Sunday, July 1, 2007
Can GPS replace ILS?
Garmin's recent upgrade to its popular GNS-430/530 navigators offers WAAS capability with vertical guidance.
For VFR pilots, WAAS offers a reliable backup to conventional navigation methods. Yes, you can put in your departure and destination and let the GPS give you a straight-line course between them, but unless it's over ground you're familiar with, you'd be wise to spend a little time with sectional charts. IFR pilots need to break out en route charts (or electronic flight-planning software) and figure out which airways to use; this is the first place where WAAS can pay off in a big way—with lower MEAs on some airways. In one example, WAAS-equipped pilots can expect to fly at 5,000 feet instead of 14,000 feet.
With the route worked out to your destination, it's time to look at the approach plates and get a preflight briefing that includes a check of NOTAMs. In the unlikely event of a systemwide WAAS failure, an areawide NOTAM will be issued. It's much more likely that you'll see site-specific NOTAMs: "…GPS IS UNRELIABLE AND MAY BE UNAVAILABLE…" or "…WAAS LNAV/VNAV AND LPV MNM UNREL…" In the first case, you can't count on using GPS in the specified area or for the specified time. If this affects you, you'll need to use some other form of navigation or change your route. If you're planning an instrument flight and your destination is in the affected area, you'll need to plan for a non-GPS approach. The second case affects which minimums instrument pilots can use for an approach.
There's one more preflight step you may need to take, depending on your equipment. Garmin supplies satellite-coverage prediction software that's built into the GNS-430/530W trainer and is available separately for the GNS-480. It requires an Internet connection and analyzes satellite coverage over your planned flight path. It then advises you of any issues that could affect your use of a navigator as a primary form of navigation along that route. It's required for any flight taking place more than 200 miles from your home airport. This means that on multiday trips you'll need to carry a laptop computer (or electronic flight bag) with the prediction software loaded, and find a place where you can connect to the Internet before launching for the day.
On instrument approaches, Garmin's GNS-480 (the first WAAS navigator certificated to TSO-C146a) can provide ILS-like vertical and horizontal guidance.
Any approach-approved GPS navigator, with or without WAAS, can fly to nonprecision straight-in lateral navigation (LNAV) or circling minimums—typically getting you down to 500 feet above the runway and requiring a mile or more visibility. If you have an approach-approved navigator with WAAS or a barometric input, you may be able to use slightly lower lateral navigation with vertical guidance (LNAV/VNAV) minimums. This looks like an ILS localizer and glideslope presentation, but may be less accurate (or have a smaller protected area at the airport you're approaching) and thus requires higher minimums—on the order of 400 or 500 feet and one mile. The most desirable approach, with the lowest minimums is localizer precision with vertical guidance (LPV), which can be as low as a category-1 ILS: 200 feet and half a mile, though 250 feet and one mile are currently more common.
In most cases, a local NOTAM will warn you if satellite coverage (or other issues) preclude certain approach minimums. However, some airports with WAAS approaches aren't included in the NOTAM reporting service. On FAA National Chart Office (NACO) plates, this is indicated by a white "W" on a black background in the briefing strip just below the WAAS channel number box. Jeppesen's presentation is a bit different but provides the same information. In these cases, you're required to plan for nonprecision LNAV or circling minimums, though lower minimums may be available when you arrive.
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