What Wikipedia Do Not Write About Global Navigation Satellite Systems
Inspired by the Radio Wave Theory series of posts, I decided on a similar post on satellite positioning systems. I work in a structure that ensures the functioning of the GLONASS system, so I will try to talk about it and its competitors from a slightly different point of view. The post will be about their device, along the way I would like to dispel several myths.
I will try to do without laying out the common truths and information that anyone can learn from Wikipedia, but sometimes they can’t do 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 Earth orbit that emit a certain signal, which allows us to determine our coordinates anywhere in the world. In fact, any GNSS contains at least three components:
All other components, such as differential correction systems are not necessary, these are just options.
At the moment, only two systems, GPS and GLONASS, are fully deployed and generally available. There are at least four more GNSS in various stages of deployment. Since not one of them has been brought to an end, we will not talk about them, although most of what has been said applies to them too.
The subsystem NKA represents a number of satellites, coordinated moving in specially selected orbits. The main condition when choosing orbits is that at least 4 satellites should be visible at any point in the world at any given time (why exactly four will be explained below). Each device has an atomic clock — cesium, rubidium, or a combination thereof, depending on the modification — synchronized with the clock on the central synchronizer of the system. Synchronized - this does not mean that they are in phase, it means that the difference in the movement of the clock is known. It is the central synchronizer that stores the so-called system timeline . Our central synchronizer is located in Moscow suburbs, American in Podvashingtonnye, which is not surprising.
Each device emits a carrier oscillation in two frequency ranges L1 and L2. All GPS satellite systems emit at common frequencies, 1575.42 MHz and 1227.60 MHz for L1 and L2, respectively, and GLONASS satellite systems emit at separated frequencies, called literals (devices located at opposite points of the orbit emit on one letter). The difference between the letters is 562.5 kHz, for the subband L1 and 437.5 kHz for L2, the zero letter has frequencies of 1602 MHz and 1245 MHz, respectively.
The carrier wave is modulated by a special code sequence in such a way that the phase of the code signal coincides with the satellite clock (if you are interested, the phase modulation). In the GPS system, each device has a unique code sequence, which makes it possible to distinguish their signals, despite the common frequency. GLONASS uses frequency division, so all devices have the same code sequence. Additionally, satellite signals are modulated by navigation messages that contain parameters of a polynomial mathematical model of satellite motion and a model for the offset of satellite clock readings relative to the system time scale.
Signal structure of GLONASS spacecraft
Navigation messages also contain ionosphere parameters (allows you 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 a lot of other useful information. Simplified, the NKA subsystem is a network of synchronized clocks moving in space with coordinates known at any moment.
The ground-based control complex is a network of ground stations that determine the parameters of the motion of spacecraft, the parameters of the course of their hours. The points measure the parameters of the planet’s rotation, the parameters of the atmosphere, they specify the characteristics of the Earth’s gravitational field and store the world coordinate system. Functionally, the NKU includes a considerable number of research institutions and laboratories. Well, of course, it is the ground-based complex that processes and lays all these data on devices that already broadcast them as part of the navigation message.
The ground-based complex includes base points with calibrated receivers, and points of the federal astronomical and geodetic network, and ultra-long base radio interferometers, laser rangefinders, and many other interesting things. In general, the functions of the ground-based complex are very diverse, its activities are too extensive to include in this article. If anyone is interested, I’ll try to write an article about this as well.
The network of stations of the ground-based GLONASS 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 propagation time of the signal to each device. And the unknown are our coordinates and the difference in the receiver’s time scale with the system time scale, that is, four unknowns. By the way, contrary to a 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 spacecraft are determined precisely in this coordinate system. There are guest equations for conversion from the parameters x, y, z, to B, L, H (latitude, longitude, height).
It is clear that to determine the four unknowns, a system of equations with four or more equations is needed. That's why we need four visible devices. It is possible to determine by three devices, for this, an additional equation of the earth's ellipsoid (which connects x, y, z by the classical ellipsoid equation) is introduced into the system. But in this case, our position will be tied precisely to an ellipsoid, that is, we can’t talk about 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 transmitting the exact time is no less urgent task than determining the coordinates. At the moment, through GNSS, it is possible to transmit the exact time to any point on the globe with an accuracy of the order of ten nanoseconds, in special cases up to several nanoseconds. In this, they have practically no competitors, all other systems for transmitting the 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 using GNSS (of course, not only GNSS), which allows us to maintain a coordinated global time scale UTC, TAI, etc. However, the transmission of time and frequency,
Of course, this is a very simplified scheme 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.
I must say right away, here I’ll just consider the most common questions and misconceptions that I constantly encounter. Well, I’ll try to explain the real state of affairs, to the extent of my competence, of course.
The most common question.
To begin with, GLONASS is not all worse than GPS.
