Tuesday, June 1, 2004
Eleven months after commissioning its GPS augmentation system, FAA is working to extend availability of the vertical component throughout the Lower 48 and in Alaska. FAA's Capstone program already is using the wide area augmentation system.
Nearly a year since commissioning the wide area augmentation system (WAAS) with initial capability, the Federal Aviation Administration (FAA) is marching ahead to maximize the availability of precision-like approach capability. The satellite-based GPS augmentation system currently provides 95 percent availability over the continental United States (CONUS) and limited availability in Alaska. A handful of vertically guided approaches with narrow lateral containment zones and relatively low minimums have been developed in the Lower 48, and many more are promised next year. There are also about 700 higher-minimum lateral navigation/vertical navigation (LNAV/VNAV) approaches, which can be flown by aircraft with WAAS receivers or flight management systems with baro VNAV.
While most of Alaska does not yet benefit from the WAAS signal's key vertical component, the system supports the high-integrity, high-availability LNAV capability crucial to achieving lower en-route altitudes while flying instrument flight rules (IFR) over the state's mountainous terrain.
LPV Performance Planned
The federal government's Capstone program is using WAAS/GPS and new regulatory authority to develop low-altitude, en-route IFR airways and create an IFR area navigation (RNAV) infrastructure from takeoff through approach. Hundreds of new WAAS avionics sets have been sold in the Lower 48, alone, and full LPV performance is planned for 2008. LPV, loosely described as localizer performance with vertical guidance, provides lateral containment areas comparable to an ILS localizer and decision heights between those of LNAV/VNAV approaches and Cat I ILS approaches. RNAV refers to the ability to fly more direct routes (at optimum altitudes) than can be achieved by flying point-to-point using ground-based navaids.
New Reference Stations
This year WAAS developer, Raytheon, will build and test four new ground reference stations in Alaska—at Fairbanks, Bethel, Barrow and Kotzebue—to generate the ionospheric data required to increase the availability of precision approach capability in the state, with the exception of the North Slope and Aleutian Islands, says Bob Jackson, Raytheon ATM's program manager for navigation and landing systems. The new stations are to be commissioned late next year.
Also under contract to FAA, Raytheon will add a third master station, upgrade the WAAS system's internal communications infrastructure, and optimize ground monitoring algorithms. Lockheed Martin will procure two replacement geostationary satellites to enhance communications coverage and Raytheon, under subcontract to Lockheed, will add uplinks to the new spacecraft. Raytheon also will upgrade reference station receivers to monitor the new geos. FAA is sticking by its original goal of Cat I-like capability, though this won't happen before 2013, when the second, aviation-protected GPS signal, L5, is to become available. GLS (GNSS landing system) promises minimums down to 200 feet. (GNSS stands for global navigation satellite system.)
Only eight LPV approaches—with decision heights down to 250 feet—so far have been approved in the CONUS, as the FAA flight inspection aircraft was not equipped with a certified WAAS receiver. But the agency expects to create 100 new LPV approach procedures in 2005 and pick up the pace thereafter, says Dan Hanlon, WAAS program manager. "We plan to put LPV and eventually GLS at every qualified runway end."
If WAAS is a safety enhancer in the Lower 48, it's crucial to safety in Alaska, where mountainous terrain blocks radar and radio transmissions and can make even the placement of traditional ground-based navaids difficult. (Most of Alaska under 10,000 feet does not have radar coverage.) Capstone already is having an impact on safety, says Jim Cieplak, Alaska site lead for Mitre Corp.'s Center for Advanced Aviation System Development (CAASD), an FAA adviser on WAAS and Capstone. He points to a recent study asserting a 20 to 25 percent reduction in the accident rate over the last three years in southwestern Alaska, the focus of activity in the program's first phase.
Making the most of existing WAAS service, Phase II "is developing a whole IFR RNAV infrastructure, based on GPS/ WAAS, from airport to airport—from departure and en route to arrival and approach," says Cieplak. With WAAS, pilots in Alaska can fly at lower altitudes than they can with VOR—if VOR is available—giving them a wider range of flight profiles. The high en-route altitudes were caused, not so much by terrain, as by the terrain's blocking terrestrial navaid signals and forcing aircraft to fly high, where ice forms. The lower altitudes increase safety for planes without deicing equipment.
What Alaskan operators want is a "usable IFR infrastructure," says James Call, an FAA aviation safety inspector in the Capstone office. "The villages they fly to don't have instrument approaches." The driver behind this aspect of Capstone is last year's special federal aviation regulation (SFAR), which allows GPS/WAAS to be used as the only means of navigation for en-route IFR operations in Alaska in areas outside the operational service volume of traditional navaids. All of Capstone's current Phase II IFR airplanes actually do have VORs and/or ADFs, although they would be "legal" without them.
