GPS and Glonass positioning accuracy. IN

Purpose

GPS (Global Positioning System) global positioning), allows you to accurately determine the three-dimensional coordinates of an object equipped with a GPS receiver: latitude, longitude, altitude above sea level, as well as its speed, direction of movement and current time.

Brief history

The GPS system was developed by the US Department of Defense. Work on this project, called NAVSTAR (NAVigation System with Timing and Ranging - navigation system for determining time and range), began back in the 70s. The first satellite of the system was launched into orbit in 1974, and the last of the 24 needed to cover the entire Earth only in 1993. Initially, GPS was intended for use by the US military (navigation, missile guidance, etc.), but since 1983, when it was shot down a Korean Airlines plane accidentally intruded into Soviet territory, the use of GPS was allowed for civilians. At the same time, the accuracy of the transmitted signal was coarsened using a special algorithm, but in 2000 this limitation was lifted. The US Department of Defense continues to maintain and upgrade the GPS system. It was this complete dependence of the system's performance on the government of one country (for example, during the first Gulf War, the civilian sector of GPS was turned off) that prompted other countries to develop alternative navigation systems (Russian - GLONASS, European - GALILEO, Chinese - Beidou).

Principles of determining coordinates

The principle of determining the coordinates of an object in the GPS system is based on calculating the distance from it to several satellites, the exact coordinates of which are known. Information about the distance to at least 3 satellites allows you to determine the coordinates of an object as the point of intersection of spheres, the center of which is the satellites, and the radius is the measured distance.

In fact, there are two points of intersection of the spheres, but one of them can be discarded because it is either deep inside the Earth or very high above its surface. The distance to each satellite is defined as the time it takes for a radio signal to travel from the satellite to the receiver multiplied by the speed of light. The problem arises of accurately determining the transit time of a radio signal. It is solved by generating and transmitting a signal from the satellite, modulated using a special sequence. Exactly the same signal is generated in the GPS receiver, and analysis of the lag of the received signal from the internal signal makes it possible to determine its travel time.

To accurately determine the signal transit time GPS watch The receiver and satellite must be synchronized as much as possible; a deviation of even a few microseconds leads to a measurement error of tens of kilometers. For these purposes, the satellite has high-precision atomic clock. It is impossible to install a similar clock in a GPS receiver (regular quartz clocks are used), so additional signals from at least one more satellite are used to synchronize time. It is assumed that if the time in the GPS receiver is precisely synchronized, then a circle with a radius equal to the distance from the fourth satellite will intersect the same point as the circles from the other three satellites. The GPS receiver adjusts its clock until this condition is met. Thus, to accurately determine the position of an object in three-dimensional space (3D), signals from at least 4 satellites are required (from 3 satellites without determining the height above the earth’s surface - 2D). In practice, with good visibility of the sky, GPS receivers receive signals from many satellites at once (up to 10-12), which allows them to synchronize clocks and determine coordinates with fairly high accuracy.

Along with the sequence by which the signal propagation time is determined, each satellite transmits binary information - an almanac and ephemeris. The almanac contains information about the current state and estimated orbit of all satellites (having received information from one satellite, it becomes possible to narrow the search sectors for signals from other satellites). Ephemeris - updated information about the orbit of a specific satellite transmitting a signal (the actual orbit of the satellite may differ from the calculated one). It is the exact data about the current position of the satellites that allows the GPS receiver to calculate its own location relative to them.

GPS Accuracy

The typical accuracy of determining coordinates by GPS receivers in the horizontal plane is approximately 1-2 meters (provided good visibility of the sky). The accuracy of determining altitude above sea level is usually 2-5 times lower than the accuracy of determining coordinates under the same conditions (i.e., in ideal conditions, 2-10 meters).

The level of signal reception from satellites, and as a result the accuracy of determining coordinates, deteriorates under dense foliage of trees or due to very heavy clouds. Also, normal reception of GPS signals can be impaired by interference from many terrestrial radio sources. However, the main factor influencing the decrease in GPS accuracy is incomplete visibility of the sky. This is especially evident when the GPS receiver is located in dense urban areas, when a significant part of the sky is hidden by nearby buildings, canopies and other obstacles. The accuracy of determining coordinates can drop to 20-30 meters, and sometimes more. Obstacles do not allow signals from some satellites potentially available at a given point on the Earth to pass through. This leads to the fact that calculations are carried out using a smaller number of signals from satellites located primarily in one sector of the sky. The displacement usually occurs in a plane perpendicular to the obstacle.

In general, if we talk about the accuracy of GPS in urban conditions, based on accumulated statistical data and our own experience, we can draw the following conclusions. The accuracy of determining coordinates when the vehicle is in an open area (parking lot, square, etc.) and when driving along major highways and multi-lane roads will be 1-2 meters. When driving along narrow streets, especially when there are closely spaced houses along them, the accuracy will be 4-10 meters. When the car is in “yard wells”, very close to high-rise buildings, etc. accuracy can drop down to 20-30 meters.

Of course, the accuracy of determining coordinates greatly depends on the quality of the GPS receiver itself, as well as the antennas used and their correct placement on the vehicle

GPS is a satellite navigation system that measures distance, time and determines location. Allows you to determine the location and speed of objects anywhere on Earth (not including the polar regions), in almost any weather, as well as in outer space near the planet. The system is developed, implemented and operated by the US Department of Defense.

