This report summarizes the joint workshop on Global Navigation Satellite Systems (GNSS) held jointly by the U.S. National Academy of Engineering (NAE) and the Chinese Academy of Engineering (CAE) on May 24–25, 2011, at Hongqiao Guest Hotel in Shanghai, China.
The workshop had three objectives. First, it sought to explore issues of enhanced interoperability and interchangeability for all civil users. Second, it aimed to consider collaborative efforts for countering the global threat of inadvertent or illegal interference to GNSS signals. Third, it intended to promote new applications for GNSS, emphasizing productivity, safety, and environmental protection.
The workshop presentations were chosen based on the following criteria: the presentations must have relevant engineering/technical content or usefulness; be of mutual interest; offer the opportunity for enhancing GNSS availability, accuracy, integrity, and/or continuity; and offer the possibility of recommendations for further actions and discussions.
Dr. Wang Liheng, member of CAE, started the opening ceremony of this workshop. Dr. Zhou Ji, president of CAE, and Dr. Charles M. Vest, president of NAE, provided opening remarks. The government delegates from both sides introduced the evolution process, current status, future development, applications, and international cooperation of the BeiDou/Compass Navigation Satellite system and the Global Positioning System (GPS). Experts and scholars from both the U.S. and Chinese delegations exchanged knowledge and discussed GNSS compatibility and interoperability, system monitoring and service improvement, satellite navigation terminal and application technology, as well as talent exchanges.
The key outcomes of the workshop follow.
SHARED HISTORY AND STATUS OF GPS AND COMPASS
Mr. David Turner, deputy director of space and advanced technology, U.S. Department of State, provided an update on GPS constellation status and current U.S. government GPS policy, programs, and international activities. His comments are now summarized.
At the time of the workshop, the GPS constellation consisted of 31 operational satellites, although the baseline is a constellation of 24 satellites. These operational satellites included: 11 GPS Block IIA satellites, 12 GPS Block IIR satellites, 7 GPS Block IIR-M satellites, and 1 GPS Block IIF satellite. The GPS IIA satellites initiated the Block II series. Nineteen GPS IIA satellites were launched—the first on November 26, 1990, and the last on November 6, 1997. The Block IIR series are “replenishment” satellites developed by Lockheed Martin. Each satellite weighs 4,480 pounds (2,030 kg) at launch and 2,370 pounds (1,080 kg) once on orbit. The first successful IIR launch was in July 1997. The Block IIR-M satellites are modernized IIR satellites that include a new military signal and a second civil signal, known as L2C. The first IIR-M satellite was launched in September 2005. The Block IIF series are “follow-on” satellites developed by Boeing. Boeing is under contract to build a total of 12 Block IIF satellites. The Block IIF provides a third frequency for civil use (L5), allowing position determinations with even higher precision. The first Block IIF satellite was launched in May 2010.
Looking forward, GPS modernization will result in a variety of new signals, including civil signals L2C, L5, and L1C, all at different frequencies. L2C contains two distinct pseudo random noise (PRN) code sequences to provide ranging information—the Civilian Moderate length code (CM) and the Civilian Long length code (CL). The CM code is 10,230 bits long, repeating every 20 ms. The CL code is 767,250 bits long, repeating every 1,500 ms. Each signal is transmitted at 511,500 bits per second (bits/s); however, they are multiplexed together to form a 1,023,000 bits/s signal. CM is modulated with the navigation messages, whereas CL does not contain any modulated data and is called a dataless sequence. The L5 signal also contains two PRN ranging codes: the in-phase code (denoted as the I5-code); and the quadrature-phase code (denoted as the Q5-code). Both codes are 10,230 bits long and transmitted at 10.23 MHz.
The L1C civil signal is a more robust signal broadcast on the legacy L1 frequency (1575.42 MHz), which contains the coarse/acquisition (C/A) signal used by all current GPS users. The PRN codes are 10,230 bits long and transmit at 1.023 Mbps. It uses both pilot and data carriers like L2C. The modulation techniques used are BOC(1,1) for the data signal and TMBOC (time multiplexed binary offset carrier) for the pilot. Of the total L1C signal power, 25 percent is allocated to the data and 75 percent to the pilot.
GPS performance is better than ever and will continue to improve. U.S policy encourages worldwide use of civil GPS and augmentation systems. International
cooperation is a priority in pursuit of systems compatibility and interoperability with GPS.