After a national RNAV rulemaking comes into play, however, "we would want the SFAR to go away," Cieplak says. "We don't want to be a `cul-de-sac,' so what we learn in Alaska needs to support national standards."
Layered Failure Modes
"Going to WAAS-based receivers and airways increases the availability of integrity," Jackson explains. GPS receivers that rely on receiver autonomous integrity monitoring (RAIM) for integrity are susceptible to RAIM dropouts. Hence, this type of equipment is approved for use only as a supplemental means of navigation.
"Layered failure modes" in Capstone GPS/WAAS avionics provide pilots additional protection—WAAS to fault detection and exclusion (FDE); FDE to fault detection (FD, i.e., RAIM); FD to GPS without integ- rity; and basic GPS to a dead reckoning mode if the GPS signal is lost.
Phase II commenced with the unveiling, in March 2003, of GPS/WAAS-based minimum en-route altitudes (MEAs) on more than 1,500 miles of legacy, non-RNAV en-route segments, adding 41,000 feet of usable airspace over Alaska. Since then, FAA has added two new air traffic control (ATC) communications stations in southeastern Alaska, which probably will lower the aggregate MEAs on these legacy routes by another 10,000 to 15,000 feet.
Capstone Phase II airplanes typically are single-engine or small twin-engine models flying commercial scheduled service. About 35 aircraft from about 17 commercial operators have been equipped for GPS/WAAS and another 15 aircraft are being readied. FAA hopes to involve up to 200 aircraft—essentially all the commercial operators in southeastern Alaska—over a one- to two-year period.
In addition to refurbishing the legacy IFR routes, Capstone is developing brand-new routes using GPS/WAAS. The new routes reach all the way from Ketchikan to Haines, connecting a dozen southeast Alaska communities, only two of which are linked to the road system. This typically will lower the MEA from the current 8,000 to 15,000 feet to 3,000 to 5,000 feet, says Call.
These approaches currently use WAAS-supported LNAV procedures, as approaches with vertical guidance were not initially available, Cieplak says. Three years ago, when planning started for Phase II, LPV vertical terminal instrument procedures (TERPS) were still under development. So, to reduce program risk, Capstone decided to create the en-route structures first.
In Alaska "WAAS isn't just LPV approaches," Cieplak stresses. "It's much more than that. It's first and foremost, for navigation, being able to define the whole end-to-end IFR route infrastructure, as well as driving ADS-B [automatic dependent surveillance-broadcast] and TAWS [terrain awareness warning system]." Capstone looks at the whole system, from aircraft certification to pilot training and from air traffic controller inputs to procedures approvals.
Capstone operators with flights between Juneau, Haines/Skagway, Hoonah and Gustavus for the first time will obtain IFR approach and departure procedures, enhancing safety. The Phase II IFR infrastructure also will connect Juneau and Ketchikan to air fields in Angoon (a float plane base), Sitka, Kake, Wrangell, Petersburg and Klawock. The target date for these locations is Sept. 30.
IFR in Remote Communities
Though LNAV does not allow precision approaches, it will provide great benefits. The current public approach into Juneau for a single-engine or twin-engine aircraft, for example, is about 2,100 feet with 4-mile visibility, Cieplak says. With special LNAV procedures, operators will get about half of those minimums: 940 feet with 2-mile visibility. And communities like Hoonah that had only visual flight rules (VFR) access now have the option of IFR with an approach at 800 feet and 5-mile visibility. Expected this summer, the Juneau and Hoonah approaches have been flight tested and are awaiting FAA final operational approval. Approaches will be upgraded once availability is improved and WAAS-based vertically guided procedures are developed.
Phase III, though not yet fully defined, will extend Capstone benefits throughout Alaska. From an RNAV perspective, that means developing GPS/WAAS MEAs on all of the existing public route structure where there is good voice communications. FAA also will chart additional public IFR RNAV en-route airways—so-called Q routes—based on GPS/WAAS. The program already is working on LPV feasibility analyses and studying the possibility of using the WAAS precision lateral signal where terrain or airport infrastructure would preclude LPV approaches with low minimums.
Availability is still incomplete, however. Only 80 percent of the continental United States achieves 99 percent availability for LPV and LNAV/VNAV service. Dropoffs exist in Maine, southern Texas and southern Florida. In Alaska LNAV/VNAV and LPV approach availability is limited to the "panhandle," according to Raytheon. There are only three WAAS reference stations in Alaska—at Cold Bay, Anchorage, and Juneau—compared with 20 in CONUS plus one each in Hawaii and Puerto Rico.
