What’s My GPS TellIng Me?

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Understand the significance of those lines and numbers on the display.

The Garmin 530 is a basic nav/com that provides a wealth of information—if the pilot knows how to access and interpret it.

Too many IFR pilots don’t know their GPS as intimately as they should. Hitting “Direct To” and their destination is as much as many can muster out of their navigator. There is a serious lack of knowledge of what the GPS is trying to tell them.

The most basic information a GPS provides to a pilot is their current location and the direction to a desired waypoint (that sacred magenta line). But it does this using terms that not all pilots fully understand.

The Basics

Most GPS navigators present a function called DTK—Desired Track. What exactly is that? In the old days of paper charts, the Desired Track was the line on the chart from point A to point B drawn while still sitting in your chair at home. We called it our Course—but it’s the same thing. That line on the chart never changed no matter how far from it you flew once airborne. It’s what you desired to do, but not necessarily what you did.

Likewise, the DTK is what you originally told the GPS that you wanted. Until you reprogram the GPS, the DTK will never change no matter how far off course. Occasionally we do get off course for one reason or another—not following the Desired Track—but are on some other Track.

This is what the GPS calls Track—abbreviated to TRK. Track is the path over the ground that your plane is actually following rather than what was planned. You would think that if the DTK and TRK are the same then you are on course. Sometimes that’s true—but not always.

Assume you drift a mile off course and then return to a heading that parallels the original desired track. At that point your DTK and TRK would be the same heading, but you would not be on course—you’d be paralleling the desired course. How far you are off the desired course is called cross track (XTK) by the GPS. Using the XTK information you can see if you are far off course—or just a little bit.

The other bit of data that the GPS reveals to guide you to the destination is Bearing (BRG) to the next waypoint. The bearing is where you would look out the windscreen to find the waypoint. This can be useful on an approach with a strong crosswind. If the DTK and the BRG don’t match while the needles are centered the reason is most likely a crosswind. The BRG tells where to look for the runway.

Note the Desired Track (DTK) of 067 degrees (red), the current Track (TRK) 112 degrees (white), and the compass Bearing (BRG) 076 degrees to KIDP (dotted white). Illustration courtesy of Fred Simonds.

RAIM

Most instrument pilots have heard of RAIM, but many don’t know the details. They assume it is a way to tell if the GPS is having a bad day. RAIM stands for Receiver Autonomous Integrity Monitoring. That’s a long-winded way of saying the receiver checks itself and tells you if there is a problem. We tend to think this is something new, but in fact, VOR receivers had a rudimentary version of RAIM. It was called a “flag.” If the VOR receiver detected a problem it would set the Nav Flag, alerting the pilot that something was wrong with the VOR data.

Likewise, if an IFR certified GPS receiver detects that something is wrong with either the unit itself or the signals it is receiving from the satellites, it will alert the pilot. The message presented to the pilot is more noticeable than the red and white barber pole in the corner of a CDI. While RAIM warnings from modern units are much less common than they were in first- and second-generation GPSs, they still exist.

What should pilots do if they get a R AIM warning? The FAA says, with good reason, that the pilot must revert to other forms of navigation. This could be VOR, NDB, or even radar vectors. You must also report the failure of a primary form of navigation.

What if the RAIM warning occurs while you are in the middle of a GPS approach? As always, revert to other sources of navigation and report the problem to ATC. But because you are close to the ground and without navigation information, the wisest course of action might be to immediately climb to the MSA for the approach. Once to a safe altitude, work with ATC on Plan B.

Some GPS navigators will suppress R AIM warnings for up to five minutes while on the final approach segment. The theory is that you are close enough to completing the approach that the receiver can extrapolate from recent past information and pretend it knows where you are. Using this in-formation will allow you to complete the approach.

This doesn’t give me the warm and fuzzies, how about you? In case you do get a RAIM warning while on the final approach segment the AIM tells us to proceed to the Missed Approach Point and then execute the Miss. But the RAIM warning will block navigation data so you may not be able to find the MAP. Thus, a climb to the MSA may be the smartest maneuver. As always, let ATC know what is happening once you have the situation in hand.

