The GLONASS system is the largest navigation system that allows you to track the location of various objects. The project, launched in 1982, is still actively developing and improving. Moreover, work is being done both on the technical support of GLONASS and on the infrastructure that allows more and more people to use the system. So, if in the first years of the complex’s existence, navigation through satellites was used mainly in solving military problems, today GLONASS is a technological positioning tool that has become mandatory in the life of millions of civilian users.
Global Satellite Navigation Systems
Due to the technological complexity of global satellite positioning, today only two systems can fully correspond to this name - GLONASS and GPS. The first is Russian, and the second is the fruit of American developers. From a technical point of view, GLONASS is a complex of specialized hardware located both in orbit and on the ground.
To communicate with satellites, special sensors and receivers are used to read the signals and generate location data based on them. To calculate time parameters, special ones are used. They are used to determine the position of an object, taking into account the broadcast and processing of radio waves. Reducing errors allows for more reliable calculation of positioning parameters.
Satellite navigation features
The range of tasks of global satellite navigation systems includes determining exact location ground objects. In addition to geographic location, global navigation satellite systems allow you to take into account time, route, speed and other parameters. These tasks are realized through satellites located at different points above the earth's surface.
The use of global navigation is not limited to the transportation industry. Satellites help in search and rescue operations, geodetic and construction work, and coordination and maintenance of other space stations and vehicles is also essential. The military industry is also not left without the support of a system for similar purposes, providing a secure signal designed specifically for authorized equipment of the Ministry of Defense.
GLONASS system
The system began full operation only in 2010, although attempts to put the complex into active operation have been made since 1995. The problems were largely related to the low durability of the satellites used.
At the moment, GLONASS consists of 24 satellites that operate at different points in orbit. In general, the navigation infrastructure can be represented by three components: a control complex (provides control of the group in orbit), as well as user navigation equipment.
24 satellites, each of which has its own constant altitude, are divided into several categories. There are 12 satellites for each hemisphere. Through satellite orbits, a grid is formed over the earth's surface, through the signals of which precise coordinates are determined. In addition, satellite GLONASS also has several backup facilities. They are also each in their own orbit and are not idle. Their tasks include expanding coverage over a specific region and replacing failing satellites.
GPS system
The American analogue of GLONASS is a GPS system, which also began its work in the 1980s, but only since 2000 has the accuracy of determining coordinates made it possible for it to become widespread among consumers. Today, GPS satellites guarantee accuracy up to 2-3 m. The delay in the development of navigation capabilities has long been due to artificial positioning limitations. Nevertheless, their removal made it possible to determine the coordinates with maximum accuracy. Even when synchronized with miniature receivers, a result corresponding to GLONASS is achieved.
Differences between GLONASS and GPS
There are several differences between navigation systems. In particular, there is a difference in the nature of the arrangement and movement of satellites in orbits. In the GLONASS complex they move along three planes (eight satellites for each), and the GPS system provides for work in six planes (about four per plane). Thus, the Russian system provides wider coverage of the ground area, which is reflected in higher accuracy. However, in practice, the short-term “life” of domestic satellites does not allow using the full potential of the GLONASS system. GPS, in turn, maintains high accuracy due to the redundant number of satellites. Nevertheless, the Russian complex regularly introduces new satellites, both for targeted use and as backup support.
Different signal encoding methods are also used - Americans use CDMA code, and GLONASS uses FDMA. When receivers calculate positioning data, the Russian satellite system provides a more complex model. As a result, using GLONASS requires high energy consumption, which is reflected in the dimensions of the devices.
What do GLONASS capabilities allow?
Among the basic tasks of the system is determining the coordinates of an object capable of interacting with GLONASS. GPS in this sense performs similar tasks. In particular, the movement parameters of ground, sea and air objects are calculated. In a few seconds vehicle, provided by an appropriate navigator, can calculate the characteristics of its own motion.
At the same time, the use of global navigation has already become mandatory for certain categories of transport. If in the 2000s the spread of satellite positioning related to the control of certain strategic objects, today receivers are equipped with ships and aircraft, public transport, etc. In the near future, it is possible that all private cars will be required to be provided with GLONASS navigators.
What devices work with GLONASS
The system is capable of providing continuous global service to all categories of consumers without exception, regardless of climatic, territorial and time conditions. Like GPS system services, GLONASS navigator is provided free of charge and anywhere in the world.
