Imagine flying a GPS approach that provides vertical as well as lateral guidance, just like an instrument landing system — and offering the same 200-foot ceiling and half-mile visibility minimums as a Category I ILS. But this new, standardized precision approach is simpler than many ILSs; it doesn't require a procedure turn or feeder routes. And loading the approach is as easy as entering a five-digit number into the GPS receiver.
Well, that day isn't here yet, but it's getting closer.
The FAA's Wide Area Augmentation System (WAAS), which will supplement the Department of Defense's constellation of Global Positioning System satellites, is designed to increase the accuracy of GPS navigation by transmitting additional position and integrity data from ground stations to aircraft — and other users — via geostationary satellites. WAAS is necessary to achieve GPS precision approach capability whether or not the Department of Defense intentionally degrades the GPS signal.
WAAS will ultimately support precision approach minimums as low as 200 feet height above touchdown (HAT) and one-half statute mile's visibility. A 1998 study by the Johns Hopkins University Applied Physics Laboratory concluded that WAAS will allow pilots to rely on GPS as a sole means of navigation — meaning that many of today's ground-based NDBs, VORs, and ILSs eventually could be decommissioned, and the associated receivers removed from aircraft.
"The Wide Area Augmentation System provides us with a substantially larger number of runways where we can provide vertical guidance," explained Don Pate, manager of the FAA's flight procedures standards branch. Today, only 635 of the nation's 5,300 public-use airports are ILS-equipped — and most of those have only one ILS — so there are considerable benefits to be realized from precision GPS approaches. The FAA hopes to commission all of the new GPS approaches by 2006.
Eventually WAAS will be joined by the Local Area Augmentation System (LAAS), which will transmit correction signals from ground stations directly to aircraft. In tests, Category II and Category III approaches have been successfully flown using LAAS.
WAAS is scheduled to become operational in September 2000, but pilots may begin to see new WAAS-compliant approach charts as early as January. The FAA recently issued a notam ( http://gps.faa.gov) explaining the changes that will be coming to government-produced instrument approach charts.
Because future air navigation will reference waypoints defined by latitude and longitude, not by ground-based radio stations, the FAA has developed a new charting format for area navigation (RNAV) instrument approach procedures. In fact, the new RNAV charts won't even include GPS as part of the approach's name; aircraft equipped with appropriate flight management systems can fly the same approaches. "This also provides us with a way of incorporating the current [TSO] C129 line of equipment," Pate observed. Because multiple approaches to the same runway will be depicted on one chart, pilots will not have to carry as many approach charts in the cockpit, he added.
Several years ago, the FAA and the National Ocean Service (NOS), the charting arm of the National Oceanic and Atmospheric Administration (NOAA), began a review of current approach charts. "We were looking for a new chart design that would be more user-friendly, and better from a human factors perspective," Pate said. The new charts represent NOS's first use of a format recommended several years ago by human-factors researchers at the U.S. Department of Transportation's Volpe National Transportation Systems Center. (See " Approaching the Approach," June 1996 Pilot. Jeppesen approach charts already include many of the Volpe enhancements.)
The new approach charts will be titled "RNAV RWY XX" and may contain as many as four lines of approach minimums: WAAS, LNAV/VNAV (for lateral navigation and vertical navigation), LNAV (lateral navigation only, comparable to today's nonprecision GPS approaches and flyable with existing IFR GPS receivers), and circling. In many cases, new approach procedures for both WAAS and unaugmented or "plain" GPS approaches — the nonprecision GPS approaches that many pilots are flying today — will follow the same ground track, and both approaches will share the same RNAV approach chart (with different minimums, of course).
In order to achieve lowest WAAS minimums of 200 and a half, a variety of requirements must be met in addition to adequate satellite availability. As with an ILS, the runway must meet precision approach criteria, including lighting and obstruction clearance. Runways without sufficient lighting will be limited to minimums no lower than 300 feet HAT and three-quarters of a mile's visibility. The associated line of minimums will be titled "GLS PA" (Global Navigation System landing system precision approach). The WAAS receiver will inform pilots if the satellite navigation requirements are not met. The term decision height will be replaced with decision altitude (DA) for approach minimums involving vertical guidance.
Atmospheric interference or poor GPS satellite geometry could cause WAAS equipment to revert to a less capable mode of operation called LNAV/VNAV. Both lateral and vertical navigation guidance are provided, but signal accuracy and/or integrity are diminished — so the minimums are higher. If alerted to this condition by onboard WAAS equipment, the pilot will have to revert to the LNAV/VNAV minimums. Because vertical guidance is still provided, the minimum altitude will be published as a DA.
An LNAV approach provides lateral guidance only, so its minimum altitude will be published as a minimum descent altitude (MDA). The LNAV minimums will also apply to existing IFR GPS receivers approved for nonprecision approaches under TSO C129, as well as to flight management systems integrating GPS sensors. (After the charts are renamed RNAV, aircraft equipped with multisensor FMSs can use the approaches.)
NOAA anticipates increased funding beginning in November for the new RNAV approach charts. "The plan is to update all of an airport's other approach charts at the same time an RNAV approach is commissioned," Pate explained. He said that the FAA is trying to design the new approaches in a standard T configuration whenever air traffic control and geographic considerations permit. "We're really working to make them straight-in," Pate said. "I think pilots are going to really like that."