Brief characteristics of GPS

The US Department of Defense satellite navigation system is GPS, also called NAVSTAR. The system consists of 24 navigation artificial earth satellites (NES), ground command-measuring complex and consumer equipment. It is a global, all-weather, navigation system that provides the determination of the coordinates of objects with high accuracy in three-dimensional near-Earth space. GPS satellites are placed in six medium-high orbits (altitude 20,183 km) and have an orbital period of 12 hours. The orbital planes are spaced at 60° intervals and inclined to the equator at an angle of 55°. There are 4 satellites in each orbit. 18 satellites is the minimum number to ensure visibility at each point on Earth of at least 4 satellites.

The basic principle of using the system is to determine location by measuring distances to an object from points with known coordinates - satellites. The distance is calculated by the delay time of signal propagation from sending it by the satellite to receiving it by the antenna of the GPS receiver. That is, to determine three-dimensional coordinates, the GPS receiver needs to know the distance to three satellites and the time of the GPS system. Thus, signals from at least four satellites are used to determine the coordinates and altitude of the receiver.

The system is designed to provide navigation of aircraft and ships and determine time with high precision. It can be used in two-dimensional navigation mode - 2D determination of navigation parameters of objects on the Earth's surface) and in three-dimensional mode - 3D (measurement of navigation parameters of objects above the Earth's surface). To find the three-dimensional position of an object, it is necessary to measure the navigation parameters of at least 4 NIS, and for two-dimensional navigation - at least 3 NIS. GPS uses a pseudo-rangefinder method for determining position and a pseudo-radial velocity method for finding the speed of an object.

To improve accuracy the determination results are smoothed using a Kalman filter. GPS satellites transmit navigation signals at two frequencies: F1 = 1575.42 and F2 = 1227.60 MHz. Radiation mode: continuous with pseudonoise modulation. Navigation signals are a public C/A code (course and acquisition), transmitted only on the F1 frequency, and a protected P code (precision code), emitted on the F1, F2 frequencies.

In GPS, each NIS has its own unique C/A code and unique P code. This type of satellite signal separation is called code separation. It allows on-board equipment to recognize which satellite a signal belongs to when they are all transmitting on the same frequency GPS provides two levels of customer service: PPS Precise Positioning Service and SPS Standard Positioning Service PPS is based on a precise code, and SPS - publicly available. The PPS level of service is provided to the US military and federal services, and SPS is provided to the mass civilian consumer. In addition to navigation signals, the satellite regularly transmits messages that contain information about the status of the satellite, its ephemeris, system time, ionospheric delay forecast, and performance indicators. Onboard GPS equipment consists of an antenna and a receiver indicator. The PI includes a receiver, a computer, memory units, control and display devices. The memory blocks store the necessary data, programs for solving problems and controlling the operation of the receiver indicator. Depending on the purpose, two types of on-board equipment are used: special and for the mass consumer. Special equipment is designed to determine the kinematic parameters of missiles, military aircraft, ships and special vessels. When finding object parameters, it uses P and C/A codes. This equipment provides virtually continuous determinations with accuracy: object location— 5+7 m, speed — 0.05+0.15 m/s, time — 5+15 ns

Main applications of GPS navigation satellite system:

• Geodesy: using GPS, the exact coordinates of points and boundaries of land plots are determined
• Cartography: GPS is used in civil and military cartography
• Navigation: GPS is used for both sea and road navigation
• Satellite monitoring of transport: using GPS, the position and speed of vehicles are monitored, and their movement is controlled
• Cellular communications: first mobile phones with GPS appeared in the 90s. In some countries, such as the USA, this is used to quickly determine the location of a person calling 911.
• Tectonics, Plate Tectonics: using GPS to observe the movements and vibrations of plates
• Active recreation: there are various games that use GPS, for example, Geocaching, etc.
• Geotagging: information, such as photographs, is “linked” to coordinates thanks to built-in or external GPS receivers.

Determination of consumer coordinates

Positioning by distances to satellites

The location coordinates are calculated based on the measured distances to the satellites. Four measurements are required to determine the location. Three dimensions are enough if you can exclude implausible solutions by some other accessible ways. Another measurement is required for technical reasons.

Measuring the distance to a satellite

The distance to a satellite is determined by measuring the amount of time it takes for a radio signal to travel from the satellite to us. Both the satellite and the receiver generate the same pseudo-random code strictly simultaneously on a common time scale. Let's determine how long it took the signal from the satellite to reach us by comparing the delay of its pseudo-random code with respect to the receiver code.

Ensuring perfect timing

Accurate timing is key to measuring distances to satellites. Satellites are accurate in time because they have atomic clocks on board. The receiver clock may not be perfect, since its drift can be eliminated using trigonometric calculations. To obtain this opportunity, it is necessary to measure the distance to the fourth satellite. The need for four measurements is determined by the receiver design.

Determining the position of the satellite in outer space.

To calculate our coordinates, we need to know both the distances to the satellites and the location of each in outer space. GPS satellites travel so high that their orbits are very stable and can be predicted with great accuracy. Tracking stations constantly measure small changes in orbits, and data about these changes is transmitted from satellites.

Ionospheric and atmospheric signal delays.

There are two methods that can be used to keep the error to a minimum. First, we can predict what the typical change in velocity would be on a typical day, under average ionospheric conditions, and then apply a correction to all our measurements. But, unfortunately, not every day is ordinary. Another method is to compare the propagation speeds of two signals having different carrier frequencies. If we compare the propagation time of two different-frequency components of the GPS signal, we can find out what kind of slowdown took place. This correction method is quite complex and is used only in the most advanced, so-called “dual-frequency” GPS receivers.