For example, in the polar regions, the GLONASS group provides better coverage, due to the more optimal configuration of the orbital group. However, in the equatorial regions the situation is the opposite for the same reason. Legs grow out of the military purpose of both systems, and the military interests of the Soviet Union and the United States were concentrated precisely in these areas.
In addition, the frequency separation of the signals really improves the noise immunity of the GLONASS system. The same frequency division brings many problems, but the fact remains that in the event of an armed conflict it will be more difficult to suppress our GNSS.
The system itself is constantly progressing. Although not as fast as we would like, even if it is accompanied by corruption scandals with some astronomical sums, the whole world admits that GLONASS is stable at a distance of four to five years behind GPS, and the gap does not widen. By the way, do not think that GPS is much cheaper, it also costs monstrous money, which is not always spent as it should.
So why is GLONASS behind? Few people know that the GLONASS system is olderGPS is several years old (formally the system itself is younger, but its prototypes appeared earlier and the development of the technology itself began earlier). The Americans, of course, watched its creation, and created their own, trying to take into account our mistakes, which were impossible to predict in any other way. Avoiding our system errors and not stopping development (unlike us, in the nineties, our entire satellite constellation almost ended up at the bottom of the Pacific Ocean) they turned from lagging behind to leading.
As you know, the NKA of both systems emit two types of signals: standard accuracy (ST code for GLONASS, C / A for GPS) and high accuracy (similar to the BT code and P / Y code). The GLONASS ST code is emitted in both frequency ranges, and the C / A GPS code is only in the L1 frequency range (with the exception of several new series of NSs). High precision signals are emitted in both frequency ranges. These signals differ in code sequence, while signals with a high-precision code have a wider band, which increases accuracy and makes it difficult to suppress.
Traditionally, high-precision signals are considered military, standard signals are considered civilian. This is only partially true. The code sequence of the P-code and BT-code is currently open for widespread use: the Americans officially published their code sequences, and at the same time ours (we will leave it behind the scenes from where they learned them). Therefore, now any manufacturer can freely create receivers that receive military signals (and they create, all precision equipment accepts all kinds of signals at all frequencies). The peculiarity is that, if necessary, these codes are changed according to a special algorithm, of course, classified. And after such a change of code sequences, only military equipment will be able to receive them, since this algorithm is initially sewn into it.
Moreover, if necessary, coding is also superimposed on the signals of standard accuracy, which does not interfere with the reception of these signals, but does not allow determining the position better than a couple of hundred meters in principle.
All these manipulations can be carried out 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 a normal GPS. Ours did the same during the conflict with Georgia, which did not cause much resonance, since GLONASS users could not be found in Georgia.
What is the system timeline I have already said. Also mentioned the world coordinated UTC timeline. Some people confuse all these concepts, I will try to separate flies from cutlets and explain what the differences are. The UTC coordinated world time scale is an analytical time scale (that is, it does not have physical implementation, it is kept “at the tip of the pen”), which is calculated by comparing time scales with the standards of time and frequency of all world time laboratories. Accordingly, the scales of the standards themselves in these laboratories are named after 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). The USNO American Naval Observatory (the world's most powerful time and frequency laboratory) maintains the UTC (USNO) scale, to which the central synchronizer of the GPS system is pulled. They pull it up, but there’s always a difference between the scales, of the order of a few nanoseconds, and this difference is transmitted in the navigation message of the GPS satellites. Thus, any GPS receiver can provide both a system timeline and a UTC (USNO) timeline. The situation is similar for the GLONASS and UTC (SU) system timelines. It’s just that the rotation of our planet is slowing down, and the UTC time scale is adjusted every second year by one second. A system timeline of the order of several nanoseconds, and this difference is transmitted in the navigation message of the GPS satellites. Thus, any GPS receiver can provide both a system timeline and a UTC (USNO) timeline. The situation is similar for the GLONASS and UTC (SU) system timelines. It’s just that the rotation of our planet is slowing down, and the UTC time scale is adjusted once every several years by one second. A system timeline of the order of several nanoseconds, and this difference is transmitted in the navigation message of the GPS satellites. Thus, any GPS receiver can provide both a system timeline and a UTC (USNO) timeline. The situation is similar for the GLONASS and UTC (SU) system timelines. It’s just that the rotation of our planet is slowing down, and the UTC time scale is adjusted once every several years by one second. A system timelineare not adjusted and the difference between system scales and world coordinated time at the moment is 16 seconds.
Thank you all for your attention, I hope it was interesting.
I will try to do without laying out the common truths and information that anyone can learn from Wikipedia, but sometimes they can’t do 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 Earth orbit that emit a certain signal, which allows us to determine our coordinates anywhere in the world. In fact, any GNSS contains at least three components:
- subsystem of navigation spacecraft (NKA)
- subsystem of the ground control complex (NKU)
- subsystem of navigation equipment of consumers (NAP)
All other components, such as differential correction systems are not necessary, these are just options.
