GPS availability--real experiences

F

Frahm

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I'm a CFI who writes occasional articles for Instructor Reports in AOPA's Flight Training magazine. I'm planning an article on teaching navigation to students. I'm addressing all the navigation methods -- pilotage, dead reckoning, VORs, and GPS. I'm aware of GPS Notams, but I'd like to include in the article some real flying experiences when a GPS signal was not available or was inaccurate. I'd also be interested in anyone's experiences if a GPS unit stopped working in flight. Thanks for your help!
 
I have had but one experience with GPS availability, and it was a non-event- but also, an eye-opener. Nearing the last phases of a GPS approach (in this instance, to Rwy 15 at ADS), the APP light on the annunciator extinguished and the needle did a wander.

My first thought was that I had mashed the wrong button and could recover with more button-pushing, but it turned out to be RAIM. It had not been predicted.

Fortunately for me, it was practice, in VMC with a safety pilot, but it could well have been the real magilla at an airport with no other useful approaches.

GPS has become so reliable, we forget it can fail.
 
Steve-

Hand-held GPS units are subject to signal loss from the wings or cockpit blanking the signals from available satellites. This depends on the locations of the satellites and the direction of flight. This can be mitigated by using an external antenna since the best signal is generally a place where the GPS is difficult to see (front of the glare shield). The enclosed image shows a loss of signal (gaps in the track) from a checkout flight in an unfamiliar plane from just last week. I can also dig out traces where I moved laterally some distance. I record nearly all my flights so have some data I can share.

Although my GPS (a Garmin V) lets me use a coax to put the antenna in a more optimal location, I don't do it since I am a renter and having too many wires is a pain to setup and take down. I note many people put their GPS on the yoke, so they probably experience this more. My GPS is on the glare sheild, or hanging on the air vent if I'm in a Cessna 150/152/172.
 

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I had an interesting experience a year or so ago. I was up in a 172 with an Apollo IFR Certified GPS with a primary student doing airwork at about 3700MSL. I usually have the GPS set to the map page to identify airspace boundaries.

The GPS gives us an airspace alert saying we were in Class B. See attached, I was somewhere in the green circle and it was indicating we were in the red circle about 15 nm south. Fortunately it was CAVU, in my own backyard, so there was no doubt where we really were. A few minutes later the GPS came to its senses and agreed with me:rolleyes:. It hasn't happened again. I don't have a clue what caused it. Ron Levy suggested I report this kind of thing to ATC but I didn't have enough info on the exact time and position to make an after the fact report. Next time though.

I also use that plane for IFR training, but we only do overlay approaches and always have the VORs tuned as backups (like we're supposed to).

Joe
 

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I've never had an IFR GPS provide erroneous information nor have I had a GPS approach that had to be aborted due to a signal integrity issue.

I have however experienced several system failures of an IFR GPS. The first was a KLN-94 that went dark (in warranty display failure) in IMC at night over Lake Michigan enroute (GPS direct) to KACB. The airport itself was in VMC. No simple VOR guidance is available in that area unless I were to fly quite a ways off the original direct route. I notified ATC of the problem and pulled my trusty GPS Pilot III out of it's bag and set it up while ATC gave me vectors to fly and it's a good thing I did because for some reason the controller had mixed up my destination and was vectoring me for a different airport about 20 miles from my actual destination. That became apparent fairly rapidly looking at the handheld and a query to ATC awakened the controller to his mistake. With a new vector (verified on the handheld) I made it to a visual approach at the correct destination without further difficulty.

A couple years ago I upgraded the KLN94 to a GNS480 to facilitate the use of LPV approaches (among other reasons). The GNS480 design has one serious mechanical flaw in that the connectors on the rear of the unit don't fully couple with the mating connectors on the tray under the best of circumstances. In addition my installation exacerbated this problem by butting the front (pilot side) of that tray to the back (engine side) of the panel further decreasing the connector engagement. The result was that turbulence (or in one case a "rough" landing) would occasionally cause the GPS to quit functioning. Fortunately this only happened in VMC and/or was detected prior to launching. It was eventually resolved by altering the tray mounting.
 
