSA itself. The structure of a standard NSA includes: radio transmitting equipment for transmitting the navigation signal and telemetry information; radio receiving equipment for receiving commands from the ground control complex; antennas; onboard EMV; onboard time and frequency standard; solar panels; rechargeable batteries; orbital orientation systems, etc. Modern NSA can carry related equipment, such as detectors for detecting ground nuclear explosions and elements of combat control systems.

The signals emitted by the NSA contain rangefinder and service components. The rangefinder component is used by consumers directly to determine the navigation parameters - the range to the satellite, the consumer's velocity vector, its spatial orientation, etc. The service component contains information about the coordinates of the satellites, the time scale, the velocity vectors of the satellite, serviceability, etc. Generally, the service information is prepared by the command-measuring complex and stored in the on-board memory of the satellite during a communication session. And only a small part of it is formed by onboard equipment. The procedure for transferring service information from the command complex to the memory of an onboard computer is often called data loading.

The rangefinder component contains standard and high precision components. Standard measurement accuracy is available to all consumers, and high - only authorized ones, i.e. authorized by military regulatory authorities. Access control is achieved by coding high precision signals.

In conditions of hostilities, attempts are possible both to set up deliberate interference in order to suppress the SNS signal (jamming), and attempts to impose (spoofing), i.e. substitution of the signal and introduction of deliberately false information into the enemy's receiving equipment using third-party transmitters. Since in the literature it is very rare to find a clear interpretation of the term "anti-spoofing" in relation to the SNS, it should be emphasized that we are talking about protection against imposition.

Control segment.

The control segment consists of a main station combined with a computing center; groups of control and measuring stations (CIS) connected with the main station and among themselves by communication channels; ground standard of time and frequency. Control and measuring stations are trying to be placed as evenly as possible on the surface of the Earth, in accordance with geopolitical factors and economic feasibility. The coordinates of the KIS (antenna phase center) are determined in three dimensions with the maximum available accuracy. When the satellite is flying in the visibility zone of the CIS, it monitors the satellite, receives navigation signals, carries out primary information processing and exchanges data with the main station. At the main station, information is collected from all KIS, its mathematical processing and the calculation of various coordinate and correcting data to be loaded into the onboard computer of the NSA.

The data to be loaded is subdivided into operational, updated with each communication session, and long-term. In the event of an abnormal situation, it is possible to conduct unscheduled communication sessions and download data, provided that the satellite is in the visibility zone of one of the CIS.

The ground-based time and frequency standard has a higher accuracy than on-board standards and is designed to synchronize all processes occurring in the SNS and to correct the on-board standards.

The combination of independence and demandlessness gives the SNS unlimited bandwidth - an arbitrary number of consumers can use SNS signals at any time.

Consumer segment.

The consumer segment can be roughly divided into three parts: military organizations; civil organizations; private persons. Regardless of the purpose of consumer equipment, there is a radio frequency path in which radio signals are received from the satellite and their primary processing, and a computer designed for secondary signal processing, navigation information extraction, implementation of an algorithm for calculating the optimal constellation and calculating the spatial coordinates and the consumer's velocity vector. Usually, the current coordinates of the satellite and the range to them are first determined, then the geographic coordinates of the consumer are calculated. The consumer velocity vector is calculated by measuring the Doppler frequency shifts of the satellite with known satellite velocity vectors. For non-critical transport applications, the velocity vector can be calculated from the coordinate difference at two fixed times. Further, depending on the purpose of the receiver, information can be sent to a display device, to a transmission channel, or to a control unit for external actuators.

Determination of the current coordinates of the satellite.

Despite some similarities with radio beacon navigation systems (no demand, rangefinder method), SNS also have significant differences. The coordinates of the radio beacons are unchanged and known in advance, while the coordinates of the satellite must be constantly found. Determination of the current coordinates of a spacecraft moving at high speeds, non-constant relative to the consumer, is a complex technical and computational problem.