At the moment, only two systems, GPS and GLONASS, are fully deployed and generally available. There are at least four more GNSS in various stages of deployment. Since not one of them has been brought to an end, we will not talk about them, although most of what has been said applies to them too.
How it works
The subsystem NKA represents a number of satellites, coordinated moving in specially selected orbits. The main condition when choosing orbits is that at least 4 satellites should be visible at any point in the world at any given time (why exactly four will be explained below). Each device has an atomic clock — cesium, rubidium, or a combination thereof, depending on the modification — synchronized with the clock on the central synchronizer of the system. Synchronized - this does not mean that they are in phase, it means that the difference in the movement of the clock is known. It is the central synchronizer that stores the so-called system timeline . Our central synchronizer is located in Moscow suburbs, American in Podvashingtonnye, which is not surprising.
Each device emits a carrier oscillation in two frequency ranges L1 and L2. All GPS satellite systems emit at common frequencies, 1575.42 MHz and 1227.60 MHz for L1 and L2, respectively, and GLONASS satellite systems emit at separated frequencies, called literals (devices located at opposite points of the orbit emit on one letter). The difference between the letters is 562.5 kHz, for the subband L1 and 437.5 kHz for L2, the zero letter has frequencies of 1602 MHz and 1245 MHz, respectively.
The carrier wave is modulated by a special code sequence in such a way that the phase of the code signal coincides with the satellite clock (if you are interested, the phase modulation). In the GPS system, each device has a unique code sequence, which makes it possible to distinguish their signals, despite the common frequency. GLONASS uses frequency division, so all devices have the same code sequence. Additionally, satellite signals are modulated by navigation messages that contain parameters of a polynomial mathematical model of satellite motion and a model for the offset of satellite clock readings relative to the system time scale.
Signal structure of GLONASS spacecraft
Navigation messages also contain ionosphere parameters (allows you 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 a lot of other useful information. Simplified, the NKA subsystem is a network of synchronized clocks moving in space with coordinates known at any moment.
The ground-based control complex is a network of ground stations that determine the parameters of the motion of spacecraft, the parameters of the course of their hours. The points measure the parameters of the planet’s rotation, the parameters of the atmosphere, they specify the characteristics of the Earth’s gravitational field and store the world coordinate system. Functionally, the NKU includes a considerable number of research institutions and laboratories. Well, of course, it is the ground-based complex that processes and lays all these data on devices that already broadcast them as part of the navigation message.
The ground-based complex includes base points with calibrated receivers, and points of the federal astronomical and geodetic network, and ultra-long base radio interferometers, laser rangefinders, and many other interesting things. In general, the functions of the ground-based complex are very diverse, its activities are too extensive to include in this article. If anyone is interested, I’ll try to write an article about this as well.
The network of stations of the ground-based GLONASS 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 (according to the code sequence for GPS and frequency for GLONASS).
- determination of the NCA clock readings at the time of radiation of the received signal by processing the code sequence. As mentioned above, the code sequence is synchronized with the onboard clock of the device.
- receiving a navigation message. This will give the following data: the position of the device and the difference in the movement of its clock and the system time scale. We can already determine the moment of emission of a signal by a satellite in the system time scale.
- determination of the receiver’s own clock at the time of receiving a signal from satellites. Thus, we determine the propagation time of the signal from the satellite to the receiver. But we will determine this time 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 propagation time of the signal to each device. And the unknown are our coordinates and the difference in the receiver’s time scale with the system time scale, that is, four unknowns. By the way, contrary to a 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 spacecraft are determined precisely in this coordinate system. There are guest equations for conversion from the parameters x, y, z, to B, L, H (latitude, longitude, height).
It is clear that to determine the four unknowns, a system of equations with four or more equations is needed. That's why we need four visible devices. It is possible to determine by three devices, for this, an additional equation of the earth's ellipsoid (which connects x, y, z by the classical ellipsoid equation) is introduced into the system. But in this case, our position will be tied precisely to an ellipsoid, that is, we can’t talk about 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 transmitting the exact time is no less urgent task than determining the coordinates. At the moment, through GNSS, it is possible to transmit the exact time to any point on the globe with an accuracy of the order of ten nanoseconds, in special cases up to several nanoseconds. In this, they have practically no competitors, all other systems for transmitting the 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 using GNSS (of course, not only GNSS), which allows us to maintain a coordinated global time scale UTC, TAI, etc. However, the transmission of time and frequency,
Of course, this is a very simplified scheme 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.
