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Volpe Journal Summer 2000Previous Section | Contents | Next Section 
On Course with GPSSometimes new technology bursts on the scene, more often it evolves. In the case of the Global Positioning System or GPS, a satellite-based system for navigation, the evolution was spurred on by the launch of Sputnik in 1957, and the realization of the opportunities created in this new arena. However, it was a tragic event in 1983 that most dramatically demonstrated the need for a worldwide navigation system, bringing GPS to the consciousness of the entire aviation community. In 1983, a Korean Air Lines 747, heading for Seoul from Alaska and carrying 269 air travelers, veered off course into Russian airspace and was shot down by USSR military fighters, killing everyone on board. Although much debate surrounded the exact circumstances of this event, one thing was certain. A better navigational system might have prevented this tragedy. At the time of the accident, the US Department of Defense was operating GPS, a navigation system that was extremely accurate. The system provided all-weather round-the-clock navigation capabilities using a constellation of satellites orbiting the earth. Unfortunately, it was only available to military ground, sea, and air forces. After the downing of Korean Flight 707, President Ronald Reagan made an historic decision, issuing a directive that guaranteed that this system would be available at no charge to the entire world. Developed more than 20 years ago, GPS is now an $8 billion industry, which is expected to double to $16 billion in the next few years. It is used in numerous civilian applications around the globe, including vehicle tracking, recreational uses such as hiking and boating, emergency response, and mapping and surveying. One of the biggest beneficiaries, however, has been aviation. Typically, civilian aircraft navigate from one ground radio beacon, or waypoint, to another.2 GPS simplifies and improves the method of guiding planes throughout all phases of flight. With a GPS receiver in the cockpit, pilots are provided with accurate position data and can fly a direct route to any destination, anywhere in the world. The result is significant cost savings and increases in overall system efficiency. The Federal Aviation Administration recently has embarked on an aggressive program to make GPS available for use throughout the US National Airspace System and beyond, creating a seamless, worldwide system. However, despite the promises that GPS brings to aviation, the system needs augmentations to meet the performance criteria for the more stringent phases of flight. In addition, GPS must be monitored to ensure that all parts of the system are working properly at all times. For the past two decades, the Volpe Center and its Center for Navigation have been instrumental in helping to achieve these goals. Because GPS is a worldwide navigation system, the Center for Navigation also is working to share its expertise with the world aviation community, including recent projects in Australia, Germany, and most recently, Chile. The involvement of the Volpe Center offers the opportunity for other nations to capitalize on advances in GPS, increasing international safety for air carriers and air travelers around the world.
How Does GPS Work?The GPS satellite constellation consists of a minimum of 24 satellites orbiting approximately 11,000 miles above the earth. The satellites, operated by the US Air Force, provide 24-hour worldwide coverage. Each satellite continuously transmits radio signals to GPS receivers.3 The signals give the location of the satellite and the precise time at which the signal was sent. When the signals arrive at a GPS receiver, the relative arrival times of the signals are measured. Using these measurements, the receiver computes or triangulates the position of the user.4 A GPS receiver requires signals from at least four satellites to accurately determine its three-dimensional geographic coordinates. The GPS system currently offers two levels of service: the Precise Positioning Service, which is available only to the Department of Defense and other authorized users, and the Standard Positioning Service, which is available free of charge to civilians worldwide. GPS and Air NavigationTo meet the performance criteria for critical safety-of-life applications such as aviation, GPS must be able to ensure integrity, accuracy, availability, and continuity.5 In other words, the system must be able to provide accurate readings and must be able to let a user know when it is unable to do so. Until recently, the Standard Positioning Service provided civilian users with a horizontal position that was 95 percent accurate to within 100 meters and a vertical position that was accurate to within 150 meters. Although GPS was capable of providing much better accuracy than this, it was degraded in the interest of national security by the use of selective availability. However, in May of this year, President Bill Clinton discontinued selective availability for the public, making the GPS system approximately 10 times more accurate. Currently, the Standard Positioning Service provides civilian users an accuracy of approximately 12 meters horizontal and 20 meters vertical (95 percent accurate). Despite this increase in accuracy, errors in the satellite signals still can be introduced by nature as the signals travel through the ionosphere. As a result, the US Department of Transportation is implementing GPS augmentations based on a technique know as Differential GPS. In Differential GPS, a reference station continuously monitors the GPS signals. Because the position of the reference station has been precisely surveyed, any errors in the satellite signals can be calculated and corrections broadcast to users. Augmentation systems for aviation that use Differential GPS include the Wide Area and Local Area Augmentation Systems (WAAS and LAAS). These systems support the Federal Aviation Administration's "Free Flight" and "Safer Skies" initiatives, which are aimed at improving the safety and efficiency of the US National Airspace System.6 Aircraft pilots continuously must be informed about the integrity of GPS signals, that is, when they cannot rely on GPS for instrument flight rule navigation. This can happen, for example, when a satellite is "out of tolerance," which could result in an inaccurate navigation solution. Currently, algorithms known as Receiver Autonomous Integrity Monitoring (RAIM) and Fault Detection and Exclusion (FDE) determine integrity. RAIM and FDE depend on the number and geometry of satellites visible to a user at a given location. RAIM requires a minimum of five visible satellites in order to detect a failure. FDE requires a minimum of six visible satellites.7 RAIM availability is predictable and can be provided to a pilot during pre-flight planning.