With the existing approach to the construction of the SNS, they try to transfer the maximum possible amount of calculations to the ground control complex. Monitoring stations are located in limited areas and do not provide continuous monitoring of the satellite. Based on the results of available observations in the computing center of the main command station, the parameters of the orbits of the NSA are calculated. They are subjected to mathematical processing using error elimination algorithms. Then, based on the processed data, a forecast of the orbit parameters is made at fixed (interrogation) times until the next forecast is generated.

The predicted parameters of the orbit and their derivatives are called ephemerides. During the communication session, the ephemeris are transmitted to the satellite, and then in the form of a navigation message containing the ephemeris and the corresponding timestamps - to the consumers. Knowing the estimated parameters of the orbit and the exact coordinates of the spacecraft at the reference points in time, the consumer can calculate the coordinates of the spacecraft at an arbitrary moment in time. In addition to ephemeris, an almanac is included in the navigation message - a set of information about the current state of the SNS as a whole, including coarse ephemeris used to search for visible satellite and select the optimal constellation.

Generally accepted units of measures of time.

Consideration of the principles of construction and operation of satellite navigation systems is impossible without preliminary acquaintance with the basic concepts related to units of time measures. These units are used to determine the spatial position of the satellite, link the satellite signals to a single time scale, etc.

It is customary to distinguish between two groups of time units:

Astronomical;

Non-astronomical.

The main astronomical unit of reference is a day, divided into 86400 seconds and equal to the time interval during which the Earth makes one complete revolution around its axis relative to a certain fixed reference point on the celestial sphere, for a stationary observer on the surface of the Earth. A characteristic feature of the astronomical day is that, depending on the selected reference point (center of the visible disk of the Sun, vernal equinox, etc.), the days have different durations and differ in name.

Stellar day. The time interval measured between two successive upper culminations of the vernal equinox is called sidereal days, or, in other words, the sidereal period of the Earth's orbital. The time measured at a particular meridian is called the local time of that meridian. Therefore, in the case of sidereal days, they speak of the local sidereal time of the meridian.

Local sidereal time is measured by the hour angle of the vernal equinox position relative to the celestial meridian. The celestial meridian is understood as the projection of the earth's meridian onto the conditional surface of the celestial sphere, therefore the hour angle is similar to geographical longitude, measured from the observer's hour meridian clockwise and measured in hours, minutes, seconds.

It is known that the Earth's axis of rotation makes slow periodic movements, consisting of movements along a cone - precessions, and small oscillations - nutations. Precession and nutations introduce an error in the determination of sidereal time, since they move the vernal equinox point. If the calculations take into account only the precession, then the average sidereal time is obtained. When nutation is taken into account together with precession, then true sidereal time is obtained. Sidereal time measured on the Greenwich meridian is called Greenwich sidereal time.

NAVIGATION RADIO SIGNALS

How the system works
navigation

NAVIGATION MESSAGE

COORDINATE SYSTEMS

FACTORS INFLUENCING ACCURACY DECLINE

TIME SYSTEMS

INCREASING NAVIGATION ACCURACY

The main elements of a satellite navigation system

Space segment

The space segment, consisting of navigation satellites, is a set of sources of radio navigation signals that simultaneously transmit a significant amount of service information. The main functions of each satellite are the formation and emission of radio signals necessary for navigational determinations of consumers and control of the onboard satellite systems.

Ground segment

The ground segment includes a cosmodrome, a command and measurement complex and a control center. The cosmodrome ensures the launch of satellites into the required orbits during the initial deployment of the navigation system, as well as periodic replenishment of satellites as they fail or when their resource is depleted. The main objects of the cosmodrome are the technical position and the launch complex. The technical position ensures the reception, storage and assembly of launch vehicles and satellites, their testing, refueling and docking. The tasks of the launch complex include: delivery of the launch vehicle with a navigation satellite to the launch pad, installation on the launch system, pre-flight tests, refueling the launch vehicle, guidance and launch.

The command and measurement complex is used to supply navigation satellites with service information necessary for conducting navigation sessions, as well as for monitoring and controlling them as spacecraft.

The control center, connected by information and control radio lines with the cosmodrome and the command and measurement complex, coordinates the functioning of all elements of the satellite navigation system.

