What is not written in Wikipedia on global navigation satellite systems
Inspired by the “Theory of Radio Waves” series of posts, I decided on a similar post about satellite positioning systems. I work in a structure that deals with the functioning of the GLONASS system, so I will try to tell about it and its competitors from a slightly different point of view. The post will be about their device, in passing, I would like to dispel a few myths.
I will try to do without laying out the truisms and information that anyone can learn in Wikipedia, but sometimes not without them, please treat with understanding.
You all know what global navigation satellite systems are. The most common opinion is that this is a number of satellites in near-Earth orbit, which emit a certain signal, which allows us to determine our coordinates at any point on the globe. In fact, any GNSS contains at least three components:
All other components, such as differential correction systems are not necessary, they are only options.
At the moment, only two systems, GPS and GLONASS, are fully deployed and publicly available. There are at least four other GNSSs at different stages of deployment. Since not one of them has been fully completed, we will not talk about them, although most of what has been said also applies to them.
The satellite subsystem is a certain number of satellites moving in concert along specially selected orbits. The main condition for choosing orbits is that at least 4 satellites should be visible at any point in the world at any given time (why four will be explained below). Atomic clocks are installed on each of the devices — cesium, rubidium, or a combination of the two; depending on the modification — synchronized with the clock on the central synchronizer of the system. Synchronized - this does not mean that they go in phase, it means that the difference of the clock is known. It is the central synchronizer that stores the so-called system time scale . Our central synchronizer is located in the Moscow region, American in Podvashingtonye, which is not surprising.
Each unit emits a carrier oscillation in two frequency bands L1 and L2. All NKA GPS systems emit at common frequencies, 1,575.42 MHz and 1,227.60 MHz for L1 and L2, respectively, and GLAASS systems emit at separated frequencies, called letters (devices located at opposite points of the orbit emit at one letter). The difference between the letters is 562.5 kHz, for the L1 subband and 437.5 kHz for the L2, the zero letter has frequencies of 1602 MHz and 1245 MHz, respectively.
The carrier oscillation is modulated by a special code sequence in such a way that the phase of the code signal coincides with the readings of the satellite clock (if anyone is interested - phase modulation). In the GPS system, each device has a unique code sequence that allows you to distinguish their signals, despite the common frequency. In GLONASS, frequency division is used, so all devices have the same code sequence. Additionally, satellite signals are modulated with navigation messages that contain the parameters of a polynomial mathematical model of satellite motion and a model of the satellite clock readings relative to the system time scale.
The signal structure of the GLONASS spacecraft
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Navigation messages also contain the parameters of the ionosphere (it allows to take into account the delay of signals in the ionosphere), the difference between the system time scale and the world coordinated time scale, and much more of any other useful information. Simplified, the subsystem of the NKA is a network of synchronized, moving in space clocks, with coordinates known at any moment.
The ground control complex is a network of ground stations that determine the parameters of the motion of spacecraft, the parameters of their clocks. At the points, measurements are made of the parameters of the planet’s rotation, the parameters of the atmosphere, they specify the characteristics of the Earth’s gravitational field and provide storage of the world coordinate system. Functionally, the GCC includes a considerable number of research institutions and laboratories. Well, of course, it’s the ground complex that processes all this data and lays it on the devices that are already broadcasting them as part of the navigation message.
The ground complex consists of base points with calibrated receivers, points of the federal astronomical and geodetic network, radio interferometers with a super-long base, and laser range finders, and many other interesting things. In general, the functions of the ground complex are very diverse, its activities are too extensive to include in this article. If someone is interested, I will try to write an article about it.
Station network of the GLONASS ground control complex
Well, actually the navigation equipment of consumers receives and processes the signals of the NKA system. Receiving a signal from all visible devices, the receiver performs the following functions (simplified scheme):
So, we have the position of each device, the time of signal propagation to each device. And the unknowns are our coordinates and the difference in the time scale of the receiver with the system time scale, that is, four unknowns. By the way, despite the common misconception, the receiver determines the coordinates not in the form of latitude, longitude and height, but in the form of x, y, z coordinates in the geocentric Cartesian coordinate system associated with the center of mass of the planet. This is due to the fact that the coordinates of the spacecraft are determined precisely in this coordinate system. There are lodged conversion equations from the parameters x, y, z, to B, L, H (latitude, longitude, altitude).