Devices that can receive satellite signals include not only on-board navigation aids and GPS receivers, but also cell phones. Data on location, direction and speed of movement are sent to a special server via GSM operator networks. Helps in using satellite navigation capabilities special program GLONASS and various applications that process maps.
Combo receivers
The territorial expansion of satellite navigation has led to the merging of the two systems from the consumer's point of view. In practice, GLONASS devices are often complemented by GPS and vice versa, which increases the accuracy of positioning and timing parameters. Technically, this is realized through two sensors integrated into one navigator. Based on this idea, combined receivers are produced that work simultaneously with GLONASS, GPS systems and related equipment.
In addition to increasing the accuracy of determination, such a symbiosis makes it possible to track location when the satellites of one of the systems are not detected. The minimum number of orbital objects, the “visibility” of which is required for the navigator to operate, is three units. So, if, for example, the GLONASS program becomes unavailable, then GPS satellites will come to the rescue.
Other satellite navigation systems
The European Union, as well as India and China, are developing projects similar in scale to GLONASS and GPS. plans to implement a Galileo system consisting of 30 satellites, which will achieve unrivaled accuracy. In India, it is planned to launch the IRNSS system, operating through seven satellites. The navigation complex is oriented towards domestic use. The Compass system from Chinese developers should consist of two segments. The first will include 5 satellites, and the second - 30. Accordingly, the authors of the project envision two service formats.
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 of operation of GPS 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- definition exact location any object. In the context of a car, this task is carried out through a special device that helps 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 in such a way that, at certain regular intervals, it updates the received data and recalculates the distance.
The advantage of modern systems is that they have the ability to remember the satellite layout 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 in 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 merged into a single system called Navstar. It started out as a military development, but then the decision was made 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 their equipment uses. 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 operates 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 large number satellites in all orbital planes, other experts still give the palm to GPS. Even with an active improvement program Russian system At the moment, the Americans 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.
- 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 goal of the designers is to ensure that each receiving device is capable of communicating 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 will determine the location of a person in trouble with greater speed and clarity.
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:
Hello!
Unfortunately, I did not find any mention on Habré of a wonderful library for processing raw measurements - RTKLib. In this regard, I took the risk of writing a little about how you can use it to get centimeters in relative navigation.
The goal is simple - to attract public attention.
I myself only recently started working with this library and was amazed at its capabilities for mere mortals. There is a lot of information on the Internet about practical examples, but I wanted to try it myself - and this is the result.
So the process is general view looks like this:
Let's say we have two GLONASS/GPS receivers from which we can receive raw measurements. They are called raw because they are the primary material for processing - pseudo-ranges, Doppler, phase measurements...
Using the STRSVR utility from the RTKLib library, we need to record two data streams - one from the base station, which will stand still, and the second from the rover, which we plan to move. It is advisable to start recording from the base in advance, 10-15 minutes before recording the rover.
In my case, the base was on the roof of a building, and with the rover it went out onto the street. I used two laptops for recording.
1) Set up Input – Serial on both laptops, this is the stream from the GNSS receiver.
2) Output – File, this will be our file of raw measurements.
3) We start the base for recording – Start and slowly go to the open area.
For a small demonstration, I printed out an A4 sheet with the letter H, which I wanted to outline with the antenna, or rather with the base for mounting on a tripod. Antenna TW3440 manufactured by the Canadian company Tallysman with a custom underlying surface of 30x30 cm.
4) We position ourselves on the pavement, set the rover to record and try to slowly circle the letter. Even though the rover has an output frequency of 5Hz, it’s better to do everything carefully.
5) Upon completion of the stroke, we fold up and go see what happened.
6) We drop both files onto one computer and begin processing.
7) First, you need to obtain standard RINEX files from the raw data. RTKCONV will help us with this:
8) We indicate the path to the file with raw data, as well as the folder where the program will place RINEX, the raw data format, in my case it is NVS BINR and in the settings we check the GPS and GLO boxes, the rest can be left untouched.
9) Click Convert and get files for the rover and then for the base; it is better to place them in the corresponding Base and Rover folders.
11) Click Options, Settings 1 tab, in the mode settings we specify Kinematic to process relative measurements. We tick the GPS and GLO boxes, you can then play with the settings.
