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In the last two months I have tackled the job of clearing away some of the underbrush that might obscure the use of GPS for IFR operations. It's now time to start on the "how to's."

The most important observation to make about using IFR GPS is also the most obvious: the instrument flight rules (IFR) remain in force: all the basic rules and procedures outlined in the FAR for instrument operations are still there. In consequence, most of the changes associated with the adoption of GPS are limited to a simple switch in navigation technology. You still fly airways a great deal of the time and execute what appear to the outside observer to be standard non-precision approaches.

Before going into more detail, it is essential to point out a few regulatory limitations that may come as a surprise. All boil down to this: surface-based nav systems still rule the roost.

The first restriction: despite the huge jump in potential accuracy beyond that offered by older systems, GPS today still does not offer sufficient precision to permit the equivalent of an ILS approach. This means that when an ILS is available and the destination weather is bad or you simply want the predictability of a precision approach, you will switch from GPS to standard VOR/Localizer (VLOC) guidance toward the end of the flight. GPS receivers may be programmed to "follow along" on such approaches, but the information they provide, including moving map displays, is limited to "informational" value and cannot be used as a primary source of navigation.

The second: "Aircraft using GPS navigation equipment under IFR must be equipped with an approved and operational alternate means of navigation appropriate to the flight (AIM 1-1-20 e.1.b).

Bingo: the FAA will not let you fly GPS IFR without something to back up the GPS receiver. In all likelihood, this backup will be VLOC, leading to the obvious conclusion that VLOC technology is far from dead, and that all the VOR skills normally taught in IFR training programs must remain in place.

The third limitation: one of the greatest benefits of GPS has been to make IFR approach procedures available at airports that were previously limited to VFR. This freedom is somewhat restricted, however, by the fact that airports used for alternate planning must still offer at least one conventional approach procedure. The result: when conditions require an alternate, pilots must be able to reach airports that offer conventional, surface-based approach procedures.

Given the legal restrictions to GPS use, you might conclude that the system isn't really that much of an improvement over what was already in place. The requirements for an alternative on-board nav system and for conventional approach procedures at alternate airports certainly hint at a lack of confidence in GPS on the part of the FAA. Coupled with GPS' inability to rival the precision of the ILS, it begins to sound as if the true navigation revolution may still be some distance in the future.

This, I think, is a reasonable conclusion: it may one day be possible to fly around using GPS for IFR flight entirely without VOR as a fall-back system, but that day has not yet come.

So, instead of a replacement nav system, IFR GPS is revealed as an added tool, and that is the way it should be used, at least in its current incarnation. With that said, here is some information on how to use it.

For pilots who have never used Area Navigation (RNAV) in any form, the first concept to master with GPS is waypoint navigation. "Waypoint" is defined by the AIM as a "predetermined geographical position used for route…definition…" Selection and activation of waypoints are prime GPS actions and may take a little getting used to. The geographic locations are airports, nav aids, intersections and other IFR fixes; in each case, an internal GPS database associates the known latitude/longitude coordinates of the fix with its official FAA name.

Instead of choosing and entering radio frequencies, GPS users dial or press two, three, four and five character alpha-numeric combinations ("names") to specify points of interest: two characters for Compass Locators, three for VORs, NDBs and some airports, four for the remaining airports (KSFO, for example), and five for intersections, reporting points, missed approach points and DME fixes.

Since properly initialized GPS receivers always know where they are, the only action necessary to start navigation from your current position is selection and activation of a desired destination waypoint. Once a waypoint has been selected and activated, the GPS unit will display course and distance information direct from your present position to the waypoint.

As discussed above, all IFR GPS units must be accompanied by an alternative nav system (probably VOR). It follows then that aircraft must be equipped with a selector switch used to determine which signals--GPS or VOR--will "drive" the nav indicator at any given moment. Careless misuse of this switch has the result of displaying position information from the wrong source and can have lethal consequences. As a result, the name "suicide switch" is gaining popularity.
With a waypoint entered and activated, there are two modes for using IFR GPS: "OBS" and "Leg." The mode most transparent for users accustomed to VLOC is "OBS," but the default industry standard is "Leg," so OBS mode selection requires a deliberate choice.

Every IFR GPS receiver offers a button to permit OBS mode selection, and once that choice is made, the GPS unit begins to act very much like a VOR receiver: when the OBS on the VOR head is twisted to a desired course, the needle and flag respond exactly as if there were a VOR station at the location of the selected waypoint. At this point, you can think of the waypoint as a "virtual VOR" As you intercept the chosen course, the needle centers, and as you approach the "station," mileage counts down.

If you use the OBS to select a course that takes you away from your waypoint, it shouldn't be surprising that mileage counts up as you make progress along your route. What may be unexpected is to learn that this is the only way to navigate "from" a GPS waypoint. In Leg mode, all GPS navigation is "to," a fact that can take a little getting used to.

Although OBS mode may appear to simply duplicate standard VOR function, there are several very significant advantages. To begin with, you are not limited to using the real VOR stations scattered about the countryside: GPS provides the possibility of establishing a "virtual VOR" (a destination waypoint) at any location of your choice, be it an airport, navaid, intermediate fix, or even a user defined location not held in the database (use of this option limited to VFR only).

A second important difference: "station" distance and associated reception problems are not an issue. Given enough fuel, you can program any waypoint destination on the planet. NOTE: IFR GPS use in other countries is governed by local regulations.

The third GPS advantage is precision. Because VOR courses get further apart as they extend out from the station, increased distance from a VOR equates to decreased accuracy. As an example, a 4° course deviation at ten miles from a VOR station indicates an off-course error of just over a half mile. At a distance of 40 miles, the same 4° error equates to an error of more than two and a half miles.

GPS, by contrast, is unfailingly accurate, no matter your distance from the waypoint. Because the system uses real time position data from satellites rather than ground-based signals, the GPS needle (CDI) can directly indicate miles off course, rather than degrees. The enroute standard for CDI sensitivity using GPS is five miles. By contrast, a VOR CDI at full deflection 40 miles from the station equates to an off-course error of almost seven miles.

The final advantage to GPS, regardless of mode, over standard VLOC is its ability to generate a moving map. This is the feature most likely to be cited by beginning users as the primary value of GPS. There will be much more on this subject in the next installment.

There is an essential aspect to mastering OBS mode for GPS flight that goes beyond its comfortable similarity to VLOC. At three critical points on every IFR flight, you may be called on to use the GPS unit in OBS mode: holding patterns, procedure turns and missed approaches. In each case, you will find there are no pre-programmed GPS legs to take care of navigation: you have to enter OBS mode and make course selections on the nav unit just as with standard VLOC equipment.

For all the reasons cited above, it is probably most sensible to start training for GPS navigation by learning OBS mode. The steps necessary to activate waypoints, and select intercept and track courses using standard VOR techniques serve to introduce and drill many standard GPS features. For the same reason that most IFR training programs stress basic orientation drills and holding patterns at an early stage, GPS instruction should also concentrate on early basics before extending into the more sophisticated realms of flight plans and approach procedures.

Next month, "Leg" mode and flight plans.


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