1 The time and frequency reference sources available for selection are used to unify the time information (year, month, minute, second) of the various devices or computers in the system or network to a standard time, or to limit the deviation of their time information from a reference time. Within a sufficiently small range, this technique is called time synchronization. Network systems in many industrial fields such as finance, radio and television, and transportation all need to maintain the time synchronization and time accuracy of computers and devices in the system within a wide range in order to work effectively.
When a synchronous broadcast network such as TV or radio broadcast in a broadcasting system is operating, a high stable reference frequency is required to lock the working frequency of all transmitters in the broadcast network. This reference frequency is the basis for realizing synchronous broadcasting, and its quality is related to the whole. Synchronous broadcast network quality of work.
Current broadcasting and television systems are shifting towards digitization. On the one hand, from program production to broadcast to office, the network is gradually being completed. On the other hand, cable television networks are undergoing SDH network transformation with high transmission speed, and they have achieved Internet connection and interconnection with the Internet. Open wireless synchronous broadcasting, TV/FM synchronous broadcasting are converting to digital. In this process, the selection of standard time and standard frequency reference sources is particularly important.
With the development of science and technology and the times, modern society demands more and more standardization and accuracy in time and frequency. The release and use of standard time and standard frequency have a decisive position in today’s society and measure the technology of a country. The level of development. At present, a complete standard time and standard frequency release system has been gradually formed in China. The reference time and reference frequency source that the current broadcast system can choose to use are:
(1) BPL - Long Wave Time Service System (LORAN or Changhe II System);
(2) BPM - short wave timing system;
(3) Ultra-short wave - standard time and standard frequency of CCTV television signal retracement;
(4) satellite positioning (time service) system;
(5) Network IRIG-B Time Code and Telephone Time Service System (ACTS).
In today's world, several well-known satellite positioning systems are widely used multi-track multi-star technology, which effectively solves the error caused by different receiving points due to different signal transmission distances on the reference time and the reference frequency. The principle is shown in Figure 1.
If both A and B receive a reference signal of a satellite (assuming A star) at the same time, because A and B are different from each other in the signal path of A-star, there is AO<AO', so that the satellite signal reaches the time of A. T1 is less than the time t2 to reach station B. The time difference between the two is: t2 − t1 = Δt so that the reference time received by station A and station B has an error and the phase difference ΔΦ occurs between the reference frequencies. These errors will give time to the two stations or Frequency synchronization poses a problem. If A and B two satellites receive the reference signal of the two satellites at the same time, it is not the same, although A station is closer to A than A station, there is AO<AO'; but A station is far from B station than B station. , and BO>BO'. If each station balances the signals of the received stars, it will make the reference signals received by the two stations converge. In other words, the more satellite signals that can be received by each point in the system, the more consistent the time and phase of the reference signal received at each point, and the synchronization will be accurate.
The world’s most widely used satellite positioning system, the GPS system, can receive signals from at least 8 satellites at any point of reception at an earth angle of 1800; at or below the tall buildings, the angle of view is 900 bad receivers can receive at least 4 satellite signals, which can effectively solve the time and frequency errors due to different receiving paths. At the same time, due to the advantages of satellite signal reception, less interference, and the advantages of terrain and geographies (as long as the sky is not obstructed or blocked), most systems in all walks of life are now launched using satellite positioning systems. Timing signals and frequency-frequency signals serve as reference sources for standard time and standard frequencies.
2 Major Satellite Positioning Systems in the World 2.1 Global Positioning System (GPS)
Completed and put into use by the United States in 1994, the space system consists of 24 satellites, of which 21 are evenly distributed over 6 orbital surfaces and 3 are on-orbit standby. The ground height is 20,000 km, the orbital inclination angle is 55°, and the orbital eccentricity is approximately It is 0 and the operation cycle is about 12h. This kind of layout is more beneficial to users at low and middle latitudes (our country is such an area). It can ensure that at least 4 satellite signals can be received at any time on any place on the earth. Positioning, navigation, ephemeris time and other information are broadcasted to the ground by two carrier frequencies of 1575.75 MHz and 1227.6 MHz, respectively. The carrier frequency is generated by two atomic clocks and two atomic clocks that are equipped on each satellite with high accuracy. CDMA is used. Distinguish the signals of each star; can provide high-precision, continuous, real-time three-dimensional speed measurement, three-dimensional positioning and timing service to any number of users in the world, and has now developed into the world's most widely used mainstream satellite positioning and navigation system.