With regard to the Compass system, both Drs. Ran Chengqi, director, China Satellite Navigation Office, and Tan Shusen, a researcher with the Beijing Global Information Center of Application and Exploitation, stated that the Compass Navigation system will follow a “three-step” deployment, from regional to global, and from active to passive. The Compass system aims to achieve regional passive service coverage by 2012 and global passive service coverage by 2020.
The first step occurred in the 1980s when China began to conduct a study on the BeiDou demonstration system under a dual-satellite active positioning mechanism. First initiated in 1994, the BeiDou demonstration system was able to provide regional active services in 2000. From 2000 to 2003, three satellites were launched. In 2010, these early satellites were replaced with newer ones.
The second step started in 1999. China began to study the BeiDou navigation satellite system under a passive positioning mechanism. System construction was formally started in 2004. The system will provide Regional Navigation Satellite Service (RNSS) for users in China and its surrounding areas by 2012.
For the third step, the BeiDou system will finally provide compatible and interoperable global coverage and also upgrade its regional service by 2020.
INTEROPERABILITY AND INTERCHANGEABILITY OF GPS AND COMPASS
Compatible constellations do not interfere with each other, and interoperable constellations gain strength from each other. Thus interoperability is a stronger condition than compatibility. If the GNSS signals are interoperable, then a receiver that combines the signals enjoys improved availability relative to a receiver that employs only one constellation. With this property, a receiver in an obstructed signal environment (urban, forest, or mountains) is more likely to have the number of satellites needed for good performance. Accuracy also improves and integrity improvements based on multiple constellations are under active investigation.
Interchangeability is a still stronger and more specific requirement. It enables the user to estimate the four estimanda (latitude, longitude, altitude, and user time offset) based on any four satellites from any of the cooperating constellations. Interchangeability requires that all constellations have a synchronized clock and operate on a common (or nearly common) carrier frequency.
To encourage interoperability and interchangeability, the service providers must enable a diverse group of receiver manufacturers to innovate and build equipment that takes advantage of the multiplicity of offered constellations. Specifically, they must provide the interface control documents (ICDs) for the civil signals to the receiver manufacturers well in advance of the constellation being operational. The GPS program has been well served by this openness, and the Chinese delegates mentioned that the ICD for Compass may be available as soon
as December of 2011. And, as this report was going to press, China did release the ICD for its civil signals in December of 2011.
In his talk, Dr. Thomas Powell, systems director of the navigation division for The Aerospace Corporation, discussed compatibility and interoperability. He described the benefits to GNSS providers, receiver manufacturers, and users. As a result of international bilateral and multilateral meetings, the advantages of GNSS interoperability have been recognized and major strides have been made toward achieving the goal. Specifically, the international community has agreed on right-hand circular polarization and code division multiple access (CDMA) modulation. In addition, GPS, Compass, Galileo, and the Japanese QZSS system use two identical GNSS carrier frequencies. GLONASS is also considering this possibility.
To achieve interoperability, multiple GNSS systems should have compatible or consistent system time and geodetic coordinate systems. The Compass/BeiDou satellite navigation system follows the rules of compatibility and interoperability defined by the International Committee of GNSS (ICG). The coordinate system of BeiDou is aligned to the China Geodetic Coordinate System 2000, which is aligned to the International Terrestrial Reference System (ITRS). BeiDou system time (BDT) is an internal, continuous navigation time scale, without leap second. BDT is linked to the national Coordinated Universal Time (UTC).
In the upcoming decade, GPS signals will be modernized, the Russian GLONASS satellite constellation will be rejuvenated, and the following new systems will become operational: Chinese Compass, European Galileo, and Japanese QZSS. These modernized or new systems will provide three GNSS frequencies. For precision applications, they will enable 10 cm accuracy over wide areas. Availability and accuracy for all forms of navigation will improve, because the number of visible satellites will more than double, assuring users will have access to GPS, Compass, and other GNSS systems.
Professor Per Enge of Stanford University provided a briefing on “Global Safety of Life Service from Multiple GNSS Constellations” (e.g., GPS + Compass). By comparing today’s single-frequency technology for detection of GPS faults and the future utility of dual frequencies and multiple constellations, he showed that multi-constellation GNSS will bring benefits to aviation, especially in terms of integrity.
MUTUAL AVAILABILITY OF SIGNAL SPECIFICATIONS
Mr. Turner described the U.S. policy promoting global use of GPS technology. There are no direct user fees for civil GPS services. The GPS service is provided on a continuous and worldwide basis. Moreover, the GPS signal definitions and structures are open and public for all civil services.
