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12-22-2017, 11:08 PM
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Join Date: Nov 2005
Location: Davis, CA
Posts: 1,156
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GRT SafeFly2020 GPS misbehavior
This will be long...
Avionics/gizmo background:
GRT Sport SX EFIS, 7 years old, but latest software, with internal GPS (was an option when I bought the unit).
7 year old Trutrak autopilot.
Garmin Aera 510 GPS in airgizmos mount.
Newly installed GRT SafeFly GPS/uAvionix Echo UAT combo, connected to the Sport SX- replacing a (gasp) NavWorx EXP box.
When I built the panel, I wired it such that either the Sport SX or the Aera could feed the Trutrak autopilot a flight plan, via a toggle switch on the panel.
Originally I also fed an RS232 line from the Aera to the Sport SX so it could serve as a second GPS in the event (however unlikely) of a failure of the internal GPS.
Since the Sport SX can only support 2 GPS devices, when I installed the SafeFy GPS I disconnected the RS232 input to the Sport SX from the Aera.
Today I made a short cross country flight and engaged the autopilot for the first time since installing the SafeFly/Echo combo, and observed some very strange (to me) behaviors, which I can hopefully summarize here.
Engaging the autopilot to track a heading with the GPS input set to Aera, everything worked as expected. It turned out the winds at my altitude were negligible, so track and heading were about the same. The track displayed on the autopilot, EFIS and Aera were all the same within a degree. I tried switching the autopilot input source to EFIS; with the internal GPS (labeled as GPS 1) selected, all continued to be well. But when I tried selecting the SafeFly GPS (AKA GPS 2) as the input source, the autopilot turned the plane about 14 degrees left. Weird.
The GRT display unit maintenance page in the setup menu has a GPS status page. Checking that indicated that both the original internal GPS unit and the Safefly GPS were returning a GPS track about 14 degrees to the right of the heading displayed on the Aera. I checked ForeFlight on my iPhone, it agreed with the Aera, and even my humble magnetic compass showed the same heading. If I selected EFIS as the input source to the autopilot and selected the SafeFly GPS as the NAV option on the EFIS, the plane turned left 14 degrees. Yet, despite the internal status page indication to the contrary, selecting the old internal GPS as the NAV source returned the plane to the original correct heading. During all of this, the heading and track indications on the EFIS PFD and map pages indicated the correct (according to Aera, compass and iPhone) headings.
So for some reason both EFIS internal and external (SafeFly) GPS units appear to be reporting an erroneous 14 degrees to the right heading, but only the SafeFly GPS causes the autopilot to veer off course, and both units seem to report the correct heading information to the PFD display.
I plan to contact GRT support about this, but given the holidays I expect it will be awhile before I hear back. Meanwhile, to anyone reading, does that make sense? Happy to add clarification. This is the first I've ever heard of a GPS reading so wildly erroneously.
What thinks the collective?
__________________
Lars Pedersen
Davis, CA
RV-7 Flying as of June 24, 2012
960+ hours as of June 30, 2020. Where did the time go?
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12-23-2017, 07:04 AM
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Join Date: Dec 2007
Location: SC & CA
Posts: 907
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Lars,
Serial port 3 on the EFIS is reserved for internal GPS 1. Did you by chance wire the Safe-Fly into these ports?
__________________
Tom Valenzia
RV8 (Sold)
RV12 Jabiru 2200 Powered (Sold)
Dues contributor since 2007
Learn from the mistakes of others. You won't live long enough to make all of them yourself...Anonymous
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12-23-2017, 08:37 AM
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Join Date: Nov 2013
Location: Austin
Posts: 28
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Coincidence?
Magnetic Variation at Davis CA = 14E.
I also see a note in the SafeFly manual advising to not connect the AHRS.
Bo
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12-23-2017, 08:54 AM
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Join Date: Jan 2005
Location: Peshtigo, Wisconsin
Posts: 768
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Have you considered baud rates? Serial in and out at each port must be the same.
__________________
Jeff Orear
RV6A N782P
Peshtigo, WI
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12-23-2017, 10:44 AM
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Join Date: Nov 2005
Location: Davis, CA
Posts: 1,156
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Thanks for the replies so far. The internal GPS hasn't been touched, still connected to serial port 3.
