SBAS Brings Category 1 ILS Ability without ILS

 - August 2, 2014, 3:10 AM

The list of FAA GPS procedures using Waas, known by ICAO as space-based augmentation system (SBAS) procedures, continues to grow steadily. These include ILS-equivalent localizer precision with vertical guidance (LPV) approaches, providing centerline and glideslope guidance down to 200 feet at more than 800 Part 139 runways in the NAS, plus another 2,600 at various heights above 200 feet at other NAS Part 139 and non-Part 139 runways. At most of the non-Part 139 runways, of course, there’s no ILS, and probably never will be. SBAS is filling that need.

So do LPV approaches work? Beautifully, is the easy answer, because you can’t tell the difference between an LPV and a Category 1 ILS on the approach, except that you won’t see ILS’s occasional little kinks and jogs with the LPV. Otherwise, since they both drive the same left/right, up/down needles with equal precision, you can’t tell them apart. But the LPV provides a valuable extra: continuous distance to the threshold. So SBAS LPV is impressive, although you almost have to fly one to believe it. Alternatively, ask anyone whose airport has upgraded from a VOR approach to an LPV one. And that’s a cost-free upgrade, because SBAS requires no ground equipment, unlike ILS or VOR.

Those LPV procedures, plus other SBAS variants, are all benefits of the FAA’s continuous upgrading of Waas, America’s SBAS. But they aren’t unique to the U.S. Canada was an early SBAS adopter, as was Europe, which has commissioned more than 100 LPV procedures. India and Japan will to follow soon.

By around 2020 the world will be fully covered by several GPS-like global and regional navigation satellite constellations–America’s GPS, Europe’s Galileo, India’s INSS, Japan’s MSAS, Russia’s Glonass and China’s BeiDou. Each of them has, or will have, three or more geostationary satellites to create its own accuracy augmentation/SBAS networks. Because America’s GPS is currently the only completely operational global navigation constellation, many nations don’t want to wait for their own satnav constellations to be fully up and running. Consequently, Europe, India and Japan have opted to link their respective Egnos, Gagan and MSAS SBAS networks to GPS. Russia and China are also developing SBAS networks for their own global satnav constellations, but have not yet offered them for public use.

SBAS Equipage

So what is required to fly SBAS approaches? Basically, an SBAS processor that interfaces with GPS and Waas units, and a method to enter the data of the desired SBAS procedure. Just having GPS and Waas won’t do it, unless the Waas unit already has SBAS functionality. On a fairly new corporate airplane, the SBAS capability might already be there, including the procedure selection function through the FMS.

If the aircraft has only GPS and Waas on board, but no FMS, or has an earlier FMS that can’t accept SBAS data inputs, then an economical option might be to simply add a remotely located SBAS unit that does all the approach path calculations and presents its guidance on the current ILS display, along with a small, panel-mounted controller that will display the desired SBAS procedure. CMC Electronics has taken that route, and describes its CMA-5024 and CMA-5025 SBAS upgrade items as a “bolt-on” pair, and these Part 25-certified units are flying on a number of airliners and several corporate jets, as well as the Airbus Beluga mega-transports. But regardless of the upgrade path, the number of available permutations on the market makes it essential to have proven SBAS interfaces to the flight-deck guidance systems and the autopilot. Reportedly, some combinations may not play well together.

So far, corporate, regional and general aviation aircraft have been the primary SBAS users, probably because the major airlines are served by ILS at almost all of their destinations and don’t feel the need for a backup system. Yet there’s always an “if only” case. The NTSB’s recently released findings on the Asiana 777 crash at San Francisco suggest that with the ILS glideslope off the air, the captain being checked appeared unable to establish a steady visual approach descent path and undershot the runway. Unmentioned in the NTSB’s report is the fact that SFO’s Runway 22L–the accident runway–was also served by an FAA-certified Waas/SBAS LPV approach procedure: a 28-nm straight in, with limits of 200 feet and three-quarters of mile, with a 53-foot TCH.