Dr. Ran stated that the Chinese Compass system will also ensure the openness and availability of its civil signals. In general, China aims to promote equal
access for user equipment manufacturing, applications development, and value-added services.
ESTABLISHING A GLOBAL PERFORMANCE MONITORING AND ASSESSING SYSTEM
GNSS would benefit from worldwide monitoring and assessment of the civil signals (i.e., open services). Such monitoring and visibility would extend the depth and breadth of GNSS applications and also provide the foundation for civil users to utilize system services safely and reliably. Therefore, monitoring of the GNSS Open Services has become a focus of attention for all providers and users of GNSS. Such open assessment is particularly valuable during the construction and development process of a particular GNSS constellation. Different GNSS providers and resources from different regions worldwide are involved in this issue, and enhancing international exchanges and cooperation is vital.
Mr. Ding Qun, a researcher with Xi’an Research Institute of Navigation, proposed an international GNSS Monitoring and Assessment System (iGMAS) for monitoring the performance of multiple GNSS systems. iGMAS is divided into the following layers: constellation state layer, spatial signals layer, navigation information layer, and service performance layer. The basic functions of iGMAS include data monitoring and collection, data transmission, data storage, data analysis, and information release.
INTEROPERABILITY OF THE U.S. AND CHINESE SPACE-BASED AUGMENTATION SYSTEMS (SBAS)
GNSS space-based and ground-based augmentation systems effectively improve the performance of satellite navigation and positioning services. Such systems are commonly composed of multiple ground stations, located at accurately surveyed points. The ground stations take multiple measurements of the GNSS satellites in view. Using these measurements and its known location, the ground system creates corrections and real-time error bounding information for broadcast to the end users. The corrections improve accuracy, continuity, and availability. The real-time error bounds protect the integrity of the navigation system. Compass and GPS would benefit from enhanced exchanges and cooperation regarding space-based and ground-based augmentation systems.
According to the Chinese delegation, the Compass system consists not only of medium Earth orbit (MEO) satellites, but also at least three Geostationary (GEO) satellites. The Chinese Compass GEO satellites will serve both as the satellites for ranging and for space-based augmentation. In contrast, the United States does not presently place any GPS satellites in geostationary orbit, nor does the United States broadcast aviation safety information from GPS satellites. Rather, it places transponders on commercial geostationary satellites for space-based augmentation.
In addition, the Chinese space-based augmentation system may provide cross-constellation integrity assistance. That is, the Chinese GEO satellites may broadcast error corrections messages for the Chinese Compass system and GPS and other GNSS systems. The United States has no immediate plans to broadcast error corrections for other constellations.
Certification of GNSS for use in aircraft navigation has already taken place for GPS L1 signals in many parts of the world. Even so, certification remains a challenging enterprise. The associated safety analyses rely on prototyping and careful analysis as starting points. These data and studies are used to generate and validate the threat models needed to support certification. New regions seeking to certify GNSS would benefit by drawing upon the lessons learned from previous experiences.
Dr. Todd Walter, senior research engineer at Stanford University, shared the lessons learned from developing the U.S. Wide Area Augmentation System (WAAS). Chief among these was the use of threat models. Threat models define the fault modes, how they manifest themselves, and how likely they are to occur. They describe the feared events that one must protect against. A well-defined threat model permits a quantitative assessment of the mitigation strategy. A quantitative assessment as opposed to a qualitative assessment is essential to establishing integrity. Certifying that WAAS is safe for instrument flight rules (IFR) (i.e., flying in bad weather) requires proving that there is only an extremely small probability that an error exceeding the requirements for accuracy will go undetected. Specifically, the probability is stated as 1 × 10–7 (or 10–7 integrity) and is equivalent to no more than 3 seconds of undetected faulty data per year.
Another key lesson is the application of the 10–7 integrity requirement to each approach. Rather than averaging over conditions with different risk levels, one must overbound the conditions describing the worst allowable time and place.
Dr. Grace Xingxin Gao, engineering research associate at Stanford University, addressed SBAS collaboration from the perspective of navigating in the Arctic and analyzed techniques to extend SBAS coverage to this critical area. She investigated three modifications to the SBAS architectures used today. First, an Arctic SBAS could use data from the existing ground networks for the SBAS in the United States, Europe, and Russia. These reference networks could be augmented by new reference stations located in the Arctic. Second, an Arctic SBAS would not be able to use geostationary satellites to broadcast the data to Arctic users, but could use low Earth orbiting satellites (LEOs, e.g., Iridium) to broadcast the corrections and integrity data to the end users. Third, multiple GNSS constellations (e.g., GPS + Compass) would significantly reduce VDOPs (vertical dilution of precision) and thus would improve vertical accuracy in the Arctic. Subject to the resolution of technical issues (e.g., high Doppler rates, synchronization of the time of transmission), the LEO satellites could also be used as ranging sources.