Multipath.

Another type of error is “multipath” errors. They occur when signals transmitted from a satellite are repeatedly reflected from surrounding objects and surfaces before reaching the receiver.

Geometric factor reducing accuracy.

Good receivers are equipped with computational procedures that analyze the relative positions of all observable satellites and select four candidates from them, i.e. best positioned four satellites.

Resulting GPS accuracy.

The resulting GPS error is determined by the sum of errors from various sources. The contribution of each varies depending on atmospheric conditions and the quality of the equipment. In addition, the accuracy can be purposefully reduced by the US Department of Defense as a result of installing the so-called S/A mode (“Selective Availability”) on GPS satellites. limited access). This mode is designed to prevent a potential enemy from gaining a tactical advantage in GPS positioning. When and if this mode is set, it creates the most significant component of the total GPS error.

Conclusion:

Measurement accuracy using GPS depends on the design and class of the receiver, the number and location of satellites (in real time), the state of the ionosphere and the Earth's atmosphere (heavy clouds, etc.), the presence of interference and other factors. “Household” GPS devices, for “civilian” users, have a measurement error in the range from ±3-5m to ±50m and more (on average, real accuracy, with minimal interference, if new models, is ±5–15 meters in plan). The maximum possible accuracy reaches +/- 2-3 meters horizontally. Height – from ±10-50m to ±100-150 meters. The altimeter will be more accurate if you calibrate the digital barometer by the nearest point with a known exact altitude (from a regular atlas, for example) on a flat terrain or by known atmospheric pressure (if it does not change too quickly when the weather changes). High-precision meters of “geodetic class” - more precise by two or three orders of magnitude (up to a centimeter, in plan and in height). The actual accuracy of measurements is determined by various factors, for example, distance from the nearest base (correction) station in the system service area, multiplicity (number of repeated measurements / accumulations at a point), appropriate quality control of work, level of training and practical experience of the specialist. Such high-precision equipment can only be used by specialized organizations, special services and the military.

To improve navigation accuracy It is recommended to use a GPS receiver in an open space (no buildings or overhanging trees nearby) with fairly flat terrain, and connect an additional external antenna. For marketing purposes, such devices are credited with “double reliability and accuracy” (referring to the simultaneously used two satellite systems, Glonass and Gypies), but the actual actual improvement in parameters (increased accuracy of determining coordinates) can amount to only up to several tens of percent . Only a noticeable reduction in the hot-warm start time and measurement duration is possible

The quality of GPS measurements deteriorates if the satellites are located in the sky in a dense beam or on one line and “far” - near the horizon (all this is called “bad geometry”) and there is signal interference (high-rise buildings blocking the signal, trees, steep mountains nearby, reflecting the signal ). On the day side of the Earth (currently illuminated by the Sun) - after passing through the ionospheric plasma, radio signals are weakened and distorted an order of magnitude stronger than on the night side. During a geomagnetic storm, after powerful solar flares, interruptions and long interruptions in the operation of satellite navigation equipment are possible.

The actual accuracy of the GPS depends on the type of GPS receiver and the features of data collection and processing. The more channels (there must be at least 8) in the navigator, the more accurately and quickly the correct parameters are determined. When receiving “auxiliary A-GPS location server data” via the Internet (via packet data transfer, in phones and smartphones), the speed of determining coordinates and location on the map increases

WAAS (Wide Area Augmentation System, on the American continent) and EGNOS (European Geostationary Navigation Overlay Services, in Europe) - differential subsystems transmitting through geostationary (at altitudes from 36 thousand km in lower latitudes to 40 thousand kilometers above middle and high latitudes ) satellites correcting information to GPS receivers (corrections are introduced). They can improve the quality of positioning of a rover (field, mobile receiver) if ground-based base correction stations (stationary reference signal receivers that already have a high-precision coordinate reference) are located and operate nearby. In this case, the field and base receivers must simultaneously track the satellites of the same name.

To increase measurement speed It is recommended to use a multi-channel (8-channel or more) receiver with an external antenna. At least three GPS satellites must be visible. The more there are, the better the result. Good visibility of the sky (open horizon) is also necessary. A fast, “hot” (lasting in the first seconds) or “warm start” (half a minute or a minute, in time) of the receiving device is possible if it contains an up-to-date, fresh almanac. In the case when the navigator has not been used for a long time, the receiver is forced to receive the full almanac and, when it is turned on, a cold start will be performed (if the device supports AGPS, then faster - up to a few seconds). To determine only horizontal coordinates (latitude / longitude), signals from three satellites may be sufficient. To obtain three-dimensional (with height) coordinates, you need at least four coordinates. The need to create our own, domestic navigation system is due to the fact that GPS is American, potential adversaries who can at any time, in their military and geopolitical interests, selectively disable, “jam”, modify it in any region or increase artificial , a systematic error in coordinates (for foreign consumers of this service), which is always present in peacetime.

Special error

The main cause of GPS data errors is no longer a problem. On May 2, 2000, at 5:05 a.m. (MEZ), the so-called Special Error (SA) was turned off. A special error is an artificial falsification of time in the L1 signal transmitted by the satellite. For civilian GPS receivers, this error led to less accurate determination of coordinates. (error of approximately 50 m within a few minutes).

