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GRT SafeFly2020 GPS misbehavior

L

Lars

Well Known Member
Sponsor
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,

Serial port 3 on the EFIS is reserved for internal GPS 1. Did you by chance wire the Safe-Fly into these ports?
 
Coincidence?

Magnetic Variation at Davis CA = 14E.

I also see a note in the SafeFly manual advising to not connect the AHRS.


Bo
 
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.
 
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.
 
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.
 
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...)
 
Droopy said:
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...)
Click to expand...

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.
 
Magnetic variation

Thanks again for all the replies.

As I understand it, GPS receivers natively read in true north, with the conversion to magnetic made based on databases.

The variation here is 14? as noted, but it's east of true, so unless I've gotten dyslexic (happens), when I was flying a magnetic heading of 311? (as reported on the PFD, compass, and Garmin Aera) my true heading was 297?. Yet on the GPS status page the SafeFly was reporting 325? (so was the internal GPS, even though it seemed to operate everything correctly).

Presumably the heading displayed on the PFD is obtained from the magnetometer, so it makes sense that it apparently reads correctly (consistent with my magnetic compass, etc).

Please note I'm not trying to refute anyone here. I appreciate all the input; I'm just trying to understand what's going on. And to all of those reading, best wishes for a Merry Christmas!
 
Oh yeah, another tidbit. The PFD screen on my Sport SX shows a spiffy purple (correction: lavender, according to my style consultant reading over my shoulder) dot on the display, aligned with the heading of the current waypoint. Thus if I'm pointed toward the waypoint, the dot is at the center of the screen. Its location is apparently tied to GPS data. I fed the Trutrak with Aera data, and set it to NAV mode. With the EFIS set to navigate with the internal GPS (but not controlling the TruTrak) said dot was at the top center of the screen. Thus the internal GPS and Aera appeared to agree. When I selected the SafeFly as the EFIS GPS source, the dot jumped to the left.
 
Backwards

Lars said:
The variation here is 14? as noted, but it's east of true, so unless I've gotten dyslexic (happens), when I was flying a magnetic heading of 311? (as reported on the PFD, compass, and Garmin Aera) my true heading was 297?. Yet on the GPS status page the SafeFly was reporting 325? (so was the internal GPS, even though it seemed to operate everything correctly).
!
Click to expand...

Magnetic heading of 311 = true heading of 325
 
BobTurner said:
Magnetic heading of 311 = true heading of 325
Click to expand...

Like I said, dyslexic. Thank you very much, Bob. Once again, if there is a 50-50 chance of me getting something reversed, there seems to be a 90% chance that I'll choose the wrong answer.

And that seems to explain it. For some reason the SafeFly data is not getting corrected to magnetic. Now to figure out why...
 
To emphasize again, the SafeFly is acting as a GPS sensor. Correction for magnetic variation happens inside the EFIS. With this in mind, start by looking at your EFIS settings.
 
Canadian_JOY said:
To emphasize again, the SafeFly is acting as a GPS sensor. Correction for magnetic variation happens inside the EFIS. With this in mind, start by looking at your EFIS settings.
Click to expand...

So I now understand. The SafeFly transmits raw true data to the EFIS which should correct it for magnetic variation but for some reason doesn?t. I have yet to find a setting option for that, my next test will be to move pins around in the EFIS D-Sub to determine if it?s serial port dependent. For some reason the internal GPS data and the Garmin GPS data (connected to serial port 1 in) do get corrected. And just to be clear I know that the Sport SX can only accept two GPS inputs. I swap them around using the setup menu.

Again, thanks to all who have replied. I have learned a lot in a short period of time.
 
Issue solved

I thought I should follow up in case anyone stumbles on this thread.

It turns out that, at least on the legacy Sport SX200 (mine dates from 2010) with software revision 14.00, there is an undocumented setting in the settings menu, Display Unit Maintenance page, for allowing external GPS data in NMEA0183 format to pass through the EFIS.

Per Jeff at GRT Avionics: "If you have an NMEA0183 GPS input and Autopilot NMEA0183 output, the EFIS takes a short cut and passes the GPS data directly from the GPS to the autopilot. In this case, the Safe-Fly doesn't compute magnetic variation so that passes through as blank. You can fix that by changing the SET MENU, Display Unit Maintenance, "APGPS NMEA0183 Passthru" setting to Off."

I turned that off and Voila! All well. Internal and SafeFly GPS devices both behave the same way when controlling my TruTrak autopilot, and all readings show magnetic.

I'm very happy in general with GRT and their support, but it's time for them to hire a tech writer and get caught up on some of their documentation!
 
Amen, Lars! Glad to hear you got this situation sorted out and are flying in the right direction again.

The GRT products are second to none. Their documentation is among the worst in the industry. At a minimum the installation manual should give a full, clear and concise definition of each and every setting available in the SET menu. This should include a definition of each of the options available, such as "setting it to OFF does this, while setting it to ON does that".

Having spent a dozen years being responsible for writing, editing and checking avionics technical publications for both military and airline customers, I'm astonished that GRT can remain in business with product documentation as inadequate and outdated as theirs is. I love their products, but man is it frustrating trying to figure out how to configure them. Please, if anybody from GRT is reading this, take this appeal to heart!
 
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