GPS in the IFR System: A Guide from the Ground Up

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Despite the proliferation of GPS in IFR applications during the past few years, many instrument pilots are afraid of anything that happens after they hit the power switch. Jeremy Jankowski explores some of the information that every pilot who uses GPS in the IFR system should know, including the importance of RAIM, what goes on behind the scenes during a GPS approach, and some of the ways to utilize GPS to its maximum potential in some less-than-ideal situations.

Aviation changes so rapidly that it's easy to make yourself feel like an old-timer without spending a lot of time as a pilot. I jumped in right before the alphabet airspace arrived a few years back, and so I have always been able to tell those who came after me: "We didn't have any of those Class B and C airspace categories; we had ARSAs and PCAs and TCAs." We didn't use METARs back then. Most of us didn't use GPS, either, and LORAN, even in its best form, pales in comparison to the GPS technologies available today, which will probably be far superseded by what is available a year from now! Using VORs and (ick!) NDBs to get around seems to be like the proverbial "walking through snow, uphill, both ways," in aviation.

Surprisingly, many in the aviation community have been more than a little afraid of the GPS movement, and it's not terribly surprising. Without a doubt, the FAA's "phase-in" program, the buzzwords, the TSOs, and the computer-like logic (not to mention typing with a rotary knob) have made the transition from a device that was relatively simple (but gave little situational awareness without extensive practice, like a VOR or NDB), to one that has, literally, hundreds of functions.

Any individual who has been charged with showing me how to use any one of these elusive units has invariably said something to the effect of, "You will never learn all of the functions that this thing is capable of." Unfortunately, it's true. I could explain to you how an ADF works in an hour (though, in practice, it will take much longer than that for almost any student to really understand how an ADF works), but explaining all of the functions of a modern GPS system would take weeks and several trips to the manual for the both of us.

What is imperative to users of the GPS system, however, is a basic understanding of how the system is set up, and the caveats and pitfalls of improper use. Unfortunately, I've found a great number of pilots, flight instructors, and even commercial operators who don't quite understand the GPS as well as they think they do.

I used to teach and evaluate students in Frasca FTDs at one of the major university flight programs. One of the most prominent things I noticed with students is a consistent fear of the "magic box." On the final examinations I gave, I'd often provide the student with the choice of executing the partial-panel GPS or partial-panel NDB approach. Surprisingly, to the best of my knowledge, nobody decided to use the GPS. Upon inquiry, after they'd stumble through the NDB, they'd typically indicate that they found the GPS difficult to understand. I also found that most of the important items in its use had been glossed over by their instructors. Whether this is due to the CFI's apathy or misunderstanding (I hope mostly the latter), the fact remains that there is a generation of flight instructors teaching the next generation of pilots how to use a device that nobody taught them how to use.

How Does GPS Work?

  Satellite Triangulation
 A GPS Receiver Triangulating Position from Three Satellites

A GPS receiver works on the principle of a very precise clock. Through transmission of a signal, which essentially contains the time that the transmission was sent from a satellite, the receiver can determine the distance to the satellite. It knows how fast the signal travels (the speed of light), it knows what time it was sent (because the satellite told it), and it knows what time it arrived (with its internal clock). It does the math, and determines the distance that satellite is from the receiver's current location. The satellites themselves follow a predictable orbit, and therefore can reliably relate their current position in the transmission as well.

With only one satellite, all the receiver is capable of doing is determining that it is a particular distance away from the first satellite. Unfortunately, the sphere around the satellite at that distance could still place the aircraft east, west, north, or south of the satellite, without another way of fixing position (you could even be on the other side of the satellite, headed for the moon, for all it knows!). With several more satellites, however, your receiver can fix your position three-dimensionally with reasonable accuracy (for enroute operations, a minimum of four satellites).

Have You Ever Seen the RAIM?

Let's say, for the sake of argument, that the internal clock on one of the GPS satellites becomes incorrect. It doesn't take much — 0.0001 seconds off equates to a position error of roughly 16 nm! Unfortunately, if there aren't enough satellites, your receiver might go on believing the broken satellite, and fix your position incorrectly by several miles. It's not a good thing to be going into Aspen, Colo., without being able to fix your position more accurately.

