Aircraft today use many systems of navigation to get to their destination. Satellite navigation (SATNAV) systems use satellites orbiting the earth to determine an aircraft's location. One such system is called the Global Positioning System (GPS). The Federal Aviation Administration (FAA) is developing two augmentations for GPS that will improve the system's performance for aircraft operations. These augmentations are called the Wide Area Augmentation System (WAAS) and the Local Area Augmentation System (LAAS). GPS, WAAS, and LAAS constitute the three fundamental components of the complete SATNAV system that is to be used by civilian aircraft for the foreseeable future. CAASD's Role in Air Navigation Current systems for radio navigation use terrestrially based navigation aids (NAVAIDs). Terrestrial NAVAIDs have limited service areas, so transmitters are needed to provide adequate navigation services throughout the United States. The MITRE Corporation's Center for Advanced Aviation System Development (CAASD) has been a leader in developing navigation system architecture alternatives that will provide a smooth transition from the current terrestrial system to a more cost-effective mix of both terrestrial and satellite navigation systems. Working with the FAA, user community, and industry, CAASD is helping to create the future NAS navigation architecture and transition strategy, and to bring public wide- and local-area differential GPS capabilities into operation. CAASD is also participating in GPS modernization activities that will further improve navigation services for civil users, and help develop standards and procedures for aviation use of the GPS. Global Positioning System GPS provides a seamless navigation service worldwide using only 24 nominal orbiting satellites. These satellites are monitored, controlled, and operated by the United States Department of Defense. The satellites provide a ranging signal and a navigation message that contains, among other things, information concerning the position of the satellite. By computing the pseudorange to four or more satellites, user equipment is able to compute 3-D position and time. GPS is used by military and civilian users worldwide. The Precise Positioning Service (PPS), which is available to the military, is an indispensable asset for U.S. and allied forces during all phases of conflict. Standard Positioning Service (SPS), though less accurate and more vulnerable to jamming, provides civil users with position and timing for aviation, marine, and land applications.
Figure 1: Main Elements of the Global Positioning System | Global Positioning System in Aviation In the aviation sector, GPS has resulted in significant cost savings and increases in overall system efficiency. Many General Aviation (GA) users and airline fleets are installing GPS airborne receivers for use in en route, terminal area, and non-precision approach operations. Likewise, many aviation authorities are taking the necessary steps to allow for more advanced use of GPS within their respective airspace. They have realized that GPS offers a navigation service that is equal to, and in some respects better than the existing ground-based systems. In the United States, the FAA and the aviation community recognize that GPS needs augmentation to achieve the accuracy, integrity, continuity, and availability required for it to be used as a primary means of navigation for the various phases of flight in the National Airspace System (NAS). For civil aviation, these performance parameters are defined as follows: - Accuracy is the degree of conformance between the measured or estimated position and the true position of an aircraft at a given time.
- Integrity is the ability of a system to provide timely warnings to users when the system should not be used for navigation.
- Continuity is the probability that a system will perform its function within defined performance limits for a specified period of time given the system is operating within the defined performance limits at the beginning of the flight operation.
- Availability is the fraction of time that the services of the system are usable by the flight crew.
The FAA is developing the Wide Area Augmentation System (WAAS) and Local Area Augmentation System (LAAS) to improve the accuracy, integrity, continuity, and availability of the satellite navigation signals so that GPS can be used as a primary means of navigation in the NAS. CAASD is assisting the FAA in developing these augmentations to GPS. CAASD is also assisting in evaluating various NAS architecture alternatives and in performing GPS modernization studies. Global Positioning System Modernization The GPS modernization program consists of several long term steps to make GPS more robust for both PPS and SPS users. The major elements of the modernization program include: - Improved signal waveforms for civil and military users.
- Increased power for PPS users.
- A third civil frequency (L5), internationally protected for safety-of-life applications, at 1176.45 MHz, with increased signal power.
- The discontinuance of intentional degradation of SPS accuracy, known as selective availability.
During 1998 and 1999, CAASD worked closely with MITRE's Department of Defense federally funded research and development center, the Federal Aviation Administration and the Department of Defense to identify a radio frequency that would be acceptable both from the viewpoint of international protection and for compatibility with existing civil and military systems. As part of this work, it was necessary to perform technical and economic analyses to identify a framework in which the new civil frequency could operate with acceptable levels of radio frequency interference from existing systems. CAASD's current work program in the GPS modernization area includes: - Experimental validation of the L5 spectrum sharing criteria.
- Validation of the impact of new PPS codes and power levels on SPS performance.
- Standards, requirements, and planning for the future GPS system through the year 2030.