Have you ever wondered why the GPS navigator took longer to sequence to the next waypoint than expected? It relates to how close to the next waypoint you have to be for the navigator to sequence. A human might look at the situation and say, “close enough.” But GPS navigators are digital ma-chines using mathematical algorithms. They determine sequencing by looking at the flight plan.

Assume that you are currently navigating from A to B and the subsequent waypoint is C. The three waypoints form a right angle. Your navigator will look at the angle formed by the three waypoints and draw an imaginary line that bisects the angle. In the case of the right angle, the bisector will be a 45-degree line that goes halfway between A and C. Once you cross the bisector of the included angle, the navigator will sequence to C. But if you have flown past B and are on the outside of the included angle, the navigator will not sequence until you cross the bisector, which may take quite a while.

OBS Mode

The scenario in the previous MFD segment is displayed here on the HSI. Illustration courtesy of Fred Simonds.

OBS mode is also often misunderstood. The simplest way to think about OBS mode, is to imagine that when you push the OBS key you are telling the navigator to treat the current waypoint (the one to which you are navigating at the moment) as if it were a VOR.

What happens with a VOR? You set a radial and the CDI indicates if you are left or right of course. But it never sequences to the next VOR all by it-self. The same thing happens with your GPS when you put it in OBS mode. It treats the current waypoint as if it were a VOR. So you can now select the ra-dial which you want to use to approach the waypoint. The CDI will show you if you are left or right of that radial. Once you cross the waypoint, the navigator does not sequence to the next waypoint. Instead it will flip the To/From flag and show that you are navigating away from the waypoint, still using the same radial from that waypoint.

This can be more than just handy. When in a holding pattern that is not in the GPS database, you must put the navigator in OBS mode and set the inbound leg on the CDI. Then once you arrive at the holding fix, the navigator will not sequence and will show you the way to remain in the holding pattern. When it’s time to leave the holding pattern, get back to the inbound leg, and hit the OBS button again. This will take the navigator out of OBS mode and allow sequencing once you cross the holding fix.

Flight Plan Locked

If you are a Garmin user have you ever gotten the message, “Flight Plan Locked” when you first power up the unit? It continues to give the same message every time you power up even after you installed a new database cycle and haven’t yet loaded a new flight plan. What’s it trying to tell you? If you dig through the dusty user manual, it’s telling you that one of your stored flight plans has a way-point that is no longer in the database. Perhaps it was a user defined way-point and you deleted the waypoint. In any case, delete that flight plan or the offending waypoint and the message will no longer appear on startup.

The Take-home

Whatever the navigator is trying to tell you, it’s important to understand exactly what it is. One of the best ways to do that is to open the user manual and start reading. It may not be the most riveting prose around, but it may one day save your life.


Ken Maples, a former Chief Instructor at a busy 141 flight school, has retired to the mountains of Colorado and splits his time between skiing and shoveling the doors to his hangar.


This article originally appeared in the January 2018 issue of IFR Refresher magazine.

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Comments (3)

Good stuff. Thanks.

Posted by: Rafael Sierra | June 28, 2018 6:59 PM    Report this comment

Ken,

Good description of the OBS function and its application to holding.

I'd add one more case to your RAIM failure on the approach scenario. When a WAAS approach is being flown it is possible to experience a WAAS integrity failure but still maintain non-WAAS integrity. On the G430/530W the navigator reverts to lateral guidance and the approach can be safely continued in the LNAV mode. I learned this quite by accident when my WAAS dropped out during an approach in IMC a couple of months ago. Now, I brief the WAAS approaches like we used to brief an ILS - when vertical guidance goes away continue to localizer minimums using the step down fixes.

Posted by: kim hunter | June 29, 2018 3:11 PM    Report this comment

In the paragraph about BRG the author uses the example of the CDI being centered but the DTK and BRG not matching during an approach in a crosswind. Can this really be the case? I thought DTK and BRG were both magnetic courses, the former being the planned course and the latter being the current live course, to the desired waypoint. In my book that means if the CDI needle is centered then your live course is equal to your planned course and thus the DTK and BRG will be the same. Am I missing something? In addition, the comment about knowing where to look for the runway implies the GPS knows how much I'm crabbing. My good ol' Garmin 430 knows nothing about my crab angle (heading). Do more modern GPS units make calculations using the plane's heading?

Posted by: Frank Arrison | June 29, 2018 8:37 PM    Report this comment

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