User segment

The user segment includes consumer equipment. It is designed to receive signals from navigation satellites, measure navigation parameters and process measurements. To solve navigation problems, a specialized built-in computer is provided in the consumer's equipment. The variety of existing consumer equipment meets the needs of land, sea, aviation and space (within near space) consumers.

How the navigation system works

Modern satellite navigation is based on the principle of no-demand rangefinder measurements between navigation satellites and the consumer. This means that information about the coordinates of the satellites is transmitted to the consumer as part of the navigation signal. Simultaneously (synchronously) measurements of distances to navigation satellites are made. The method for measuring ranges is based on calculating the time delays of the received signal from the satellite in comparison with the signal generated by the consumer equipment.

The figure shows a scheme for determining the location of a consumer with coordinates x, y, z based on distance measurements of up to four navigation satellites. The colored bright lines show circles with satellites in the center. The radii of the circles correspond to the true ranges, i.e. true distances between satellites and consumer. Colored faint lines are circles with radii corresponding to the measured ranges that differ from the true ones and are therefore called pseudo-ranges. The true range differs from the pseudo-range by an amount equal to the product of the speed of light and the clock departure b, i.e. the value of the consumer clock offset in relation to the system time. The figure shows the case when the consumer's clock drift is greater than zero - that is, the consumer's clock is ahead of the system time, so the measured pseudo-ranges are less than the true ranges.

Ideally, when the measurements are made accurately and the clock readings of the satellites and the consumer coincide, to determine the position of the consumer in space, it is enough to measure up to three navigation satellites.

In fact, the readings of the clocks that are part of the user's navigation equipment differ from the readings of the clocks on board the navigation satellites. Then, to solve the navigation problem, one more parameter should be added to the previously unknown parameters (three coordinates of the consumer) - the offset between the consumer's clock and the system time. Hence it follows that in the general case, to solve the navigation problem, the consumer must "see" at least four navigation satellites.

Coordinate systems

For the operation of navigation satellite systems, data on the parameters of the Earth's rotation, the fundamental ephemeris of the Moon and planets, data on the Earth's gravitational field, on atmospheric models, as well as high-precision data on the coordinate and time systems used are required.

Geocentric coordinate systems are coordinate systems whose origin coincides with the center of mass of the Earth. They are also called terrestrial or global.

To build and maintain common terrestrial coordinate systems, four main methods of space geodesy are used:

• very long baseline radio interferometry (VLBI),
• spacecraft laser ranging (SLR),
• doppler measuring systems (DORIS),
• navigation measurements of GLONASS and other GNSS spacecraft.

The International Terrestrial Coordinate System ITRF is the reference for the terrestrial coordinate system.

In modern navigation satellite systems, various, usually national, coordinate systems are used.

Time systems

In accordance with the tasks to be solved, two types of time systems are used: astronomical and atomic.

Astronomical time systems based on the Earth's diurnal rotation. The standard for constructing astronomical time scales are solar or sidereal days, depending on the point of the celestial sphere, along which the time is measured.

Universal Time UT (Universal Time) is the mean solar time on the Greenwich meridian.

Coordinated Universal Time (UTC) synchronized with atomic time and is the international standard on which civil time is based.

Atomic time (TAI) - time, the measurement of which is based on electromagnetic oscillations emitted by atoms or molecules during the transition from one energy state to another. In 1967, at the General Conference of Weights and Measures, the atomic second represents a transition between the hyperfine levels F \u003d 4, M \u003d 0 and F \u003d 3, M \u003d 0 of the 2S1 / 2 ground state of the cesium-133 atom, not perturbed by external fields, and that the frequency this transition is assigned the value 9 192 631 770 Hertz.

The satellite radio navigation system is a space-time system with a coverage area covering the entire near-earth space and operates in its own system time. An important place in GNSS is given to the problem of time synchronization of subsystems. Time synchronization is also important to ensure the specified sequence of emission of signals from all navigation satellites. It makes it possible to use passive rangefinder (pseudo-range) measurement methods. The ground-based command and measurement complex provides synchronization of the time scales of all navigation spacecraft by means of their verification and correction (direct and algorithmic).