It is clear that a system of equations with four or more equations is necessary to determine four unknowns. That's why we need four visible apparatus. It is possible to determine by three devices; for this, an additional equation of the terrestrial ellipsoid is introduced into the system (which connects x, y, z with the classical equation of the ellipsoid). But in this case our position will also be tied to an ellipsoid, that is, we cannot speak of height.
In any case, the result of solving this system of equations will be our coordinates and the position of the system time scale. The latter is sometimes forgotten, although the transfer of the exact time is no less relevant than determining the coordinates. At the moment, by means of GNSS it is possible to transmit the exact time to any point on the globe with an accuracy of about ten nanoseconds, in special cases up to a few nanoseconds. They have practically no competitors in this, all other systems of transfer of exact time are either much more expensive or much worse. All world time laboratories, all national standards of time and frequency (including ours) are compared by means of GNSS (of course, not only GNSS), which allows us to conduct the world coordinated time scale UTC, TAI, etc. However, the transmission of time and frequency World time scales are a separate conversation.
Of course, this is a greatly simplified diagram of the operation of navigation systems; you can talk about any component for a very long time. So if anyone is interested, I’m ready to delve into any aspect of GNSS work.
At once I will say, here I’ll just consider the most common questions and misconceptions that I constantly encounter. Well, I will try to explain the real state of affairs, to the best of my competence, of course.
The most common question.
To begin with, GLONASS is not at all worse than GPS.
For example, in the polar regions, the GLONASS grouping provides better coverage, due to the more optimal configuration of the orbital grouping. But in the equatorial regions, the situation is reversed for the same reason. Legs grow from the military purpose of both systems, and the military interests of the Soviet Union and the United States were concentrated in precisely these areas.
In addition, the frequency separation of signals really improves the noise immunity of the GLONASS system. The same frequency division pulls along with itself and a lot of problems, but the fact remains that in case of armed conflict it will be more difficult to suppress our GNSS.
The system itself is continuously progressing. Let it not be as fast as we would like, let it be accompanied by corruption scandals with some astronomical sums, but the whole world recognizes that GLONASS steadily keeps at a distance of four to five years behind the GPS, and the gap does not widen. By the way, do not think that GPS is much cheaper, it is also worth the monstrous money, which is not always spent properly.
So why is GLONASS behind? Few people know that the GLONASS system is older than GPS by several years (formally, the system itself is younger, but its prototypes appeared earlier and the technology itself began to work earlier). Americans, of course, watched its creation, and created their own, trying to accommodate our mistakes, which it was impossible to predict in any other way. Having avoided our system errors, and without stopping the development (as opposed to us, in the nineties, our entire satellite constellation nearly ended up at the bottom of the Pacific Ocean) they turned from lagging to leading.
As is known, the NKA of both systems emit two types of signals: standard accuracy (CT code for GLONASS, C / A for GPS) and high accuracy (similar to BT code and P / Y code). The GLONASS CT code is radiated in both frequency ranges, and the GPS C / A code only in the L1 frequency range (except for a few new satellite series). High precision signals are emitted in both frequency ranges. These signals are distinguished by a code sequence, while signals with a high precision code have a wider bandwidth, which improves accuracy and makes suppression more difficult.
Traditionally, high precision signals are considered military, standard signals are considered civil. This is only partly true. The code sequence of the P-code and the BT-code is currently open for wide use: the Americans officially published their code sequences, and at the same time ours (from where they learned them, we leave behind the scenes). Therefore, now any manufacturer is completely free to create receivers that receive military signals (and create, all precision equipment receives all types of signals at all frequencies). The peculiarity is that, if necessary, these codes change according to a special algorithm, of course a secret one. And after such a change in the code sequences, only military equipment will be able to receive them, since this algorithm is initially sewn into it.