SUPPRESSION OF WORLDWIDE CIVIL SIGNAL JAMMING DEVICES
The Compass and GPS satellites are in medium Earth orbit, and so the received signals are very weak. These signals are easily overcome by radio frequency interference (RFI) from terrestrial sources. During RFI events, the safety and efficiency benefits of GNSS are lost. Thus, the worldwide community must cooperatively pursue an interference-free environment for the GNSS signals. Technical discussions were conducted regarding interference identification and localization. Jamming resistant solutions were explored from system design, terminal applications, and other broader aspects.
Civil signals face two kinds of interference: intentional and unintentional. Unintentional interference sources can be further categorized:
- Radio systems in or near the GNSS frequency band.
- Harmonics or intermodulation products produced by radio broadcasting and communication emitters.
- Stray signals from electronic equipment into the navigation frequency band.
Of particular concern, the delegates discussed the January 2011 conditional authorization by the U.S. Federal Communication Commission for Light-Squared (LSQ) Corporation to deploy 40,000 powerful (up to 15 kW) broadband transmitters in radio spectrum previously reserved for weak space-to-Earth signals adjacent to the faint GPS signal. With a transmitter spacing of 400 to 800 m in cities, the LSQ signal would be more than 5 billion times more powerful than the GPS signal. Hence, the proposed LSQ signals are dangerous to the current GNSS system. Moreover, the LSQ band is even closer to the Compass B1 band, and so it may cause even more serious interference to Compass.
As discussed by Dr. A.J. Van Dierendonck of AJ Systems, intentional jammers, especially personal privacy devices (PPDs), have become an important issue for GNSS. PPDs are small and cheap jammers that can plug into cigarette lighter sockets in a vehicle. They are designed to “jam” or “mask” GPS (GNSS) receiver/navigators that report, through other means, the position of the host vehicle when the user does not want his or her position known. Use of PPDs is illegal in the United States, although they are usually available for purchase via the Internet. Unknown (or known) to the owner, PPDs can jam other users of GPS (GNSS), including Safety-of-Life (SOL) operations. At Newark Liberty International Airport (EWR) in New Jersey, the RFI from such a device affected the operation of the Ground-Based Augmentation System (GBAS). GBAS performance interruptions have been observed since November 2009, impacting GBAS service availability. The PPDs do not seem to directly interfere with the airborne avionics, but as they interrupt the data from the GBAS ground system, they could interrupt the continuity of the aircraft approach operation.
Mr. Leo Eldredge, manager of the GNSS Group of the U.S. Federal Aviation Administration (FAA), presented the need for robust radio navigation and potential architectures for alternative position, navigation, and timing (APNT). The FAA has concentrated on three categories of solutions that appear promising, while inviting input from the public and industry at meetings, symposia, and conferences on other potential areas of research. The three categories currently being considered are Optimized Distance Measuring Equipment (DME) Network, Wide-Area Multi-lateration, and a Pseudolite Network.
Mr. Du Xiaodong, a researcher at the Beijing Research Institute of Telemetry, stated that governments have the responsibility to protect the navigation frequency band against illegal interference. China has clear legal provisions for the usage and protection of radio spectrum resources and the investigation, production, distribution, and importation of radio equipment. Relevant laws and regulations are: Real Right Law of the People’s Republic of China, Criminal Law of the People’s Republic of China, Radio Regulations of the People’s Republic of China, and Radio Station License Regulations. The protection of navigation signals spectrum resources relies on technology and the effective implementation of spectrum management. The China Radio Administration Bureau of Industry and Information Ministry has overall responsibility for administration and coordination. Its responsibilities are frequency spectrum monitoring, interference detection and investigation, matters of electromagnetic interference coordination, and maintenance of the transmission of radio waves in the air.
Receivers with the capability of tracking multiple GNSS systems may achieve better accuracy and integrity compared to those that track a single GNSS system. Dr. Han Shaowei from Unicore Communications Inc. showcased the Compass/GPS dual constellation receiver developed by his company. The receiver is a deeply integrated receiver, which has been used in China in precise positioning and also navigation applications. Dr. Han demonstrated the advantages of the Compass/GPS coupled receiver for sky-impaired users, such as those in an urban environment.
