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From: "Alan Adamson" <aadamson@highrf.com>
Subject: Which GPS source for MVP50, etc??
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Ok, need to pick your brains...Seeing another question on the
cheltonefispilots forum about the service bulletin from Chelton on GPS
hookup to the TT autopilot got me to thinking and now I'm stuck as well.
It's obvious which GPS info to provide to the AP, but what about other
devices that need GPS - MVP50, Vertical Power Display Unit, etc?

I went back and found Roberts DuRall's original description of the issue:

------------------
There has been some questions about the GPS data feeding the TruTrak
autopilot from the Chelton GADAHRS. This post is intended to address this
issue. 

The TruTrak autopilots take the GPS track data from a GPS source directly
off of the RS-232 NMEA string. There is no correction for magnetic deviation
used by the autopilot. The GPS sensor in the Chelton GADAHRS outputs the
standard GPS track without magnetic deviation correction, but includes this
deviation later in the RS-232 string. Since the autopilot applies the GPS
track directly, there will be a difference between the GPS track in the EFIS
and the GPS track in the autopilot. This difference is equal to the magnetic
deviation at the current aircraft location. 

To correct this difference, it is suggested that the GPS data for the
autopilot be fed from COM09 of the EFIS instead of the GADAHRS. Chelton has
issued a Service Information Letter describing this issue and the use of
COM09 for GPS drive. In addition, a new revision to the EFIS-SV Installation
Manual will be released shortly showing the suggested aircraft wiring. 

Safe flying.
Robert DuRall 
Technical Support Manager 
Chelton Flight Systems 
rdurall@cheltonfs.com
-------------------

Ok, so what GPS from the Chelton system do I want to share with the other
devices that need GPS (MVP50, VP Display Unit for example).

The one of COM09 (FlightPlan data per the service bulletin from Chelton) or
the GADAHRS GPS from the PinPoint? These 2 values of heading are different
and I'm suspecting that I want to send the COM09 value, not the Pinpoint
value????  Or better put, which source provides typical NMEA type output as
it appears there are 2 versions.

Thanks in advance,
Alan Adamson 
Atlanta, GA 
Lancair Legacy FG-6 N60AL (fixed gear, carbon fiber, IO-550)