Moreover, if necessary, coding is superimposed on standard accuracy signals, which does not interfere with receiving these signals, but does not allow determining the position better than a couple of hundred meters in principle.
All these manipulations can be made not globally, but only over a certain region of the globe, which the Americans demonstrated during the war in Iraq, depriving the entire Middle East of normal GPS. Ours did the same during the conflict with Georgia, which did not cause much resonance, since no GLONASS users could be found in Georgia.
What is the system time scale I have already told. Also mentioned the world coordinated time scale UTC. Some people confuse all these concepts, I will try to separate flies from cutlets and explain the differences. The UTC World Coordinated Time Scale is an analytical time scale (that is, it has no physical implementation, is conducted “at the tip of the pen”), which is calculated by comparing time scales from time standards and frequency of all world time laboratories. Accordingly, the scales of the standards themselves in these laboratories are called by the name of the country or institution. For example, the scale of our national standard is called UTC (SU) (SU, because almost all countries of the former Soviet Union live on the same scale), the scale of the American Institute of Standards NIST is called UTC (NIST). At the USNO US Naval Observatory (the most powerful time and frequency laboratory in the world), the UTC (USNO) scale is maintained, to which the central synchronizer of the GPS system is pulled. They pull up, but the difference between the scales is always there, on the order of several nanoseconds, and this difference is transmitted in the navigation message of the GPS satellites. Thus, any GPS receiver can output both the system time scale and the UTC time scale (USNO). The situation is similar for the GLONASS and UTC (SU) system time scale. The only thing is that the rotation of our planet is slowing down, and the UTC time scale is adjusted once every few years for one second. And the system time scales are not adjusted and the difference between the system scales and the world coordinated time at the moment is 16 seconds.
Thank you all for your attention, I hope that was interesting.
I will try to do without laying out the truisms and information that anyone can learn in Wikipedia, but sometimes not without them, please treat with understanding.
System structure
You all know what global navigation satellite systems are. The most common opinion is that this is a number of satellites in near-Earth orbit, which emit a certain signal, which allows us to determine our coordinates at any point on the globe. In fact, any GNSS contains at least three components:
- subsystem of navigation spacecraft (NCA)
- subsystem of ground control complex (NKU)
- subsystem of consumer navigation equipment (NAP)
All other components, such as differential correction systems are not necessary, they are only options.
At the moment, only two systems, GPS and GLONASS, are fully deployed and publicly available. There are at least four other GNSSs at different stages of deployment. Since not one of them has been fully completed, we will not talk about them, although most of what has been said also applies to them.
How it works
The satellite subsystem is a certain number of satellites moving in concert along specially selected orbits. The main condition for choosing orbits is that at least 4 satellites should be visible at any point in the world at any given time (why four will be explained below). Atomic clocks are installed on each of the devices — cesium, rubidium, or a combination of the two; depending on the modification — synchronized with the clock on the central synchronizer of the system. Synchronized - this does not mean that they go in phase, it means that the difference of the clock is known. It is the central synchronizer that stores the so-called system time scale . Our central synchronizer is located in the Moscow region, American in Podvashingtonye, which is not surprising.
Each unit emits a carrier oscillation in two frequency bands L1 and L2. All NKA GPS systems emit at common frequencies, 1,575.42 MHz and 1,227.60 MHz for L1 and L2, respectively, and GLAASS systems emit at separated frequencies, called letters (devices located at opposite points of the orbit emit at one letter). The difference between the letters is 562.5 kHz, for the L1 subband and 437.5 kHz for the L2, the zero letter has frequencies of 1602 MHz and 1245 MHz, respectively.
The carrier oscillation is modulated by a special code sequence in such a way that the phase of the code signal coincides with the readings of the satellite clock (if anyone is interested - phase modulation). In the GPS system, each device has a unique code sequence that allows you to distinguish their signals, despite the common frequency. In GLONASS, frequency division is used, so all devices have the same code sequence. Additionally, satellite signals are modulated with navigation messages that contain the parameters of a polynomial mathematical model of satellite motion and a model of the satellite clock readings relative to the system time scale.
