What not to write in Wikipedia on global navigation satellite systems
Inspired by the series of posts "the Theory of radio waves", I decided to take a similar post about the systems of satellite positioning. I work at the institution, which is engaged in the operation of the GLONASS system, so I will try to tell about it and its competitors with a slightly different point of view. Will post it on their device, simultaneously I would like to dispel a few myths.
will Try to do without putting some truths and information that anyone can gather at Wikipedia, but sometimes they cannot be avoided, please understand.
You all know what is global navigation satellite system. The most widely believed that is a number of satellites orbiting the earth which emit a signal that allows us to determine your coordinates at any point of the globe. In fact, any GNSS contains at least three components:
the
All other components such as system differential corrections are not necessary, it is only option.
Now fully deployed and available to the public only two systems, GPS and GLONASS. There are at least four GNSS are in various stages of deployment. As the end none of them brought, to talk about them we won't, although most of it is written to them too.
Subsystem NKA is the number of satellites, moving in concert on a specially selected orbits. The main condition when choosing the orbits in any point of the planet at any point in time should be seen at least 4 satellites (why four, will be explained below). On each of apparatuses installed atomic clock — cesium, rubidium, or a combination thereof, depending on the modification is synchronized with clock on Central system synchronizer. Synchronized — it does not mean that they are common mode, it means that the difference of the clock. Is the Central synchronizer and stores so-called timeline. Our Central synchronizer is in the suburbs, in American Pdvalentine, which is not surprising.
Each device emits a carrier oscillation in the two frequency bands L1 and L2. All NKA GPS system to radiate at a common frequency 1575,42 MHz and 1227,60 MHz for L1 and L2, respectively, and NKA of the GLONASS system radiating at spaced frequencies, called letters (devices located on the opposite points of the orbit radiate to one liter). The difference between the letters is 562,5 kHz, for sub-band L1 and 437,5 kHz, for L2, the null character has the frequency of 1602 MHz 1245 MHz respectively.
A carrier oscillation is modulated by a special code sequence such that the phase of the code signal coincides with the clock of the satellite (if anyone is interested — modulation phase). In the GPS every phone has a unique code sequence that allows to distinguish between their signals, despite the General frequency. GLONASS also uses frequency division, so that all devices have the same code sequence. Additionally, the satellite signals are modulated by the navigation message, which contains the polynomial parameters of the mathematical model of the satellite and model displacement readings satellite clock relative to the system time scale.
the structure of the signal space vehicles GLONASS
The navigation messages also contain parameters of the ionosphere (allows to take into account the delay of signals in the ionosphere), the difference between system time scale and global coordinated time scale many sorts of other useful information. Simplistically, the system NSV is a network of synchronized, moving in the space of hours, is known at any point coordinates.
Ground control is a network of ground stations, providing the definition of the parameters of motion of space vehicles, parameters of the progress of their hours.The points are measurements of the planet's rotation, atmospheric parameters, there are precise characteristics of the Earth's gravitational field and provide storage for the world coordinate system. Functionally part of the TCU includes a considerable number of research institutions and laboratories. Well, of course, that is the ground of all these processes and provides data on devices that already broadcast their navigation messages.
Ground is and the basic points with grooved receivers, and the offices of the Federal astronomic-geodetic network, and radio interferometers with very long base, and laser rangefinders, and a lot of other interesting things. In General, the functions of the ground complex is very diverse, his work is too vast to include it in this article. If someone interested — I will try to write a post about it.
station Network of ground control complex of GLONASS
Well, actually user navigation equipment receives and processes signals NKA system. Receiving a signal from all the visible apparatus, the receiver performs the following functions (simplified diagram):
the
So we have the position of each apparatus, the signal propagation time to each device. And the unknown are our coordinates and the difference of the time scale of the receiver with the system time scale, i.e. four unknown. By the way, contrary to popular belief, the receiver determines coordinates are the latitude, longitude, and altitude, and in the form of x,y,z coordinates in the geocentric Cartesian coordinate system connected with the center of mass of the planet. This is due to the fact that the coordinates of the spacecraft are determined in this coordinate system. There are on GOST equations of recalculation of the parameters x,y,z, B,L,H (latitude, longitude, altitude).
