-T / T / +T | Comment(s)

Friday, February 1, 2002

WAAS: Back in Step

Auigmented satellite navigation stumbled after questions arose of its meeting all requirements. But now the U.S. system appears to be off and running and so, too, may be its European counterpart, EGNOS.

David Jensen

After two years of relative obscurity and delay, the Federal Aviation Administration’s (FAA’s) Wide Area Augmentation System (WAAS) program is gaining renewed prominence, as it approaches for a major milestone. The agency and WAAS prime contractor Raytheon have ostensibly resolved the very challenging issue of verifying the system’s integrity. In addition, the U.S. Congress has agreed to fund the WAAS program fully in 2002, allocating $75.9 million, plus an additional $5 million toward securing another communications satellite for the system. By May, FAA plans to begin a rigorous test and evaluation program of WAAS, leading to contractor acceptance in March 2003. Nine years after former FAA Administrator David Hinson initiated the program, WAAS would then be commissioned for use as a primary means of en-route navigation. And that will be a major step toward the long sought goal to establish a single, integrated global navigation satellite system (GNSS). It also will be an essential step toward National Airspace (NAS) modernization, according to FAA.

A Matter of Integrity

But first the issue of verifying WAAS integrity (the assurance that navigation information is sound) must be addressed. Light was shed on the issue about two years ago, just when Raytheon was beginning acceptance testing. A review document from the General Accounting Office (GAO) stated the WAAS integrated product team (IPT) "did not work with the contractor [Raytheon] to determine how this requirement could be proven" and "that the contractor’s chosen approach to demonstrate integrity was inadequate."

The FAA established the WAAS Integrity Performance Panel (WIPP) to determine the data needed to verify integrity. Subordinate to a 15-member independent review board (established to review program activities and provide advice), the WIPP comprises FAA and Raytheon officials, specialists from Mitre Corp. and professors at institutions such as Stanford University and the University of Ohio.

Responding to concerns about system integrity, Raytheon has been monitoring WAAS performance, gathering data for certification and redesigning its integrity monitoring software at its Fullerton, Calif., facility. The WIPP has been making monthly visits to oversee the manufacturer’s progress, according to Dan Hanlon, FAA’s WAAS program manager. The integrity monitoring software–which is being developed using DO-178B, level B, software assurance standards–must prove WAAS can demonstrate a no more than a one-in-10 million chance, or 10—7, of broadcasting misleading navigational data.

"We currently are ahead of schedule," says Hanlon, confident the redesigned monitoring software will be installed and operational in the WAAS system’s two wide area master stations by May 2002.

Supporting Hanlon’s confidence, John Britigan, Raytheon’s director-satellite navigation systems, reports, "So far, the WIPP has given technical concurrence to virtually all the algorithms" for the monitoring software.

The integrity requirement for WAAS certification stipulates that, within 47 years, no more than one instance of misleading information may be transmitted without a consequent warning within six seconds. It is one of four requirements, the others being accuracy, availability and continuity.

"Accuracy is no problem," says Britigan. "We’ve consistently proven WAAS’ accuracy down to 2 to 3 meters [6.5 to 10 feet] vertically and down to about a meter [3.25 feet] horizontally."

The system corrects signals refracted by the ionosphere, an electronically distorting set of layers in the Earth’s atmosphere. By comparing the simultaneously transmitted signals from the two GPS bands, L1 (1575.42 MHz) and L2 (1227.60 MHz), WAAS can minimize the error caused by ionospheric time delays.

The WAAS team also believes it has satisfied the continuity of navigation requirement, which calls for 0.99999 (or 99.999 percent) per hour continuity during en-route flight and 0.999945 continuity per approach during precision approaches. Continuity assures the WAAS service performs without interruption throughout a phase of flight–for example, a precision approach.

WAAS availability also must eventually be 0.99999, which means it can be non-operational only a few minutes a year. "Essentially, if you have continuity, integrity and accuracy, you have availability," says Britigan.

"For availability, we also need considerable redundancy, and we need more geosynchronous satellites," Britigan adds. FAA points out that, with only two geosynchronous satellites serving the United States, WAAS currently "is a single failure away from reducing coverage by 50 percent."

How It Works

A better understanding of FAA’s plans to establish redundancy for a fully operational WAAS calls for a brief description of how the system works.

Generally, WAAS will provide differential corrections to GPS signals that will allow users to calculate their positions more accurately. To augment GPS accuracy in providing three-dimensional directional signals, the FAA has established 24 wide area reference stations scattered throughout the United States. These stations compare the ranging data from the 24-satellite GPS constellation with their own precisely surveyed positions. Each station has triple redundancy, with three antennas, three receivers and three data processors to collect the basic GPS data. It also includes discrete connections to each of two WAAS master stations in the United States, in Leesburg, Va., and Palmdale, Calif.