The signal structure of the GLONASS spacecraft
')
Navigation messages also contain the parameters of the ionosphere (it allows to take into account the delay of signals in the ionosphere), the difference between the system time scale and the world coordinated time scale, and much more of any other useful information. Simplified, the subsystem of the NKA is a network of synchronized, moving in space clocks, with coordinates known at any moment.
The ground control complex is a network of ground stations that determine the parameters of the motion of spacecraft, the parameters of their clocks. At the points, measurements are made of the parameters of the planet’s rotation, the parameters of the atmosphere, they specify the characteristics of the Earth’s gravitational field and provide storage of the world coordinate system. Functionally, the GCC includes a considerable number of research institutions and laboratories. Well, of course, it’s the ground complex that processes all this data and lays it on the devices that are already broadcasting them as part of the navigation message.
The ground complex consists of base points with calibrated receivers, points of the federal astronomical and geodetic network, radio interferometers with a super-long base, and laser range finders, and many other interesting things. In general, the functions of the ground complex are very diverse, its activities are too extensive to include in this article. If someone is interested, I will try to write an article about it.
Station network of the GLONASS ground control complex
Well, actually the navigation equipment of consumers receives and processes the signals of the NKA system. Receiving a signal from all visible devices, the receiver performs the following functions (simplified scheme):
- separation of the signal from each satellite (by code sequence for GPS and by frequency for GLONASS).
- determination of the readings of the NKA clock at the time of emission of the received signal by processing the code sequence. As mentioned above, the code sequence is synchronized with the vehicle’s onboard clock.
- receive navigation message. This will give the following data: the position of the device and the difference in the course of its clock and system time scale. We can already determine the moment of radiation of a signal by a satellite in the system time scale.
- determination of the receiver’s own clock readings at the time of receiving the signal from the satellites. Thus, we determine the propagation time of the signal from the satellite to the receiver. But this time we will determine with an error equal to the difference in the clock of the receiver and the system time scale. Obviously, this error will be the same for all devices.
So, we have the position of each device, the time of signal propagation to each device. And the unknowns are our coordinates and the difference in the time scale of the receiver with the system time scale, that is, four unknowns. By the way, despite the common misconception, the receiver determines the coordinates not in the form of latitude, longitude and height, but in the form of x, y, z coordinates in the geocentric Cartesian coordinate system associated with the center of mass of the planet. This is due to the fact that the coordinates of the spacecraft are determined precisely in this coordinate system. There are lodged conversion equations from the parameters x, y, z, to B, L, H (latitude, longitude, altitude).
It is clear that a system of equations with four or more equations is necessary to determine four unknowns. That's why we need four visible apparatus. It is possible to determine by three devices; for this, an additional equation of the terrestrial ellipsoid is introduced into the system (which connects x, y, z with the classical equation of the ellipsoid). But in this case our position will also be tied to an ellipsoid, that is, we cannot speak of height.
In any case, the result of solving this system of equations will be our coordinates and the position of the system time scale. The latter is sometimes forgotten, although the transfer of the exact time is no less relevant than determining the coordinates. At the moment, by means of GNSS it is possible to transmit the exact time to any point on the globe with an accuracy of about ten nanoseconds, in special cases up to a few nanoseconds. They have practically no competitors in this, all other systems of transfer of exact time are either much more expensive or much worse. All world time laboratories, all national standards of time and frequency (including ours) are compared by means of GNSS (of course, not only GNSS), which allows us to conduct the world coordinated time scale UTC, TAI, etc. However, the transmission of time and frequency World time scales are a separate conversation.
Of course, this is a greatly simplified diagram of the operation of navigation systems; you can talk about any component for a very long time. So if anyone is interested, I’m ready to delve into any aspect of GNSS work.
Disruption of integument
At once I will say, here I’ll just consider the most common questions and misconceptions that I constantly encounter. Well, I will try to explain the real state of affairs, to the best of my competence, of course.
Why is GLONASS so bad?
The most common question.
To begin with, GLONASS is not at all worse than GPS.