And Pate doesn't mean just big-airplane pilots. Although he has been in-volved in the flight evaluation of new instrument procedures during his more than 20 years with the FAA, he has been flying even longer than that. A commercial pilot with multiengine and helicopter ratings, Pate owns a Beech Bonanza.
When will we see WAAS receivers?
There will be two certification standards for the manufacturing and installation of GPS/WAAS receivers in aircraft. FAA Technical Standard Order (TSO) C145, which is for flight management system sensors, has been out for some time, while TSO C146 — for panel-mount receivers — will be issued soon. "It has gone through the entire coordination process, including public comment, and it's on schedule to be issued in mid-October," said Bruce DeCleene, the FAA's navigation program manager.
Avionics manufacturers have been involved since work on the minimum operational performance specifications and TSOs began in 1993.
"I would say that the interest has been quite strong recently," DeCleene said. All current TSO C129 GPS and sensor manufacturers have participated in the process — and although that can sometimes cause frustrating delays, he said, it is rewarded by better results. "I think it definitely facilitates the end product."
DeCleene anticipates that certified receivers will be available in the marketplace when WAAS is implemented next September, adding that by the time you read this, some manufacturers may already be displaying uncertified prototype WAAS receivers.
What will future panel-mount navigators look like? That, too, will be left in large part to the manufacturers. "The TSO requirements define what you need to do, but don't mandate how you do it," DeCleene said. "Certainly, there are much better requirements in this area than there were for the C129 TSO." He credits the improvement to the extra work and industry collaboration that went into the process. The requirements are pretty specific in one area, however: Expect the deviation information on your WAAS precision approaches to be displayed on something that looks very much like an ILS indicator, with course deviation indicator-like scaling.
So, what should I buy?
Don't rip current TSO C129-approved, nonprecision approach-certified GPS receivers from your panel, advises Doug Helton, AOPA's vice president of air traffic services and technology. "We actually have a lot of capability in a TSO C129 box right now," he said. Not only can it be used to fly new GPS approaches, but it is approved for use in lieu of an ADF or DME on traditional approaches. WAAS approaches will eventually provide lower minimums, he added, but if existing nonprecision GPS approach minimums are adequate for your needs, the receiver has many more years of service remaining. It can also provide redundancy for a WAAS receiver when the final transition to GPS is made.
"People who don't have an IFR GPS at this point could go ahead and buy a [TSO C] 129 box now," Helton observed, adding that today's GPS navigators may for some time offer the best bang for the buck — especially if early TSO C146 re-ceivers are expensive, as was the case when the C129 units were introduced.
"In September 2000, a GPS/WAAS receiver will provide the ability to use some limited precision approaches and avoid existing requirements to have a non-GPS approach at your required alternate airport," Helton said. "But the big advantages to equipping with a WAAS box at this point will be the availability of better interface designs, to include enhanced moving maps, and the knowledge that you wouldn't have to reequip for at least 15 years."
WAAS precision approaches may not be available in most of Alaska, Helton said, but LAAS ground stations will be installed there beginning about two years after WAAS comes on line.
Pulling the plug
Under the current schedule, the decommissioning of ground-based navaids — NDBs, VORs, and ILSs — could begin as early as 2008. However, that is based on proven system performance, the implementation of WAAS and LAAS capabilities, and user acceptance. Raytheon Corporation, the FAA's primary contractor to develop the augmented GPS system, passed a significant milestone on August 16. During an eight-day trial, WAAS demonstrated a horizontal 95-percent accuracy of better than three meters and a vertical accuracy of better than four meters. (To meet precision approach requirements, the system must provide an accuracy of 7.6 meters.) The test was the last of three key milestones in WAAS development prior to commencement of system certification. That, combined with renewed FAA commitment based on an updated cost/benefit analysis and satellite acquisition strategy, clears the way for full WAAS implementation by 2006. An owner could replace all existing nav radios with a WAAS-capable GPS as early as 2003, but the number of GPS approaches may be limited.
"If the FAA sticks to these plans and implements all the new GPS approach procedures by 2006, a GPS/WAAS box would then provide all the capability of VOR, ILS, ADF, and DME, as well as additional airport access that comes with all the new GPS approaches," Helton said. "AOPA is also working with the FAA to implement other operational benefits like VFR and IFR transition routes through Class B and C airspace based on GPS. These are the types of benefits that will make this transition worthwhile for the GA community. Aircraft owners could replace their VOR and ILS receivers with a WAAS box as early as 2003, but more likely beginning in 2006. By that time all the approaches should be in place, and avionics prices will have dropped."
When decommissioning begins, Helton said that it will be done in a phased approach and may indefinitely include some level of redundancy. Loran may be retained for GA, as well as for nonaviation users. AOPA is currently working with the FAA, the Air Transport Association, and other aviation user groups to settle such transition issues.
As the future of GPS navigation continues to take shape, the central question for pilots comes into focus. As the example above suggests, the issue for pilots who value utility and navigational flexibility isn't whether to invest in WAAS-compatible receivers, but when to do so.
Links to additional information about the Wide Area Augmentation System and other GPS issues can be found on AOPA Online ( www.aopa.org/pilot/links/links9910.shtml). E-mail the author at [email protected].
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