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Volpe Journal Spring 98

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Notifying Airplane Pilots of Navigational System Outages

Currently, the finest navigation aid available is the Global Positioning System (GPS), which is based on a constellation of 24 satellites that orbit the earth approximately 11,000 miles above its surface. The satellites emit radio signals that a GPS receiver uses to calculate its location with great accuracywithin 100 meters or closer 95 percent of the timeand far more precisely with receivers that incorporate differential GPS corrections.

The U.S. Air Force designed GPS in the mid-1960s to guide missiles to their targets. Block I test satellites were launched in the late 1970s through the mid-1980s; the first Block II operational satellites went into orbit in 1989.

The GPS system was declared operational for civilian use in late 1993, and it now provides navigational positioning to users ranging from airplane pilots to backpackers. Several automobile manufacturers even equip their vehicles with GPS receivers that, in combination with CD-ROM maps, can guide drivers to their destinations. If one of these cars breaks down or runs out of gas, a service vehicle can be dispatched to its location even if the driver is lost. (View Photo: One of 24 GPS satellites orbitting the earth approximately 11,000 miles above its surface.)

GETTING A NAVIGATIONAL "FIX"
To determine its location from satellite signals, a receiver must be able to "see" at least four of the GPS satellites. Moreover, the geometric relation among the five positionsfour satellites and the receivermust be within certain parameters; otherwise, the receiver cannot calculate the receiver's location with acceptable accuracy. Using computational analysis and other technological solutions, the position of the satellites can be determined through a capability known as RAIM, or receiver autonomous integrity monitoring. Performing a consistency check among the measurements from the satellites, RAIM determines whether it can "see" five or more satellites, because detection of four satellites is insufficient for GPS to operate correctly. Using measurements from these satellites, RAIM automatically determines when GPS is unreliable because a satellite is "out of tolerance," that is, when its position relative to the other visible satellites makes it impossible to calculate the receiver's position accurately. Figure 1 shows a GPS receiver display informing the pilot of his position and indicating that RAIM is available. (View Figure 1)

The Federal Aviation Administration (FAA) has decreed that all GPS receivers certified for aircraft navigation incorporate the RAIM capability. The Volpe Center has assisted the FAA in developing specific guidelines for certifying aircraft use of GPS receivers.

RAIM has enabled the FAA to certify GPS for primary means navigation in transoceanic flights as well as for supplemental navigation in domestic flights en route or in nonprecision approaches. (In a nonprecision approach, a navigation system provides only the horizontal positioning of the aircraft. GPS does not determine a plane's altitude with sufficient accuracy to guide it safely to the ground, although a system for providing differential corrections for precision approaches is being developed.)

Even though there are 24 satellites aloft, locations exist where the system cannot read five appropriately placed satellites. Coverage is further reduced when a satellite goes out of service, either for scheduled maintenance or because of a malfunction. Unfortunately, the zones where RAIM is unavailable are not fixed; they move around the globe as the satellites orbit. Even if users were informed that a certain satellite was to be out of service, they would not necessarily know where RAIM would be affected and, therefore, where GPS would be unavailable. In some locationseven when all satellites are operationalRAIM might be inoperational for as long as 50 minutes.

It's easy to imagine how aircraft pilots would feel if they were approaching an airport and relying on GPS to provide the correct distance and bearing to that airport when all of a sudden the RAIM algorithms informed them that GPS was not available. For safe flying, pilots must be able to find out during the pre-flight planning phase that RAIM will not be available during the approach. With that information, they can prepare to use other navigational procedures.

PREDICTING OUTAGES
The U.S. Air Force Flight Standards Agency asked the Volpe Center to develop a system to predict RAIM availability for selected military airfields, so that as pilots planned their flights to these airfields, they would know when they could not rely on GPS. Based on its standards for carrying fuel reserves aboard aircraft, the Air Force required notification of any outage lasting more than 20 minutes. The Volpe Center responded by developing a computer program that calculated RAIM outage information for each specified airfield, predicting beginning and end times of each outage within one minute.

The program's normal output is based on the availability of all operational satellites; however, if an operator enters the information that a certain satellite is out of service, the program recalculates the outage locations and times accordingly. When a satellite malfunctions or is scheduled for routine maintenance, the Department of Defense's GPS master control station in Colorado Springs, CO, sends a fax to the FAA office that issues Notices to Airmen (NOTAMs), a service used by both military and civilian aviation, and to the U.S. Coast Guard's GPS Information Center. The personnel staffing the NOTAM office 24 hours a day enter the information into the RAIM computer program so that it can calculate new outage predictions. The Volpe Center currently is automating the link between the master control station and the NOTAM office so that the notification can be entered directly into the NOTAM computer.