For example, in the polar regions, the GLONASS grouping provides better coverage, due to the more optimal configuration of the orbital grouping. But in the equatorial regions, the situation is reversed for the same reason. Legs grow from the military purpose of both systems, and the military interests of the Soviet Union and the United States were concentrated in precisely these areas.
In addition, the frequency separation of signals really improves the noise immunity of the GLONASS system. The same frequency division pulls along with itself and a lot of problems, but the fact remains that in case of armed conflict it will be more difficult to suppress our GNSS.
The system itself is continuously progressing. Let it not be as fast as we would like, let it be accompanied by corruption scandals with some astronomical sums, but the whole world recognizes that GLONASS steadily keeps at a distance of four to five years behind the GPS, and the gap does not widen. By the way, do not think that GPS is much cheaper, it is also worth the monstrous money, which is not always spent properly.
So why is GLONASS behind? Few people know that the GLONASS system is older than GPS by several years (formally, the system itself is younger, but its prototypes appeared earlier and the technology itself began to work earlier). Americans, of course, watched its creation, and created their own, trying to accommodate our mistakes, which it was impossible to predict in any other way. Having avoided our system errors, and without stopping the development (as opposed to us, in the nineties, our entire satellite constellation nearly ended up at the bottom of the Pacific Ocean) they turned from lagging to leading.
Military codes
As is known, the NKA of both systems emit two types of signals: standard accuracy (CT code for GLONASS, C / A for GPS) and high accuracy (similar to BT code and P / Y code). The GLONASS CT code is radiated in both frequency ranges, and the GPS C / A code only in the L1 frequency range (except for a few new satellite series). High precision signals are emitted in both frequency ranges. These signals are distinguished by a code sequence, while signals with a high precision code have a wider bandwidth, which improves accuracy and makes suppression more difficult.
Traditionally, high precision signals are considered military, standard signals are considered civil. This is only partly true. The code sequence of the P-code and the BT-code is currently open for wide use: the Americans officially published their code sequences, and at the same time ours (from where they learned them, we leave behind the scenes). Therefore, now any manufacturer is completely free to create receivers that receive military signals (and create, all precision equipment receives all types of signals at all frequencies). The peculiarity is that, if necessary, these codes change according to a special algorithm, of course a secret one. And after such a change in the code sequences, only military equipment will be able to receive them, since this algorithm is initially sewn into it.
Moreover, if necessary, coding is superimposed on standard accuracy signals, which does not interfere with receiving these signals, but does not allow determining the position better than a couple of hundred meters in principle.
All these manipulations can be made not globally, but only over a certain region of the globe, which the Americans demonstrated during the war in Iraq, depriving the entire Middle East of normal GPS. Ours did the same during the conflict with Georgia, which did not cause much resonance, since no GLONASS users could be found in Georgia.
GPS, GLONASS, UTC scales
What is the system time scale I have already told. Also mentioned the world coordinated time scale UTC. Some people confuse all these concepts, I will try to separate flies from cutlets and explain the differences. The UTC World Coordinated Time Scale is an analytical time scale (that is, it has no physical implementation, is conducted “at the tip of the pen”), which is calculated by comparing time scales from time standards and frequency of all world time laboratories. Accordingly, the scales of the standards themselves in these laboratories are called by the name of the country or institution. For example, the scale of our national standard is called UTC (SU) (SU, because almost all countries of the former Soviet Union live on the same scale), the scale of the American Institute of Standards NIST is called UTC (NIST). At the USNO US Naval Observatory (the most powerful time and frequency laboratory in the world), the UTC (USNO) scale is maintained, to which the central synchronizer of the GPS system is pulled. They pull up, but the difference between the scales is always there, on the order of several nanoseconds, and this difference is transmitted in the navigation message of the GPS satellites. Thus, any GPS receiver can output both the system time scale and the UTC time scale (USNO). The situation is similar for the GLONASS and UTC (SU) system time scale. The only thing is that the rotation of our planet is slowing down, and the UTC time scale is adjusted once every few years for one second. And the system time scales are not adjusted and the difference between the system scales and the world coordinated time at the moment is 16 seconds.
Thank you all for your attention, I hope that was interesting.
Source: https://habr.com/ru/post/164185/
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