LPV Acceptance Marks End of ‘Dive and Drive’

 - November 2, 2012, 12:35 AM

We often think of the FAA as a cumbersome organization that usually–but not always–gets the job done, often in its own sweet time. But with its Waas space-based augmentation system (Sbas) program, the agency has shown it can also move quickly.

About 60 years ago, the FAA commissioned its first Category I ILS precision approach aid, and today the agency has 1,280 Category I systems across the U.S., plus 160 Category IIs and 113 Category IIIs, for a total of 1,553. ILS has been a great system, and will be with us–at least in the form of Category IIs and IIIs, and a number of Category Is in critical locations–for many years ahead.


About six-and-a-half years ago, the FAA approved its first approach procedures for Category I-capable Sbas localizer performance with vertical guidance (LPV), and by September this year had published a total of 2,989 procedures, of which about 1,300 have been for Part 139 airports and 1,700 for non-Part 139s. More than 650 of the total have a 200-foot decision height. The FAA plans to produce approximately 5,200 LPV procedures by 2018.

Fortunately, FAA Category I services aren’t a political issue, or else we’d have some wingless but statistically minded politician asking why, on average, the FAA has commissioned just one Category I ILS every 1.8 months since the 1960s, while the equivalent Category I LPVs are appearing at an average of one every five days or less. It’s a bogus comparison, of course, yet a telling one, because to provide precision approach guidance, the installation of the ILS localizer and glideslope transmitting antennas and their associated monitors and controls is an exacting and lengthy undertaking that can start at $500,000 and escalate rapidly, depending on the terrain and other factors. In comparison, all the guidance smarts they provide now come for a lot less money with a Waas Sbas avionics upgrade or, in newer units, integrated within the GPS receiver.

And that, of course, is the key to the rapid acceptance of Sbas by the FAA, airports and operators, because beside the cost factors, the rock-steady Sbas localizer and, more important, glideslope indications provide greatly enhanced operator confidence and safety over the “dive and drive” NDB or VOR non-precision approaches. In fact, Sbas dramatically improves GPS approach accuracy, by reducing its “legacy” non-Waas +/-10-meter positioning to +/-2 meters. As Universal Avionics demonstration pilot Jerry Harkin told AIN, “GPS will always bring you down somewhere on the runway. With Sbas, it always puts you on the center line.”


Now, all non-U.S. navigation satellite system operators are moving to incorporate Sbas service. For example, Europe’s GPS-like Galileo has Egnos, its Waas equivalent, in operation, although it is currently providing accuracy corrections only to GPS over Europe while the Galileo constellation builds up its satellite complement. When that is complete, Egnos will automatically provide Sbas service to either GPS or Galileo, depending on which offers better signal quality and/or better satellite fix geometry at the aircraft’s position. It has also been proposed that Egnos may also provide Sbas service to Russia’s Glonass while its SCDM service builds up. And to complete the world picture, we must include the fourth fully global satnav system–China’s Compass–and its BeiDou Sbas component. There are also two regional navigation satellite systems (RNSS) under way, with the Indian RNSS fielding its Gagan Sbas, and Japan’s RNSS building up its QZSS Sbas.

A key attribute of the ICAO Global Navigation Satellite System (GNSS) is interoperability of satellites and Sbas components of the different constellation–GPS, Galileo, Glonass and so on–to create a truly seamless navigation system. For example, while it is geographically unlikely, it would be possible in the future for a satellite navigation receiver to calculate its position from the four required satellites by taking one each from, say, the GPS, Galileo, Glonass and Compass constellations. And, by the way, the receiver automatically selects which satellites to use, as it does with GPS today.

Not surprisingly, acronyms also change outside the U.S. The FAA selected the acronym LPV to emphasize the equivalence of LPV and ILS. Nevertheless, ICAO has adopted APV, for approach precision with vertical guidance, and it seems likely that APV will become the standard term.

In Europe, France is the APV pacemaker. At an Sbas meeting in Madrid in June, France’s GNSS program manager, Benoit Roturier, said, “We plan to provide APVs for about 200 of our IFR runway ends, along with vertical guidance everywhere and the ability to cut back on the dense network of ILS Category I systems by providing an equivalent capability.” Roturier spoke of major benefits in safety, ILS maintenance cost reductions and improved ATC, and he also described the mix of aircraft that are already using APV approaches on some of the first 20 airports for which APV procedures are either certified or in development. These include business and airline twinjets, a Dassault Falcon demonstrator and the Airbus Beluga. Several other European nations noted that they are also planning APV procedures at their airports.

The End of ILS?

Mark Twain wrote, “Reports of my death are greatly exaggerated,” and so it is with ILS. Certainly, new Category I installations seem unlikely in the future and, as France has indicated, LPV will cause some current installations to be de-commissioned as the Sbas user community increases. But it will be a gradual process over several years, and system experts expect that some could still be operating beyond 2040.

It will also be a long process for the inevitable satellite replacements for Category II and Category III ILS, since Waas doesn’t, and isn’t expected to, reach the higher signal integrity levels of those more demanding procedures. An alternative concept has been proposed: the ground-based augmentation system (Gbas) would process the fine-grain signal measurements at the airport rather than in space and send satellite error corrections to individual aircraft on the approach via datalink. But development is only just getting under way and seems likely to result in a hybrid Gbas/inertial solution. Currently, a number of private Category I Gbas units are installed at test locations in the U.S. and other parts of the world, but only one, in Germany, is understood to have been certified for exclusive use by a specific airline. It is unknown whether, in the fullness of time, that effort will continue in light of the rapidly growing LPV/APV user community.