In turn, the wide area master stations determine nationwide corrections for the GPS data and the correction data’s integrity. They then format and transmit the corrected data via four ground communications stations to the geosynchronous communications satellites. (Lockheed Martin is prime contractor for the satellite communications.)

The communications satellites each house a transponder with a C-band uplink and both an L-band and a C-band downlink to transmit to aircraft. Because it simply relays the data that comes up and then goes down again, the transponder is referred to as a "bent pipe."

Presently, two geosynchronous satellites serve WAAS. Both are Inmarsat 3 communications satellites that have space to lease for the WAAS transponder. The satellites are positioned over the two oceans bordering the United States. WAAS officials refer to the satellites as POR (Pacific Ocean Region) and the AOR-W (Atlantic Ocean Region-West). There exists an AOR-East satellite, as well; it serves the European Geostationary Navigation Overlay Service (EGNOS), Europe’s counterpart to WAAS.

AOR-West provides coverage throughout the continental United States (CONUS), while POR covers Hawaii, much of Alaska and parts of the West Coast. There exists little overlap of coverage, which is why, for redundancy, FAA seeks a third communications satellite for WAAS, to be positioned over the central United States.

Third Satellite

The congressional allocation of $5 million for the third satellite will "cover non-recurring engineering costs to design the transponder and to pay the upfront cost to lease satellite space," says Hanlon. Satellite space could be leased from Inmarsat; however, FAA is eyeing other possibilities, including leasing from a Canadian satellite TV service. (In 2006, the agency will have to negotiate for still more satellite space, as its lease with Inmarsat will have been expired.)

The third satellite, to be "deployed and broadcasting hopefully by 2004," will be the first with the capability of transmitting the L5 (1176.45 MHz) signal, according to Hanlon. L5 will be the third civil GPS signal, which was made available by the World Radio Conference in June 2000. (The second civil signal, to be on L2, is intended largely for ground use. Unlike L5, it is not ARNS-protected and thus can not be used for safety-of-life applications.)

The third satellite, which would be located over the center of the CONUS, will provide the WAAS program with dual redundancy over the continental United States. However, the WAAS program calls for nationwide triple redundancy, and that will require more than three geosynchronous communications satellites. How many? "It depends on lease opportunities and orbital location; it could be a total of three to five," says Hanlon.

Infrastructure expansion to include more communications satellites will follow WAAS commissioning, according to Hanlon. To achieve triple redundancy and "risk mitigation," the expansion also calls for two more wide area master stations, in Chicago and Atlanta.

FAA plans to add to its 24 ground reference stations, as well, though the agency has not yet determined how many will be required. "We initially thought we would need up to 50 stations," says Hanlon, "but it appears we may only need five or six more."

The infrastructure buildup would lead to the establishment in 2009 of what FAA calls an "end-state WAAS." The system then will provide a Cat I precision approach capability at all airports, not just those with instrument landing systems (ILS). WAAS will be fully operational at that time.

WAAS also will provide precision guidance for curved approaches and for missed approaches, neither of which are possible with ILS. Airports in mountainous regions and near urban environments will welcome these features, as they will help enhance safety and reduce noise pollution.

FAA was able to establish a phased approach toward Cat I and the end-state WAAS. "Time was when you had only three options for navigational standards: en route, non-precision approach and precision approach," says Britigan. "But with the precision of satellite navigation, we have many more options." And this allows FAA to advance WAAS at a prudent pace. FAA won’t confine the phases within a strict schedule, but Hanlon says the agency hopes to enter a new phase every two years.

In 1995, the FAA’s Office of Aviation Systems Standards in Oklahoma City began the WAAS approval process by first certifying non-precision area navigation (RNAV) approaches using GPS. Little more than a year ago, FAA took a giant step forward by approving lateral/vertical navigation (LNAV/VNAV) approaches using GPS, which represents the first phase of WAAS.

According to Jim Snow, Aviation Systems Standards-GPS program manager for FAA, the minima for LNAV/VNAV approaches are a 400-foot decision height (DH) and 1-mile visibility at airports without approach lights and a 300-foot DH and 0.5-mile visibility at those with approach lights. "We’re close to Cat I [capability] with LNAV/VNAV," adds Snow, who describes Cat I minima as 300-foot DH and 0.75-mile visibility where there is no approach lighting and 200-foot/0.5-mile where approach lighting exists.

Capstone Bound

"We have more than 3,500 sites approved for GPS approaches," says Snow. "About 300 of those are for LNAV/VNAV." The next step will be approvals for the more accurate approach with precision vertical guidance (APV), leading up to the GNSS landing system (GLS) approach.

WAAS capabilities are most urgently desired in Alaska, which has a large, bustling aviation community but few navigational aids. "We’ll test WAAS procedures in Alaska," says Britigan.