Developing a GPS RAIM Outage Prediction SystemIn the early 1990s, the US Air Force asked the Volpe Center to develop a system for predicting RAIM availability for selected military airfields. The predictions were intended to aid pilots in planning their flights to these airfields because they would know whether they could rely on GPS. The system was based on a computer program that calculates RAIM availability for each specified airfield and predicts the beginning and end time of each outage to within one minute. In fact, this US Air Force system was so well received that the Volpe Center went on to develop a similar program for the Federal Aviation Administration. The program determines the availability of all operational satellites. If any of the satellites are out of service, the program recalculates the outage locations and duration accordingly. When a satellite malfunctions, or is scheduled for routine maintenance, the Master Control Station in Colorado Springs, Colorado, sends a fax to the Federal Aviation Administration office that issues Notices to Airmen (NOTAMs), a service used by both military and civilian aviation. The personnel, who staff the NOTAM office 24 hours a day, enter information into the RAIM computer program so that it can calculate GPS availability. The Volpe Center program for GPS outage reporting was presented at a 1994 International Civil Aviation Organization (ICAO) meeting. The Australian delegate at the meeting realized that Australia was going to need this type of system and arranged a meeting with the US delegate to discuss it. A year later, in 1995, Karen Van Dyke from the Center for Navigation, traveled to Brisbane, Australia, to present details of the US system at a South Pacific Air Traffic Working Group meeting. Volpe Develops GPS RAIM Outage Reporting System for AustraliaThe Australian decision to implement a GPS RAIM outage reporting system was prompted largely by the 1994 partial shutdown of the Australian domestic Distance Measuring Equipment network, (DME A), which was their primary air navigation system.8 After this shutdown, the Australian Civil Aviation Authority approved the use of GPS as a supplemental Instrument Flight Rule en route navigation aid for Australian operations. The following year, Australia approved the use of GPS as a primary means en route navigation aid. The Australian plan was to develop non-precision instrument approaches for GPS, initially as overlays of the existing approaches and eventually as stand-alone GPS approaches. After learning of the Volpe work at the ICAO meeting, Captain Ian Mallett, the Satellite Program Operations Manager for Airservices Australia, visited the Volpe Center to discuss Australian navigation requirements and plans in light of the DME shutdown. An agreement with the Volpe Center for Navigation to develop a satellite outage reporting system for Australia was signed soon thereafter in 1996.
In preparation for deploying the Australian system, Karen Van Dyke met with representatives from Airservices Australia and the Australian Civil Aviation Safety Authority at a meeting in Canberra, Australia. During this meeting, Van Dyke visited with all interested parties in Australia and also flew trial GPS non-precision approach flights to several Australian airfields. In 1998, Van Dyke and Jon Parmet, also of the Center for Navigation, traveled to Australia for the installation of a RAIM prediction and outage reporting system at the Australian International Notice to Airmen (NOTAM) Office in Brisbane, Australia. The three most important inputs to the prediction system are the GPS almanac, satellite outages, and airfield locations. The almanac data provide the precise positions of all orbiting GPS satellites at a particular time. This information is downloaded once a day from a GPS receiver and, should that receiver fail, the US Coast Guard Internet Web site is used as a backup. GPS satellite outage information (data on satellites that are malfunctioning or down for routine maintenance) is received via the Aeronautical Fixed Telecommunications Network as NOTAMs from the United States. The Australian system, which runs on an IBM RISC 6000 workstation, provides pilots with GPS outage information during pre-flight planning and predicts satellite availability for non-precision approaches. Prediction information is available for access by pilots and by Airservices Australia staff via the National Aeronautical Information Processing System and also is dispatched to the Airservices Australia Internet Web server. The Australian system now serves a total of 178 airports within Australia. In addition, Australia currently is providing RAIM predictions to New Zealand and Tonga as part of a joint South Pacific RAIM prediction system. Canada recently has joined the group on a trial basis. Volpe Introduces GPS RAIM Outage Reporting System to EuropeWhen other countries learned of the Volpe Center program in Australia, they began to inquire about assistance in setting up their own GPS outage reporting systems. Germany was the first European nation to step up to the plate. In October 1998, the Volpe Center completed installation of a RAIM prediction system at the airport in Frankfurt, Germany. Testing and training also was provided. This work was performed for DFS Deutsche Flugsicherung, the organization responsible for air traffic control in Germany. Working with Germany was a new challenge for the Volpe Center because they no longer were dealing with an English-speaking nation. For example, German aviation officials understandably wanted the agreement written in German; the Volpe Center understandably wanted the agreement written in English. The obvious solution was to develop one agreement in both languages. "We had both parties agree to the English version, and then we had to translate it word for word into German," explains Karen Van Dyke. "After that, we had to develop another paper that stated that both agreements essentially