Communications satellite coverage will be enhanced in CONUS and Alaska by 2007, when better positioned geostationary communications satellites are added to the system. Lockheed Martin has selected Telesat and PanAmSat to supply the spacecraft. With the current pair of Inmarsat communications satellites, about two-thirds of CONUS and all of Alaska is subject to single-point failure, explains Daniel O'Laughlin, WAAS project team manager with Mitre CAASD. The replacement geos will provide dual coverage over all of CONUS and a good portion of Alaska. There also is an option to procure an additional satellite, which would ensure that two spacecraft would be running at all times.
Four new reference monitoring stations in Canada and five in Mexico will further increase LPV availability on the edges of the U.S. coverage area and enable LPV and LNAV/VNAV approach capability in significant portions of those countries, as well. LPV approaches will be a "real benefit" in Alaska, Raytheon's Jackson predicts, "because [LPV] allows you to conduct very tightly flown approaches and accommodate the terrain." LNAV, which provides no vertical guidance, involves a lateral containment area of 556 meters, or 0.3 nautical mile, compared with 40 meters (131 feet) laterally and 50 meters (164 feet) vertically for LPV. The narrower an approach's horizontal containment zone, the fewer the obstacles that can interfere, and hence the lower the minimums.
Raytheon also will add a third master station and upgrade the WAAS terrestrial communications system with additional routers and more and better internal links. Having three master stations will ensure that two of them will be available when one is taken offline for maintenance. The reference stations and new master station are to come on-line during 2005 and 2006.
Raytheon will make a series of improvements to the WAAS grid ionospheric vertical error (GIVE) algorithm. The vast quantity of data amassed since WAAS began operations in 2000 has extended understanding of the ionosphere to the point where FAA feels that the efficiency of ground monitoring algorithms can be safely increased. The end result of this step-by-step optimization process will be to increase the availability of LPV service, a key goal of WAAS full operational capability. (The goal is 99 percent or better availability over CONUS and most of Alaska.)
Current ground monitoring algorithms for this first-of-a-kind system were designed to be conservative. When they detect any kind of ionospheric disturbance, they assume it's the worst type, Jackson explains. This built-in conservatism can occasionally remove an aircraft's vertical guidance unnecessarily during mild ionospheric disturbances, he says. The receiver—based on integrity information from the WAAS system—would no longer have confidence that the aircraft was within the "alert limits," or containment zone, established for an approach procedure. In that case, the receiver would no longer provide vertical guidance and the pilot would revert to lateral navigation with its large horizontal footprint and less desirable stair-step descent.
Gradual improvements to the GIVE algorithm ultimately will increase the availability of precise vertical guidance. Because the aircraft receiver, based on the enhanced algorithms, will be getting more precise data about the ionosphere's actual state, it will calculate correspondingly lower "protection levels," the estimates of how large its position error might be.
The lower the protection levels, the less likely it is that they would ever exceed the "alert limits" set for a particular procedure and hence the greater the availability will be. The improved GIVE algorithm will increase LPV availability by generating lower vertical protection levels in the receiver and fewer alarms about the state of the ionosphere.
Does FAA feel it can safely tweak the ground monitoring algorithms? At system startup, "you had to estimate conservatively," Hanlon says. But much more data is available now than then. "We're looking at the data, looking at how the system works and making adjustments to some of the original assumptions. We feel comfortable with that." FAA will be aided in the process by a team of experts from the Jet Propulsion Laboratory, Stanford University and Mitre CAASD, who constantly monitor the data and assess the health of the system.
Some operators participating in the instrument flight rules (IFR), navigation-intensive Phase II of the Capstone program are affiliated with Alaska Airlines. For its larger aircraft—already capable of area navigation (RNAV) and required navigation performance (RNP)—the carrier is interested in the automatic dependent surveillance-broadcast (ADS-B) side of Capstone, says Jim Cieplak, Alaska site lead for Mitre Corp. Alaska Airlines is weighing the benefits it could achieve—if it had an ADS-B-based, air traffic control (ATC) surveillance environment—in reducing delays into Juneau.
Because Juneau is a non-radar airport, Alaska Airlines delays its own airplanes coming into the state capital. The improved situational awareness provided by ADS-B for both pilots and controllers might enable the carrier to reduce these delays and land the planes using regular radar-type separation standards. Under Capstone, ADS-B was approved as a surveillance source for ATC services, including radar-type separation.
FAA initially will put ADS-B surveillance displays in the Juneau tower and connect the ADS-B back to the Anchorage air traffic control center, Cieplak says. Thus, the Anchorage center will be able to see ADS-B-equipped aircraft on a radar display. Over the next 12 months, the agency plans to install 14 ADS-B remote ground-based transceivers to improve surveillance coverage throughout southeastern Alaska even at low altitudes.