2.2 Global Navigation Satellite System (GLONASS)
Constructed by the former Soviet Union to compete with the US GPS system, it was completed in 1996. Its space system is also composed of 24 satellites, including 3 on-orbit satellites, 21 working satellites, and evenly distributed on 3 orbital planes. Each orbit is equally distributed with 8 satellites. The orbital planes form an angle of 120 degrees with each other. With an orbit height of 19,100 km, an orbital inclination angle of 64.8°, a track eccentricity of approximately 0.01, and an operating period of 11h15min, this layout is more suitable for users at high latitudes. Its design also requires that at least 4 satellite signals be received at any time on any location on the globe. However, due to many reasons such as the replacement of satellites due to the end of their working lives, they cannot be fully realized. And all GPS satellites use the same carrier frequency, GLONASS broadcasts two carrier frequency bands: F1=1602+0.5625N (MHz); F1=1246+0.4375N (MHz); N=1～24 for each satellite number, each star's The carrier frequency is different and FDMA is used to distinguish the signals of each star.
2.3 Beidou-1 satellite navigation and positioning system China's own established and improved satellite navigation and positioning system. Launched in 2000, the first satellite was launched. To date, three satellites in orbit have formed a complete navigation and positioning system to provide regional navigation and timing services throughout the day and throughout the day. The satellite is located in a geosynchronous orbit at a height of 36,000 km and has a brief digital message communication function. The service scope of Beidou-1 is limited to China and its surrounding areas, and it has adopted a positioning method with a small number of stars and a unique economy. Both GPS and GOLNASS are global positioning systems, and all of them are three-dimensional unidirectional systems. Ground users require receiving the position information of four satellites and solving their own three-dimensional coordinates. The Beidou 1 is itself a two-dimensional system. Ground users should send positioning signals to two satellites at the same time. Both satellites are forwarded to the ground control center, and the ground control center calculates the coordinates of the satellites and forwards them back to the users via satellites. In other words, the system has a certain two-way communication function.
2.4 Galileo Global Navigation Satellite System (GNSS)
It is a global navigation satellite system with the highest precision and technical level that is being developed by the European Union. The "Galileo Plan" was put forward in 1999. Since many parties in the United States prevented the opposition, it was only officially launched in 2003 and the entire system will be officially completed in 2008. China was invited to sign the "Galileo Plan" in 2003. Now the plan has been implemented in China.
The Galileo Space System consists of 30 satellites, of which 27 are operational satellites and 3 are redundant. The satellites are positioned on 3 circular middle orbital planes, with a height of 15,000 miles and an orbital inclination angle of 56°. After the Galileo system is completed, it will have greater advantages than the current GPS system. Not only will the global coverage be more comprehensive, but the positioning accuracy will be more than 10 times that of GPS.
According to the agreement between the European Union and the United States and Russia, the Galileo system is compatible with the GPS and GLONASS systems. GPS and GLONASS equipment can be translated to receive Galileo signals and users' investments can be protected. Another advantage of the Galileo system is that unlike the GPS and GLONASS systems, which are controlled by a national defense department, they will be controlled and managed by multinational civil organizations, and the risk of political use is small.
3 Comparison of GPS and GLONASS Satellite Positioning Systems 3.1 Advantages and Disadvantages of GPS Systems GPS is the earliest global satellite positioning system built in the world. Because of its experience in the Internet, it began with a dual-use policy, focusing on civilian market development. Over the years, it has been supported and sought after by almost all world-famous technology service companies and device manufacturers. Various types of technical support and cost-effective chips at the bottom of the form are available everywhere, and application software and hardware are available everywhere. It has been well maintained and has been able to maintain stable and reliable operation; coupled with its leading technical indicators, many localities are alone in such a favorable situation, and thus have covered more than 90% of the global market. Its position in the satellite positioning field is similar to that of Intel. Chips in the CPU area, Microsoft's Windows Forms form the bottom of the form operating system in the field of home computers, almost can not shake, the word GPS has become the name of the entire satellite positioning navigation system.