This kind of error on an IFR approved GPS is very difficult to understand and if someone else told me this story I
probably wouldn't believe it (I have no doubt that Joe's experience is exactly as he stated). Without WAAS or RAIM it's indeed plausible for the signal from a single SV to be in error (transmitted wrong) or misread (receiver error) in a way that would translate your position several miles but RAIM should easily catch this.

Ignoring some kind of weird bug in the GPS operating firmware I could see a couple of plausible possibilities though. One is that the GPS position solution was correct but the airspace boundary wasn't correct in the database and the other is that the failure would have been caught by RAIM but didn't last long enough to get that warning. It's my understanding that some of the older IFR GPS units (like that Apollo) wouldn't flag a RAIM error in the enroute mode unless the position solution was inconsistent for a significant period of time (like several minutes).

Newer designs that use a dozen or more parallel "receiver" channels would probably puke up a warning much sooner.

Areeda said:
I had an interesting experience a year or so ago. I was up in a 172 with an Apollo IFR Certified GPS with a primary student doing airwork at about 3700MSL. I usually have the GPS set to the map page to identify airspace boundaries.

The GPS gives us an airspace alert saying we were in Class B. See attached, I was somewhere in the green circle and it was indicating we were in the red circle about 15 nm south. Fortunately it was CAVU, in my own backyard, so there was no doubt where we really were. A few minutes later the GPS came to its senses and agreed with me:rolleyes:. It hasn't happened again. I don't have a clue what caused it. Ron Levy suggested I report this kind of thing to ATC but I didn't have enough info on the exact time and position to make an after the fact report. Next time though.

I also use that plane for IFR training, but we only do overlay approaches and always have the VORs tuned as backups (like we're supposed to).

Joe
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Areeda said:
The GPS gives us an airspace alert saying we were in Class B. See attached, I was somewhere in the green circle and it was indicating we were in the red circle about 15 nm south.
Click to expand...

Interesting, Joe... looks like it thought you were exactly 15 degrees south of where you were. Math error in the GPS, or signal error from the satellites?

How many here think the software in their GPS unit is 100% bug free? :rofl:
 
lancefisher said:
I notified ATC of the problem and pulled my trusty GPS Pilot III out of it's bag and set it up while ATC gave me vectors to fly and it's a good thing I did because for some reason the controller had mixed up my destination and was vectoring me for a different airport about 20 miles from my actual destination. That became apparent fairly rapidly looking at the handheld and a query to ATC awakened the controller to his mistake. With a new vector (verified on the handheld) I made it to a visual approach at the correct destination without further difficulty.
Click to expand...

Wow, if you hadn't caught the controller's error, that could have made for a mental discontinuity at the "destination", with you looking for visual cues or navaid reception and indications related to airport "A" when he actually sent you to "B".

Proves you have to trust, but verify, not just GPS signals, but ANY nav method, including controller vectors... lest they plow you into a thunderstorm, mountainside, or send you the wrong direction.
 
lancefisher said:
Newer designs that use a dozen or more parallel "receiver" channels would probably puke up a warning much sooner.
Click to expand...
I hope so. I've seen RAIM failures on 430/530 enroute lasting a couple of minutes.

A few more details. I probably was not even in enroute mode (no flight plan or direct to) only displaying current position on the map. The failure was definitely in position not Class B boundaries as it had us near airports 15-20 miles away from where we were.

At the time I didn't think of checking RAIM or going to one of the websites afterwards to see if the constellation was unfavorable near that time. There was no (obvious) RAIM failure indication.

Joe
 
My worst GPS incident was about 5 hours after my checkride. I was flying a crappy Cessna 150 with some old GPS. I departed KAUM for KFKA following the GPS and glancing at my sectional. I eventually noticed that my course and my window view did not match the sectional. I turned the GPS off and went by my eyes.

The next day I did some more experimenting trying to figure out what the issue was. It turns out that there was a problem with the database. KFKA had the exact same coordinates as KRST.
 
The only time a panel-mount GPS has stopped working for me is when a panel-mount GPS slid out of its tray on takeoff and I caught it in mid-fall. :hairraise: Someone musta forgotten their allen wrench at home I guess!

Handheld GPS? My issues have been solved by using the external (corded) antenna and putting the antenna on the glareshield. Yet one more annoying wire, though.
 