Disruption of the integument
I must say right away, here I’ll just consider the most common questions and misconceptions that I constantly encounter. Well, I’ll try to explain the real state of affairs, to the extent of my competence, of course.
Why is GLONASS so bad?
The most common question.
To begin with, GLONASS is not all worse than GPS.
For example, in the polar regions, the GLONASS group provides better coverage, due to the more optimal configuration of the orbital group. However, in the equatorial regions the situation is the opposite for the same reason. Legs grow out of the military purpose of both systems, and the military interests of the Soviet Union and the United States were concentrated precisely in these areas.
In addition, the frequency separation of the signals really improves the noise immunity of the GLONASS system. The same frequency division brings many problems, but the fact remains that in the event of an armed conflict it will be more difficult to suppress our GNSS.
The system itself is constantly progressing. Although not as fast as we would like, even if it is accompanied by corruption scandals with some astronomical sums, the whole world admits that GLONASS is stable at a distance of four to five years behind GPS, and the gap does not widen. By the way, do not think that GPS is much cheaper, it also costs monstrous money, which is not always spent as it should.
So why is GLONASS behind? Few people know that the GLONASS system is olderGPS is several years old (formally the system itself is younger, but its prototypes appeared earlier and the development of the technology itself began earlier). The Americans, of course, watched its creation, and created their own, trying to take into account our mistakes, which were impossible to predict in any other way. Avoiding our system errors and not stopping development (unlike us, in the nineties, our entire satellite constellation almost ended up at the bottom of the Pacific Ocean) they turned from lagging behind to leading.
Military codes
As you know, the NKA of both systems emit two types of signals: standard accuracy (ST code for GLONASS, C / A for GPS) and high accuracy (similar to the BT code and P / Y code). The GLONASS ST code is emitted in both frequency ranges, and the C / A GPS code is only in the L1 frequency range (with the exception of several new series of NSs). High precision signals are emitted in both frequency ranges. These signals differ in code sequence, while signals with a high-precision code have a wider band, which increases accuracy and makes it difficult to suppress.
Traditionally, high-precision signals are considered military, standard signals are considered civilian. This is only partially true. The code sequence of the P-code and BT-code is currently open for widespread use: the Americans officially published their code sequences, and at the same time ours (we will leave it behind the scenes from where they learned them). Therefore, now any manufacturer can freely create receivers that receive military signals (and they create, all precision equipment accepts all kinds of signals at all frequencies). The peculiarity is that, if necessary, these codes are changed according to a special algorithm, of course, classified. And after such a change of code sequences, only military equipment will be able to receive them, since this algorithm is initially sewn into it.
Moreover, if necessary, coding is also superimposed on the signals of standard accuracy, which does not interfere with the reception of these signals, but does not allow determining the position better than a couple of hundred meters in principle.
All these manipulations can be carried out 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 a normal GPS. Ours did the same during the conflict with Georgia, which did not cause much resonance, since GLONASS users could not be found in Georgia.
GPS, GLONASS, UTC scales
What is the system timeline I have already said. Also mentioned the world coordinated UTC timeline. Some people confuse all these concepts, I will try to separate flies from cutlets and explain what the differences are. The UTC coordinated world time scale is an analytical time scale (that is, it does not have physical implementation, it is kept “at the tip of the pen”), which is calculated by comparing time scales with the standards of time and frequency of all world time laboratories. Accordingly, the scales of the standards themselves in these laboratories are named after 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). The USNO American Naval Observatory (the world's most powerful time and frequency laboratory) maintains the UTC (USNO) scale, to which the central synchronizer of the GPS system is pulled. They pull it up, but there’s always a difference between the scales, of the order of a few nanoseconds, and this difference is transmitted in the navigation message of the GPS satellites. Thus, any GPS receiver can provide both a system timeline and a UTC (USNO) timeline. The situation is similar for the GLONASS and UTC (SU) system timelines. It’s just that the rotation of our planet is slowing down, and the UTC time scale is adjusted every second year by one second. A system timeline of the order of several nanoseconds, and this difference is transmitted in the navigation message of the GPS satellites. Thus, any GPS receiver can provide both a system timeline and a UTC (USNO) timeline. The situation is similar for the GLONASS and UTC (SU) system timelines. It’s just that the rotation of our planet is slowing down, and the UTC time scale is adjusted once every several years by one second. A system timeline of the order of several nanoseconds, and this difference is transmitted in the navigation message of the GPS satellites. Thus, any GPS receiver can provide both a system timeline and a UTC (USNO) timeline. The situation is similar for the GLONASS and UTC (SU) system timelines. It’s just that the rotation of our planet is slowing down, and the UTC time scale is adjusted once every several years by one second. A system timelineare not adjusted and the difference between system scales and world coordinated time at the moment is 16 seconds.
Thank you all for your attention, I hope it was interesting.