The biggest criticism of the GPS system is that the U.S. government has adopted a priority for its own military. GPS broadcasts two independent signals: C/A code and P code. The P code is used for precision services and can achieve nanosecond time synchronization and is used only by the military or authorized users. The C/A code is used for ordinary services and achieves microsecond time synchronization for use by general users. What is more, in order to reduce the application accuracy of C/A codes, the United States adds SA measures to satellites, and SA (Selection Adaptive) selects suitability measures to artificially reduce the positioning and timing accuracy of C/A codes. However, under the strong opposition of people recently, GLONASS, Beidou and other systems have been catching up. India and Japan have accelerated the development of their own satellite positioning systems. Especially under the enormous pressure of the Galileo plan, the United States has been forced to relax and promise to cancel. SA measures.
3.2 The situation of the GLONASS system In the former Soviet Union, the GLONASS system was designed purely for military use. Russia later switched to an open-minded and non-encrypting open policy. GLONASS satellite broadcasts S-code and P-code at the same time. P-code is fine-grained and used for high-precision positioning of Russian military and scientific research. S-code is coarse-grained and used to provide standard positioning to civilians. No artificial reduction of positioning accuracy is adopted. Measures.
The economic difficulties caused by the disintegration of the Soviet Union caused the backwardness of GLONASS development. The lifespan-expired satellites (with an average design life of only 3 to 5 years, originally shorter than GPS) were not replaced in time, equipment could not be updated, maintenance was poor, and technical indicators declined. There are only 9 to 10 satellites in orbit that can be normally used in a few years and cannot be networked independently. In recent years, with the recovery of the Russian economy, Russia decided to revitalize the GLONASS system and use it for four years to update it to the world. By 2006, the entire system had recovered to 17 satellites. However, this change may be too late, and the main factors constraining its development are: (1) It is relatively late to put into operation, and it is not backed by extensive and excellent technical services (Never find relevant Chinese technical materials on the Internet at present), related products With a small number of devices and a lack of competitiveness at the bottom of the form, it is difficult for subsequent manufacturers to invest more money and technology to develop corresponding hardware and software for technical support; (2) Poor recognition. Due to the twists and turns in the development of the GLONASS system, coupled with the system's operational records indicating that its stability and reliability were not satisfactory, the market lacked confidence in it; (3) In the predominant position of the existing GPS, Galileo was put into operation two years later. Under the situation where China, India, and Japan are committed to developing their own systems, there is limited room for GLONASS system development. In light of the above, the prospects for the development of this system are not optimistic. GLONASS systems may not be able to perform big tasks other than the backup of GPS systems in some countries in the former Soviet Union.
3.3 Comparing the timing accuracy of several timing systems For several major timing systems that have been used for a period of time, we only consider the accuracy of their clocks or frequency bands, and do not care about the accuracy of other parameters such as their positioning, navigation, etc. The synchronization clock accuracy is compared as follows.
Table 1 Key Timing System Synchronization Clock Accuracy Comparison System Synchronization Clock Error Usage Features LORAN (BPL) 1~3μs Receiver Antenna Size Large OMEGS 2 ms Error Large GOES 0.1 to 0.3 ms Unstable GPS 1μs Easy to receive and stable GLONASS 10μs Inadequate technical support 4 Problems with dual-system work Standard time and frequency stability The stability or continuity of the reference source is an important aspect that one needs to consider, especially for applications such as broadcasting systems that place heavy emphasis on down-hit rate. Because the satellite signal is transmitted through the ionosphere, the ionosphere is affected by the complex and varied ion, and the signal transmission quality will be affected and may even be interrupted. To this end, it was proposed a dual system work program, that is, using two satellite timing system as the reference source of time and frequency, the market also introduced some dual-system OEM board, the most popular is the GPS+GLONASS dual system. The synchro broadcasting activator previously launched by our company has also adopted this dual-system compatible incentive working method, but our practice has proved that the dual-system working solution does not have much practical value in the current working mode of the broadcasting system network. as follows.
4.1 The practical significance of dual-system work If a single GPS system, the number of visible satellites above the horizon is generally 8 to 11 stars, in the harsh environment at the foot of the mountain or tall buildings, visibility of only 900 viewing angle to ensure the incumbent At least 4 satellite signals can be received at a time. For GPS+GLONASS dual systems, the number of visible satellites above the horizon can reach 14 to 20, but in the case of less than 900 viewing angles, the number of visible satellites can only be properly increased by 1 to 2.
