Land-based PNT System Program Suitable for Small and Medium-sized Countries
Land-based PNT System Program Suitable for Small and Medium-sized Countries
In 2004, the United States released a document entitled "US National Star-based Positioning, Navigation and Timing (PNT) Policy" to replace the decision of its president on the U.S. "Global Positioning System" (GPS) policy, replaced PNT with GPS, marking a new era for satellite navigation systems that consider PNT as a basic element.
Positioning, navigation, and timing (PNT) is a key technology to describe time and space. It is an important national space information infrastructure of a country!
The key role of the PNT facility is to provide time / space references as well as all kinds of real-time dynamic information related to the time / location. It is a major technology platform for economic security, national defense security, homeland security and public security and basic resource for strategic deterrence and it is also an important tool for national economic construction.
The GPS we are familiar with is one of the typical PNT facilities, except that it is established and controlled by the United States. GPS is a typical space-based (star-based) PNT facility, and what corresponds to it is a ground-based PNT facility.
A country's PNT system should make full use of various PNT means, overcome the shortcomings of a single solution through multi-source information fusion and ensure accurate, consistent and reliable information on time, location and speed. The construction of China's PNT system is shifting from "focusing on BeiDou satellite navigation" to the ambitious goal of "integrated service of a multi-information carrier composed of satellite-based PNT and civil mobile communications / networks". The national PNT system is a more advanced stage of PNT capacity building and development and represents the direction for the development of national PNT.
PNT technology is a strategic indicator of comprehensive national strength and has a strong service function in national economy, national security and military fields. It has become a vital parameter in a country's science, economy, military and social life. Its application range covers basic research (physics theory and basic physical constants) and applications of engineering technologies (high-speed digital communications, power transmission and distribution, deep space exploration, space travel, weapons experiments, geology and mineral exploration, metrology testing, smart city, etc.), concerning various important sectors related to national economy and the people's livelihood (transportation, financial securities, telecommunications, energy, etc.).
For example, in national defense construction:
Only with high accuracy and high stability in time and space can we achieve the cooperative combat among forces, arms and various kinds of weapons and equipment so as to give full play the effectiveness of military command, communications, radar, electronic warfare and electronic equipment. For national defense scientific research, such as the missile and aerospace test, the launch time of the missile or the launcher is highly important and the accurate launching moment (also called zero hour of launch) is even related to the success of the whole experiment. This moment is signified by a unified PNT facility. In addition, missile and spacecraft guidance also requires PNT facilities to provide position and speed information.
In another example, in the process of national economic construction, the power industry is related to the national energy security and the lifeline of the national economy. Time (T) is the basis of real-time data collection, control, protection, communication, tele-control, monitoring, automation, accident recall and analysis. It is an important part of the safe operation of the power grid and an important basis for grid-connection, settlement and transactions of power plants. In the field of communications, high-speed digital communications are inseparable from the support of high-precision time and frequency. Taking the mobile network as an example, poor accuracy of time synchronization between base stations will affect inter-cell community switchover and call interruption, cause interference between base stations at the same time, and make access difficult for users. In the future, 5G requires that time synchronization accuracy should be measured by nanoseconds and will be more dependent on the ultra-high-precision timing information given by PNT facilities.
With the development of the times, science and technology, people’s reliance on PNT is beyond any period in history. It is extensively associated with various industries and deeply integrated with the communications industry and the internet industry. It has effectively penetrated many areas of the national economy and people's daily life and has become a high-growth booster for high-tech industries as a new economic growth point of global electronic industry after mobile communications and the internet.
GPS began with a project by the U.S. military in 1958. The main objective was to provide real-time, all-weather and global navigation services to the three major territories of land, sea and air. After more than 20 years of research and experiment, it has cost more than 30 billion U.S. dollars. In order to allow other countries to abandon the development of this strategic technology, the United States puts the GPS system into civilian use and opens it to the whole world for free, and even foreign troops can use it. However, the United States may interfere with free signals at any time and affect positioning accuracy. Today, GPS is adopted by all countries in the world and the United States has made a huge profit from it.