It is clear that to determine the four unknowns require a system of equations with four or more equations. That's why we need four visible apparatus. It is possible to identify three devices, this system introduces an additional equation of the earth's ellipsoid (which binds x,y,z equation of the ellipsoid). But in this case and our situation will be tied to the ellipsoid, that is about the height can not speak.
In any case, the result of solving this system of equations will be our coordinates system and the position of the timeline. Sometimes I forget about the past, although transferring precise time the task is no less relevant than the definition of coordinates. At the moment, through GNSS, it is possible to transfer accurate time to any point of the globe with precision of the order of tens of nanoseconds, in special cases, to a few nanoseconds. In this competition they have almost no other systems provide an accurate time either much more expensive or much worse. All the world of laboratory time, all national standards of time and frequency (including ours) sluchautsya by GNSS (of course, not only GNSS) that enables the coordinated world time scale UTC, TAI, etc. However, the transmission time and frequency, the world timeline — a separate conversation.
Of course this is a highly simplified diagram of the operation of the navigation systems about any component you can talk for very long. So if anyone is interested, I'm willing to delve into any of the aspects of GNSS.
I must say, then I'll just consider the most common questions and misconceptions faced constantly. Well, I will try to explain the real state of Affairs, to the extent of his competence, of course.
The most common question.
Start with the fact that GLONASS is not at all the worse GPS.
For example, in polar areas, the GLONASS constellation provides better coverage due to more optimal configuration of the orbital constellation. However in Equatorial regions the situation is reversed for the same reason. The legs grow out of military use both systems, and military interests of the Soviet Union and the United States were concentrated in these areas.
In addition, the frequency division of signals indeed improves the immunity system GLONASS. This is frequency division and pulls a lot of problems, but the fact remains — in the event of armed conflict to suppress our GNSS will be more difficult.
The system itself is continuously progressing. Though not as fast as we would like, let it is accompanied by corruption scandals, with some astronomical amounts of money, but the whole world recognizes that GLONASS is consistently kept at a distance of four or five-year lag from the GPS and the gap is not increasing. By the way, do not think that the GPS is much cheaper, it also costs a monstrous money is not always spent as it should.
So why GLONASS lagging behind? Few people know that the GLONASS system over GPS for several years (formally, the system itself is younger, but her prototype came first and the development of the technology started earlier). The Americans of course watched her create, and have created their own, taking into account our mistakes, that other way was impossible to predict. Avoiding our system errors, and not stopping the development (in contrast to us, in the nineties all of our satellite fleet was almost at the bottom of the Pacific) they went from lagging to leading.
As is known, the RNA of both systems emit two types of signals: standard precision (ST-code for GLONASS C/A for GPS) and high precision (similar to the W-code and P/Y-code). ST-code of GLONASS is emitted in both frequency bands, and C/A code only GPS frequency band L1 (with the exception of a few NKA new series). Signals are emitted by high accuracy in both frequency ranges. The difference between these signals with the code sequence, the code signals with high accuracy have a wide band, which increases the accuracy and makes it difficult to suppress.
Traditionally, signals of high precision are considered war, standard signals are considered to be civil. This is only partly true. The code sequence of the P-code and W-code is currently open for wide applications: the Americans have officially published their code sequence, and at the same time our (where they learned, leave behind the scenes). So now any manufacturer is absolutely free to create receivers, receiving military signals (and create all precision instrument accepts all types of signals at all frequencies). The feature is that if necessary, these codes vary according to a special algorithm, of course classified. And after this change code sequences only military equipment will be able to accept them, because it is initially the algorithm is sutured.