In addition, the received data was transmitted with less accuracy, which means that the transmitted position of the satellite was not correct. Thus, within a few hours, there is an inaccuracy of 50-150 m in position data. In the days when the special error was active, civilian GPS devices had an inaccuracy of approximately 10 meters, and today it is 20 or usually even less. Turning off sampling error mainly improved the accuracy of the elevation data.

The reason for the special error was safety. For example, terrorists should not be able to detect important construction sites using weapons on remote control. During the first Gulf War in 1990, the special error was partially disabled because... American troops lacked military GPS receivers. 10,000 civilian GPS devices (Magellan and Trimble) were purchased, which made it possible to freely and accurately navigate desert terrain. The special error has been deactivated due to the widespread use of GPS systems around the world. The next two graphs show how the accuracy of determining coordinates has changed after turning off the special error. The length of the boundary of the diagrams is 200 meters, the data were obtained on May 1, 2000 and May 3, 2000, within a period of 24 hours each. While coordinates with a special error are within a radius of 45 meters, without it, 95 percent of all points are within a radius of 6.3 meters.

"Geometry of satellites"

Another factor that affects the accuracy of coordinate determination is the “geometry of the satellites.” Satellite geometry describes the satellites' positions relative to each other from the receiver's point of view.

If the receiver sees 4 satellites and they are all located, for example, in the northwest, then this will lead to “bad” geometry. In the worst case, location detection will be completely impossible when all detected distances point in the same direction. Even if the location is recognized, the error can reach 100 - 150 m. If these 4 satellites are well distributed across the sky, then the accuracy of the determined location will be much higher. Let's assume that the satellites are located in the north, east, south and west, forming angles of 90 degrees with respect to each other. In this case, distances can be measured in four different directions, which characterizes “good” satellite geometry.

If two satellites are in the best position relative to the receiver, then the angle between the receiver and the satellites is 90 degrees. The signal travel time cannot be absolutely certain, as discussed earlier. Therefore, possible positions are marked with black circles. The intersection point (A) of the two circles is quite small and is indicated by a blue square field, which means that the determined coordinates will be quite accurate.

If the satellites are located almost in one line relative to the receiver, then, as you can see, we will get a larger area at the crosshairs, and therefore less accuracy.

The geometry of the satellites also depends a lot on tall cars or whether you are using the instrument in a car. If any of the signals are blocked, the remaining satellites will try to determine the coordinates, if this is possible at all. This can often happen in buildings when you are close to windows. If location determination is possible, in most cases it will not be accurate. The more part of the sky is blocked by any object, the more difficult it becomes to determine the coordinates.

Most GPS receivers not only show the number of satellites "caught", but also their position in the sky. This allows the user to judge whether a particular satellite is being obscured by an object and whether the data will become inaccurate when moving just a couple of meters.

Manufacturers of most instruments provide their own formulation of the accuracy of the measured values, which mainly depends on various factors. (which the manufacturer is reluctant to talk about).

DOP (Dilution of Precision) values ​​are primarily used to determine the quality of satellite geometry. Depending on what factors are used to calculate DOP values, different options are possible:

• GDOP(Geometrical Dilution Of Precision); Full precision; 3D coordinates and time
• PDOP(Positional Dilution Of Precision) ; Position accuracy; 3D coordinates
• HDOP(Horizontal Dilution Of Precision); Horizontal accuracy; 2D coordinates
• VDOP(Vertical Dilution Of Precision); Vertical accuracy; height
• TDOP(Time Dilution Of Precision); temporal precision; time

HDOP values ​​below 4 are good, above 8 are bad. HDOP values ​​become worse if the "caught" satellites are high in the sky above the receiver. On the other hand, VDOP values ​​get worse the closer the satellites are to the horizon, and PDOP values ​​are good when there are satellites directly overhead and three more spread out on the horizon. For accurate location determination, the GDOP value should not be less than 5. The PDOP, HDOP and VDOP values ​​are part of the NMEA GPGSA data.

The geometry of the satellites does not cause error in position determination, which can be measured in meters. In fact, the DOP value amplifies other inaccuracies. High DOP values ​​increase other errors more than low DOP values.

The error that occurs in position determination due to the geometry of the satellites also depends on the latitude at which the receiver is located. This is shown in the diagrams below. The diagram on the left shows the height uncertainty (the curve is shown with a special error at the beginning) which was recorded in Wuhan (China). Wuhan is located at 30.5° north latitude and is the best place where the constellation of satellites is always perfect. The diagram on the right shows the same recorded interval taken at Kasei station in Antarctica (66.3°S latitude). Due to the less than ideal constellation of satellites at this latitude, more severe errors occurred from time to time. In addition, the error occurs due to the influence of the atmosphere - the closer to the poles, the greater the error.

Satellite orbits

Although the satellites are in fairly well-defined orbits, slight deviations from the orbits are still possible due to gravity. The Sun and Moon have little influence on the orbits. The orbit data is constantly adjusted and corrected and is regularly sent to the receiver in the empirical memory. Therefore, the impact on accuracy location determination is quite small and if an error occurs, it is no more than 2 meters.

Effects of signal reflections

The effect occurs due to the reflection of satellite signals from other objects. For GPS signals this effect mainly occurs in the vicinity of large buildings or other objects. The reflected signal takes longer to complete than the direct signal. The error will be only a few meters.