The main benefits of increasing the number of visible satellites are: (1) It can improve the reliability and accuracy of observations. In the foregoing, we mentioned that the reliability of the satellite system's measurement and positioning depends mainly on the number of satellites that are located and calculated. Of course, the reliability and accuracy of satellites increase, and (2) the probability of satellite signal degradation and interruption is reduced. Poorly transmitted satellite signals may be compensated by satellite signals in other orbital positions of other systems; in positioning and navigation, a single system may sometimes fail to work when a moving target enters an area where obstacles obstruct a part of satellite sight; (3) Improves efficiency . The time required for GPS measurement depends on the time required to solve the integer ambiguities of the carrier phase; the more satellites that can be observed increase in fast positioning, real-time dynamic measurements, or post-processing dynamic measurements, solves the carrier phase integer ambiguity. The time required will be shortened. It can be seen that the dual-system work plan is mainly applied in the good receiving environment where the visibility is greater than 900. It is of great practical value for those applications that do not allow instantaneous signal loss, fast-moving target navigation, precision radio measurements, and other applications. .
4.2 The single GPS system is sufficient to meet the requirements of the synchronous working mode of today's broadcasting systems. In today's broadcasting systems, whether it is a wired network or a wireless synchronized broadcasting network, the working mode is generally a fixed point of the standard frequency or standard time, and generally does not move; When satellite signals are required to be received at synchronous points, the information modulated on the carrier is removed to recover the carrier, and the required 10 MHz or other desired frequency is obtained as a reference for locking its time or frequency through frequency division processing or DDS. In other words, the current synchronization mode only requires the simplest function of the satellite positioning system to receive several satellite signals sufficient to eliminate the phase difference at each point (even less than the four satellite signals required by the GPS positioning accuracy). The GPS satellite carrier frequency is generated by the high stability atomic clock. Each satellite uses the same carrier frequency and synchronizes with each other. It is ideal to use it as the reference source of the synchronous broadcast network.
Since the current broadcast network's synchronization system uses information such as positioning and navigation on the satellite system, even the satellite's timing signal is not used in most cases (currently, network time synchronization is locked by using a GPS or PPS pulse obtained from GPS. Or correct the clock forming circuit of each synchronization point to achieve a unified time accuracy, so there is no need to care about its demodulation and other processing processes, and the requirement for satellite signal reception is relatively low. Judging from the actual work, the situation that the GPS signal is interrupted or degraded to be unusable at a fixed point of view with good visibility rarely occurs. Only one system is required to work in a dual system, and most of the time in another system does not work. Basically become furnishings.
4.3 Simultaneous Switching Problems When Dual Systems Are Working The other big problem that solves the dual system operation is the problem of synchronous switching. Synchronous network work is not like a single target's positioning and navigation. There are many stations or servers in the system working in different places at the same time. Each point is a few hundred meters apart and several hundred kilometers apart. The signal receiving environment is quite different, and quite often it is A GPS signal is good, B GLONASS signal is strong, inevitably there will be a point switch to the GPS system, point B to switch to work in the GLONASS system. If A and B switch between two systems in different systems, although the frequencies of the frequency bands generated by different systems can be unified, the initial phases of the frequency bands formed by the GPS and the GLONASS systems are different, and the initial phases of the frequency bands are different. The stability index is also different, which will make the phase difference between two frequency points form a cycle cycle, which cannot be eliminated, and will make the broadcast network in a state of non-precise synchronization and loss of lock.
In order to solve the problem that the GPS signal may be interrupted, we envisage that a sample-and-hold circuit can be set at the synchronization point. The sample-and-hold circuit samples the average value of one or more days when the GPS is in normal operation. It is taken out when the GPS signal is momentarily interrupted to maintain the normal operation of the PLL.
5 Recommendations To sum up, try to make the following recommendations.
(1) At present, it is appropriate to use the signals transmitted by the satellite positioning system for the standard time and standard frequency reference sources of the broadcasting system network. In the period before the maturity of the Beidou system and the opening of the Galileo system, if the political use risk is not emphasized, considering the technical indicators, economic costs, operation, maintenance and other factors, the satellite positioning system should be the best choice for selecting the GPS system;
(2) Taking into account the development and changes of satellite positioning systems, the incentive system of the broadcasting synchronization network should adopt a modular structure, dividing the reference source part and the performance function part into two independent units, changing and upgrading. Changes in any part will not affect each other;
(3) The dual-system work plan is actually a costly and practically-effective solution to the current broadcast synchronization network;
(4) It is strongly recommended that relevant national authorities carry out research on relevant standard time and standard frequencies, and provide relevant industry guidance as early as possible, formulate corresponding standards and laws and regulations on this basis, and change the current choice of standard time and standard frequency. The use of various aspects of their own, their own state of disorder, in order to avoid confusion in the future, causing unnecessary losses.
