The FAA’s decision to relegate the GPS Cat 1 local-area augmentation system (LAAS) to a mere R&D program (AIN, March 2004, page 1) came as little surprise to either administration officials or industry. However, while ILS stands to remain the precision approach aid of choice for the major airlines for the rest of this decade, LAAS’s big brother, WAAS (wide-area augmentation system), continues to make strides toward implementation across the U.S. and is of much greater potential benefit to the helicopter community–as it is for GA operators and even regional carriers.
Reflecting this impetus, FAA WAAS program manager Dan Hanlon was at the FreeFlight booth at March’s Heli-Expo in Las Vegas, answering questions on the satellite/ ground-station precision-approach technology that is designed to replace conventional approach aids such as VOR and ILS. He had a full house for both his formal briefings.
Hanlon briefed his audiences that, using ground-station-corrected GPS signals, WAAS can deliver horizontal position accuracy within a meter–five times more accurate than ILS (GPS vertical accuracy is plus or minus three meters, depending on the sensor’s position relative to the satellite). And that is to any point on the ground, not to where the ILS signal dictates.
Apart from purloining the name given to the whole concept, FreeFlight’s place in this lies in its product line. It designs and builds avionics hardware, including a 12-channel GPS sensor, designated the 1201, which is one of only a few cards to be certified as meeting the adopted WAAS standard (FAA TSO-145A). The device is being used as part of the FAA’s Capstone program, which has a wide brief to improve flight safety in Alaska. The Waco, Texas-based company, led by Steve Williams, took over Trimble’s aviation GPS business in 2001 and has since more than tripled its turnover. Garmin AT (formerly UPS Aviation Technology) and Chelton Flight Systems have also received certification for WAAS avionics with horizontal navigation capability.
Theoretically, pilots can already descend to 350 feet above the runway end under IFR, using satellite navigation to provide stable vertical guidance. Later this year, a new procedure will be published for the full capability of WAAS, resulting in approaches down to 250 feet above the target point, which could just as easily be a hospital helipad as a runway threshold.
En route navigation is already a reality. In March last year, Capstone conducted the first commercial flight with data from a FreeFlight 1201 GPS/WAAS receiver fed into the Chelton primary flight display of a L.A.B. Flying Service Piper Seneca. Captained by Chuck Thompson, the airplane flew from its base at Juneau, Alaska, along an “optimized” Rnav route through airspace that would not have been accessible using conventional avionics. Special FAR 97 now provides authorization for suitably trained pilots to use TSO 145/146 receivers as the sole means for en route navigation in Alaska. Trials have already resulted in an additional 41,000 feet of airspace being made available, along more than 1,500 nm of the existing route structure in southeast Alaska.
WAAS Critical, FAA Says
The FAA says WAAS is a “critical component” of its objective to create a “seamless” satellite navigation system for civil aviation. It will improve the accuracy, availability and integrity of GPS, thereby improving the capacity and safety currently provided by the National Airspace System. Ultimately, WAAS will allow GPS to be used as a primary means of navigation, from takeoff through to a Cat I precision approach. The only bit missing, it would appear, is some sort of surveillance service for aircraft plowing their lonely airborne furrow. (Automatic dependent surveillance-broadcast [ADS-B] equipment does that job for Capstone.)
Unlike traditional ground-based navigation aids, WAAS covers a much more extensive service area. Wide-area ground reference stations (WRS) are linked to form a WAAS network (see illustration). These precisely surveyed stations receive signals from GPS satellites and identify any errors in the signals. Each station in the network relays the data to one of two wide-area master stations (WMS), where correction information for specific geographical areas is computed.
GUS Calling GEO
A correction message is then prepared and uplinked, via a ground uplink station (GUS), to a geo-stationary earth-orbit (GEO) satellite. This message is broadcast on the same frequency as GPS (L1, 1575.42 MHz) to GPS/WAAS receivers aboard suitably equipped aircraft flying within the broadcast coverage area.