said the same thing before anyone would sign them." And that was only a small portion of the paperwork involved. "Sometimes," she says, "It seemed like the agreement never would be signed." In the end, however, it was well worth the effort. The Volpe program in Germany initially enabled the DFS to commission stand-alone GPS, non-precision approaches at Munich, Augsberg, and Braunschweig airfields. Now, the system provides service to 42 airfields, including two in the Netherlands. Other European countries such as Switzerland, Austria, Sweden, Norway, Finland, and the Netherlands have expressed interest in working with Germany to expand this capability to their countries. Developing a Desktop GPS RAIM/FDE Outage Reporting System for ChileWithin the past year, the Volpe Center worked with the Chilean Aviation Authority, Direccion General de Aeronautica Civil, to establish a GPS outage reporting system for the Chilean Flight Service Centers. This work was initiated by "word of mouth" when a delegation of Chilean air navigation officials, who were invited to the Federal Aviation Administration Air Traffic Command Center in Herndon, Virginia, expressed interest in GPS and outage reporting. The Chilean system is unique in that it is a Windows-based system. It is the first RAIM system that the Volpe Center has developed for a desktop computer. In addition to satisfying the air navigation needs of Chile, the system also can be used as a marketing tool both by Chile and by the Volpe Center. "When something is developed on a workstation, you only can show slides," says Karen Van Dyke, "but this system can go right on a laptop." A similar desktop system now has been developed for Germany and both the Federal Aviation Administration and Australia also have expressed interest. Charting the Future: the European Market and System CompatibilityEarly this year, the European Union announced that it plans to go ahead with the development of its own $2.4 billion to $3.2 billion satellite navigation system called Galileo. A similar system called GLONASS or the Global Orbiting Navigation Satellite System is operated by Russia. It is similar in composition and function to GPS, but it is not fully operational. Galileo is designed to be compatible with GPS so that future receivers, and related technology, will be able to use signals from both systems. Deployment of the Galileo constellation, which will include 25 to 36 satellites, will occur between 2005 and 2007. Galileo is scheduled to become fully operational in 2008. Although some people feel that there is competition between GPS and Galileo, Van Dyke points out, "More satellites will provide redundancy and additional availability." Recent Developments: GPS ModernizationGPS is undergoing a Modernization Program that is a joint effort between the Department of Defense and the Department of Transportation. Working with the Department of Defense, the civilian agencies of the federal government plan to add two more civilian signals to future GPS satellites in the 2010 to 2015 timeframe. The future GPS will have a total of three civilian GPS signals. Two are protected for safety-of-life applications, such as aviation, and the other will be available for non-critical civilian uses. When all three civilian GPS signals are broadcast from a sufficient number of satellites, the accuracy of GPS will approach the accuracy now only possible using Differential GPS. In addition, the improved service will be worldwide, not only where the Differential GPS service exists. Additional civilian GPS signals will enable receivers to reduce ionospheric errors with signal-processing techniques. With more than one signal, GPS also is less susceptible to unintentional interference.9 The benefits that GPS currently provides to aviation users are just the beginning. With air travel continually on the rise, GPS can assist in providing high levels of safety, while reducing delays, and increasing airway capacity. The future of GPS promises to revolutionize navigation. Endnotes
GlossaryTypes of Navigation Systems Primary means navigation is a system that, for a given operation or phase of flight, must meet the accuracy and integrity requirements, but need not meet full availability and continuity of service requirements. Safety is achieved by either limiting flights to specific time periods, or through appropriate procedural restriction and operational requirements. Sole means navigation is a system that, for a given phase of flight, must allow the aircraft to meet all four navigation system performance requirements--accuracy, integrity, availability, and continuity of service. Supplemental means navigation is a system that must be used in conjunction with a sole means navigation system. Phases of Air Navigation En route is a phase of navigation covering operations between a point of departure and termination of mission. For airborne missions, the en route phase of navigation has two subcategories--en route oceanic and en route domestic. Oceanic en route is the phase of flight between the departure and arrival terminal phases, with an extended flight plan over an ocean. Domestic en route is the phase of flight between the departure and arrival terminal phases, with departure and arrival points within the same country. Terminal is a phase of navigation covering operations required to initiate or terminate a planned mission or function at appropriate facilities. For airborne missions, the terminal phase is used to describe airspace in which approach control service or airport traffic control service is provided. Types of Approaches Non-precision approaches are standard instrument approach procedures in which the aircraft is provided with only horizontal guidance and position information. Precision approaches are standard instrument approach procedures where the aircraft is provided with vertical and horizontal guidance and position information. |
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