Atmospheric effects

Another source of inaccuracy is a decrease in the speed of signal propagation in the troposphere and ionosphere. The speed of signal propagation in outer space is equal to the speed of light, but in the ionosphere and troposphere it is less. In the atmosphere at an altitude of 80 - 400 km, the energy of the sun creates a large number of positively charged ions. Electrons and ions are concentrated in the four conductive layers of the ionosphere (D-, E-, F1-, and F2 layers).
These layers refract electromagnetic waves emanating from satellites, which increases the travel time of signals. Basically, these errors are corrected by the computational actions of the receiver. Various speed options when passing through the ionosphere for low and high frequencies are well known for normal conditions. These values ​​are used when calculating location coordinates. However, civilian receivers are unable to adjust for unexpected changes in signal transmission, which can be caused by strong solar winds.

It is known that during the passage of the ionosphere, electromagnetic waves slow down in inverse proportion to the area of ​​their frequency (1/f2). This means that low frequency electromagnetic waves slow down faster than high frequency electromagnetic waves. If the high and low frequency signals that reached the receiver allowed the difference in their arrival times to be analyzed, then the time of passage through the ionosphere would also be calculated. Military GPS receivers use signals of two frequencies (L1 and L2), which behave differently in the ionosphere, and this eliminates another error in the calculations.

The influence of the troposphere is the next reason why the signal travel time increases due to refraction. The causes of refraction are different concentrations of water vapor in the troposphere, depending on the weather. This error is not as large as the error that occurs when passing through the ionosphere, but it cannot be eliminated by calculation. To correct this error, an approximate correction is used in the calculation.

The next two graphs show the ionospheric error. The data shown on the left was obtained with a single-frequency receiver, which cannot correct for ionospheric error. The graph on the right was obtained with a dual-frequency receiver that can correct for ionospheric error. Both diagrams have approximately the same scale (Left: Latitude -15m to +10m, Longitude -10m to +20m. Right: Latitude -12m to +8m, Longitude -10m to +20m). The right graph shows higher accuracy.

Using WAAS and EGNOS you can set up "maps" of weather conditions over different regions. The corrected data is sent to the receiver and significantly improves accuracy.

Clock inaccuracy and rounding errors

Even though the receiver time is synchronized with the satellite time during position determination, there is still a time inaccuracy, which leads to a 2m error in position determination. Rounding and receiver computational errors have an error of approximately 1m.

Relativistic effects

IN this section there is no complete explanation of the theory of relativity. In everyday life we ​​are not aware of the importance of the theory of relativity. However, this theory affects many processes, including the proper functioning of the GPS system. This influence will be briefly explained below.

As we know, time is one of the main factors in GPS navigation and should be equal to 20-30 nanoseconds to ensure the necessary accuracy. Therefore, it is necessary to take into account the speed of the satellites (approximately 12,000 km/h)

Anyone who has ever encountered the theory of relativity knows that time flows more slowly at high speeds. For satellites, which move at a speed of 3874 m/s, the clock runs slower than for the earth. This relativistic time results in a time inaccuracy of approximately 7.2 microseconds per day (1 microsecond = 10-6 seconds). The theory of relativity also states that time moves slower the stronger the gravitational field. For an observer on the earth's surface, the satellite's clock will run faster (since the satellite is 20,000 km higher and is subject to less gravitational forces than the observer). And this is the second reason for this effect, which is six times stronger than the inaccuracy that was mentioned a little earlier.

In general, the clocks on the satellites seem to move a little faster. The time deviation for an observer on Earth would be 38 microseconds per day and would result in a total error of 10 km per day. To avoid this mistake there is no need to constantly make adjustments. The clock frequency on the satellites was set to 10.229999995453 Mhz instead of 10.23 Mhz, but the data is used as if it had standard frequency at 10.23 MHz. This trick solved the problem of the relativistic effect once and for all.

But there is another relativistic effect that is not taken into account when determining location using the GPS system. This is the so-called Sagnak effect and is caused by the fact that the observer on the surface of the Earth is also constantly moving at a speed of 500 m/s (speed at the equator) due to the fact that the planet rotates. But the influence of this effect is small and its adjustment is difficult to calculate, because depends on the direction of movement. Therefore, this effect is taken into account only in special cases.

GPS system errors are shown in the following table. Partial values ​​are not constant values, but are subject to differences. All numbers are approximate values.

Article about GLONASS and GPS systems: characteristics of satellite systems, their features and comparative analysis. At the end of the article there is a video about the principles GPS operation and GLONASS.

Now the spheres of influence are divided between the Russian GLONASS, the American GPS (Global Positioning System) and the Chinese BeiDou, which is gradually gaining momentum. The choice of a system for your own car may be determined by patriotic motives, or it may be based on a competent weighing of the advantages and disadvantages of these developments.

Basics of Satellite Communications

The purpose of each satellite system is to determine the exact location of any object. In the context of a car, this task is accomplished by special device, helping to establish coordinates on the ground, known as a navigator.

Satellites interacting with a particular navigation system send it personal signals that are different from each other. To clearly determine spatial coordinates, the navigator needs information from 4 satellites. Thus, this is not a simple automotive gadget, but one of the elements of a complex space positioning mechanism.

As the car moves, the coordinates continuously change. Therefore, the navigation system is designed so that at certain regular intervals it updates the received data and recalculates the distance.

Advantage modern systems is that they have the ability to remember the location of satellites even when turned off. This significantly increases the efficiency of the device, when there is no need to re-find the satellite’s orbit each time. For motorists who regularly access the navigator, the developers have provided a “hot start” function - the fastest possible connection between the device and the satellite. If you rarely use the navigator, the start will be “cold”, that is, in this case, the connection with the satellite will take longer, taking from 10 to 20 minutes.