Wide Area Augmentation System The Wide Area Augmentation System (WAAS) is a combination of ground- and space-based equipment to augment the standard positioning service of the GPS. It is being designed as a cornerstone of the next-generation civil aviation navigation service. The primary mission of WAAS is to provide a primary means of navigation for en route, terminal, nonprecision and precision approach phases of flight in the National Airspace System. The functions being provided by WAAS are: differential corrections (to improve accuracy), integrity monitoring (to ensure that errors are within tolerable limits with a very high probability and thus ensure safety), and ranging (to improve availability). Separate differential corrections are broadcast by WAAS to correct GPS satellite clock errors, ephemeris errors, and ionospheric errors. Ionospheric corrections are broadcast for selected Ionospheric Grid Points (IGPs), which are lattice points of a virtual grid of lines of constant latitude and longitude at the height of the ionosphere. WAAS consists of Wide Area Reference Stations (WRSs), Wide Area Master Stations (WMSs), Ground Earth Stations (GESs), Geostationary Earth Orbiting Satellites (GEOs) and a Terrestrial Communications Subsystem (TCS). (See Figure 2). In WAAS Phase 1, the GPS satellites' data are received and processed at 25 WRSs, which are distributed throughout the US. Data from the WRSs are forwarded to the WMSs, which process the data to determine the differential corrections and bounds on the residual errors for each monitored satellite and for each ionospheric grid point (IGP). The bounds on the residual errors are used to establish the integrity of the ranging signals. The corrections and integrity information from the WMSs are then sent to each GES and uplinked along with the GEO navigation message to the GEOs. The GEOs downlink this data to the users via the GPS L1 frequency with GPStype modulation. Each groundbased station or subsystem communicates via the TCS.
Figure 2: WAAS Architecture (Generic) | CAASD performed early WAAS feasibility studies including participating in the early flight tests of an experimental WAAS in the early to mid 1990's. Working with the FAA Technical Center, CAASD helped analyze the accuracy of the experimental system when it was used to provided guidance to aircraft performing approaches on the east and west coasts of the United States. Feasibility studies included performance tests of alternative ionospheric correction and integrity (error bounding) algorithms. Later, CAASD helped establish performance requirements for the WAAS and provided technical information to the FAA team that evaluated contractor responses to the WAAS Request for Proposal (RFP). After contract award, CAASD assisted in the transfer of technology to the prime contractor. CAASD has also provided technical advice to FAA on the WAAS design in the areas of performance and safety since contract award. CAASD has also been involved in the modeling and simulation of WAAS availability performance. CAASD's Satellite-Based Augmentation System (SBAS) Worldwide Availability Tool (SWAT) has been used in sensitivity analyses to help determine the optimal mix and location of resources (such as WRSs and GEOs), and to determine the impacts of design changes that alter equipment performance or location. It has also been used as a tool to demonstrate to air traffic planners the behavior of a space-based navigation system (i.e., orbiting sensors), and to help determine operational strategies for dealing with low performance areas. In 2000, the FAA organized a WAAS Integrity Performance Panel (WIPP) to assess the feasibility of achieving GNSS Landing System (GLS) performance from WAAS. CAASD was involved in performing numerous analyses and trade studies in support of the WIPP's GLS efforts and helped determine the feasibility of, and to define an implementation roadmap to GLS. Local Area Augmentation System CAASD also is working with the FAA to develop a ground system specification for the Local Area Augmentation System (LAAS), a Ground-Based Augmentation System (GBAS) that will provide all-weather approach capabilities to aircraft within airport-line-of-sight distances via a very high frequency (VHF) data broadcast (see Figure 3). CAASD's role in specification development and validation includes analyses of signal integrity and availability. In addition to LAAS specification activities, CAASD is providing technical expertise in FAA's partnership with industry to develop LAAS equipage and ensure the international standardization of GBAS applications. Finally, CAASD also is working with the FAA to ensure the integrity and safety of a future LAAS system that will support approaches and landings to operations during the most demanding weather conditions allowed for aircraft in the foreseeable future.
Figure 3: LAAS Architecture (Generic) | National Airspace System Architecture Alternatives As GPS gains acceptance and WAAS and LAAS are introduced into the NAS, there will be a transition from terrestrial navigation systems to satellite navigation systems. The transition period will probably span less than two decades, but it is expected that eventually most aircraft will use satellite navigation for all phases of flight. To help the FAA plan the transition to future precision approach and landing systems, CAASD is evaluating candidate system architectures using a hybrid of existing systems, including GPS, WAAS, and LAAS. CAASD is working with the FAA and aviation users through a Satellite Navigation User Group (SNUG) to define policy considerations for the transition from conventional NAVAIDs to satellite navigation. CAASD also supports the FAA on the Interagency GPS Executive Board (IGEB) working to determine the long-term ramifications to GPS users of improvements to the design, operation, and maintenance of the basic satellite constellation. The results of CAASD's work in support of the FAA's GPS augmentation programs, the SNUG, and the IGEB are helping to shape the country's future civil navigation architecture by a method that provides key benefits to users at reasonable costs.
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