When a synchronous broadcast network such as TV or radio broadcast in a broadcasting system is operating, a high stable reference frequency is required to lock the working frequency of all transmitters in the broadcast network. This reference frequency is the basis for realizing synchronous broadcasting, and its quality is related to the whole. Synchronous broadcast network quality of work.
Current broadcasting and television systems are shifting towards digitization. On the one hand, from program production to broadcast to office, the network is gradually being completed. On the other hand, cable television networks are undergoing SDH network transformation with high transmission speed, and they have achieved Internet connection and interconnection with the Internet. Open wireless synchronous broadcasting, TV/FM synchronous broadcasting are converting to digital. In this process, the selection of standard time and standard frequency reference sources is particularly important.
With the development of science and technology and the times, modern society demands more and more standardization and accuracy in time and frequency. The release and use of standard time and standard frequency have a decisive position in today’s society and measure the technology of a country. The level of development. At present, a complete standard time and standard frequency release system has been gradually formed in China. The reference time and reference frequency source that the current broadcast system can choose to use are:
(1) BPL - Long Wave Time Service System (LORAN or Changhe II System);
(2) BPM - short wave timing system;
(3) Ultra-short wave - standard time and standard frequency of CCTV television signal retracement;
(4) satellite positioning (time service) system;
(5) Network IRIG-B Time Code and Telephone Time Service System (ACTS).
In today's world, several well-known satellite positioning systems are widely used multi-track multi-star technology, which effectively solves the error caused by different receiving points due to different signal transmission distances on the reference time and the reference frequency. The principle is shown in Figure 1.
If both A and B receive a reference signal of a satellite (assuming A star) at the same time, because A and B are different from each other in the signal path of A-star, there is AO<AO', so that the satellite signal reaches the time of A. T1 is less than the time t2 to reach station B. The time difference between the two is: t2 − t1 = Δt so that the reference time received by station A and station B has an error and the phase difference ΔΦ occurs between the reference frequencies. These errors will give time to the two stations or Frequency synchronization poses a problem. If A and B two satellites receive the reference signal of the two satellites at the same time, it is not the same, although A station is closer to A than A station, there is AO<AO'; but A station is far from B station than B station. , and BO>BO'. If each station balances the signals of the received stars, it will make the reference signals received by the two stations converge. In other words, the more satellite signals that can be received by each point in the system, the more consistent the time and phase of the reference signal received at each point, and the synchronization will be accurate.
The world’s most widely used satellite positioning system, the GPS system, can receive signals from at least 8 satellites at any point of reception at an earth angle of 1800; at or below the tall buildings, the angle of view is 900 bad receivers can receive at least 4 satellite signals, which can effectively solve the time and frequency errors due to different receiving paths. At the same time, due to the advantages of satellite signal reception, less interference, and the advantages of terrain and geographies (as long as the sky is not obstructed or blocked), most systems in all walks of life are now launched using satellite positioning systems. Timing signals and frequency-frequency signals serve as reference sources for standard time and standard frequencies.
2 Major Satellite Positioning Systems in the World 2.1 Global Positioning System (GPS)
Completed and put into use by the United States in 1994, the space system consists of 24 satellites, of which 21 are evenly distributed over 6 orbital surfaces and 3 are on-orbit standby. The ground height is 20,000 km, the orbital inclination angle is 55°, and the orbital eccentricity is approximately It is 0 and the operation cycle is about 12h. This kind of layout is more beneficial to users at low and middle latitudes (our country is such an area). It can ensure that at least 4 satellite signals can be received at any time on any place on the earth. Positioning, navigation, ephemeris time and other information are broadcasted to the ground by two carrier frequencies of 1575.75 MHz and 1227.6 MHz, respectively. The carrier frequency is generated by two atomic clocks and two atomic clocks that are equipped on each satellite with high accuracy. CDMA is used. Distinguish the signals of each star; can provide high-precision, continuous, real-time three-dimensional speed measurement, three-dimensional positioning and timing service to any number of users in the world, and has now developed into the world's most widely used mainstream satellite positioning and navigation system.