As with any other facet of aviation, GPS comes with its own set of alphabet soup. Among the most important items in GPS use is the concept of RAIM, or Receiver Autonomous Integrity Monitoring. RAIM's function is to utilize an additional satellite to identify a discrepancy between the satellites that are being used, thus ensuring that one satellite's slight error is detected and a potential disaster in position accuracy is averted. In other words, your GPS receiver can detect a disagreement between the positions that the satellites indicate, and toss the incorrect one out of the equation. The catch is that the receiver requires more visible satellites to execute this function. Since it is of the utmost importance that your receiver be accurate during the approach phase, the FAA requires that you acquire RAIM before executing an approach.

The existence of RAIM is one of several reasons that handheld GPS units are not permitted to be utilized for IFR operations. Though arguably a rare occurrence, the FAA is concerned that the handheld units could be miles off course without any indication of a problem. We'll get into the other requirements later.

Some Other Buzzwords

I don't care what type of GPS you're using, it's got all of the next functions available in some form. However, one of the principle problems in working with the abundance of information available is that it can be difficult to isolate what's really useful in a tight situation, especially to new users of the system. Here's a breakdown of the best things to look for on your particular unit:

 
The Relationship Between Track, Desired Track, and Bearing

Track (abbreviated TRK or TK, generally): The single most useful (and neglected) function of the GPS receiver. Essentially, the track display provides the pilot with the aircraft's precise track along the ground. This isn't heading, folks, this is the actual direction the aircraft is moving, corrected for wind, deviation, and variation, the whole nine yards. The best part is that you don't need to put in a flight plan, execute an approach, or push many buttons to use it.

Desired Track (abbreviated DTK): The course line to your next waypoint that you told the GPS you wanted to go. Basically, if VOR A is directly south of VOR B, then your desired track would be 360°. This is often confused with bearing, described next.

Bearing (abbreviated BRG): The direction you'd need to go to get to your next waypoint from where you are now. If VOR A were directly south of VOR B, as described above, then your DTK is 360°, whether you're on it or not. However, if you're east of the DTK (the course), your bearing would be some number less than 360° (like 340°, for instance). If you turned left until your track was 340°, you'd fly directly to the station.

It's important to note the difference between DTK and BRG. If you're flying airways from VOR to VOR, you want to stay on your DTK. In the above example, if you wanted to get back on the 180° radial of VOR B from the position where the bearing to VOR B is 340°, you'd need to turn left to a heading of 330° or even 320°, depending on how far off course you are, how quickly you want to return, and how much wind there is. A common error is to make the track line up with the bearing, which will work fine, so long as you don't mind being off course! The classic ADF "homing error" is the result of consistently changing the track to match the bearing, which is not the course you actually want to fly.

Distance: This may seem like a no-brainer, but it can get confusing, because DME distances are based on slant distance. For example, if the aircraft is 12,000 feet (roughly 2 nm) above the elevation of the station, when the GPS says you're 5 nm from the station, the DME will read 5.4 nm, because DME measures the distance between the aircraft (which is 2 nm in the air) and the station (on the ground). This error gets larger as you get closer to the station or higher from the elevation of the station; for instance, when you cross over the station, the DME will read 2 nm! GPS distances do not have these errors built into them, so you will not pass over a GPS waypoint until the distance reads zero, although the GPS may sequence to the next waypoint earlier. You may use GPS as a substitute for DME fixes, but you should be cautious due to the differences between the distances given by GPS and those by DME. More on that in a minute.

OBS or Non-Sequential Mode: When you enter a series of waypoints in your GPS, it assumes that, as you cross each one, it should automatically switch to the next waypoint to reduce your workload. This is a great feature, unless you need to do something like a hold or a procedure turn. In these instances, you will cross a particular waypoint more than once before you want to activate the next waypoint. As a result, every GPS has a method of turning off the auto-sequencing function. Typically, the "holding" mode is activated by a big pushbutton on the unit itself or on an auxiliary panel for easy activation. One more thing: When you get to your missed-approach point (MAP), the GPS will not automatically sequence to the missed-approach segment. You must activate the missed-approach segment by taking the unit out of the OBS mode, which may require a significant amount of user intervention on some models, just at the moment you should be following the missed-approach procedure very carefully!

