WAAS Hits The Street

In Garmins 295, it delivers gnats whisker accuracy. Then again, we havent needed GPS to find the driveway yet.

With all the pomp and circumstance of a Taco Bell Grand Opening, the FAA last fall announced that its long- awaited wide area augmentation system for GPS was finally here. The WAAS satellites were in geosync orbit, the signal was up to snuff and, with a grand flourish, the FAA pronounced WAAS suitable for VFR navigation.

VFR navigation? Huh? The government has chewed through the better part of a billion dollars to field a system thatll get hikers to within sniffing distance of a night latrine or keep a drunken power boat driver in center channel, but for pilots, WAAS is, in the words of Cactus Jack Garner, about as useful as a bucket of warm spit.

Nonetheless, through a minor software revision, Garmin has forged ahead and made WAAS available for its popular GPS 295 color moving map. We recently flight and road tested the system and, sure enough, it cranks the accuracy of GPS down to the couple-of-feet level. Impressive. But itll be awhile before that does anything significant for pilots.

A Primer
In case you dozed through the 200 or so articles written on WAAS, heres the nutshell explanation of what it does. Although basic GPS is accurate-especially since selective availability has been switched off-because of ionospheric and other errors, its not accurate enough for precision approaches. Moreover, unlike VOR, which trips a flag the instant the stations signal integrity falls apart, GPSs navigation errors can drift subtly awry without the receiver necessarily detecting the fault. WAAS does two things: It broadcasts over a wide area a differential correction which damps iono errors and it sends an integrity message, alarming the receiver if any GPS satellites wander out of spec. In addition, the WAAS satellites-there are two-also transmit a standard GPS navigation message.

In its current configuration as a trial system only, WAAS has 25 ground-based signal sampling or reference stations which collect data for transmission to two master stations, which uplink the WAAS corrections to two geostationary satellites. Signal analysis is so-called bent pipe, meaning that the data is massaged by the ground stations, the corrections are calculated and the results uplinked for distribution by the satellites. Eventually, three geostationary WAAS satellites are planned to cover the full U.S. Presently, there are only two, thus WAAS coverage can be spotty at times. you’ll likely see better coverage from Colorado west than you will on the east coast.

Garmins First
Ironically, with WAAS limited to VFR only, even if a manufacturer has a WAAS-capable IFR-certified receiver-none do, as far as we know-it couldnt sell one to you. Enabling WAAS in the receiver would invalidate the IFR TSO and youd have a $10,000 box incapable of flying approaches. (No one said this stuff has to make sense.) No such problems in the 295. All you need is a software upgrade to version 2.02 and, poof, instant WAAS. Augmentation can be toggled on or off on the 295s system set-up page. Thats a good idea, too, because WAAS processing slows the receiver a little and itll burn through batteries even quicker than it already does. We can see situations where youd want to toggle the WAAS option off.

It takes a few minutes-perhaps up to 20-for the receiver to download all the data it needs to process WAAS. Even though WAAS satellites are similar to plain-vanilla GPS sats, they have their own almanacs and emphemerides. Once the data is loaded, WAAS comes on line quickly. When its locked in, the signal bars on the sat status page display a small D for differential.

Well, at least thats how its supposed to work. As we noted, the WAAS satellites are stationary in the sky which is good if youre right under one, but bad if the sat is on the horizon. In experiments with the 295 on the east coast, we noted that the WAAS signal is quite tender. While flying or driving on the open road, the WAAS differential locks in relatively reliably. In a wooded area or around structures that block the signal, forget it; you’ll be WAASless.

Garmin provided us with a WAAS-capable GPS 295 but if you have your own, you can download the software from Garmins Web site and flash it into the receivers memory using the instructions provided. Once thats done, select WAAS enabled and youre in business. When the receiver is seeing the WAAS satellite-not always the case, as noted-performance gains are most dramatic in repeatable horizontal accuracies of two or three feet, versus twice that or more with WAAS off. We also noticed that GPS altitude accuracy improves noticeably, although perhaps not to the point youd descend using only it as a reference.

Amazing as this accuracy is, its of little practical use in the air, although we could see it being of benefit to boaters trying to find a buoy at night or when using a GPS for land navigation, when the width of street makes a difference.

Otherwise, the real benefit of WAAS, indeed, its primary purpose, awaits the FAA declaring the system operational, which is at least a year or two off.

Meanwhile, while the FAA dithers, none of the manufacturers are yet sure that current IFR-approved panel mounts will be WAAS capable without at least some modification, meaning it may not be just a software upgrade.

But thats all it takes to get a 295 WAASable. And since the price is right-its free-we think its worth the effort, if only for the gee-whiz factor. Contact Garmin at www.garmin.com.

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