2.2 Global Navigation Satellite System (GLONASS)
Constructed by the former Soviet Union to compete with the US GPS system, it was completed in 1996. Its space system is also composed of 24 satellites, including 3 on-orbit satellites, 21 working satellites, and evenly distributed on 3 orbital planes. Each orbit is equally distributed with 8 satellites. The orbital planes form an angle of 120 degrees with each other. With an orbit height of 19,100 km, an orbital inclination angle of 64.8°, a track eccentricity of approximately 0.01, and an operating period of 11h15min, this layout is more suitable for users at high latitudes. Its design also requires that at least 4 satellite signals be received at any time on any location on the globe. However, due to many reasons such as the replacement of satellites due to the end of their working lives, they cannot be fully realized. And all GPS satellites use the same carrier frequency, GLONASS broadcasts two carrier frequency bands: F1=1602+0.5625N (MHz); F1=1246+0.4375N (MHz); N=1～24 for each satellite number, each star's The carrier frequency is different and FDMA is used to distinguish the signals of each star.
2.3 Beidou-1 satellite navigation and positioning system China's own established and improved satellite navigation and positioning system. Launched in 2000, the first satellite was launched. To date, three satellites in orbit have formed a complete navigation and positioning system to provide regional navigation and timing services throughout the day and throughout the day. The satellite is located in a geosynchronous orbit at a height of 36,000 km and has a brief digital message communication function. The service scope of Beidou-1 is limited to China and its surrounding areas, and it has adopted a positioning method with a small number of stars and a unique economy. Both GPS and GOLNASS are global positioning systems, and all of them are three-dimensional unidirectional systems. Ground users require receiving the position information of four satellites and solving their own three-dimensional coordinates. The Beidou 1 is itself a two-dimensional system. Ground users should send positioning signals to two satellites at the same time. Both satellites are forwarded to the ground control center, and the ground control center calculates the coordinates of the satellites and forwards them back to the users via satellites. In other words, the system has a certain two-way communication function.
2.4 Galileo Global Navigation Satellite System (GNSS)
It is a global navigation satellite system with the highest precision and technical level that is being developed by the European Union. The "Galileo Plan" was put forward in 1999. Since many parties in the United States prevented the opposition, it was only officially launched in 2003 and the entire system will be officially completed in 2008. China was invited to sign the "Galileo Plan" in 2003. Now the plan has been implemented in China.
The Galileo Space System consists of 30 satellites, of which 27 are operational satellites and 3 are redundant. The satellites are positioned on 3 circular middle orbital planes, with a height of 15,000 miles and an orbital inclination angle of 56°. After the Galileo system is completed, it will have greater advantages than the current GPS system. Not only will the global coverage be more comprehensive, but the positioning accuracy will be more than 10 times that of GPS.
According to the agreement between the European Union and the United States and Russia, the Galileo system is compatible with the GPS and GLONASS systems. GPS and GLONASS equipment can be translated to receive Galileo signals and users' investments can be protected. Another advantage of the Galileo system is that unlike the GPS and GLONASS systems, which are controlled by a national defense department, they will be controlled and managed by multinational civil organizations, and the risk of political use is small.
3 Comparison of GPS and GLONASS Satellite Positioning Systems 3.1 Advantages and Disadvantages of GPS Systems GPS is the earliest global satellite positioning system built in the world. Because of its experience in the Internet, it began with a dual-use policy, focusing on civilian market development. Over the years, it has been supported and sought after by almost all world-famous technology service companies and device manufacturers. Various types of technical support and cost-effective chips at the bottom of the form are available everywhere, and application software and hardware are available everywhere. It has been well maintained and has been able to maintain stable and reliable operation; coupled with its leading technical indicators, many localities are alone in such a favorable situation, and thus have covered more than 90% of the global market. Its position in the satellite positioning field is similar to that of Intel. Chips in the CPU area, Microsoft's Windows Forms form the bottom of the form operating system in the field of home computers, almost can not shake, the word GPS has become the name of the entire satellite positioning navigation system.
The biggest criticism of the GPS system is that the U.S. government has adopted a priority for its own military. GPS broadcasts two independent signals: C/A code and P code. The P code is used for precision services and can achieve nanosecond time synchronization and is used only by the military or authorized users. The C/A code is used for ordinary services and achieves microsecond time synchronization for use by general users. What is more, in order to reduce the application accuracy of C/A codes, the United States adds SA measures to satellites, and SA (Selection Adaptive) selects suitability measures to artificially reduce the positioning and timing accuracy of C/A codes. However, under the strong opposition of people recently, GLONASS, Beidou and other systems have been catching up. India and Japan have accelerated the development of their own satellite positioning systems. Especially under the enormous pressure of the Galileo plan, the United States has been forced to relax and promise to cancel. SA measures.