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I heard from a guy who departed VFR from the runway under the 1900 foot MSL (~1100 AGL) Bravo shelf, noticed that the GPS was not getting a signal (You know. Needed to know when clear of the Bravo.) and wiggled and jiggled the connectors while the rapidly climbing plane entered about a hundred feet into the Bravo.

That the kind of story you want? :rolleyes:
 
I was on the GPS-B approach in IMC into 1C5. I was just shy of the FAF and still inthe clouds when I got a RAIM failure annunciation. Discussed it with my copilot for about 10 seconds, then went missed and requested vectors from Chicago Approach to the only other approach (VOR-A) into the same airport. Any equipment can fail!
 
skyflyer8 said:
Handheld GPS? My issues have been solved by using the external (corded) antenna and putting the antenna on the glareshield. Yet one more annoying wire, though.
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Kate,

My student has a C205 and a Garmin 496. The high wing seems to consistently interrupt the XM and GPS signals.

What sort of corded antenna and mount are you using?
 
jesse said:
The next day I did some more experimenting trying to figure out what the issue was. It turns out that there was a problem with the database. KFKA had the exact same coordinates as KRST.
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I had the same experience with a handheld -- coordinates for 2G9 were identical to KJST.
 
I've seen a couple different panel-mounted GPSs lose their mind as evidenced by flags on the HSI and some sort of error message. One was a KLN-90B and the other was a Trimble 2101 I/O. I never found out what the problem was but CTRL-ALT-DEL (turning them off then on) seemed to work for both units.
 
I've had two handheld GPS failures, both when the unit was being used on the highway. None in the air.

The first was with a Lowrance airmap 100 right after I got it. I was travelling west from Little Rock on I-40 and the GPS showed me on the south side of the river near state highway 10. It gave no indication that it didn't know where it was. It is supposed to flash the data numbers whenever it has lost position. I called the company and they had me re-initialize it using some key combination not documented in the hand book and then download a software update.

The second time was quite a bit later and was either the same Lowrance Airmap 100 or an Airmap 1000. I can't remember which. I was traveling north in Kansas on the interstate that runs north from Oklahoma City to Salina. It just went nuts for awhile showing ridiculous speeds jumping up to well over a 100 and back down to low numbers. I just shut it off since I didn't need it. I called flight service after I got to my destination to check for GPS Notams at that time but there were none.

Using two handhelds from different manufacturers to eliminate common mode failures from hard ware or software would provide a significant confidence increase. Being able to verify from a ground based reference also helps the confidence.
 
As long as we're mentioning automotive GPS failures too, I'll say that Leslie MB has a GPS that has begun totally failing to acquire satellites, so it sometimes puts her 30 miles away. There is a satellite indicator that clues one in that it doesn't have a lock, though.
 
Graueradler said:
I've had two handheld GPS failures, both when the unit was being used on the highway. None in the air.

The first was with a Lowrance airmap 100 right after I got it. I was travelling west from Little Rock on I-40 and the GPS showed me on the south side of the river near state highway 10. It gave no indication that it didn't know where it was. It is supposed to flash the data numbers whenever it has lost position. I called the company and they had me re-initialize it using some key combination not documented in the hand book and then download a software update.

The second time was quite a bit later and was either the same Lowrance Airmap 100 or an Airmap 1000. I can't remember which. I was traveling north in Kansas on the interstate that runs north from Oklahoma City to Salina. It just went nuts for awhile showing ridiculous speeds jumping up to well over a 100 and back down to low numbers. I just shut it off since I didn't need it. I called flight service after I got to my destination to check for GPS Notams at that time but there were none.

Using two handhelds from different manufacturers to eliminate common mode failures from hard ware or software would provide a significant confidence increase. Being able to verify from a ground based reference also helps the confidence.
Click to expand...
I figured one thing when using my 295 on the road and in the air in the wrong mode: LAND mode means it has to put your "car" on the nearest road. You couldn't possibly be traveling across the middle of a field, right? So when you're in an area where the map data doesn't have all of the roads, it "places" you on the nearest road it knows about, even if that's a few miles away from where you are AND gives distances and bearing from where that position on the map is, not where you really are. It makes finding stuff in the country a bit of a challenge.