12) Output tab – you can set the output data format, for example NMEA.
13) An important point is the Positions tab, here you need to specify the coordinates of the base station, either take them from the header, or by averaging over the recording period. The more accurately we know the coordinates of the base, the more accurate the absolute coordinates of the rover will be.
For example, let's indicate RINEX Header Position - taken from the file header.
14) Click OK and go to the main window, there in the Rover field we indicate the path to the RINEX file of the rover, and for the database the path to the corresponding file. Click Execute and wait for the result. After processing, we can see the result by clicking on Plot.
15) From the figure below you can see that 97.3% of solutions with centimeter accuracy were obtained, the rest is a floating solution, the accuracy of which is much worse.
That's all for now.
If anyone is interested, I can write how to implement RTK mode.
It would also be nice to know your opinion: in what non-obvious applications can solutions with centimeter navigation be used?
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 widespread use GPS systems s all over 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 throughout 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 90 degree angles 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 data of the GPGSA.
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
This section does not provide a 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 a standard frequency of 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 errors systems are shown in the following table. Partial values are not constant values, but are subject to differences. All numbers are approximate values.
GLONASS/GPS for everyone: tests for accuracy and accessibility of positioning of a single-chip receiver in difficult operating conditions
Philip Mattos (Philip Mattos)
Translation: Andrey Rusak
support@site
Victoria Bulanova
[email protected]
The single-chip GNSS receiver, which has now entered mass production, was tested in dense urban environments to demonstrate the benefits of multi-system (GLONASS and GPS) operation as a consumer receiver. The use of the combined GLONASS/GPS system began with several tens of thousands of receivers for geodetic surveys; millions of such consumer devices are currently operating. Thanks to the growth in the number of personal satellite navigation devices, the emergence of automotive OEM systems and mobile phones, it was possible to achieve significant market volumes in 2011. Confidence in the prospects for the development of the market for navigation user devices is pushing manufacturers of high-frequency specific components, such as antennas and SAW filters, to increase production volumes and optimize the cost of goods. One of the first Russian companies to market modules based on the STM receiver was NAVIA. NAVIA GLONASS modules have already proven themselves as reliable, convenient modules for the production of ready-made navigation terminals and control of moving objects. Various module tests have shown that ML8088s and GL 8088s meet all the manufacturer’s stated characteristics and can be successfully used in monitoring devices.
Tests of a single-chip GLONASS/GPS receiver in London, Tokyo and Texas were carried out to show that the joint use of all visible GLONASS satellites coupled with GPS provides better positioning availability in dense urban areas, and in the case of poor positioning availability - better positioning. accuracy.
It is obvious that multi-system receivers are in great demand in the consumer market. They can ensure operation over a larger number of satellites in conditions of “urban canyons”, where only part of the celestial hemisphere is visible in the visibility zone and high reliability in filtering out unnecessary signals is required, when the quality of useful signals is greatly degraded due to multiple reflections and attenuations. The following briefly describes the difficulties of integrating the GLONASS system (and subsequently GALILEO), on the basis of which cost-effective devices are produced for the mass consumer. For such a market, on the one hand, cost comes first, and on the other hand, there are high performance requirements associated with low signal levels, limited power consumption, short cold start times and positioning stability.
The goal was to use all available satellites to improve the performance of consumer navigation devices in indoor and urban environments. 2011 passed under the auspices of GLONASS support; the development of this satellite system is approximately three years ahead of GALILEO. When designing receivers, it was important to overcome the problems of incompatibility of hardware support for GLONASS and GPS. That is, the frequency-modulated GLONASS signal required a wider frequency band than pulse-code modulation signals used by GPS, bandpass filters with different frequency centers and at different speeds transmission of signal elements. And all this without significantly increasing the cost of the receiver.
Under ideal operating conditions, satellites from additional constellations will be ineffective, since positioning availability I get close to 100 percent using only GPS. The presence in the ionosphere of seven, eight or nine satellites used for positioning in fixation mode minimizes the total error and gives correct coordinates.