Moreover, if necessary, on the signals of standard precision processing and the encoding, which does not prevent to take these signals, but does not allow to determine the position better than a couple of hundred meters in principle.
All these manipulations can not occur globally, but only over a certain region of the globe, as demonstrated by the Americans during the war in Iraq, depriving the entire middle East a normal GPS. Similarly, there were our during the conflict with Georgia, a special resonance is not caused because the users of GLONASS in Georgia could not be found.
What is the system time scale I have told. As mentioned coordinated world time scale UTC. Some confuse all these concepts, I will try to separate flies from cutlets and to explain what the differences are. The coordinated world time scale UTC, the analytical time scale (that is, it has no physical implementation is "on the tip of the pen"), which is calculated by comparison of time scales with the standards of time and frequency all laboratories world time. Accordingly, the scale itself references in these laboratories are referred to by country name or institution. For example, the scale of our national standard is called UTC(SU) (SU, because on this scale live in almost all countries of the former Soviet Union), the scale of the us standards Institute NIST, called UTC(NIST). In the us naval Observatory USNO (the world's most powerful laboratory of time and frequency) scale is UTC(USNO), which tighten the Central synchronizer the GPS system. Tighten, but the difference between the scales is still always there, in the order of several nanoseconds, and this difference is transmitted in the navigation message of GPS satellites. Thus, any GPS receiver may issue as the system time scale and the time scale UTC(USNO). Similarly, things for the system time scale of GLONASS and UTC(SU). That's just our planet's rotation is slowing, and the time scale to UTC every few years to adjust for one second. A system timeline not corrected and the difference between the system scale and global coordinated time is currently 16 seconds.
Thank you all for your attention, I hope it was interesting.
Article based on information from habrahabr.ru
will Try to do without putting some truths and information that anyone can gather at Wikipedia, but sometimes they cannot be avoided, please understand.
Structure of system
You all know what is global navigation satellite system. The most widely believed that is a number of satellites orbiting the earth which emit a signal that allows us to determine your coordinates at any point of the globe. In fact, any GNSS contains at least three components:
the
subsystem navigation space vehicles (NSV)
subsystem ground control complex (GCC)
subsystem navigation equipment consumers (NAP)
All other components such as system differential corrections are not necessary, it is only option.
Now fully deployed and available to the public only two systems, GPS and GLONASS. There are at least four GNSS are in various stages of deployment. As the end none of them brought, to talk about them we won't, although most of it is written to them too.
How it works
Subsystem NKA is the number of satellites, moving in concert on a specially selected orbits. The main condition when choosing the orbits in any point of the planet at any point in time should be seen at least 4 satellites (why four, will be explained below). On each of apparatuses installed atomic clock — cesium, rubidium, or a combination thereof, depending on the modification is synchronized with clock on Central system synchronizer. Synchronized — it does not mean that they are common mode, it means that the difference of the clock. Is the Central synchronizer and stores so-called timeline. Our Central synchronizer is in the suburbs, in American Pdvalentine, which is not surprising.
Each device emits a carrier oscillation in the two frequency bands L1 and L2. All NKA GPS system to radiate at a common frequency 1575,42 MHz and 1227,60 MHz for L1 and L2, respectively, and NKA of the GLONASS system radiating at spaced frequencies, called letters (devices located on the opposite points of the orbit radiate to one liter). The difference between the letters is 562,5 kHz, for sub-band L1 and 437,5 kHz, for L2, the null character has the frequency of 1602 MHz 1245 MHz respectively.
A carrier oscillation is modulated by a special code sequence such that the phase of the code signal coincides with the clock of the satellite (if anyone is interested — modulation phase). In the GPS every phone has a unique code sequence that allows to distinguish between their signals, despite the General frequency. GLONASS also uses frequency division, so that all devices have the same code sequence. Additionally, the satellite signals are modulated by the navigation message, which contains the polynomial parameters of the mathematical model of the satellite and model displacement readings satellite clock relative to the system time scale.
the structure of the signal space vehicles GLONASS
The navigation messages also contain parameters of the ionosphere (allows to take into account the delay of signals in the ionosphere), the difference between system time scale and global coordinated time scale many sorts of other useful information. Simplistically, the system NSV is a network of synchronized, moving in the space of hours, is known at any point coordinates.
