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Date: Fri, 02 Dec 2011 07:49:51 -0500
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From: Tim Olson <Tim@myrv10.com>
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Subject: Re: [LML] Re: cheltons losing gps signal
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The GPS on the Chelton isn't integrated into the circuitry of the
AHRS but is a stand-alone GPS (If you're talking about the
pinpoint/Chelton GADAHRS).   The software revisions really
should have nothing at all to do with any GPS issues.

If someone really wants to know if the GPS is working or
if the problem is further into the system or not, it's
actually really pretty simple.  All you'd have to do is
set up a laptop running a serial port with pins 2 and 5
being the only ones that matter.  Hook the ground on
the pin 5 on the PC to the same ground or airframe
ground as the AHRS uses.   Then for the pin 2 connection,
unplug the harness from your screen and figure out
which pin (by wiring diagram) your GPS is coming into
the system on, and make a jumper wire from that pin
(using a pin/socket crimped onto the wire) running
from that wire of your J1 harness (likely pin 1 on J1)
to your laptop.

Then just fire up the laptop and watch using hyper term
or any comm program set to 9600 baud N81 and watch
what you get.

This all sounds way more complex than it really is,
but it's very simple...just 2 wires and you can
"listen" to the GPS and if you see messages with
Lat/Long and other info in there you can log it
and many folks can tell you if it looks normal or
not.  (do this in clear view of the sky with
satellites in view)

I had to do this to find out what the GPS messages
coming from a GPS looked like, into my new ELT
early this year.  Once I got the messages it was
easy to see that I was missing one type of message
due to the particular GPS I was using, and so I got
a different one and it had the necessary string
being passed.  The theory is the same though...if
you see no good data, the GPS is bad.  If it's an
intermittent problem, then of course you'd have to
monitor it while it was bad.  Then it's a bit
trickier because you're going to have to splice
that wire so you can actually fly and have
the cable ready to plug into your laptop...and
you would only want to have this in place for
when you are diagnosing that problem.

At any rate, your issue is likely either going
to be:

1) a wiring or connector issue
2) an antenna or ground plane issue
3) a failing GPS
4) a failing serial port on the EFIS

It's really not likely that it's a software
issue at all.

Tim


On 12/1/2011 12:05 PM, Dr. Weinsweig wrote:
> i don't know when the software came out but tech support at cobham
> emailed it to me and we installed it.  i'm not sure if that was a
> great idea as shortly after that one screen went belly up requiring
> an "overhaul" and i never had a problem with the gps signal before.
> perhaps the software made the units too sensitive.  i should mention
> that the gps signal on the cnx80 and my ipad foreflight was not lost-
> only the cheltons intermittently and i have not seen it lost on the
> ground only in flight temporarily.  i should  also mention during the
> time of overhaul i had a gtx 327 transponder installed in the panel
> and the remote sl70 removed so lots of "minor" wiring changes.
> however, i was told that the cheltons are much more sensitve and
> require a "better signal" than typical gps.  i wonder if the update
> just made mine too sensitive but we'll be troubleshooting and if/when
> we figure it out i'll let you know.  one fellow told me he saw the
> gps signal totally disappear requiring a new receiver so maybe that's
> it though intermittent problems are always more dfficult to track
> down.
>
> thanks,
>
> david weinsweig 304-633-5221 weinsweigd@gmail.com n750dw propjet
>
>
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