In extreme operating conditions, the use of only GPS allows one to determine the position, but the use of only three, four, five satellites concentrated in a narrow part of the celestial hemisphere leads to poor DOP values. Increasing the number of satellites significantly improves accuracy, thereby improving DOP and averaging multipath errors. Limiting the number of positioned satellites leads to the imposition of multipath errors on the determination of the coordinates of the amplified DOPs. Adding a second or third satellite constellation involves expanding the number of visible satellites, and thus more satellites are involved in the coordinate determination process, which leads to a reduction in errors.
Therefore, in extreme conditions, where the use of GPS alone is not enough, the additional use of GLONASS satellites (and subsequently GALILEO) increases the availability of positioning to 100% (with the exception of underground tunnels).
In fact, accessibility is a self-improving loop of positive feedback: since the satellites are constantly tracked, even if they are rejected from participating in the current solution to the positioning problem using the RAIM / fault and FDE algorithms, there is no need to search for them again - they have already become available for use earlier. If the positioning process is not interrupted, then it is possible to continue to accurately predict phases for satellites with closed obstacles, which allows for instant use when leaving the shadows, since there is no need for reception additional information to find and record them.
Additional visible satellites are very important for the consumer, in particular - as an example, with “self-assistance”, when the minimum group is represented by five satellites, rather than three or four, in order to autonomously establish that all satellites are “correct” ,using receiver autonomous integrity monitoring (RAIM) techniques. “Self-service” has even more significant advantages for GLONASS: there is no need for any infrastructure such as assisted servers, which always lead to a delay in service. The GLONASS method of transmitting satellite orbit parameters in the Keplerian format is also very suitable for the “self-service” algorithm.
Test value
Previous attempts to characterize the benefits of multi-system devices in urban environments have been stalled by the need to use professional receivers not designed for such signal levels, and would have to obtain separate results for each group or sacrifice one of the satellite measurements to measure time. These circumstances did not allow us to continue testing the devices that were planned for release into the mass market.
The release of a new multi-system solution is of great importance, since the receiver under test is a truly mass-produced device if it has increased sensitivity and is completely ready for both measurement and calculation. Thus, the author of this article reports for the first time absolutely reliable test results.
Background
Tests were carried out on a single-chip GNSS receiver Teseo-II (STA-8088). Brief history: This is a 2009 product manufactured by STM, based on Cartesio+ with GPS/GALILEO and Digital Signal Processor (DSP) already included, it was ready to be implanted with GLONASS functionality, which led to the creation of the Teseo-II chip (2010 product ). Test results with real satellite signals were obtained on a Baseband chip in FPGA implementation already at the end of 2009, and in 2010 - already using a ready-made chip.
The current design required the introduction of additional minor circuit modifications. The required DSP hardware and software changes were minor and are included in the next scheduled TeseoII circuit update. The implementation of the RF part circuit required much more attention than the two-channel circuit with an intermediate frequency (IF) stage and an analog-to-digital converter (ADC), with additional frequency conversion and a wider bandwidth IF filter. But, since the area of the crystal with the RF part located on it is very small in the total volume, even a 30% increase in the circuit is insignificant for the entire circuit. According to the fact that the chip design is for a common single-chip system (RF and BB, from antenna to positioning, velocity and timing (PVT)), so the total die area for the 65nm process is very small.
From a commercial point of view, the inclusion of all three satellite constellations (GPS/GLONASS andGALILEO) into one chip is new for the consumer. Many of the companies present on the Russian market have settled on a two-system approach, just to satisfy the requirements of the Russian government about the need to work in the GLONASS system. They did not think about the global future, when there will be several positioning groupings in the world and perhaps each of the countries participating in this process will further put forward demands for the predominant use of their own system.
In this regard, the solutionTeseo— II is revolutionary because prepared in advance for such a scenario and can already receive GLONASS systems/ GPS/ GALILEO/ QZSSAndSBAS.
Technically, the inclusion of independent channels for receiving and processing the GLONASS system in a group is also new, while the GPS/GALILEO combination is already standard practice. Achieving such flexibility also required new technical solutions that take into account differing RF hardware delays and differences in signal transmission speeds. In addition to this, there are the now well-known Coordinated Universal Time (UTC) correction and the geoid correction problem.
A direct transition to a single-chip solution (RF + Baseband + CPU) is rare: this is an important technological breakthrough. Confidence in this step is due to the experience of using the RF part and the proven Baseband circuit of the processor. The basis was the external RF interface STA5630 and a modified GPS/GALILEO DSP, which were previously used in Cartesio+.