Navigation radio signals

Navigation radio signals

When choosing the types and parameters of signals used in satellite radio navigation systems, a whole range of requirements and conditions are taken into account. The signals must ensure high accuracy in measuring the time of arrival (delay) of the signal and its Doppler frequency and a high probability of correct decoding of the navigation message. Also, the signals must have a low level of cross-correlation in order for the signals of different navigation spacecraft to be reliably distinguished by the navigation equipment of consumers. In addition, GNSS signals should use the allocated frequency band as efficiently as possible with a low level of out-of-band radiation and have high noise immunity.

Almost all existing navigation satellite systems, with the exception of the Indian NAVIC system, use the L-band for signaling. The NAVIC system will also transmit signals in the S-band as well.

Ranges occupied by various navigation satellite systems

Modulation types

With the development of satellite navigation systems, the types of modulation of radio signals used have changed.
Most navigation systems initially used exclusively signals with binary (two-position) phase modulation - PM-2 (BPSK). Currently, in satellite navigation, a transition has begun to a new class of modulating functions called BOC (Binary Offset Carrier) -signals.

The fundamental difference between BOC signals and signals with PM-2 is that the baseband symbol of the BOC signal is not a rectangular video pulse, but a segment of a meander waveform, including a certain constant number of periods k. Therefore, BOC-modulated signals are often referred to as square wave noise-like signals.

The use of BOC modulated signals increases potential measurement accuracy and delay resolution. At the same time, the level of mutual interference is reduced in the joint operation of navigation systems using traditional and new signals.

Navigation message

Each satellite receives navigation information from ground control stations, which is transmitted back to users as part of a navigation message. The navigation message contains the different types of information needed to locate a user and synchronize their timeline with a national standard.

Navigation message information types

• Ephemeris information needed to calculate satellite coordinates with sufficient accuracy
• The error in the discrepancy between the onboard time scale relative to the system time scale to take into account the spacecraft time offset during navigation measurements
• The discrepancy between the time scale of the navigation system and the national time scale for solving the problem of synchronizing consumers
• Indicators of suitability with information about the state of the satellite for prompt exclusion of satellites with identified failures from the navigation solution
• Almanac with information about the orbits and state of all vehicles in the constellation for long-term rough forecast of satellite movement and planning of measurements
• Ionospheric model parameters required for single-frequency receivers to compensate for navigation measurement errors associated with the propagation delay of signals in the ionosphere
• Earth rotation parameters for accurate conversion of consumer coordinates in different coordinate systems

The suitability indicators are updated within seconds when a failure is detected. Ephemeris and time parameters, as a rule, are updated no more often than once every half hour. At the same time, the update period for different systems is very different and can reach four hours, while the almanac is updated no more often than once a day.

According to its content, the navigation message is divided into operational and non-operational information and is transmitted as a digital information stream (DI). Initially, all navigation satellite systems used a superframe / frame / line / word structure. With this structure, the DI stream is formed in the form of continuously repeating superframes, a superframe consists of several frames, and a frame consists of several lines.
In accordance with the "superframe / frame / line / word" structure, the signals of the BEIDOU, GALILEO systems (except for E6), GPS (LNAV data, L1), GLONASS signals with frequency division were formed. Depending on the system, the sizes of superframes, frames and lines may differ, but the principle of formation remains the same.

Most signals now use a flexible string structure. In this structure, the navigation message is formed as a variable stream of strings of various types. Each line type has its own unique structure and contains a specific type of information (listed above). The NAP extracts the next line from the stream, determines its type and, in accordance with the type, extracts the information contained in this line.