3.2 The situation of the GLONASS system In the former Soviet Union, the GLONASS system was designed purely for military use. Russia later switched to an open-minded and non-encrypting open policy. GLONASS satellite broadcasts S-code and P-code at the same time. P-code is fine-grained and used for high-precision positioning of Russian military and scientific research. S-code is coarse-grained and used to provide standard positioning to civilians. No artificial reduction of positioning accuracy is adopted. Measures.
The economic difficulties caused by the disintegration of the Soviet Union caused the backwardness of GLONASS development. The lifespan-expired satellites (with an average design life of only 3 to 5 years, originally shorter than GPS) were not replaced in time, equipment could not be updated, maintenance was poor, and technical indicators declined. There are only 9 to 10 satellites in orbit that can be normally used in a few years and cannot be networked independently. In recent years, with the recovery of the Russian economy, Russia decided to revitalize the GLONASS system and use it for four years to update it to the world. By 2006, the entire system had recovered to 17 satellites. However, this change may be too late, and the main factors constraining its development are: (1) It is relatively late to put into operation, and it is not backed by extensive and excellent technical services (Never find relevant Chinese technical materials on the Internet at present), related products With a small number of devices and a lack of competitiveness at the bottom of the form, it is difficult for subsequent manufacturers to invest more money and technology to develop corresponding hardware and software for technical support; (2) Poor recognition. Due to the twists and turns in the development of the GLONASS system, coupled with the system's operational records indicating that its stability and reliability were not satisfactory, the market lacked confidence in it; (3) In the predominant position of the existing GPS, Galileo was put into operation two years later. Under the situation where China, India, and Japan are committed to developing their own systems, there is limited room for GLONASS system development. In light of the above, the prospects for the development of this system are not optimistic. GLONASS systems may not be able to perform big tasks other than the backup of GPS systems in some countries in the former Soviet Union.
3.3 Comparing the timing accuracy of several timing systems For several major timing systems that have been used for a period of time, we only consider the accuracy of their clocks or frequency bands, and do not care about the accuracy of other parameters such as their positioning, navigation, etc. The synchronization clock accuracy is compared as follows.
Table 1 Key Timing System Synchronization Clock Accuracy Comparison System Synchronization Clock Error Usage Features LORAN (BPL) 1~3μs Receiver Antenna Size Large OMEGS 2 ms Error Large GOES 0.1 to 0.3 ms Unstable GPS 1μs Easy to receive and stable GLONASS 10μs Inadequate technical support 4 Problems with dual-system work Standard time and frequency stability The stability or continuity of the reference source is an important aspect that one needs to consider, especially for applications such as broadcasting systems that place heavy emphasis on down-hit rate. Because the satellite signal is transmitted through the ionosphere, the ionosphere is affected by the complex and varied ion, and the signal transmission quality will be affected and may even be interrupted. To this end, it was proposed a dual system work program, that is, using two satellite timing system as the reference source of time and frequency, the market also introduced some dual-system OEM board, the most popular is the GPS+GLONASS dual system. The synchro broadcasting activator previously launched by our company has also adopted this dual-system compatible incentive working method, but our practice has proved that the dual-system working solution does not have much practical value in the current working mode of the broadcasting system network. as follows.
4.1 The practical significance of dual-system work If a single GPS system, the number of visible satellites above the horizon is generally 8 to 11 stars, in the harsh environment at the foot of the mountain or tall buildings, visibility of only 900 viewing angle to ensure the incumbent At least 4 satellite signals can be received at a time. For GPS+GLONASS dual systems, the number of visible satellites above the horizon can reach 14 to 20, but in the case of less than 900 viewing angles, the number of visible satellites can only be properly increased by 1 to 2.