After completing the RAIM outage-reporting program for the Air Force, the Volpe Center developed a similar program for the FAA, complying with the FAA's requirement to report RAIM outages lasting more than 5 minutes. Civilian pilots can request the RAIM outage information produced by this program from the Automated Flight Service Stations.

IMPROVING THE REPORT'S FORMAT AND AVAILABILITY
The Volpe Center is working on ways to make information about GPS unavailability more readily available and more user-friendly. Figure 2 shows a prototype map plotted from the data generated by the existing computer program. It shows RAIM availability throughout the world in terms of the duration of the predicted outages5-minute intervals up to 25 minutes, plus outages lasting from 25 to 50 minutes. The map would be supplemented by a table showing dates and times of day when the outages occur, because these vary daily. As the figure indicates, RAIM availability is greatest near the equator, where more satellites are visible. (View Figure 2)

Although the worldwide availability map is interesting, localized maps are more useful when planning an individual flight. The computer program generates outage maps of any area, down to a 20-mile radius around a single airport, and can produce any desired time intervals and outage-zone sizes. Examination of the RAIM availability for Logan International Airport in Boston, MA, shows first a 37-minute outage, then 10 hours later, another outage lasting 7 minutes. The actual time of the outages varies daily, because outages on a given day occur 4 minutes earlier than they did the prior day.

To date, outage maps have been created for demonstration purposes only; no procedure exists for distributing them to pilots. The FAA's Flight Service Stations, which distribute the text-based NOTAMs, do not have the capability of providing graphics. Several alternatives are under consideration:

• The FAA's Flight Information Advisory Service currently is in the prototype stage. This service is experimenting with distributing weather data over the Internet and determining how effective this distribution is in reaching pilots. If it proves successful, adding information about GPS availability is a logical service extension.
• The direct user access terminal (DUAT) is another computer-based, remote-access system, but it is operated by private vendors using FAA data and generating value-added services, such as weather maps. There are advantages to combining weather and GPS-availability information on the same map, because a pilot flying visual flight rules in clear weather would not be concerned about GPS availability. Conversely, a pilot flying into bad weather would want to know whether GPS would be available.

As part of its work in delivering GPS information to the aviation community, the Volpe Center has just started a project with the FAA's Prototype Oper-ational Control Center (POCC) in Nashua, NH, one of nine POCCs located throughout the country. The project merges graphical output capabilities with the Washington-based, real-time aeronautical information system that produces the text output. The intent is to produce various types of graphical overlays and to see how they mesh with other FAA functions, such as air traffic control.

Although the current emphasis is on supplying information about GPS availability during nonprecision approaches to an airport, additional developments are already under consideration. Figure 3 shows a flight-plan graphic that would be highly useful to pilots: the entire proposed flight path is subdivided into segments according to RAIM availability. Prediction at this high level is far more complex than that needed for nonprecision approaches and is currently at the conceptual level only. (View Figure 3)

ANTICIPATING FUTURE DEVELOPMENTS
Although GPS currently is available only as a supplemental navigational system for flights over the United States, the Volpe Center is investigating the enhancements necessary to enable pilots to use GPS as their primary system. It also is investigating the enhancements needed to use GPS for precision approaches.

Aviation use of GPS is growing rapidly outside the United States as well. The Volpe Center signed a contract to provide Australia with the software and training to establish an Australian RAIM-prediction system. Figure 4 shows Karen Van Dyke, the Volpe Center's program manager for the RAIM outage-reporting system, meeting in February 1996 with officials from Airservices Australia to discuss this project. In preparation for deploying RAIM capabilities, Volpe Center staff not only visited with all interested parties in Australia, but they also flew trial GPS non-precision approach flights to several Australian airfields. The Australian RAIM-prediction capability is expected to become operational in mid-1998, when the system will begin providing GPS outage information to the Brisbane and Melbourne flight briefing centers. (View Figure 4.)

Although the Center's only international GPS-related contract to date is with Australia, several other countries, including Germany and Chile, have contacted the Center to investigate the capability. Fiji and New Zealand have held detailed discussions about obtaining the system from the Center and are now considering the possibility of joining with Australia's system to form a joint South Pacific RAIM prediction system. This system would be capable of serving other South Pacific nations as well. Regardless of whether the three countries now interested proceed independently or jointly, they anticipate operating GPS RAIM predictions as part of their own air traffic control systems. (View Figure 5: Map showing Australia, New Zealand and Fiji.)

Given the fact that RAIM prediction is so newit was instituted in May 1995 for military pilots and in November 1995 for the FAAthe level of international interest suggests that the Volpe Center will be in the forefront of international GPS work for years to come. It might even become known as the Volpe International Transportation Systems Center.

Contributor: Karen Van Dyke

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