WAAS also provides indications to receivers if the GPS system becomes temporarily unusable due to system errors or other effects. Users can be notified within six seconds of any misleading data that might cause an error in the GPS position estimate.
In July last year, the FAA commissioned WAAS as an operational navigation aid within U.S. national airspace. From that date, and given the appropriate sensors, aviators were able to use WAAS as a navigation aid for en route and lateral navigation/vertical navigation (Lnav/Vnav) approaches down to 400 feet. Last September the first near precision approaches (known as LPV) were provided, enabling pilots to descend safely to a decision height of 250 feet agl.
Williams told AIN that by year-end 750 WAAS approach plates will be FAA-approved across the U.S., but the potential is for “at least 4,500.” He said that, by removing the requirement for approach path hardware at individual airfields and helipads, there are enormous savings to be made–anywhere aircraft need to make instrument approaches.
“The technology also removes the final obstacle to the goal of achieving precision helicopter approaches to the hover over a given point in space,” he added. At 250 feet agl, it may not be an in-ground-cushion hover over the pad, but it will at a stroke expand the operating envelope of every IFR-capable helicopter.
“Remember, though, that WAAS doesn’t make every airfield ILS redundant,” said FreeFlight engineering v-p Jim Davis. “WAAS Class 3 is roughly comparable with a Cat 1 ILS. To get Cat 3 accuracy you would still need something like LAAS.”
Unavoidable and continuing fluctuations in the ionosphere otherwise erode the goal of pinpoint WAAS accuracy. Future refinements could include tie-ins between GPS and EGNOS–the European geostationary navigation overlay service, due realistically to come on line between 2010 and 2013–combined on two different frequencies.
Japan has also established a network of WRSs but lost a vital satellite in a launch-vehicle accident. Earlier this year, India signed a contract with Raytheon to develop GNSS, its own version of WAAS. Eventually, GPS users around the world will have access to precise position data using these and other compatible systems.
Williams has recently returned from the Fidae show in Chile, where WAAS interest is said to be high, and where presentations were made to airlines, the military and the DGAC, the country’s regulatory authority.
Step by Step
As with any new technology, WAAS will be developed incrementally. The next phase, planned for 2006, will support LPV approaches over a much greater area of North America. System component additions will serve to expand WAAS service to northeastern Maine, southwest California and southwest Texas. These additional stations will also greatly increase WAAS coverage in Alaska, as well as providing a service in Canada and Mexico. The FAA also plans to upgrade the software and hardware to make the system more efficient and yield increased performance.
This phase will serve as a springboard on the path to achieving ICAO standards for a global landing system (GLS), which will allow pilots to descend safely to 200 feet using GPS/WAAS. This is expected to be achieved in 2013, concurrent with the commissioning of the second (L2) frequency in the Galileo constellation.
Meanwhile, the FAA and HAI have been discussing how best to achieve helicopter ILS approaches to 100-foot minimums at Category 1 airports. The agency’s Ken Knopp said the ultimate aim is still zero-ceiling, zero-visibility approaches. At press time, a group of helicopter pilots were starting to gather data on a series of approaches while undergoing routine training at FlightSafety International’s West Palm Beach, Fla. facility.
ILS signal integrity, currently the largest issue in achieving such minimums, is likely to become moot as the National Airspace System moves away from such ground-based navaids. Having the technology does not mean that the access to airspace will follow just as night follows day. WAAS in particular, said Knopp, will be the nearer-term solution to a helicopter-specific low-altitude route structure that can reduce pressure on the national ATC system.
For now, though, it is safer to back both runners. The time when helicopters take a place in the wider transport infrastructure–flying non-conflicting approaches to airports and corporate helipads with equal verve and aplomb–might not be as far away as we all feared. Ironically, as it turns out, WAAS could deliver even greater accuracy if only it were teamed with a ground station, such as LAAS. But it seems that, to paraphrase the Rolling Stones, you can’t always get everything you want.