Creation of systems

Although the first Earth satellite was a Soviet development, it was the American GPS. Scientists have noticed changes in satellite signals, depending on its movement along the orbit. Then they thought about a method for calculating not only the coordinates of the satellite itself, but also the earthly objects attached to it.

In 1964, an exclusively military navigation system called TRANZIT went into operation, becoming the world's first development of this level. It facilitated the launch of missiles from submarines, but calculated the accuracy of the location of the object only at a distance of 50 meters. In addition, this object had to remain absolutely motionless.

It became clear that the first and at that time only navigator in the world could not cope with the task of constantly determining coordinates. This was due to the fact that while passing in low orbit, the satellite could send signals to Earth only for an hour.

The next, modernized version appeared 3 years later, along with the new satellite Timation-1 and its brother Timation-2. Together they rose to a higher orbit and united into unified system, called "Navstar". It started out just like a military development, but then it was decided to make it publicly available for the needs of the civilian population.

This system is still in operation, with 32 satellites in its arsenal, providing complete coverage of the Earth. Another 8 devices are in reserve for some unforeseen event. Moving at a significant distance from the planet in several orbits, the satellites complete their revolution in almost a day.

Over domestic GLONASS system began to work back in the days of the Union - a powerful power with outstanding scientific minds. Injection into orbit artificial satellite launched design work for a positioning system.

The first Soviet satellite, born in 1967, was supposed to be the only one sufficient to calculate coordinates. But soon a whole system equipped with radio transmitters appeared in space, known to the population as the Cicada, the military called it the Cyclone. Its task was to identify objects in distress, which it did until the advent of GLONASS in 1982.

The Soviet Union was destroyed, the country was in dire straits and could not find reserves to bring the high-tech system to fruition. The entire system included 24 satellites, but due to financial difficulties, almost half of them did not function. Therefore, at that time, in the 90s, GLONASS could not even come close to competing with GPS.

Today, Russian developers intend to catch up and overtake their American colleagues, which already confirms the faster revolution of our satellites around the Earth. Although historically the Russian satellite system has lagged significantly behind the American one, this gap is shrinking from year to year.

Advantages and Disadvantages

At what level are both systems now? Which one should the average person prefer for their everyday tasks?

By and large, many citizens do not care what kind of satellite navigation uses his technique. They are both available without restrictions or fees to the entire civilian population, including for use in a car. If we look from a technical point of view, the Swedish satellite company has officially announced the merits of GLONASS, which works much better in northern latitudes.

GPS satellites practically do not appear north of the 55th parallel, and in the southern hemisphere, accordingly, further south. Whereas, with an inclination angle of 65 degrees and an altitude of 19.4 thousand km, GLONASS satellites deliver excellent, stable signals to Moscow, Norway and Sweden, which is so appreciated by foreign experts.

Although both systems have a large number of satellites in all orbital planes, other experts still give the palm to GPS. Even with an active program to improve the Russian system, the Americans currently have 27 satellites versus 24 Russian ones, which gives greater clarity to their signals.

The reliability of GLONASS signals is 2.8 m compared to 1.8 m for GPS. However, this figure is quite average, because satellites can be lined up in orbit in such a way that the error rate increases several times. Moreover, such a situation can befall both satellite systems.

For this reason, manufacturers are trying to equip their devices with dual-system navigation that receives signals from both GPS and GLONASS.

An important role is played by the quality of ground equipment that receives and decrypts the received data.

If we talk about the identified shortcomings of both navigation systems, they can be distributed as follows:

GLONASS:

• changing celestial coordinates (ephemerides) leads to inaccuracy in determining coordinates, reaching 30 meters;
• fairly frequent, albeit short-term, interruption of the signal;
• tangible influence of relief features on the clarity of the obtained data.

GPS:

• receiving an erroneous signal due to multipath interference and atmospheric instability;
• a significant difference between the civilian version of the system, which has too limited capabilities compared to military development.

Two-system

In total, more than five dozen satellites of both world powers are constantly spinning in orbit. As already mentioned, to obtain reliable coordinates, a good “view” of 4 satellites is sufficient. On flat ground, in the steppe or in a field, any receiver will be able to simultaneously detect up to a dozen signals, while in a forest or mountainous area the connection quickly disappears.

Thus, the design goal is for each receiving device to be able to communicate with as many satellites as possible. This again returns to the idea of ​​combining GLONASS and GPS, which is already practiced in America for rescue services. No matter how the relations between states develop, human life comes first, and a dual-system chip with higher speed and clearly determine the location of the person in trouble.

Such a synthesis will also save motorists from the inability to find their way in unfamiliar areas due to the fact that the navigator is too slow to establish a connection and takes too long to process information. The reason for this is the loss of a satellite due to banal interference: a tall building, an overpass, or even a large truck in the neighborhood. But if the car navigator is equipped with a dual-system chip, the likelihood of it freezing will be significantly reduced.

When this practice becomes widespread, the navigator will not care about the country of origin of the system, because it will be able to simultaneously track up to 40 satellites, giving a fantastically accurate location determination.

Video about the principles of operation of GPS and GLONASS:

Lecture on the anatomy of mobile devicesV. Navigation (GPS, GLONASS, etc.) in smartphones and tablets. Sources of errors. Testing methods.

Until recently, it was possible to buy devices called “Navigators” in retail chains. The main function of these devices fully corresponded to their name, and they usually performed it well.