Moving Map: There are GPSs out there that don't provide a moving map display, but they're becoming scarce, and those are typically used as receivers to feed data to a larger display package. The information provided by any particular moving map display varies by manufacturer, but there are generally a few common ways of using the map. The Track-Up display rotates the map around to your direction of flight, making what is in front of you in the airplane at the top of the map. The Desired Track Up display will put the direction you're supposed to go at the top of the map, but this can be dangerous, since you could feasibly fly off course enough for your aircraft to depart the map display. Finally, the North-Up display places north at the top of the map (and east at the right, south at the bottom, and west at the left), more like a chart.

GPS Approaches and the Phase-in Program

Open up a book of instrument charts, and you'll find that there are a variety of types of GPS approaches. Don't be intimidated, though; if you have an IFR-certified GPS, you can probably shoot the approach, no matter what its title.

When the FAA began authorizing GPS for use in instrument approach procedures (IAPs), it decided that authorizing the use of GPS as the sole source of navigation was not prudent before reliability of the system had been established. At first, the GPS had to be supported by another means of navigation; then, the aircraft was required to have other means of navigation installed and operational, though not necessarily monitored. Now, GPS approaches may be executed without any reference to any other navigational system — in fact, many GPS approaches exist now that cannot be executed through any other means, giving many airports the opportunity to have an IAP without incurring the costs of ground-based navigational equipment.

GPS approaches can typically be subdivided into three types: the overlay approach, the GPS-only approach, and the area-navigation approach.

 
(Click charts for hi-res versions)
GPS Overlay Approach
GPS Overlay Approach
GPS Only Approach
GPS Only Approach
RNAV Approach
RNAV Approach

Overlay approaches were the first GPS approaches to be created, and allow you to mirror a previously established IAP without utilizing the traditional navigational equipment at all (VOR, NDB, etc.) These approaches are found as "VOR or GPS," for example, in the title of the IAP.

A popular "gotcha" comes from the overlay approaches that involve a DME from a station while heading toward the airport. For instance, a VOR approach where your MAP is 10 DME from the station can be confusing when using the GPS, since the GPS will always count down to the next waypoint (see chart). In this scenario, your MAP, contrary to the approach diagram, will occur at 0.0 nm (from the MAP). The only way to make the GPS display the distance from the VOR is to activate the OBS/Non-Sequential Mode. However, as you'll see, this will preclude the GPS from ensuring its accuracy through RAIM, and would not be a legal or safe way to execute the approach.

GPS-only approaches are just that — they can only be executed with a properly certified IFR GPS system. No other type of equipment may be used (or is needed) to execute these approaches. They are identified by a title like "GPS RWY 22" (see chart).

Area-navigation approaches, or RNAV approaches, also stand alone without reference to any specific ground-based stations. One thing that makes these types of approaches confusing is that, previously, an "RNAV" approach implied use of a special piece of avionics, such as the Bendix/King KNS80. Now, however, the FAA has decided that an RNAV approach requires only a certain level of precision, no matter what type of navigational system the aircraft is using. The Required Navigational Performance, or RNP, typically required is 0.3, which means a CDI sensitivity of 0.3 nm from the center of the indicator to the edge of the case. You'll see how that comes into play a little further into the article.

Why the fuss? Well, many larger aircraft use multi-sensor navigational systems, such as a Flight Management System (FMS). The concept behind these systems is to augment any particular navigational source (VOR, DME, GPS, LORAN, Omega, Inertial, etc.) with as many other sources as possible. In other words, it compares the GPS position with positions of any VOR stations in range, any DME values it can find, etc. As a result, the failure of any one particular system does not necessarily prohibit it from achieving the required degree of accuracy. It is possible that these aircraft, even without GPS, could execute an RNAV (though not a GPS-only) approach. The RNAV approaches are identified by the title "RNAV (GPS) RWY 36L," and there's typically a note about an RNP or GPS required (see chart).

What's next? The FAA is currently in the process of implementing the Wide-Area Augmentation System, or WAAS. Through largely separated ground-based stations (fake, or pseudo-satellites), GPS accuracy can be greatly improved — down to an error as small as 7 meters (23 feet). With that degree of accuracy, the FAA will begin developing precision-approach procedures utilizing GPS receivers, so that hundreds of airports that may never have anticipated precision approach capability may have it soon.

Shooting a GPS Approach

Before you can go out and start blasting through terminal airspace with your own "magic box," you'll need to have a basic understanding of how to use the particular model of GPS that you have. At the bare minimum, you should know how to go direct to a waypoint, enter a flight plan, load an approach, and use the moving map. These steps can vary greatly between models of GPS.