Over the years, the effective operation and service of many business systems in national defense, military, communications, electric power, finance, transportation, radio and television sectors of many countries rely on GPS. For example, in timing service of a mobile communication system, strict time synchronization between base stations must be maintained. Otherwise, a call drop occurs or even communication is interrupted. The current 4G network commonly uses GPS timing synchronization. Another example is the power system, power companies extensively use GPS to provide timing signals to power automation equipment and monitoring system. If the GPS fails, the power system's power generation and supply network will mess up. Once the grid fails, the entire internet will not be spared.
The development of satellite navigation system involves many technical barriers, including rocket technology, satellite technology, payload technology, atomic clock technology and even land area factors (GPS monitoring stations should be globally distributed, which is easy for the United States). At present, only a few countries in the world can independently develop and produce satellite navigation systems.
Satellite navigation system is not only facing insurmountable technical barriers, it is time-consuming and costly. China's BeiDou navigation satellites cost nearly one hundred billion yuan, and they are still constantly optimized and perfected.
Even if the national power allows, there are many political barriers. The Missile Technology Control Regime (MTCR), established in 1987, it is a group export control system developed by seven western countries led by the United States to prevent the proliferation of missiles of capable of carrying weapons of mass destruction (WMD), unmanned aerial vehicles and related technologies. Powers are competing for the development of PNT technology from the space to the ground for the purpose of national security. In order to gain the upper hand, big powers even draft regulations to limit development by other countries, so MTCR basically eliminate opportunities of other small and medium-sized countries to be engaged in space development!
Therefore, there are huge technical and political barriers in the space-based navigation system. Even if money allows, it is fundamentally impossible for small and medium-sized countries to do so, that is why they are forced to rely solely on GPS. The exact location of any goal on earth can only be mastered by the United States, other countries can only know about "probably" where it is even the goal itself is within their own country. Even worse, the United States can cut off the signals sent to certain countries whenever it needs and paralyze the navigational applications in those countries. Anything uncontrollable will always lead to being controlled by others.
The limitations of satellite navigation system are mainly reflected in the following aspects:
(1) Weak anti-interference ability, poor reliability and safety: due to the use of radio transmission, it is highly vulnerable to electromagnetic interference and missile attack from the enemy in case of war;
(2) Greatly affected by the weather: ionosphere and lightning, thunder and lightning, dark clouds, and rainfall will affect the satellite signal transmission and reception;
(3) Only the open space can be covered: the satellite signal cannot be used in indoor, underground and tunnel environments due to obstructions of satellite signals;
(4) The timing accuracy is hard to be continuously improved. Since the atomic clock on the satellite applies a wireless method to check the time with the ground, and it provides service in a wireless way. The time terminal solves and compensates the delay according to the spatial information, and it is difficult to improve accuracy by algorithm optimization as a result of the inherent characteristics of wireless links.
In addition to the above, there are major strategic risks with the satellite navigation systems, such as:
(1) The satellite navigation system (GPS / GLONASS / BeiDou) can be completely disabled by transmitting interference noise.
The United States, Russia and some other countries have aircrafts equipped with special devices that emit noise to blocks the entire satellite signals in terrestrial space. At the outbreak of the war, satellite signals coming from space will be interfered with by the "noise" emitted from these special devices, rendering the satellite navigation system unusable. According to the Yonhap News Agency, North Korea is independently developing a new type of electronic warfare weapon - GPS jammer, and its operating range is more than 100 km.
(2) The U.S. anti-satellite (ASAT) missile program can destroy BeiDou satellites.
On September 12, 1985, the United States launched ASM-135A anti-satellite missiles and destroyed the decommissioned Solwind P78-1 observation satellite. On February 20, 2008, the U.S. Navy launched a SM-3 missile from USS Lake Erie cruise (CG-70) and shot down the uncontrolled USA-193 satellite.
(3) China's anti-satellite missile test can destroy GPS satellites.
On January 11, 2007, China conducted an anti-satellite missile test and destroyed the scrapped Fengyun 1 meteorological satellite. On January 11, 2010, China launched a SC-19 anti-satellite missile and destroyed a CSS-X-11 medium range ballistic missile fired from the Jiuquan Satellite Launch Center.