At that time, practically the only normally functioning navigation system in the world was the American GPS (Global Positioning System), and it was enough for all needs. Actually, the words “navigation” (navigator) and GPS were synonymous at that time.

Everything changed when manufacturers of PDAs (handheld computers), and then smartphones and tablets, began to build navigation support into their devices. Physically, it was implemented in the form of built-in receivers of navigation signals. Sometimes navigation support could be found even in push-button phones.

From that moment on, everything changed. Navigators, as separate devices, have almost disappeared from both production and sale. Consumers have switched en masse to using smartphones and tablets as navigators.
In the meantime, two more navigation systems were successfully put into operation - the Russian GLONASS and the Chinese Beidou (Beidou, BDS).

But this does not mean that the quality of navigation has improved. The navigation function in these devices (smartphones and tablets) has no longer become the main one, but one of many.

As a result, many users began to notice that not all smartphones are “equally useful” for navigation purposes.

This is where we come to the problem of identifying the sources of errors in navigation, including the question of the role of dishonesty of device manufacturers in this matter. Sad but true.

But before blaming manufacturers for all their sins, let’s first look at the sources of errors in navigation. For producers, as we will find out later, are not to blame for all sins, but only for half. :)

Navigation errors can be divided into two main classes: caused by reasons external to the navigation device, and internal.

Let's start with external reasons. They arise mainly due to the unevenness of the atmosphere and the natural technical error of measuring instruments.

Their approximate contributions are:

Signal refraction in the ionosphere ± 5 meters;
- Satellite orbit fluctuations ± 2.5 meters;
- Satellite clock error ± 2 meters;
- Tropospheric unevenness ± 0.5 meters;
- The influence of reflections from objects± 1 meter;
- Measurement errors in the receiver ± 1 meter.

These errors have a random sign and direction, so the final error is calculated in accordance with probability theory as the root of the sum of squares and is 6.12 meters. This does not mean that the error will always be this way. It depends on the number of visible satellites, their relative position, and most of all, on the level of reflections from surrounding objects and the influence of obstacles on the weakening of satellite signals. As a result, the error may be either higher or lower than the given “averaged” value.

Signals from satellites may weaken, for example, in the following cases:
- when indoors;
- when located between closely spaced high objects (between high-rise buildings, in a narrow mountain gorge, etc.);
- while in the forest. Experience shows that dense, tall forest can make navigation significantly more difficult.

These problems are due to the fact that high-frequency radio signals travel like light - that is, only within a line of sight.

Sometimes navigation, albeit with errors, can also work on signals reflected from obstacles; but when repeatedly reflected, they become so weak that navigation stops working with them.

Now let's move on to the "internal" causes of errors in navigation; those. which are created by the smartphone or tablet itself.

Actually, there are only two problems here. Firstly, poor sensitivity of the navigation receiver (or problems with the antenna); secondly, the “crooked” software of a smartphone or tablet.

Before looking at specific examples, let's talk about ways to check the quality of navigation.

Navigation testing methods.

1. Testing navigation in “static” mode (with the smartphone/tablet in a stationary position).

This check allows you to determine the following parameters:
- speed of initial determination of coordinates during a “cold start” (measured by the clock);
- a list of navigation systems that this smartphone/tablet works with (GPS, GLONASS, etc.);
- estimated accuracy of coordinate determination;
- speed of determining coordinates during a “hot start”.

These parameters can be determined using the usual navigation programs, and using special test programs (which is more convenient).

The rules for static testing are very simple: testing must be done in open space(wide street, square, field, etc.) and when the Internet is turned off. If the last requirement is violated, the “cold start” time can be significantly accelerated due to direct downloading of satellite orbits from the Internet (A-GPS, assisted GPS) instead of determining them from signals from the satellites themselves; but it will no longer be “fair”, since this will no longer be the pure work of the navigation system itself.

Let's look at an example of how the AndroiTS navigation testing program works (there are analogues):

(click to enlarge)

The picture just presented shows that the smartphone works with three navigation systems: American GPS, Russian GLONASS and Chinese Beidou (BDS).

At the bottom of the screenshot you can see the successfully determined coordinates of the current location. The value of one degree in latitude is approximately 100 km; accordingly, the price of a unit of the lowest rank is 10 cm.

The value of one degree in longitude is different for different geographical locations. At the equator it is also about 100 km, and near the poles it decreases to 0 (at the poles the meridians move closer together).

To the right of the column indicating the nationality of the satellites is a column with satellite numbers. These numbers are strictly attached to them and do not change.

Next come columns with colored bars. The size of the bars indicates the signal level, and the color indicates whether the navigation system is using them or not. Unused satellites are indicated by gray bars. The color of those used depends on their signal level.

The next column is also the signal level from navigation satellites, but in numbers (“conventional units”).

Then there is a column with green checkmarks and red dashes - this is a repetition of information about whether the satellite is being used or not.

In the top line, the word "ON" indicates the status of the navigation state; in this case, this means that the determination of coordinates is allowed in the smartphone settings and they are determined. If the status is "WAIT", then determination of coordinates is allowed, but the required number of satellites has not yet been found. The "OFF" status means that coordinate determination is prohibited in the smartphone settings.

Then, a circle with concentric circles and the number 5 indicates the estimated accuracy of determining the coordinates at the moment - 5 m. This value is calculated based on the number and “quality” of satellites used and assumes that the processing of data from satellites in a smartphone is done without errors; but, as we will see later, this is not always the case.