What does not vary is the GPS logic used to shoot an approach. GPS units will automatically step up the sensitivity of the CDI during an approach to achieve the required degree of accuracy to shoot the approach. During enroute operations, each dot on the CDI corresponds to 1 nm off course, meaning, if it reads off the scale, you are more than 5 nm off course!

If you have loaded an approach, the first step is to Arm/Enable that approach. This procedure can also differ depending on units. Some I have used will do it automatically, unless you say otherwise (such as the KLN90B, which has a toggle-switch installation), whereas others will prompt you to enable the approach (such as the Apollo or II Morrow GX50 series). In either case, once you've enabled the approach, the first sensitivity change will occur when you come within 30 nm of the destination airport. At this point, the CDI will start to change sensitivity incrementally toward 1.0 nm (full scale), or 0.2 nm per dot on the CDI.

  CDI Sensitivity Changes
CDI Sensitivity Changes During GPS Approach

At 2 nm from the FAF, the GPS receiver will run a quick check to ensure that it has enough satellites to ensure RAIM during the approach. If it believes that the accuracy can be guaranteed, it will activate the approach, which should trigger a message or annunciation on the panel to alert you that the approach has been "activated." If this RAIM check fails, a message will alert you to this. If the approach does not activate, you may not shoot the approach.

After activation, the CDI will progressively step up sensitivity again, this time to 0.3 nm for a full-scale deflection (approximately 360 feet per dot). It will achieve this sensitivity by the time you cross the FAF.

It's important to mention that this change in CDI sensitivity is precisely why you must "load" and "activate" an approach rather than just typing in the names of the fixes on the approach plate. Without the approach being armed and activated, the CDI sensitivity could stay at +/- 5 nm, which means you would have to keep the CDI deflected less than one half of one dot from the center to ensure obstacle clearance! So, it is not acceptable to shoot an approach unless it is in the database and you've loaded it properly.

Another thing to keep in mind is that, during the changes in sensitivity, a common mistake is to overcorrect for any deviations from course. For instance, let's say that at 2.5 nm from the FAF, you're one dot (1 nm) off course, so you put in a heading correction of thirty degrees. After you cross the two mile mark, the approach activates, so the sensitivity starts to increase. As you move toward your course, the sensitivity increases, so the needle doesn't move. Keep in mind that you are getting closer to being on course; however, many pilots who don't understand this change in sensitivity assume that they need a larger correction angle ("geez, this is one heck of a crosswind!"), and start putting in a very large heading change to compensate. All of the sudden, the sensitivity stops changing, and before you know, it the needle is pegged on the other side of the dial.

Also worth discussing is the RAIM check that the GPS performs. By itself, the GPS needs five satellites to guarantee accuracy of the system during the approach. However, all IFR-approved GPS systems have a sensor connected to the encoding altimeter — this gives the GPS information about the aircraft's altitude, thus giving one positive fix on the aircraft's location. As a result, the GPS will only require four satellites to achieve RAIM and execute the approach. This is called Baro-Aiding. This will only work if you set the internal altimeter (which the GPS should prompt you to do when you get near the terminal environment). I've met at least three people from completely different facets of flying who believed that you only needed to set the altimeter if you're utilizing the vertical navigation functions of the GPS — not true!

If, at any point during the approach, your GPS loses its capability to achieve RAIM, then you must not descend to the MDA for the approach. If you already have started down, you should execute a missed approach immediately. You should still overfly the MAP to guarantee clearance with obstacles, but you should begin your climb to a safe altitude immediately. Note that once the approach has been activated, the GPS receiver will give up to five minutes without the necessary satellites before giving an annunciation, to give itself the opportunity to reestablish communication with the satellites. If a warning pops up, you could already be considerably off course.

As discussed previously, the GPS will not automatically sequence past the MAP to the missed-approach segment without user interaction. Typically, this involves taking the GPS out of the OBS mode and putting it back into sequencing mode, but for some models, the user will have to manually alter the flight plan to go to the next waypoint. There is typically more than one way to achieve the desired result, but, either way, low, slow, and busy is not a good time to be giving all of your attention to the GPS. Point yourself in the right direction, start climbing, clean it up, do anything else you need to do, and then worry about sequencing the GPS.