"WAAS will be part of phase two of southeast Alaska’s Capstone Project," adds John Hallinan, Capstone program manager. "We intend to receive the first 150 to 200 [WAAS] display systems and receivers for the project." He adds WAAS will be "one slice of the pie" for Capstone phase two. Other "slices" will include data link communications and automatic dependent surveillance-broadcast (ADS-B).

Chelton Flight Systems, Boise, Idaho, was selected in late 2001 to provide a navigational display and primary flight display, as well as WAAS receivers, for Capstone phase two. A demonstration of the systems is scheduled for March 2002 in Juneau.

"The driving need [for WAAS] here is for en-route navigation, though we’ll take whatever approach capability we can get," says Hallinan. Alaska has few ground navaids and those that exist are "Lord almighty expensive to maintain," he adds, explaining the need for en-route WAAS.

Thousands of Users

Capstone may be WAAS’ entry into aviation, but the system already has found many users–more than 200,000, according to Britigan. Certified for non-safety-of-life missions about a year ago, WAAS has gained popularity among operators of marine craft and crop-spraying aircraft. Two wireless phone makers selected WAAS for the positioning solution that the Federal Communications Commission (FCC) requires for emergency 911 use.

"It was selected because you can pick up signals even in the canyons of high-rise buildings," Britigan explains. "And, at the World Trade Center, rescue teams used WAAS to survey the site during the recovery program," he adds, referring to the aftermath of the Sept. 11 terrorist attacks.

The LAAS Program

The Wide Area Augmentation System (WAAS) proposes to allow GPS approaches using Category I minima. But attaining greater precision–down to Cat II, III or IIIb minima–requires the capabilities of another FAA development program: the Local Area Augmentation System (LAAS).

While general aviation desires the universality of WAAS, the airlines prefer the precision of LAAS, which would transmit correction data to aircraft via a VHF data link from ground facilities near airports, rather than from communications satellites. The U.S. Congress, too, must like LAAS, as it allocated $43.2 million for FY2002, more than the FAA requested for the program.

A request for proposals (RFP) is expected by early March to acquire LAAS ground facilities from a single source. Three bidders are expected to respond: Honeywell, Thales ATM and Raytheon Systems.

"We get about 98 percent availability from the ground navaids," says Hallinan. "Raw GPS gives us 99.9 percent availability, and with WAAS, we expect it will be better yet."

The third geosynchronous satellite will be key to WAAS precision approach performance in Alaska. Currently, mountains can shadow the transmitted corrections from the lone POR satellite, as its signal reaches north and begins to hug the horizon. But the second source of signals from the third satellite should provide more complete coverage, even to Juneau’s airport, which is shadowed by tall peaks and where much of Capstone phase two will take place.

What’s Happening with EGNOS?

WAAS isn’t the only space-based augmentation system (S-BAS) making strides. Europe’s counterpart, the European Geostationary Navigation Overlay Service (EGNOS), was to enter critical design review (CDR) in late January or early February 2002. If the review goes as expected, production to establish the EGNOS infrastructure will receive a green light, leading to an operational readiness review in the first quarter of 2004.

However, a service provider and EGNOS system operator must first be selected, and it should be done "as soon as possible," according to John Storey, Eurocontrol’s program manager for global navigation satellite systems (GNSS). The provider/operator is to be selected during the first quarter of this year, following a competition, says Storey. It then will take over EGNOS from the program developer, the European Space Agency (ESA).

Like WAAS, EGNOS would initially be served by two geosynchronous communications satellites, one in the Atlantic Ocean Region East (AOR-E) and the other in the Indian Ocean Region (IOR). The signal footprint from two satellites, both owned by Inmarsat, would provide considerable overlap for redundancy.

Also, as planned for WAAS, ESA has launched a third satellite, called Artemis. Its primary mission is intersatellite communications, but it also carries an EGNOS transponder. However, the French rocket launched from French Guiana last July delivered the satellite to an altitude about 11,200 miles lower than expected, so ESA must position Artemis into its correct orbit before the satellite can be used. The EGNOS infrastructure also is to include 34 ground-based monitoring stations and four master control stations at London-Gatwick airport, Frankfurt-Langen airport, Rome and Madrid.

The EGNOS program office is based in Toulouse, France. So, too, are the EGNOS satellite test bed (ESTB) and the station that uplinks navigational corrections to AOR-E, the only satellite now serving the system. In addition to AOR-E, the ESTB has 10 ranging integrity monitoring stations scattered throughout Europe and a processing center in Norway to compute the navigational corrections.

Among the ESTB’s recent exercises was a series of approaches, including curved approaches, to the airport in Nice, France. Conducted for operational validation, the trials employed a Citation II research aircraft owned by the Netherlands’ Nationaal Lucht-en Ruimtevaartlaboratorium (NLR).

EGNOS is part of GNSS-1, an initial program that is developing satellite navigation using the current GPS constellation. A subsequent, GNSS-2 program will be based on a civilian controlled satellite constellation, Galileo.

[X] Dismiss Ad
Live chat by BoldChat