By J.R. Wilson
FULLERTON, Calif.-The first effort to make the Global Positioning System a major part of air traffic control will combine commercial off-the-shelf (COTS) hardware with cutting-edge software algorithms that program officials say will dramatically reduce existing system costs and open many remote parts of the world to scheduled airline and air cargo service.
The Wide Area Augmentation System (WAAS) will provide GPS data for all phases of commercial flight. This has the potential not only to improve en-route times, fuel use, and costs, but also to enable pilots to fly Category 1 precision approaches at a wide range of airports throughout most of North America by the turn of the century.
Engineers from Hughes Information Technology Systems of Fullerton, Calif., (http://www.hughes-hits.com) are building WAAS under a $483.5 million contract from the U.S. Federal Aviation Administration (http://www.faa.gov) in Washington.
The technology involved in the project will be almost exclusively commercial off-the-shelf - a COTS computer, router, and all the basic components.
Several earth stations will provide WAAS coverage to the United States, including Alaska, Hawaii, and Puerto Rico in the initial "domestic" phase, with additional stations going into Canada and Mexico in the second two phases of the six-year WAAS effort, which ends on November 1, 2002.
The ground stations - reference, master, and ground uplink station (GUS) - will share common computers, receivers, and routers, thus dramatically reducing hardware maintenance, training, and spare parts costs. And with nearly all hardware elements coming from existing COTS sources, WAAS will have an open architecture that will allow quick and relatively inexpensive updates as new technologies become available.
"There are some advanced receivers, but they are still receivers that are on sale in a COTS environment," says John Britigan, the Hughes WAAS program manager. "Almost every part of the system is truly open architecture, which gives great advantage to the FAA long-term because, as technology advances, we can update the system with almost no change in the system as developed."
In Phase 3, which runs from November 1999 to November 2001, FAA officials plan to replace a great deal of hardware with state-of-the-art equipment. This includes upgrades to the AIX operating system, "which is the only constant," Britigan says. "The FAA is even planning for upgrades to the algorithms, as Stanford and JPL and others work on advancing them."
While each of the three phases runs roughly 24 months, they actually overlap. Early activity on Phase 2, for example, begins in May 1998. And although Phase 3 ends in 2001, the Hughes WAAS contract actually runs through November 1, 2002, providing a year of interim contractor maintenance and logistics support.
This will provide the FAA with guaranteed contractor system logistics support once agency officials have taken over full operational control of the North American WAAS.
Open systems
Each WAAS reference station has two 19-inch racks and each master station has three. The ground uplink stations have additional equipment along with roughly the equivalent of two 19-inch racks.
Each has the same router and computer, but the GUS also has a receiver for a return signal, a router, computer, signal generator, and "safety processor" - a small computer fully separated from the others - that performs safety critical functions such as validating. This independent processor provides an extra margin of safety for critical functions should anything go wrong in the main computer.
Reference stations have three receive channels from the GPS satellite, so if one fails there will be a fallback. For the same reason, the stations also have three independent receivers and antennas.
There are two different requirements for receivers, although the same equipment may be used for both applications.
The reference station receiver brings in the raw signal from the GPS satellites. After the ground uplink station computer corrects that data, it transmits to a special communications satellite, which relays it back down to the GUS, partly to confirm the validity of the signal.
That satellite looks like another GPS satellite to the receiver, so it also serves to augment the number of satellites in view at any given time. That helps WAAS adjust the timing on the ranging signal, thus improving its overall accuracy.
Unique features
While there will be some unique features to the receiver, which will be required to perform a considerable amount of processing, Britigan says the subcontractors providing them may use an existing front end with a new processor - although that processor is expected to be COTS. The FAA has already used prototype receivers in flying test aircraft against the National Satellite Test Bed.
The companies supplying WAAS elements, from the relay satellites to the ground components, may not be the ones providing the same elements by the time the program reaches the end of Phase 1 - or anywhere else along the line.
At this early stage, Hughes is using an IBM computer with the AIX operating system; the initial router is being competed between two companies - Cisco Systems Inc. of San Jose, Calif., (http://www.cisco. com) and Bay Networks of Santa Clara, Calif., (http://www.baynetworks.com) - and connects to a standard links interface from MCI to a satellite from Comsat. But all of those are open to change.
In addition to the new algorithms, one of the most important aspects of WAAS will be safety, Britigan says. "We have to do some rather sophisticated safety analysis to assure we have the required availability, continuity of service, integrity of information put out, and so on. So even though this system is put together with COTS hardware, the overall system design has to assure that even if you have a failure in the system it will not cause bad information to be used by the aircraft. All of the software will be proven on the functional verification system prior to ever being put into the operational system."
The secret to WAAS is the ability to use a differential system to augment and improve the accuracy of existing GPS data aircraft already can receive. The ground reference stations pick up signals from the entire GPS satellite constellation, as well as from within their own Earth-bound grid. These data are then sent to the GUS for differential analysis and correction. The corrected data then are sent up to a communications satellite and back down to the aircraft, which will receive them through new avionics systems.
Awaiting WAAS format
Those aircraft systems have yet to be built because the manufacturers are awaiting a WAAS message format that has not yet been issued by the RTCA (formerly the Radio Technical Commission for Aeronautics) in Washington - an independent federal advisory group (http://www.rtca.org/).
"So the real heart of the system is the software, the algorithms we use to calculate and remove the errors, including atmospheric, ionospheric, and other errors," Britigan says. "We also will detect when you need to ignore a satellite, which eventually starts to go bad. The overall driving point in the system is safety. And one of the advantages we bring to the table is being able to detect when we have erroneous data and can tell the aircraft to ignore it.
"There are many sets of algorithms out there used to do various types of corrections," Britigan continues. "Some, such as at airports, may be very simplistic because they have a known position reference station."
For such a major step forward in technology and technique for the FAA, WAAS also is a fast-paced program by traditional U.S. government standards. "The agency`s new acquisition management system gives us the flexibility to put contracts in place quickly, saving time and money, while delivering quality products to the aviation community," FAA Administrator David Hinson said in announcing the October 29 contract award to Hughes.
Signal in space
"We will have a signal in space in the fourth quarter of 1998, possibly earlier," Britigan says. "We have a set of requirements that the system will meet at the end of `98, but we have nothing that would really be equivalent to an IOC milestone. We will have validated every component in the system by the time we sell it off to the customer and it will go operational at the end of 1998.
En route stations will cover all U.S. - and shortly thereafter Canadian and Mexican - airspace, much of that by the 1998 end of Phase 1. Category 1 precision approach for selected high-traffic airports will come with the additional reference stations included in Phases 2 and 3 to provide the required level of accuracy, says Henry Winkler, assistant manager and technical director for the Command and Control Division at Hughes Information Technology Systems.
"Once the system is in place, you`ll be able to tap in anywhere you want," he says.
Aircraft independent
"The aircraft won`t know the difference - they will be able to navigate around the world using the same (on-board) receiver. There is a plan for what I call a `North Atlantic Bridge` that will give the capability for aircraft to fly across the Atlantic with the same technology. They certainly could do the same thing in South America, Africa, Asia, and so on."
The actual size of the grid covered by any given reference station has not yet been established, but Britigan and Winkler say the differential data provided at any point along the route will be more accurate than pure GPS data, even if the U.S. government`s "selective availability" (a built-in error) has been turned off.