The flexible string structure of the navigation message allows much more efficient use of the bandwidth of the data transmission channel. But the main advantage of a navigation message with a flexible string structure is the possibility of its evolutionary modernization while observing the principle of backward compatibility. For this purpose, the ICD for NAP developers specifically states that if an NAP in a navigation message encounters strings of unknown types, it must ignore them. This allows new line types to be added to pre-existing string types during GNSS upgrades. The NAP, released earlier, ignores lines with new types and, therefore, does not use the innovations that are introduced in the process of modernizing the GNSS, but at the same time its performance is not disturbed.
GLONASS code division messages have a string structure.

Factors Affecting Accuracy Degradation

The accuracy of the consumer's determination of his coordinates, speed and time is influenced by many factors, which can be divided into categories:

1. System errors introduced by the space complex equipment

   Errors associated with the operation of the onboard equipment of the satellite and ground control complex for GNSS are mainly due to imperfection of the time-frequency and ephemeris support.

2. Errors arising on the signal propagation path from the spacecraft to the consumer

   Errors are due to the difference in the propagation speed of radio signals in the Earth's atmosphere from the speed of their propagation in vacuum, as well as the dependence of the speed on the physical properties of various layers of the atmosphere.

3. Errors arising in consumer equipment

   Hardware errors are subdivided into the systematic error of the hardware delay of the radio signal in the AP and fluctuation errors due to noise and consumer dynamics.

In addition, the relative position of navigation satellites and the consumer significantly affects the accuracy of navigation-time determination.
The quantitative characteristic of the error in determining the position and correction of the clock readings associated with the peculiarities of the spatial position of the satellite and the consumer is the so-called geometric factor Γ Σ or geometry coefficient. In the English language literature, the designation GDOP is used - Geometrical delusion of precision.
Geometric factor Γ Σ shows how many times the measurement accuracy decreases and depends on the following parameters:

• Г п is the geometric factor of the accuracy of determining the location of the GNSS consumer in space.
  Compliant with PDOP - Position delusion of precision.
• Г г - geometrical factor of horizontal positioning accuracy of GNSS consumer.
  Complies with HDOP - Horizontal delusion of precision.
• Г в - the geometric factor of the accuracy of determining the location of the GNSS consumer vertically.
  Complies with VDOP - Vertical delusion of precision.
• Г т - geometrical factor of accuracy of determination of correction of readings of GNSS consumer's clock.
  Conforms to TDOP - Time delusion of precision.

Improving navigation accuracy

Currently existing global navigation satellite systems (GNSS) GPS and GLONASS can meet the needs for navigation services for a wide range of consumers. But there are a number of tasks that require high navigation accuracy. These tasks include: takeoff, landing approach and landing of aircraft, navigation in coastal waters, navigation of helicopters and cars, and others.

The classical method for increasing the accuracy of navigation definitions is the use of a differential (relative) definition mode.

Differential mode involves the use of one or more base receivers located at points with known coordinates, which simultaneously with the consumer's receiver (mobile or mobile) receive signals from the same satellites.

An increase in the accuracy of navigation determinations is achieved due to the fact that the measurement errors of the navigation parameters of the consumer and base receivers are correlated. When the differences in the measured parameters are formed, most of these errors are compensated.

The differential method is based on the knowledge of the coordinates of a reference point - a control and correcting station (CCS) or a system of reference stations, relative to which corrections to the determination of pseudo-ranges to navigation satellites can be calculated. If these corrections are taken into account in the consumer's equipment, then the accuracy of the calculation, in particular, of the coordinates, can be increased tenfold.

To ensure differential mode for a large region - for example, for Russia, European countries, the United States - the transmission of correcting differential corrections is carried out using geostationary satellites. Systems that implement this approach are called wide-gap differential systems.

Essential elements

The main elements of a satellite navigation system:

• An orbital constellation consisting of several (from 2 to 30) satellites emitting special radio signals;
• Ground control and monitoring system (ground segment), which includes units for measuring the current position of satellites and transmitting the received information to them to correct information about orbits;
• Receiving client equipment ("satellite navigators") used to determine coordinates;
• Optionally: a ground-based radio beacon system that significantly improves the accuracy of positioning.
• Optionally: an information radio system for transmitting corrections to users, which can significantly improve the accuracy of determining coordinates.