Potpourri of Pitfalls

One classic dilemma with GPS is the fascination factor. During critical phases of flight, the GPS seems to demand the most attention, with blinking annunciators, inquiries for input, and constantly changing information. Unfortunately, these phases of flight are the most important times to look for traffic when in VMC. Also, since many aircraft have the GPS retrofitted on the other side of the panel from the pilot, the pilot's attention can be diverted away from the most important instruments on the panel. On the bright side, you'll know your exact latitude and longitude after you lose control of the airplane in this manner. The best idea is to enter information into the GPS in stages and well ahead of the time that you need it.

Consider the study done by Diane Damos, Richard John, and Elizabeth Lyall in 1999, which studied the amount of time crews spent looking for traffic in highly automated and computer-dependant cockpits, versus non-sophisticated versions of the same aircraft. They found that the pilots in automated aircraft (which, arguably, should lessen pilot workload!) spent significantly less time looking for traffic during approach and landing. Why? Because they were concentrating on setting up the computerized cockpit to do what it should do properly, rather than looking around outside.

Most importantly, don't ever let yourself become so wrapped up in the GPS that you lose control of the airplane. It's happened more than once that I've flown with students who get lost or confused in the myriad of functions and forget that you "navigate" after you "aviate." If you find yourself asking, "Why is it doing that?!?" turn it off, turn it on again, and get yourself back to the beginning. Remember: The GPS doesn't care if it makes it home safely.

Partial Panel

Here's food for thought — you could shoot an entire approach using only the GPS. I don't mean in lieu of a VOR; I'm saying that you don't need any of your instruments at all. Of course, I'd never recommend that you ignore other instrument indications (and please don't try this without a safety pilot on a beautiful day), but it's worth noting that the GPS can provide everything you need: heading (actually, track, which is even better), airspeed (indirectly through groundspeed indications), altitude (through the encoding altimeter input), and course information. To prove the point, I took an appropriately rated safety pilot up with me in a Cessna 172R with the KLN90B, covered up the airspeed indicator, the attitude indicator, the altimeter, the turn and bank indicator, the directional gyro, and the vertical speed indicator. I did not utilize any other radios, and permitted myself only the inclinometer and the tachometer in addition to the GPS. The result? A passable (although not entirely smooth) GPS approach under the most bizarre of circumstances. I call it a "no panel" approach.

It's pretty unlikely that you'll be faced with such an emergency, but with all that information available, how bad can a more traditional vacuum failure be? Think about trying to shoot an NDB approach, partial panel, with a heavy crosswind, to minimums. Bring back bitter memories? It should be a snap with a GPS receiver, because if your inbound course is 90°, just make your track 90°. That's it. No need to think about wind correction angles, no worrying about leading the needle or pushing the needle or any other such nonsense. If you're north of course, make the track 95° for a nice correction angle. What could be simpler?

The real benefit to this is that track information is available without any input of the pilot. You don't need to enter a flight plan, load an approach, or navigate any bizarre menus. There are typically several pages that display track information, to boot. The only catch is a lag during turns of a significant bank angle, but with practice and patience, you have something that's a lot easier to handle than the whiskey compass!

Other Neat Stuff

I couldn't begin to list everything that's available with these units. The nearest airport, VOR, intersection, etc., is generally a few button pushes away from you with any newer model. The airport information pages, while they shouldn't be used solely for navigation, can be a great way to find information about an airport in an emergency, or to find a nearby FSS or radar facility. Some units have flight timers, or stop/start timers, or countdown timers (set it up for the time inbound from FAF to MAP, start it, and wait until it says "0," rather than trying to remember that bizarre time from the approach plate). It can also tell you your estimated time enroute, time of arrival, and time of departure, calculate the current winds aloft, and provide a plethora of other information.

GPS has already become the most remarkable advance in aircraft technology since the advent of the VOR, and as the system continues to prove itself, along with lowering costs and improved receiver interfaces, it will undoubtedly become the norm rather than the exception. If you're working on your instrument rating, try to find a way to incorporate a little GPS work into your course of study. If you have the rating and are thinking about getting a GPS unit for your own aircraft, or renting one that already does, grab hold of a good CFI and give yourself the opportunity to learn how to use the GPS as another tool in all of your instrument flying. When the soup is thick, the instruments are shaky, and the vertigo starts to set in, you'll be glad it's there (but only if you know how to use it!)