Interesting thought about the magnetic variation. But all other GPS devices and the magnetic compass indicated correctly. Although as I mentioned above, the EFIS GPS status page seems to be telling me that the internal GPS is also reporting incorrectly, even though it apparently steers the airplane just fine.
As for baud rate, all are set per GRT guidance, though I woke up wondering about it. The Trutrak requires 9600, which is what the Aera is set to, as well as the serial out data stream from the EFIS. The baud rate from the SafeFly into the EFIS is 115200, which I would think should have no effect on the data stream to the Trutrak. In any case, the GPS status page seems to show that the SafeFly is reporting an incorrect track to the EFIS, as I mentioned above.
Now I'm wondering if something is wrong with the GPS antenna supplied with the SafeFly. I have recent experience with a SafeFly in another aircraft where the supplied GPS antenna was DOA.
__________________
Lars Pedersen
Davis, CA
RV-7 Flying as of June 24, 2012
960+ hours as of June 30, 2020. Where did the time go?
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12-23-2017, 11:23 AM
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Join Date: Feb 2006
Location: Baltimore, MD
Posts: 1,039
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I find it hard to believe it is a baud rate issue (sorry Painless  ). GPS by itself doesn't provide track - at the most basic level is just gives you a 3D point in space (lat/lon/altitude) at an interval (5 times a second). Speed and track are derived from connecting the points over a time period. Speed is derived by Rate = Distance between points / Time between points. Track is derived by forming a vector with the most recent points and deriving a direction.
If the baud rate was incorrect, the EFIS would simply not understand anything the GPS is outputting.
Is the general position and speed correct on the EFIS charts/map? Also, have you tried to isolate the GPS? Try powering off each GPS and see if the issue follows a particular GPS receiver. Antennas can also interfere with each other. I doubt that is the issue, but isolating the GRT GPS's will certainly be the next thing I would do.
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12-23-2017, 01:27 PM
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Join Date: Dec 2011
Location: Livermore, CA
Posts: 6,797
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Quote:
Originally Posted by Bo1000
Magnetic Variation at Davis CA = 14E.
Bo
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Hard to imagine this is a coincidence. Does adsb send out heading info based on true north?
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12-23-2017, 01:34 PM
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Join Date: Aug 2007
Location: Ontario, Canada
Posts: 2,291
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If the SafeFly GPS antenna were dead you wouldn't be seeing any heading info being provided to the A/P since the EFIS wouldn't know where it was relative to the next waypoint. An antenna problem is highly unlikely to cause a position error which, ultimately, would be what's driving the heading error.
The real question is whether or not the Sport is properly compensating for magnetic variation since that seems to match up nicely with the heading error you've observed. In this instance the SafeFly should be acting solely as a source of GPS position / rate information. The Sport EFIS should be taking that data and plugging it into its own navigation algorithm to produce the necessary "waypoint is over THERE" info to the autopilot. Having a heading error of 14 degrees would suggest the Sport EFIS isn't applying the necessary correction to change from True to Magnetic Bearing to Waypoint.
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12-23-2017, 02:01 PM
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Join Date: Aug 2005
Location: AZ
Posts: 43
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Mag Deviation
Just wondering since the mag deviation happens to be the same as the track/heading change.
Perhaps the Safeflight AND the EFIS are applying the mag deviation change to the track? (ie Track + 14deg + 14deg = heading change from the others of the additional 14 degrees...)
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12-23-2017, 03:50 PM
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Join Date: Aug 2007
Location: Ontario, Canada
Posts: 2,291
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Quote:
Originally Posted by Droopy
Just wondering since the mag deviation happens to be the same as the track/heading change.
Perhaps the Safeflight AND the EFIS are applying the mag deviation change to the track? (ie Track + 14deg + 14deg = heading change from the others of the additional 14 degrees...)
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The SafeFlight GPS is just a position sensor rather than a navigator, so it has no clue what the bearing is to the next waypoint. As such it cannot apply magnetic deviation.
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