Ground control is a network of ground stations, providing the definition of the parameters of motion of space vehicles, parameters of the progress of their hours.The points are measurements of the planet's rotation, atmospheric parameters, there are precise characteristics of the Earth's gravitational field and provide storage for the world coordinate system. Functionally part of the TCU includes a considerable number of research institutions and laboratories. Well, of course, that is the ground of all these processes and provides data on devices that already broadcast their navigation messages.
Ground is and the basic points with grooved receivers, and the offices of the Federal astronomic-geodetic network, and radio interferometers with very long base, and laser rangefinders, and a lot of other interesting things. In General, the functions of the ground complex is very diverse, his work is too vast to include it in this article. If someone interested — I will try to write a post about it.
station Network of ground control complex of GLONASS
Well, actually user navigation equipment receives and processes signals NKA system. Receiving a signal from all the visible apparatus, the receiver performs the following functions (simplified diagram):
the
-
the
- separation of signal from each satellite (for a code sequences for GPS and frequency for GLONASS). the
- definition of the clock of NKA at the time of emission of received signal by processing a code sequence. As mentioned above, the code sequence is synchronized with the onboard clock of the device. the
- reception of navigation messages. This will give the following data: the position of the vehicle and the difference of the progress of its hours and the system time scale. We can determine the time of signal emission in the satellite system time scale. the
- definition of evidence own hours receiver at the time of signal reception from satellites. Thus, we define the signal propagation time from the satellite to the receiver. But this time we will determine with an accuracy equal to the difference of the clock receiver and the system time scale. It is obvious that this error will be the same for all vehicles.
So we have the position of each apparatus, the signal propagation time to each device. And the unknown are our coordinates and the difference of the time scale of the receiver with the system time scale, i.e. four unknown. By the way, contrary to popular belief, the receiver determines coordinates are the latitude, longitude, and altitude, and in the form of x,y,z coordinates in the geocentric Cartesian coordinate system connected with the center of mass of the planet. This is due to the fact that the coordinates of the spacecraft are determined in this coordinate system. There are on GOST equations of recalculation of the parameters x,y,z, B,L,H (latitude, longitude, altitude).
It is clear that to determine the four unknowns require a system of equations with four or more equations. That's why we need four visible apparatus. It is possible to identify three devices, this system introduces an additional equation of the earth's ellipsoid (which binds x,y,z equation of the ellipsoid). But in this case and our situation will be tied to the ellipsoid, that is about the height can not speak.
In any case, the result of solving this system of equations will be our coordinates system and the position of the timeline. Sometimes I forget about the past, although transferring precise time the task is no less relevant than the definition of coordinates. At the moment, through GNSS, it is possible to transfer accurate time to any point of the globe with precision of the order of tens of nanoseconds, in special cases, to a few nanoseconds. In this competition they have almost no other systems provide an accurate time either much more expensive or much worse. All the world of laboratory time, all national standards of time and frequency (including ours) sluchautsya by GNSS (of course, not only GNSS) that enables the coordinated world time scale UTC, TAI, etc. However, the transmission time and frequency, the world timeline — a separate conversation.
Of course this is a highly simplified diagram of the operation of the navigation systems about any component you can talk for very long. So if anyone is interested, I'm willing to delve into any of the aspects of GNSS.
Breakdown cover
I must say, then I'll just consider the most common questions and misconceptions faced constantly. Well, I will try to explain the real state of Affairs, to the extent of his competence, of course.
Why GLONASS so bad?
The most common question.
Start with the fact that GLONASS is not at all the worse GPS.