Attacks and interferences on the satellite itself are the greatest threats to the satellite navigation system (GPS / GLONASS). The way in which the enemy directly uses various anti-satellite weapons to destroy and interfere with satellites can completely disturb and undermine the normal operation of the satellite navigation system of the enemy.
To conclude, when the war breaks out, space-based satellite navigation systems will not be reliable or even usable! As a result, the military forces will become blind and deaf. Important sectors such as communications and electricity will not be able to function effectively and the national economy will also suffer heavy losses.
"Positioning, navigation and timing" (PNT) services are just as essential to a nation's national defense and national economy just as oxygen to human beings. The United States has long before recognized the security and vulnerability issues inherent in GPS. The "Presidential Directive No. 39 on National Security" promulgated in 2004 required the Ministry of Transportation and the Department of Homeland Security to carry out GPS interference monitoring and mitigation plans and to develop PNT (positioning, navigation and timing) backup capabilities. In 2012, the U.S. Department of Homeland Security released the "The National Risk Evaluation of GPS on U.S. Critical Infrastructures", according to which: (1) increasing reliance on GPS in providing PNT services increases the risk of critical infrastructures; (2) There are various threats to the GPS service which will influence infrastructure operations. In April 2014, the GLONASS system in Russia suffered a series of major operational accidents, with the longest interruption of up to 11 hours. The EU also pays close attention to the vulnerability of satellite services. Within the European Union, the UK hopes to play a leading role in the EU and even global GNSS vulnerability and response research activities and will launch a backup PNT system.
To achieve land-based PNT system, long-distance, long-span, multi-node time transfer is the core. Germany, France, Italy, the Netherlands and other developed countries have embarked on the establishment of optical fiber time frequency transmission networks. In the Medium and Long-term Planning of Major National Sci-tech Infrastructure Construction (2012-2030) promulgated by the State Council in 2013 (GF  No. 8 File), China explicitly included the construction of a "high-precision ground-based timing system" in the planning. "High-precision ground-based timing system" is also included in the ten construction projects launched as key sci-tech infrastructures as stipulated in the "13th Five-Year Development Plan" released by the National Development and Reform Commission.
In the information age, a sovereign state must have an autonomous and controlled space-time information infrastructure. The construction of land-based PNT systems is one of the major infrastructures related to national security. Therefore, in order to meet the strategic needs, satisfy the practical application, support the scientific development, it is highly essential to build a mutually complementary, supported, and backed up high-precision ground-based PNT system that is relatively independent of the space-based PNT system. In order to realize ground-based PNT, it is necessary to rely on the high-precision ground-based networks. First we have to solve the timing problem, and then the positioning and navigation problems.
To achieve the land-based PNT system, we first need to address the issue of timing. Two conditions are needed to solve the problem of timing:
1. Time source
2. Timing technology
Clock source is readily available. Each country has a standard time release organization, such as NTSC, NIST and so on. Of course, it could be a punctual lab with atomic clocks or a self-built atomic time source.
The higher the timing accuracy, the higher the location and navigation accuracy in the future, that is true for timing technology. According to the relationship between timing accuracy and positioning accuracy, in order to achieve the positioning accuracy of 2 meters, the timing accuracy shall be at least better than ± 10ns (nanosecond). To achieve such a high timing accuracy, you need to use optical networks. Usually domestic communications operators or power, broadcasting, military systems have ready-made optical networks. However, to realize long-distance, long-span and multi-node high-precision time transmission based on fiber, the timing technology is the core.
After solving the problem of time source and timing technology, a fiber network capable of precise time transmission can be established on the ground, and then the positioning and navigation functions can be accomplished by using the base station transmitting station and the receiving terminal.
Figure 1: Schematic diagram of ground-based PNT
As shown in the figure, it is easy to see that the components of the entire land-based PNT system include: time source, fiber optic network, timing technology, positioning navigation transmitter / receiver.
As mentioned above, the basis of ground-based PNT is long-distance, large-span and multi-node high-precision time transmission based on optical fiber network.