Principle of operation

The principle of operation of satellite navigation systems is based on measuring the distance from an antenna on an object (whose coordinates must be obtained) to satellites, the position of which is known with great accuracy. The table of positions of all satellites is called almanacwhich any satellite receiver should have before starting measurements. Typically, the receiver stores the almanac in memory since the last shutdown, and if it is not out of date, it instantly uses it. Each satellite transmits the entire almanac in its signal. Thus, knowing the distances to several satellites in the system, using conventional geometric constructions, based on the almanac, you can calculate the position of an object in space.

The method of measuring the distance from the satellite to the receiver antenna is based on the certainty of the propagation speed of radio waves. To make it possible to measure the time of the propagated radio signal, each satellite of the navigation system emits precise time signals using an atomic clock precisely synchronized with the system time. When a satellite receiver is operating, its clock is synchronized with the system time, and upon further reception of signals, the delay between the emission time contained in the signal itself and the time of signal reception is calculated. With this information, the navigation receiver calculates the coordinates of the antenna. All other parameters of movement (speed, course, distance traveled) are calculated based on measuring the time that the object spent moving between two or more points with certain coordinates.

In reality, the work of the system is much more complicated. Below are some of the problems that require special techniques to solve them:

• Lack of atomic clocks in most navigation receivers. This disadvantage is usually eliminated by the requirement to obtain information from at least three (2-dimensional navigation at a known altitude) or four (3-dimensional navigation) satellites; (If there is a signal from at least one satellite, you can determine the current time with good accuracy).
• Inhomogeneity of the Earth's gravitational field, affecting the orbits of satellites;
• Inhomogeneity of the atmosphere, due to which the speed and direction of propagation of radio waves can vary within certain limits;
• Reflections of signals from ground objects, which is especially noticeable in the city;
• The impossibility of placing high-power transmitters on satellites, which is why the reception of their signals is possible only in direct line of sight in the open air.

Application of navigation systems

In addition to navigation, the coordinates obtained through satellite systems are used in the following industries:

• Geodesy: using navigation systems, the exact coordinates of points are determined
• Cartography: navigation systems are used in civil and military cartography
• Navigation: using navigation systems, both sea and road navigation is carried out
• Satellite monitoring of transport: with the help of navigation systems, the position, speed of vehicles, control over their movement is monitored
• Cellular: The first mobile phones with GPS appeared in the 90s. In some countries (for example, the USA) it is used to quickly determine the location of a person calling 911. In 2010, a similar project, Era-GLONASS, was launched in Russia.
• Tectonics, Plate Tectonics: Navigation systems monitor plate movements and vibrations
• Outdoor activities: there are various games that use navigation systems, for example, Geocaching, etc.
• Geo-tagging: information, such as photographs, are "snapped" to coordinates thanks to built-in or external GPS receivers

State of the art

Currently, the following satellite navigation systems are in operation or are being prepared for deployment:

GPS

Owned by the US Department of Defense. This fact, according to some states, is its main drawback. Devices supporting GPS navigation are the most widespread in the world. Also known as NAVSTAR.

GLONASS

Belongs to the Russian Ministry of Defense. The system, according to the developers of ground equipment, will have some technical advantages over GPS. After 1996, the satellite constellation was reduced and by 2002 it almost completely fell into decay. It was fully restored only at the end of 2011. There is a low prevalence of client equipment. A deep modernization of the system is planned by 2025.

Beidou

The GNSS subsystem deployed by China is intended for use in that country only. The peculiarity is the small number of satellites in geostationary orbit. At the moment, eight navigation satellites have been launched into Earth's orbit. According to plans, by 2012 it will be able to cover the Asia-Pacific region, and by 2020, when the number of satellites will be increased to 35, the Beidou system will be able to operate as a global one. The implementation of this program began in 2000. The first satellite entered orbit in 2007.

Galileo

European system at the stage of satellite constellation creation. It is planned to fully deploy the satellite constellation by 2020.

IRNSS

Indian Navigation Satellite System, under development. Intended for use in that country only. The first satellite was launched in 2008.