If you leave it land mode when your in the plane you see it jump from road to road while it tries to figure how you're traveling down them. :rolleyes:

The lesson? Leave the older GPS's in aviation mode, except it gives distances in NM and speeds in knots. :dunno:
 
I was fling in IMC approaching Fort Wayne, IN. and was planning on landing at Smith Field using the GPS 13 approach. I was cleared down to 2500'MSL and was told in go direct to the IAF.

I had the approach loaded and RAIM was good. At about .8NM from the FAF I lost all position information on the GPS. I had just been handed off to the CTAF. As I held my heading I made a quick couple of attempts to reset the GPS but to no avail. Knowing that there some very tall towers just to my right I did not dare bumble around in IMC so a switch back to approach as soon as possible was important. I did that and told approach that I needed vectors to the missed and the GPS had broken.

Approach allowed my to go down to 2000'MSL. I know ceilings we just about 2000'MSL with visibility that was several miles. I opted for the lower altitude and when I just barely broke out at 2000' I could see the airport. I requested a visual approach and landed safely. I never did figure out why the lose of the GPS. There were no GPS NOTAMS for the day either.

While there was a VOR 13 approach it was not an overlay to the GPS approach. But if I had at least the VOR set up to go to the IAF on that approach I would have had better situational awareness. Other than that there is nothing I would have changed.
 
dmccormack said:
Kate,

My student has a C205 and a Garmin 496. The high wing seems to consistently interrupt the XM and GPS signals.

What sort of corded antenna and mount are you using?
Click to expand...

Well, it is the kind of antenna that is meant to be used inside the plane. I have a Garmin 295. This corded antenna replaces the built-in swiveling antenna. The cord has a standard BNC connector.

There are a couple remote antennas on the accessories list for the 496. The one I am using is like the "GA 26C low-profile remote automobile antenna." I don't suction cup it to anything.... I just put it on the glareshield as far forward as I can get it. The cord is very skinny.
 
Areeda said:
I had an interesting experience a year or so ago. I was up in a 172 with an Apollo IFR Certified GPS with a primary student doing airwork at about 3700MSL. I usually have the GPS set to the map page to identify airspace boundaries.
Click to expand...

If I remember your report correctly, you were in 85Q at the time, not 4FH. 85Q had a VFR GX-55 and not an IFR GX-50. On top of that, 85Q's GPS antenna wasn't in the best of locations. It was next to the dorsal fin, which blanked half it's field of view of the sky.

Though 4FH's GPS is starting to get wonky too.

--Carlos V.
 
Unbeliever said:
If I remember your report correctly, you were in 85Q at the time, not 4FH. 85Q had a VFR GX-55 and not an IFR GX-50. On top of that, 85Q's GPS antenna wasn't in the best of locations. It was next to the dorsal fin, which blanked half it's field of view of the sky.

Though 4FH's GPS is starting to get wonky too.

--Carlos V.
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Thanks Carlos. I was remembering 4FH.

Joe
 
Jesse was there during my one issue with GPS.

Leaving KMIC, headed under the Class B around KMSP, and the GPS suddenly stops showing anything useful. Luckily, Jesse knew the area pretty well, so we worked together to make sure I stayed clear of the bravo. Seems like just as we exited the bravo, the GPS came back. One of those little tests.

Was a bit freaky at first, since at the time I heavily relied on GPS for navigation. It opened my eyes to keeping backup with a sectional, because without Jesse, I'm not so sure I'd have remained clear of the bravo successfully.
 
Ooh - there was a 2nd issue once too.

I was flying between Questa, NM and Taos, NM. Taos VOR is TAS, the airport is KSKX. I accidentally inputted KTAS, and the GPS told me I had like 3 hours and 400nm to go. It alerted me right away that something was wrong, and I changed the identifier to KSKX. When I got home, I got curious. There is no KTAS anywhere....so why did this happen? I never did find out, as we updated the GPS database before I got a chance to get back into the plane to see where the GPS was taking me.
 
final Approach in to KCXY in a Lance w/ a 430 . Approach course on the 430 had us way far off to the right of final. Lots of solar activity, sunspots etc. Have no idea if that had any role in it but couldn't think of anything else.
 
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