In 2014, Sichuan Taifu Ground BeiDou Technology Co., Ltd. adopted the originally developed "ground-based BeiDou" technology consisting of the core technologies of "optical fiber time delay three-point-in-one accurate measurement technology" and "optical fiber time delay servo lock and precise control technology", successfully solved the technical problem of high-precision long-distance transmission of ground optical fiber network. The "ground-based timing core equipment - TFT series high-precision time and frequency transmission equipment", has passed the verification by the Chinese national professional testing organization and the test and appraisal organized by Sichuan Science and Technology Department on October 7: "The key technical indexes of this equipment are as follows: relative time interval ≤ ± 1ns, relative deviation compared with time source ≤ ± 10ns, relative frequency deviation ≤ ± 5E-14 / day, the performance indicators are significantly better than similar products at home and abroad." This achievement has filled the gaps between home and abroad and reached the world's leading level.
The technology has been tested for 2000 km and 21 nodes at the "Second Metrology Test Research Center of National Defense Science and Technology Industry", "CTLL Laboratory of China's Ministry of Industry and Information Technology" and "China Electric Power Research Institute", and the timing accuracy is ± 7ns or so. The technology has been tested for 5G networking on more than 1300 kilometers of China Mobile, and the timing accuracy is ± 5ns.
In September 2016, Sichuan Taifu Ground BeiDou Technology Co., Ltd. won the second prize of the 5th China Innovation and Entrepreneurship Competition held by the Ministry of Science and Technology of China, ranking the top 4 of China's military technology industry in 2016.
Figure 2: Principle of technology of ground BeiDou
As shown in the above figure, "Ground BeiDou" technology can make the time information pass zero-delay on the optical fiber; at the same time, the timing accuracy of each node is guaranteed to be less than 10 ns relative to the time source (relative to UTC, and that of BeiDou second generation is 50ns).
Because time is the basis of PNT, the time accuracy is positively correlated with the positioning accuracy. Simply put, the positioning accuracy of 10 nanosecond timing accuracy corresponds to a positioning accuracy of up to 2 meters, so that the stereo positioning can be realized without any difference method. At the same time, since the time delay is 0, the master-slave time synchronization performance is consistent, which can greatly improve the positioning performance and reach the level of less than one meter.
Technological breakthroughs have led to revolutionary changes in PNT that have not only made the assumption of land-based (including sea-based) PNT possible, but also laid the foundation of the building of the world's leading time-space information infrastructure independent of satellites.
The high cost of space-based satellite navigation systems makes it difficult for development by a country of small and medium size. Therefore, it is necessary to establish and own a ground-based PNT system that is independent of satellite navigation systems and has better performance than satellite navigation systems.
Therefore, the overall objective of the project is to study and build an autonomous and controlled land-based PNT system and establish an integrated PNT spatial information infrastructure suitable for small and medium-sized countries. Cooperating with the Belt and Road Initiative, it can be used as a business card. While the space-based BeiDou is available for globalized use, small and medium-sized countries are still seeking independent and controllable backup solutions for strategic and security concerns. Spaced-based PNT system (ground-based BeiDou), once completed, can be transferred to the respective country.
 Regional demonstration
Select a pilot area (with a radius of 50km), use terrestrial optical fiber (consider joint local communications operators), adapt "ground BeiDou" technology to deliver the standard time source (the pilot may purchase a cesium clock) to each positioning launch platform (transform the existing communication base station if necessary), see the figure below:
Figure 3: Schematic diagram of regional demonstration area
The time transfer indicator of the star-shaped topological structure as shown above will be better than ± 2.5ns, and the corresponding positioning accuracy is up to 0.5 meters.
 Network of regional demonstration area
Through the full verification of the demonstration area, the coverage of the PNT can be further expanded. Each demonstration area is connected with the optical fiber to transmit the time signal to each other and form PNT service for a big area.
Figure 4: Schematic diagram of regional demonstration network
 Wide area network
Connect large areas across the country, unify the timing, form an effective time transfer network (hierarchical structure) and combine with the positioning and navigation signals of the base stations to form ground-based PNT systems independent of satellites.
Figure 5: schematic diagram of wide area network
Although the timing network is the foundation, there are many positioning and navigation options, such as pseudolite technology, Locata technology and communication base station transformation technology.
