Garmin GPSmap 496 False Terrain Alert

Do any of you fly into KADS with a Garmin GPSmap 496? I'm getting a
false terrain alert when flying the Rwy 15 ILS and am wondering if
anyone else has this problem. I'm working with Garmin to resolve this,
but so far what they have suggested has not solved the problem.

It might be the calculation used to determine the terrain. All it has is
the distance from the satellite and formulae to determine terrain distance.


Actually the GPS receiver should have terrain (assume in MSL feet) in
the database (unknown accuracy of data). The GPS receiver as part of
its position determination can calculate the aircraft altitude in the
same units and coordinate system and then compare using whatever
method the receiver programmers decided upon.

Ron Lee

Here's the latest information on the issue with the Garmin GPSmap 496
reporting a false terrain alert when flying into KADS along the runway 15
ILS:



Garmin provided me with a current terrain database update, in an effort to
see if corrections that have been made in that database since June 2006
would correct the problem. It did not, though I certainly appreciated their
efforts.



Another pilot who occassionally flies into ADS who also utilizes his 496 for
improved positional awareness and has his 496 audio connected to his
intercom for terrain /obstacle warnings (not IFR operations) has reported to
me that the problem also occurs with his unit when he lands on runway 15 at
Addison. As with my 496, the problem is easily reproduced and occurs 100% of
the time. (Thanks! This was an important point of information for Garmin
technical support.) Another ADS pilot/owner has offered to plug his 496 into
my panel and permit me to fly with him and his unit to confirm that the
problem occurs with his unit in my installation. I'll do that on Saturday,
but we have every expectation that this will repeat, given the prior report.
It will provide a second confirmation. (In his installation, the audio alert
feature is not utilized.)



Garmin agrees that this evidently points to an errant data point within the
terrain database close to the approach end of runway 15. They admit that the
elevation data has errors which they continue to correct. No other airport
approach has been reported by anyone on the Internet as having this problem,
in response to my queries. (If you have experienced such a problem in a
repeatable fashion, Garmin is interested in the details.)



Garmin have asked me to download the recent tracks that demonstrate the
problem from my 496 and email them to them. They will use this information
as they document the problem to the FAA subcontractor who produces the
database that they then license, reformat, and resell for use in the GPSmap
units. I've done that, yesterday.



Thanks to those of you who assisted with the analysis and troubleshooting of
this problem thus far. The rest of you can continue chatting



-Ron Gordon

"Ron Gordon" wrote in message
t...
Here's the latest information on the issue with the Garmin GPSmap 496
reporting a false terrain alert when flying into KADS along the runway 15
ILS:
Garmin provided me with a current terrain database update, in an effort to
see if corrections that have been made in that database since June 2006
would correct the problem. It did not, though I certainly appreciated
their efforts.

Ron, have you asked Garmin if they're willing to give you a copy of the raw
datapoints of the terrain elevation around ADS? I'm thinking the erroneous
datapoint(s) will be terrain data since an obstacle should be easily
displayed as such on the 496 map. Since you know the area, you might be able
to spot the errant datapoint and help them out.

Marco

That's a very good suggestion. I will make the request on Monday. Since I
can carry the 496 to the elevation data points leading to and around the
approach path, these should be easy to confirm. And as you point out, the
terrain around KADS is very flat. It is likely that an erroneous data point
may be self evident. Thanks! -Ron Gordon

Ron, have you asked Garmin if they're willing to give you a copy of the
raw datapoints of the terrain elevation around ADS? I'm thinking the
erroneous datapoint(s) will be terrain data since an obstacle should be
easily displayed as such on the 496 map. Since you know the area, you
might be able to spot the errant datapoint and help them out.

Marco