GPS problem, unexpected solution

Jim Percy · #1 08-19-2009, 03:10 PM

In the interest of increasing the knowledge base, I'll share a recent problem and solution.

I have a CNX-80 (GNS-480), but it seems possible other GPS's might be subject to the same thing. Some time ago I started getting messages saying "Warning, Loss of Integrity, Cross Check NAV". In the beginning they were intermittent- every few flights it would happen, and the GPS would "recover" quickly. As time went on the warnings became more frequent (most every flight) and the loss lasted longer. Eventually I started getting "insufficient satellites" warnings, where I could bring up the status page and it would be totally blank. In these cases it would sometimes take 20+ minutes to reacquire sufficient satellites. Not good.

Skip to the solution. After a couple of months (VFR...) of head scratching and testing everything I could think of, replacing the antenna cable, and making numerous calls to Apollo support, Stark, and Stein, I started getting low voltage alarms on the EIS. Turned out to be a bad rectifier on the alternator. Maybe others smarter than I could have thought of that, but without the low voltage alarms I'd probably still be scratching my head. It's now been more than 35 flight hours since replacing the alternator, including a trip to OSH and back, with no warnings. I'm pretty confident that the bad rectifier was the cause of my GPS problem.

GPS antennas are powered, in my case by something like 5 volts sent from the CNX-80 through the coax. My guess that the alternator was sending out spikes of AC that were strong enough to knock the GPS antenna off line, breaking the satellite lock and requiring reacquisition.

An additional note- soon after I started getting low voltage alarms (after checking belt tension, wiring, etc) I took the alternator to Autozone for testing ("it's from a 1989 Samurai, sir"

). I had it tested a total of 3 times at 2 stores, each time it tested GOOD. Fortunately someone had a spare that I was able to bolt up for a ground test (sometimes you just gotta love KCNO!)- that was all I needed.

My hope in sharing is that I might save someone else from the kind of frustration I went through on this!

Geico266 · #2 08-19-2009, 03:20 PM

Great catch!

N395V · #3 08-20-2009, 08:19 AM

This really brings home the idea that when your airplane talks to you, you need to listen and be persistent in seeking out and rectifying the problem.

Mechanical issues never fix themselves , never improve with time. They only get worse and finally bite you at the worst possible moment.

TrutrakTech · #4 08-20-2009, 11:33 AM

Not aviation related, but a similar problem on a 2003 Ford Mustang GT (I'm a Mustang nut). A bad rectifier in the alternator was causing AC spikes to corrupt the camshaft position sensor signal. This caused a fault code for the CPS circuit and made the engine run poorly and backfire at startup. A change of the alternator and voila! The problem is gone.

It's strange sometimes what one component failure can do to another component.

Glad you got everything worked out!

az_gila · #5 08-20-2009, 01:59 PM

...for military testing (jamming) of their GPS systems.

Typical ones

GPS 12/051 ZDC NAV GPS IS UNRELIABLE
AND MAY BE UNAVAILABLE WITHIN A 35
NM RADIUS OF 381541N / 762601W
(LOCATED WITHIN PATUXENT RIVER
NAVAL AIR STATION) AT AND BELOW
FL400 1300-2100 MON-FRI. WEF
0812151300-081219210.

GPS 04/009 ZDC GPS IS UNRELIABLE AND
MAY BE UNAVAILABLE WITHIN A 275 NM
RADIUS OF 2958N/0791500W AT FL400.
DECREASING IN AREA WITH A DECREASE
IN ALTITUDE TO 229 NM RADIUS AT FL250,
163 NMR RADIUS AT FL100, AND 120 NM
RADIUS AT 4000 FT AGL. THE TEST AREA
IMPACTS THE WASHINGTON, JACKSONVILLE
AND MIAMI ARTCC AIRSPACE.
WEF 0604201800-0604202100

Two weeks ago there was one in the Mojave, CA area that had a radius of several hundred miles, even at 10,000 ft.

It may not be your equipment....

Jim Percy · #6 08-20-2009, 03:46 PM

Quote:

Originally Posted by az_gila

Check Notams...for military testing (jamming) of their GPS systems.

Agree. And yes, I did do that at someone's suggestion (can't remember who).

splevy · #7 08-21-2009, 10:07 AM

And what happens to the new ADS-B system when the jamming is active?

Inquiring minds want to know. I have never received an answer to this question other than "they won't de-commission ALL of the VORs".

Stan

N67SL (reserved)
RV-7A (fuse)
KCCB

Ron Lee · #8 08-21-2009, 01:51 PM

Stan, you may know that the VOR answer appears bogus. You won't get a position accuracy similar to GPS from VORs if at all. I doubt that ADS-B Out units will be VOR equipped as a backup.

I am strongly opposed to ADS-B Out mandated equipage in the air spaces we fly.