The main benefits of increasing the number of visible satellites are: (1) It can improve the reliability and accuracy of observations. In the foregoing, we mentioned that the reliability of the satellite system's measurement and positioning depends mainly on the number of satellites that are located and calculated. Of course, the reliability and accuracy of satellites increase, and (2) the probability of satellite signal degradation and interruption is reduced. Poorly transmitted satellite signals may be compensated by satellite signals in other orbital positions of other systems; in positioning and navigation, a single system may sometimes fail to work when a moving target enters an area where obstacles obstruct a part of satellite sight; (3) Improves efficiency . The time required for GPS measurement depends on the time required to solve the integer ambiguities of the carrier phase; the more satellites that can be observed increase in fast positioning, real-time dynamic measurements, or post-processing dynamic measurements, solves the carrier phase integer ambiguity. The time required will be shortened. It can be seen that the dual-system work plan is mainly applied in the good receiving environment where the visibility is greater than 900. It is of great practical value for those applications that do not allow instantaneous signal loss, fast-moving target navigation, precision radio measurements, and other applications. .
4.2 The single GPS system is sufficient to meet the requirements of the synchronous working mode of today's broadcasting systems. In today's broadcasting systems, whether it is a wired network or a wireless synchronized broadcasting network, the working mode is generally a fixed point of the standard frequency or standard time, and generally does not move; When satellite signals are required to be received at synchronous points, the information modulated on the carrier is removed to recover the carrier, and the required 10 MHz or other desired frequency is obtained as a reference for locking its time or frequency through frequency division processing or DDS. In other words, the current synchronization mode only requires the simplest function of the satellite positioning system to receive several satellite signals sufficient to eliminate the phase difference at each point (even less than the four satellite signals required by the GPS positioning accuracy). The GPS satellite carrier frequency is generated by the high stability atomic clock. Each satellite uses the same carrier frequency and synchronizes with each other. It is ideal to use it as the reference source of the synchronous broadcast network.
Since the current broadcast network's synchronization system uses information such as positioning and navigation on the satellite system, even the satellite's timing signal is not used in most cases (currently, network time synchronization is locked by using a GPS or PPS pulse obtained from GPS. Or correct the clock forming circuit of each synchronization point to achieve a unified time accuracy, so there is no need to care about its demodulation and other processing processes, and the requirement for satellite signal reception is relatively low. Judging from the actual work, the situation that the GPS signal is interrupted or degraded to be unusable at a fixed point of view with good visibility rarely occurs. Only one system is required to work in a dual system, and most of the time in another system does not work. Basically become furnishings.
4.3 Simultaneous Switching Problems When Dual Systems Are Working The other big problem that solves the dual system operation is the problem of synchronous switching. Synchronous network work is not like a single target's positioning and navigation. There are many stations or servers in the system working in different places at the same time. Each point is a few hundred meters apart and several hundred kilometers apart. The signal receiving environment is quite different, and quite often it is A GPS signal is good, B GLONASS signal is strong, inevitably there will be a point switch to the GPS system, point B to switch to work in the GLONASS system. If A and B switch between two systems in different systems, although the frequencies of the frequency bands generated by different systems can be unified, the initial phases of the frequency bands formed by the GPS and the GLONASS systems are different, and the initial phases of the frequency bands are different. The stability index is also different, which will make the phase difference between two frequency points form a cycle cycle, which cannot be eliminated, and will make the broadcast network in a state of non-precise synchronization and loss of lock.
In order to solve the problem that the GPS signal may be interrupted, we envisage that a sample-and-hold circuit can be set at the synchronization point. The sample-and-hold circuit samples the average value of one or more days when the GPS is in normal operation. It is taken out when the GPS signal is momentarily interrupted to maintain the normal operation of the PLL.
5 Recommendations To sum up, try to make the following recommendations.
(1) At present, it is appropriate to use the signals transmitted by the satellite positioning system for the standard time and standard frequency reference sources of the broadcasting system network. In the period before the maturity of the Beidou system and the opening of the Galileo system, if the political use risk is not emphasized, considering the technical indicators, economic costs, operation, maintenance and other factors, the satellite positioning system should be the best choice for selecting the GPS system;
(2) Taking into account the development and changes of satellite positioning systems, the incentive system of the broadcasting synchronization network should adopt a modular structure, dividing the reference source part and the performance function part into two independent units, changing and upgrading. Changes in any part will not affect each other;
(3) The dual-system work plan is actually a costly and practically-effective solution to the current broadcast synchronization network;
(4) It is strongly recommended that relevant national authorities carry out research on relevant standard time and standard frequencies, and provide relevant industry guidance as early as possible, formulate corresponding standards and laws and regulations on this basis, and change the current choice of standard time and standard frequency. The use of various aspects of their own, their own state of disorder, in order to avoid confusion in the future, causing unnecessary losses.