The benefits of pseudosatellites are simple and straightforward, the same as the principles of satellite navigation, but the high power of pseudolitesignals can have some impact, such as the distance problem; interference with other common receivers. And the currently, pseudolites are relatively costly.
Developed by Locata, an Australian GPS manufacturer, Locata technology provides high-precision positioning capabilities when GNSS is not available. Locata technology includes the time synchronization pseudo-satellite transceiver (LocataLite), LocataNet composed of a number of LocataLits, the transmitted signal is equivalent to the GPS signal, and its technical essence is also similar to the pseudolite.
At the same time, taking into account the needs of indoor coverage, the current indoor positioning technology for the public has not yet succeeded in large-scale promotion and application, it is difficult for any of the existing single positioning technology indoor positioning solutions to meet the large-scale practical applications, such as WIFI, Ultra Wide Band (UWB), RFID, MEMS and even pseudolite. However, the development of mobile communication brings an opportunity to this. Mobile communication network has many advantages of indoor positioning. First, seamless coverage of mobile communication network makes positioning continuity good. Second, it is applicable for ordinary mobile intelligent terminals. Meanwhile, terminals have a high popularization and penetration rate. Indoor positioning based on communication network is the mainstream trend in the future judging from the aspects of standardization degree, application object, deployment and application cost, and development potentials and can cover universal application groups.
Therefore, with the positioning of communication base station as a universal support, after a reasonable transformation, seamless indoor and outdoor high-precision location services can be provided.
Transformation of existing communication base stations need to consider the cost factor and the appropriate cost is estimated to be 20,000 RMB / station or less.
Due to the fact that time transfer requires fiber resources, the use of optical cable resources of telecommunications operators is the most appropriate. Therefore, the smooth progress of the project requires the close cooperation of telecommunication operators, and the communication operators' own networks also have time synchronization and positioning requirements. Of course, in addition to communications operators, cooperation from power, broadcasting, military and other units or departments that have basic networks is also feasible.
To establish a regional demonstration area mentioned in Article VII (with a radius of 50km), or phase :
Investment composition and budget (unit: 100,000 yuan) in Phase 1 (not as final basis)
Ground BeiDou equipment cost
One medium（TFT1001）and four small (TFT1003) equipment
Communication base station renovation cost
Time source cost
Commercial cesium atomic clock (for demonstration in phase 1)
Travel and conference fees for technical support
Note: The investment budget for phases  and  is still unpredictable and planning shall be based on actual conditions.
Ground-based BeiDou navigation technology is brought to the ground, that is, Ground Bei Dou, briefed as GBD technology, can effectively overcome the shortcomings of space-based satellite navigation systems, truly achieving terrestrial system-based positioning, navigation and timing service (PNT) independent of satellites. The positioning precision of "Ground BeiDou" is five times more precise than the current space-based navigation and positing. The position precision is within 2 meters, better than any of existing satellite navigation systems (as shown in the following table), and among the world's leading class.
Figure 6: Integrated PNT
The system, once completed, can be used in coordination with the space-based navigation systems (including GPS, etc.) to provide military and civilian systems with timing, navigation and positioning services. The "ground BeiDou" ground-based PNT system is completely autonomous and controllable. What is more unique is that underground optical cables can form a network of underground optical transmission extending in all directions. It is characteristic of the best anti-interference and anti-destruction performance, and it is tough, durable and cannot be completely destroyed even if a war breaks out.
Space-based timing system (GPS / BeiDou)
Ground-based system (Ground BeiDou)
High (10ns, even picosecond)
Timing precision consistency
Generally speaking, ground-based PNT system has many advantages, e.g. they are free of distraction and destruction. After the network is formed, the system has the function of protection and self-healing. Meanwhile, the timing precision is very high (BeiDou has a timing accuracy of 50 nanoseconds, and that of the spaced-based system is 10 nanoseconds or higher), and the signal is stable (based on fiber optic transmission). It can provide reliable PNT services to military and civilian systems around the clock and help to get rid of reliance on satellite PNT systems.
Optical fiber timing leads the world
Ground BeiDou creates a future
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