QZSS

Originally Japan's QZSS was conceived in 2002 as a commercial system with a suite of services for mobile communications, broadcasting and widespread use for navigation in Japan and neighboring regions of Southeast Asia. The first satellite launch for QZSS was planned for 2008. In March 2006, the Japanese government announced that the first satellite would not be intended for commercial use and would be launched entirely on budget funds to test the decisions made in order to ensure the solution of navigation problems. Only after the successful completion of the tests of the first satellite will the second stage begin and the next satellites will fully provide the previously planned volume of services.

Main characteristics of navigation satellite systems

parameter, way	SRNS GLONASS	GPS NAVSTAR	TEN GALILEO
Number of NS (reserve)	24 (3)	24 (3)	27 (3)
Orbital planes	3	6	3
The number of NS in the orbital plane	8	4	9
Orbit type	Circular (e \u003d 0 ± 0.01)	Circular	Circular
Orbit height, KM	19100	20183	23224
Orbit inclination, degrees	64.8 ± 0.3	~55 (63)	56
Nominal solar mean orbital period	11h 15min 44 ± 5s	~ 11h 58 min	14 hours 4 minutes and 42 s.
Method of separating NS signals	Frequency	Code	Code-frequency
Carrier frequencies of radio signals, MHz	L1 \u003d 1602.5625… 1615.5 L2 \u003d 1246.4375… 1256.5	L1 \u003d 1575.42 L2 \u003d 1227.60 L5 \u003d 1176.45	E1 \u003d 1575.42 E5 \u003d 1191.795 E5A \u003d 1176.46 E5B \u003d 1207.14 E6 \u003d 12787.75
repetition period of the ranging code (or its segment)	1 ms	1 ms (C / A code)	no data
rangefinder code type	M-sequence (CT-code 511 characters)	Gold code (С / А-code 1023 characters)	M-sequence
rangefinder code clock frequency, MHz	0.511	1.023 (C / A code) 10.23 (P, Y code)	E1 \u003d 1.023 E5 \u003d 10.23 E6 \u003d 5.115
Digital information transmission rate (SI and D code, respectively)	50 z / s (50Hz)	50 z / s (50Hz)	25, 50, 125, 500, 100HZ
Superframe duration, Min	2,5	12,5	5
Number of frames per superframe	5	25	no data
Number of lines per frame	15	5	no data
Time reference system	UTS (SU)	UTS (USNO)	UTS (GST)
Coordinate reference system	PZ-90 / PZ90.2	WGS-84	ETRF-00
Ephemirid type	Geocentric coordinates and their derivatives		Modified Keplerian elements
Sector of radiation from direction to the center of the earth	± 19 to 0	L1 \u003d ± 21 v 0 L2 \u003d ± 23.5 v 0	no data
Sector of the Earth	± 14.1 to 0	± 13.5V 0	no data

Technical details of the systems

Let's consider some of the features of the main operating satellite navigation systems (GPS and GLONASS):

Differential measurement

Separate models of satellite receivers allow the production of the so-called. "Differential measurement" of distances between two points with high precision (centimeters). For this, the position of the navigator is measured at two points with a short time interval. At the same time, although each such measurement has an accuracy of the order of 10-15 meters without a ground-based correction system and 10-50 cm with such a system, the measured distance has a much smaller error, since the factors interfering with the measurement (error of satellite orbits, atmospheric inhomogeneity in a given place Lands, etc.) in this case are mutually subtracted. In addition, there are several systems that send clarifying information ("differential correction to coordinates"), which makes it possible to increase the accuracy of measuring the coordinates of the receiver up to ten centimeters. Differential correction is sent either from geostationary satellites, or from ground base stations, it can be paid (signal decoding is possible only by one specific receiver after paying for a "service subscription") or free.

see also

	Satellite navigation system at Wikimedia Commons

• Pseudo-satellite

Notes

Links

International Satellite Navigation Forum Satellite Navigation Event

Mobile GIS for forestry enterprises GPS navigation, control of forest inventory data using satellite images, forestry map, taxation description in a mobile phone.

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