The reliability of the STA5630/Cartesio+ was proven in mass production in the form of separate circuits even before the release of 3-in-1 SoC solutions.
Unlike dual-chip solutionsGPS/GLONASS modules present on the Russian market, single-chip solution fromSTMicroelectronics (Teseo— II) S.T.A.8088 FG has much greater reliability, noise immunity, lower power consumption and, of course, smaller dimensions (module M.L.8088 shas dimensions 13 x 15 mm).
Support for GLONASS and GALILEO is a step forward compared to the previous generation of RF hardware. GALILEO is compatible with GPS and therefore the existing scheme could be used, but GLONASS required additional changes. See Figures 1 and 2.
Figure 1.
Figure 2.ChangesBaseband parts to support GLONASS
In the RF part, the LNA, RF amplifier and first mixer were combined into one channel. This allowed us to save on the number of chip pins and minimize power consumption. Moreover, this allowed to maintain external costs for equipment manufacturers. The GLONASS signal, reduced in the first mixer to 30 MHz, enters the secondary processing channel (shown in brown) and, mixed to 8 MHz, is fed to a separate ADC and then to the Baseband part.
The Baseband part provides an additional preliminary processing stage (indicated in brown), which converts the signal to 8 MHz, which is necessary for feeding into the Baseband and passes the resulting signal through an anti-interference filter, and also reduces the sampling frequency to the standard value of 16, suitable for processing in DSP hardware.
Existing acquisition devices and tracking channels can choose where and when to receive GPS/GALILEO or GLONASS signals, which makes the distribution of channels in relation to satellite constellations very flexible.
Less noticeable, but very important point in relation to system performance is the software that controls these hardware resources, firstly to close the PLL tracking loops and take measurements, and secondly, the Kalman filter, which converts the measured data into PVT data required by the user.
All this has undergone a structural modification to provide support for working with many satellite constellations, and not just GLONASS. In this case, the extension software to receive future global navigation systems will be a stage of evolutionary development, and will not require major modifications to the crystal itself.
The software had been running on a real chip since 2010, but using signals from any simulator or static roof-mounted antennas, only GPS data was available, which was so good that it did not allow any maneuvers for research to improve the system. At the beginning of 2011, pre-production samples of chips and development boards with antennas in the housing became available, which made it possible to conduct mobile field tests all over the world.
Actual results
Before the birth of the crystal with multi-system reception, the results were already visible from preliminary tests carried out using professional receivers with separate GPS and GLONASS measurements. However, these tests did not provide good data for a consumer receiver because they showed low sensitivity. The receivers required a sufficiently clean signal to drive the PLL, but this could not be done in an urban environment, and most importantly, the receivers created two separate solutions with a constant additional satellite to deal with inter-system timing differences. Unrelated solutions did not allow one to predict the position of satellites in one constellation by calculating their position based on coordinates calculated using another, which is one of the main advantages of multi-system GNSS receivers.
The simulation of visible satellites was carried out in 2010 in dense urban conditions in Italy, the center of Milan. The results, averaged every minute for a full 24 hours, are presented in Table 1. The average number of visible satellites increased from 4.4 with GPS only, to 7.8 for GPS+GLONASS, with the number of No Fix points equal to zero . Moreover, in the “GPS only” mode, 380 false points were received, which amounted to about 26% of the total reception time.
Table 1.Accuracy and AvailabilityGPSAndGPS+GLONASS, on average over 24 hours
However, the availability of satellites was not an end in itself. Having more satellites in the same small area of the celestial hemisphere over urban areas may not be sufficient due to the geometric reduction in accuracy. To examine this data, the geometric precision represented by HDOP. When using GLONASS and GPS together, the result was 2.5 times better.
Previous studies have shown that in individual test cities, two to three additional satellites were available, but one of them was used for timing. When using a highly sensitive receiver combined on one chip, we assumed that four or five additional satellites would be involved.
The actual results far exceeded our expectations. First, signals from many other satellites appeared, since all previous tests and simulations excluded reflected signals. Having additional signals, the receiver significantly improved DOP performance. The effect of reflections on accuracy was significantly reduced, firstly due to better positioning geometry, and secondly due to the ability of the FDE/RAIM algorithms to maintain satellite tracking stability. In addition, the number of false signals that can distort coordinate data has decreased.