APPLICATIONS FOR GNSS
GNSS applications have made significant contributions to science, engineering, and commerce. They require the public availability of ICDs for the underlying GNSS systems. The success of GPS is based on industry-funded innovation combined with the open availability of these public documents. With ICDs, the forthcoming set of interoperable satellite constellations will improve the utility of current applications and open new sectors for exploration. Without ICDs, this expansion would have been limited.
Dr. Penina Axelrad, professor at the University of Colorado, Boulder, presented the application of GNSS to environmental studies. GNSS signals are influenced by the transmission media and interaction with surfaces near the receiving antenna. Observation of the modified signals from the ground and
from airborne and space-borne platforms allows for scientific study of the ionosphere, atmosphere, and the Earth’s surface. With appropriate signal processing, standard ground-based receivers provide estimates of atmospheric water vapor and soil moisture. Specialized receivers can measure occulted signals and enable high-resolution estimates of atmospheric density. Receivers measuring reflected signals are used to infer surface roughness and reflectivity, which can be related to surface conditions like ocean winds, soil moisture, and ice type. Modern receivers that can use multiple GNSS constellations will provide a rich global data set for environmental study.
Dr. Michael O’Connor of O’C and Associates briefed attendees on the recent advances of applying GNSS for precision agriculture. Precision agriculture (or precision farming) uses technology to better measure and control crop production on a site-specific basis to improve efficiency. Examples include more efficient application of seed and fertilizer and more effective utilization of tillage equipment. GNSS has been a key enabling technology for precision agriculture since the mid-1990s. Dr. O’Connor indicated that adoption of precision agriculture for seed and fertilizer management will improve when three key challenges have been overcome: (1) the improvement of GNSS signal availability, (2) the improvement of the efficiency of soil measurements, and (3) the wider adoption of simultaneous data analysis across multiple farms.
Dr. Liu Jingnan, CAE member, presented China’s effort to use Precision Point Positioning (PPP) to monitor earthquakes. With high-rate GNSS data, both seismic waveforms and permanent offsets can be observed. Combining high-rate GNSS data and seismometer data, seismologic research on earthquake parameter determination and fault rupture modeling will be more scientific and reliable. Combined data can be applied for real-time earthquake monitoring, tsunami warning, and other engineering deformation monitoring.
Dr. Tan presented a variety of Compass applications. First, he discussed a dispatching system for the marine fishery. China has realized offshore real-time monitoring of about 2,000 fishing vessels by taking advantage of Compass for positioning, location reporting, and short message communication. Other applications include: a hydrological data collection system at the Three Gorges of the Yangtze river; a detection system for forest fires; an inspection and monitoring system for high-voltage power lines in remote areas; a system for environmental protection and energy conservation in shipping on the Yangtze River; a vehicle navigation system for all vehicles to share location information and reduce the severity of traffic jams; and a two-way timing service that provides a standard of high accuracy time and frequency for upgrading the old power grid.
The U.S. and Chinese delegations discussed the benefits that could accrue to both sides by promoting a deep and multilevel cooperation and exchange
program regarding GNSS. Key observations from the workshop include the following: (1) it is beneficial for technology advancement and construction of the BeiDou system and GPS system to strengthen technical exchanges and cooperation between CAE and NAE in the GNSS field; (2) it is valuable to enhance academic exchanges and mutual understanding between technical personnel on both sides to improve technologies together; and (3) such academic exchange activities may be organized regularly or irregularly as needed.
Talent exchanges could include: consulting services; technology and scientific exchange, such as organizing technical conferences; and training from both sides. Specific activities may include: China inviting U.S. experts to give seminars to Chinese researchers and engineers; the United States inviting Chinese experts to visit U.S. institutions and enterprises and to communicate with U.S. experts; and the sending and receiving of visiting students and scholars.
In conclusion, the U.S. and Chinese delegates experienced a very useful exchange of knowledge and experience in the field of GNSS. As Dr. Vest stated in his opening remarks, “We have one world, and only one set of global resources. It is important to work together on satellite navigation. Competing and cooperation is like Yin and Yang. They need to be balanced.” This NAE-CAE Workshop on GNSS achieved the balance between such Yin and Yang of the U.S. and Chinese sides and has hopefully provided a basis for mutually beneficial and fruitful cooperation in the future.
The reader is invited to review papers from the conference presented in the following pages.