AviationBanter - View Single Post - Unusual Honeywell KLN94 aviation GPS failure

**Ross Richardson[_2_]**

Peter wrote:

There were no errors showing on the KLN94, other than the lack of a
fix.

I think this failure is unusual because - as far as I can tell - the
SV (space vehicle) signal indications are all normal, but the GPS
isn't producing a fix (STATE NAV D).

It is as if there was a disconnection between the GPS receiver and the
rest of the unit - all this is internal to the KLN94 so ARINC buses
don't come into it; the KLN94 doesn't have an ARINC bus anyway, and
the KMD550 MFD doesn't use ARINC, it gets the data via RS232.

It took an hour to get a fix, which is too long for any kind of slow
acquisition. The previous time it was on was about 2 days earlier; a 5
hour flight.

If the problem was just showing on the KMD550 then one could
understand a bad connection, but this is entirely internal to the
KLN94.

Satellite geometry can't be the issue because the two other GPSs (one
of which was using the same type of rooftop antenna as the KLN94) were
receiving fine.

The internal KLN94 battery should not be relevant once the power is
on. It was also changed a few months ago.

Presently I have no idea what this could be. I believe the KLN94
contains a separate GPS receiver module which emits an NMEA stream to
the rest of the unit. This stream is in fact accessible via a
connector pin on the back of the unit. A break in this stream would
account for this issue - but how does the main processor get the SV
reception data? I thought that stuff comes down the same NMEA data
stream, which is how (nearly) every GPS on the market is able to
display the constellation details.

I even changed the DME frequencies, while still on the ground, in case
it was a DME harmonic messing with the GPS signal (that's a well known
one). However, I may not have done that for long enough. The trouble
with that theory is that such interference should show up in the SV
signals too.

The selected VOR/DME would have been 113.4 MHz. I don't know if that
is a harmonic of the GPS signal.

Also the failure persisted for about an hour, by which time I would
have long been tracking a different VOR.

Dave S wrote

Ooops. Only looked at pic number 1, not the ones showing the sat
reception. Only time I've had a slow position computation is when the
unit has been powered down for a LONG (weeks to months) time. Takes up
to 15 mins for it to "find itself" while remaining still.

An intermittent connection on the internal battery (there is a small
watch battery on the board inside - at least the KLN-89/B does) to keep
last known position and user waypoints stored. You didn't have any
error/hardware error messages on startup?

Dave

Dave S wrote:

I have seen that indication when the antenna did not have a clear view
of the sky.

The installed aircraft was an EADS/Socata TB-9 with gull wing doors.
Coming out of the pad, the pilot's door when up (as it was in the
summer) would obscure much of the sky and not allow the GPS to get a
position. Resolved by removing the blockage by repositioning the plane
or lowering the door. The antenna in this case was on the roof of the
cabin between the two gull-wing-doors.

Peter wrote:

http://www.peter2000.co.uk/aviation/kln94-failure/

This unit has been working perfectly for four years, and works fine
now.

Peter.
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You said the battery was changed a few months ago, then you started
having the problem. I wonder if the "repair" was not done well.

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Regards, Ross
C-172F 180HP
KSWI