As the satellites move, all this data should change, but the coordinates (in the bottom line) should change slightly.

Unfortunately, this application does not show the time spent on the initial determination of coordinates ("cold start"), and neither do other similar applications. This time must be “timed” manually. If the “cold start” time was less than a minute, then this is an excellent result; up to 5 minutes – good; up to 15 minutes – average; more than 15 minutes – bad.

To determine the “hot start” speed, just exit the testing program and log in again after a few minutes. As a rule, during the launch of the test program, it manages to determine the coordinates and immediately presents them to the user. If the delay in presenting coordinates during a “hot start” exceeds 10 seconds, then this is already suspiciously long.

The effect of quickly determining coordinates during a “hot start” is due to the fact that the navigation system remembers the last calculated satellite orbits and does not need to determine them again.

So, we’ve sorted out testing navigation in “static” mode.

Let's move on to the 2nd point of testing navigation - in motion.

The main purpose of navigation is to lead us to the right place while moving, and without testing while moving, the test would be incomplete.

In the process of movement, from a navigation point of view, there are three types of terrain: open terrain, urban areas and forest.

Open areas are ideal navigation conditions; there are no problems here (except for very “sucky” devices).

Urban development in most cases is characterized by the presence of a high level of reflections and a slight decrease in the signal level.

The forest “works” the other way around – a significant weakening of the signal and a low level of reflections.

First, let's look at a sample of an almost "ideal" track:

The picture shows two tracks: there/back (this will continue to be the case in almost all pictures). Such pictures allow us to make a reliable conclusion about the quality of navigation, since we can compare two almost identical tracks with each other and with the road. Everything is fine in this picture - the track vibrations are within the limits of natural error. In the upper part, the passage on different sides of the roundabout is adequately drawn. In some places, there is a noticeable discrepancy between the tracks, probably caused by signal reflections from the water surface and from the metal structures of the bridge over the river. And in some - an almost perfect coincidence.

Now let's look at several typical cases of "problem" tracks.

Let's look at the GPS track of a smartphone, which was affected by a decrease in signal level in a high forest:

The divergence of the tracks from each other and from the road is noticeable, but far from catastrophic. In this case, the accuracy of smartphone navigation decreased within the limits of “natural decline” for such conditions. Such a smartphone must be considered suitable for navigation purposes.

On the right side of the screenshot, the discrepancies between the tracks and the road are clearly visible. Such discrepancies in the conditions of such a “well-shaped” development are almost inevitable, and in this case they do not in any way indicate against the smartphone being tested.

Theoretically, than more systems navigation is supported by a smartphone (tablet), the more satellites it uses for navigation and the smaller the error should be.
In practice, this is not always the case. Quite often, due to crooked software, a smartphone cannot correctly connect data from different systems and, as a result, abnormal errors occur. Let's look at a few examples.

Take, for example, this track:

The screenshot just shown shows a needle-shaped ejection, which could not be the result of any interference: the path passed through a low-rise building without dense forested plantings. This release is entirely on the conscience of the “crooked” software.

But these were still “flowers”. There are smartphones where abnormal navigation errors are no longer flowers, but berries:

When recording this track, anomalous errors in the “crooked” software were combined with weakening of the signals in the high forest. The result is a track from which it is simply impossible to guess that the path there and back was taken along the same path by a sober person. :)
And the thick bunch of lines at the top is the “path” of a motionless smartphone during a rest stop. :)

There is another type of anomalous error associated with a pause in the data flow coming from the navigation receiver to the computing part of the smartphone:

This picture shows that part of the path (about 300 m) passed in a straight line, and partly directly through the water. :)

In this case, the smartphone simply connected the points where the coordinate stream disappeared and appeared with a straight line. Their loss could be associated either with a decrease in the number of visible satellites below a critical number, or with “crooked” software and even hardware problems (although the latter is unlikely).

In the event of a complete loss of signals from satellites, navigation programs usually do not connect the points of loss and reappearance with straight lines, but simply leave an “empty space” (this results in a gap in the track):

This picture shows a break in the track in the place where part of the path passed through an underground passage with a complete loss of visibility of all satellites.

After studying the causes and typical navigation errors, it’s time go to conclusions.

The best navigation, as you would expect, is found in smartphones and tablets of “high” brands. Problems in the form of anomalous errors have not yet been detected with them. And, of course, the more navigation systems a device supports, the better. True, support for the Chinese Beidou still makes sense when using the device in regions and countries located near the Middle Kingdom. The Chinese navigation system is not global, but “local” (for now). So support for GPS and GLONASS will be quite enough.

If a smartphone or tablet is not of very “renowned” origin, then there may or may not be problems with navigation. Before using it in combat, it is recommended to test it both statically and in motion in different environments, so that later it does not present any unpleasant surprise. In most cases mobile devices those with GPS support alone bring fewer problems, although their accuracy is lower than that of multi-system systems.

Unfortunately, when choosing a smartphone (tablet) with good navigation, it is quite difficult to navigate through reviews of devices on the Internet. The overwhelming number of IT portals ignore checking navigation on the move and in difficult conditions. This check is done only on this portal () and literally on a couple of others.

In conclusion It must be said that not only smartphones and tablets, but also many other devices are now equipped with navigation aids. They are installed, for example, in cameras, video cameras, GPS trackers, car video recorders, smart watches, some specialized types of devices, and even in electronic system taxation of drivers of Russian heavy trucks "Platon".

Your Doctor.
20.01.2017