The results presented here are obtained from a fully integrated high-sensitivity receiver such as the NAVIA ML8088s receiver, based on the STA8088s chip. It is optimized to detect even very low-level signals and obtain results directly from all satellites in view, regardless of constellation. This ensures 100% satellite availability and greatly improves accuracy in difficult urban environments.
Availability
The use of highly sensitive receivers that are independent of phase-locking loops (PLLs) ensures full accessibility in modern cities, even when reflected from glass surfaces in modern buildings. Therefore, some other definitions of availability than “four satellites are available” are now required. For example, tracking satellites at a given level of signal quality, the result of which depends on DOP. Even the DOP can be difficult to estimate because the Kalman filter assigns different weights to each satellite, which are not taken into account when calculating the DOP. And also, in addition to instant measurements, this filter uses the historical position and current speed, which leaves the positioning accuracy unchanged.
Figure 3 shows satellite availability in tracking mode. Testing took place in London's financial district in May 2011.
Tracked Satellites –GPS, GLONASS,GPS+GLONASS
Figure 3.GPS(marked in blue) against GLONASS (marked in red) and all tracked satellitesGNSS(marked in green).
As can be seen in Fig. 3, in total there are 7-8 GLONASS satellites and 8-9 GPS satellites, that is, multi-GNSS - about 16 satellites. There was a period when satellite signals were not picked up: during the passage of the Blackfriars Underpass tunnel, time stamp approximately 156400 seconds. In other areas of the city, at approximately 158,500 and 161,300 seconds, visibility dropped to four satellites, but their total number was never less than eight. It should be noted that the testing took place in the old city, where there are mainly stone buildings, so the reflective signals are weaker than from glass and metal buildings.
Although satellite availability is 100% outside tunnels, it may be limited by DOP or positioning accuracy. As can be seen in Figure 4, from other tests in London, multi-GNSS DOP remains below 1, as it should be with 10-16 visible satellites, while GPS-only DOP is often above 4, with no distortion Due to reflections and weak signals, the DOP is significantly increased to 10 at peak.
GPScompared toGNSS
Figure 4.OnlyGPSagainst combinedGPS/GLONASS accuracy reduction indicators
Since the tests conducted in May 2011 were light enough to create stressful conditions under which GPS would need multi-GNSS support, new testing was conducted in August 2011. As shown in the aerial photograph (Fig. 5), the tests were carried out in the modern high-rise part of the city, Canary Wharf. Additionally, the roads in the city are very narrow, which made the city's challenges even more difficult. Glass and metal buildings in the modern part of the city tend to give better reflection than stone buildings, causing the RAIM and FDE algorithms to go off the charts.
Figure 5. GPS vs GNSS, London, Canary Wharf
Obtaining GPS only results was difficult (shown in green), especially in the closed part of Docklands station, center left, bottom track.
Figure 6 shows the same real test results displayed on a schematic road map.
Figure 6. GPS vs GNSS, London, Canary Wharf, sketch map
Multi-GNSS testing (blue) showed very good results, especially on the northern (eastbound) part of the loop (driving in the UK is on the left, so clockwise creates a one-way loop).
Figure 7. a) Tests in Tokyo: Teseo-I (GPS) versus Teseo-II (GNSS); b) DOP when tested in Tokyo
Further testing was carried out at STMicroelectronics offices around the world. Figure 7a shows tests in Tokyo, where yellow indicates the test results of the previous generation of chips without GLONASS, and red indicates Teseo-II with GPS+GLONASS.
Figure 7b provides some clarification of the accuracy definition by showing the DOP over the course of the test. It can be seen that the Teseo-II DOPs were rarely higher than 2, but the GPS-only (Teseo-I) DOPs were between 6 and 12 in the circled northern compound.
We repeat that the test algorithm is simple for GPS, but the accuracy of the determination is difficult.
Further testing in Tokyo was carried out on narrower city streets under the same testing conditions, shown in Figure 9. Blue - GPS only, red - GPS+GLONASS, significant improvement in results is observed.
Figure 9 uses the same color scheme to display the Dallas test results, this time with a competitor's GPS receiver versus Teseo-II in a GPS+GLONASS configuration, again seeing very good results.
Figure 8. OnlyGPS(blue) vs multi-GNSS(red), Tokyo.