For example, in polar areas, the GLONASS constellation provides better coverage due to more optimal configuration of the orbital constellation. However in Equatorial regions the situation is reversed for the same reason. The legs grow out of military use both systems, and military interests of the Soviet Union and the United States were concentrated in these areas.
In addition, the frequency division of signals indeed improves the immunity system GLONASS. This is frequency division and pulls a lot of problems, but the fact remains — in the event of armed conflict to suppress our GNSS will be more difficult.
The system itself is continuously progressing. Though not as fast as we would like, let it is accompanied by corruption scandals, with some astronomical amounts of money, but the whole world recognizes that GLONASS is consistently kept at a distance of four or five-year lag from the GPS and the gap is not increasing. By the way, do not think that the GPS is much cheaper, it also costs a monstrous money is not always spent as it should.
So why GLONASS lagging behind? Few people know that the GLONASS system over GPS for several years (formally, the system itself is younger, but her prototype came first and the development of the technology started earlier). The Americans of course watched her create, and have created their own, taking into account our mistakes, that other way was impossible to predict. Avoiding our system errors, and not stopping the development (in contrast to us, in the nineties all of our satellite fleet was almost at the bottom of the Pacific) they went from lagging to leading.
Military codes
As is known, the RNA of both systems emit two types of signals: standard precision (ST-code for GLONASS C/A for GPS) and high precision (similar to the W-code and P/Y-code). ST-code of GLONASS is emitted in both frequency bands, and C/A code only GPS frequency band L1 (with the exception of a few NKA new series). Signals are emitted by high accuracy in both frequency ranges. The difference between these signals with the code sequence, the code signals with high accuracy have a wide band, which increases the accuracy and makes it difficult to suppress.
Traditionally, signals of high precision are considered war, standard signals are considered to be civil. This is only partly true. The code sequence of the P-code and W-code is currently open for wide applications: the Americans have officially published their code sequence, and at the same time our (where they learned, leave behind the scenes). So now any manufacturer is absolutely free to create receivers, receiving military signals (and create all precision instrument accepts all types of signals at all frequencies). The feature is that if necessary, these codes vary according to a special algorithm, of course classified. And after this change code sequences only military equipment will be able to accept them, because it is initially the algorithm is sutured.
Moreover, if necessary, on the signals of standard precision processing and the encoding, which does not prevent to take these signals, but does not allow to determine the position better than a couple of hundred meters in principle.
All these manipulations can not occur globally, but only over a certain region of the globe, as demonstrated by the Americans during the war in Iraq, depriving the entire middle East a normal GPS. Similarly, there were our during the conflict with Georgia, a special resonance is not caused because the users of GLONASS in Georgia could not be found.
the Scale of GPS, GLONASS, UTC
What is the system time scale I have told. As mentioned coordinated world time scale UTC. Some confuse all these concepts, I will try to separate flies from cutlets and to explain what the differences are. The coordinated world time scale UTC, the analytical time scale (that is, it has no physical implementation is "on the tip of the pen"), which is calculated by comparison of time scales with the standards of time and frequency all laboratories world time. Accordingly, the scale itself references in these laboratories are referred to by country name or institution. For example, the scale of our national standard is called UTC(SU) (SU, because on this scale live in almost all countries of the former Soviet Union), the scale of the us standards Institute NIST, called UTC(NIST). In the us naval Observatory USNO (the world's most powerful laboratory of time and frequency) scale is UTC(USNO), which tighten the Central synchronizer the GPS system. Tighten, but the difference between the scales is still always there, in the order of several nanoseconds, and this difference is transmitted in the navigation message of GPS satellites. Thus, any GPS receiver may issue as the system time scale and the time scale UTC(USNO). Similarly, things for the system time scale of GLONASS and UTC(SU). That's just our planet's rotation is slowing, and the time scale to UTC every few years to adjust for one second. A system timeline not corrected and the difference between the system scale and global coordinated time is currently 16 seconds.
Thank you all for your attention, I hope it was interesting.
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