Figure 9. OnlyGPS(blue, competitor manufacturer's receiver) compared toGNSS(red), Dallas.
Other satellite constellations
Although the hardwareTeseo— IIsupports andGALILEO, no satellites available yetGALILEO(as of September 2011), so devices based on this chip in use around the world still do not have the software loaded to service this satellite constellation. However, if the time comes to use GALILEO, there is always the opportunity to update the software.
The Japanese QZSS system has one satellite available, transmitting traditional GPS-compatible signals, SBAS signals and L1C BOC signals. Teseo-II, with the help of the functions of the currently loaded software, can handle the first two of them, and while the use of SBAS is useless in urban environments, since signal reflections and interference are local and undetectable, the purpose of the QZSS system is to provide a satellite with a very high angle so that this satellite was always available in urban areas.
Figure 10 shows a test in Taipei (Taiwan) using GPS (yellow) versus multi-GNSS (GPS plus one QZSS satellite (red)) and ground truth (purple).
Figure 10. OnlyGPS(yellow) versus multi-GNSS (GPS+
QZSS (1 satellite, red)), true value -lilac, Taipei
Further work
Testing will continue to obtain more accurate quantitative results. Testing will take place in the UK, where there are road maps with vector data to display real travel directions. It is planned to modify the hardware to support the Compass system and GPS-III (L1-C), in addition to the existing GALILEO. Finding and tracking these signals has already been demonstrated using pre-recorded broadcast script samples on GNSS signal simulators.
Compass was not available in 2011. In this regard, work on the silicon implementation of Teseo-II was focused mainly on maximum flexibility in the conditions of different code lengths, for example, BOC or BPSK, which made it possible, with one or another loaded software for configuring the DSP hardware functions, gain compatibility between different satellite constellations.
Compatibility work on the current version of the multi-GNSS CHIP has been weak: Because the Compass system's 1561 MHz center frequency can only be maintained using a voltage-controlled oscillator and PLL, the Compass system cannot operate simultaneously with other satellite constellations. In addition, the code transmission rate in the Compass system is 2 million bps, which is also not supported by Teseo-II and can be brought to standard through the use of external alternative circuits, which means serious signal losses.
So Compass support work is only relevant for research and software development, for a single system solution, or using a separate RF chip.
The worldwide Compass signal, which is in GPS/GALILEO signal format at carrier frequency and at code length and rate, will be fully compatible within a single multi-GNSS circuit, but most likely not before 2020.
Tests in urban conditions will be repeated as the group developsGALILEO. If there are 32 channels, you can use 11/11/10 division (GPS/ GALILEO/GLONASS), in the presence of a full complement of all three groups, but within the framework of modern requirements for navigation services, the combination 14/8/10 is more than sufficient.
Conclusion
A multi-system receiver can include GPS, GLONASS and GALILEO at minimally increased cost. With 32 tracking channels and up to 22 visible satellites, even in the harshest urban environments, 100% availability and acceptable positioning accuracy can be ensured. During testing, 10–16 satellites are typically visible. Multiple measurements make RAIM and FDE algorithms much more effective at eliminating poorly reflected signals, while also minimizing the geometric effects of remaining signal distortion.
Recently, with the development of the Russian GLONASS, the needs of the navigation market for multi-system receivers are only growing. A number of domestic companies use single-chip chips STM to develop your own GLONASS modules and ready-made packaged devices. In particular, in 2011, the NAVIA company released 2 combined GLONASS/ GPS/ Galileomodules, tests of which showed very good results.
Instant or integral availability(English) Availability – represents the % of time during which the PDOP condition is satisfied<=6 при углах места КА >= 5 degrees. A simple example: in the old days, before 2010, GLONASS availability in some areas of the globe was no higher than 70-80%, but now it is 100% everywhere!)
Reduced accuracy or Geometric Accuracy Reduction(English) Dilution of precision, DOP, English Geometric Dilution of Precision (GDOP)
RAIM(English) Receiver Autonomous Integrity Monitoring Autonomous Receiver Integrity Monitoring (ARIC), a technology designed to evaluate and maintain the integrity of the GPS system and GPS receiver. This is especially important in cases where the correct operation of GPS systems is necessary to ensure an adequate level of safety, for example in aviation or maritime navigation.