What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

GPS Signal for Trutrak Digitrak

Snap

Well Known Member
Hi all,

unfortunately our winglever won't accept the NMEA GPS Signal from out Holux GPS reciever. We got a 12V NMEA Signal but the autopilot head won't use the signal. In track hold mode the autopilot will wander away in no time and can't be used for real flying. Is this normal?

What kind of protocol is needed for the Trutrak devices?

Thanks ahead guys.
 
You can wire your gps to the serial port of a pc. Using a terminal program you can see the data stream. This will allow you to validate if your gps is capable of supporting 4800 baud and a data rate of once per second or better.


From the manual:

We require a data output rate of once per second for best performance. Some handhelds output data at longer intervals than once per second. These handhelds will cause the autopilot not to perform well in turns and it may cause overshooting and hunting. If the baud rate is selectable the optimum setting is 4800. Most handhelds will require a data cable that plugs into the handheld and provides a medium for data output. This is an accessory and is available from your GPS manufacturer. The setup procedures are in your GPS manual. If your GPS is not listed here consult your GPS manual for NMEA output setup. The autopilot must have a direct connection with the handheld GPS to provide the autopilot with RS-232. To allow the handheld GPS to be removed easily you should add a connector in your panel. We recommend that you use a 9 pin D subminature connector in your panel that will mate to the harness from the GPS. This will also allow you to wire power and ground and use the aircraft electrical system to power your handheld GPS.
 
A GPS handheld that has some navigation capability will make the autopilot much more useful.

A used Garmin 296 isn't very expensive.
 
Hi Bob, we did exactly what you suggested last week. We used a microcontroller to get the data the GPS receiver puts out: It is a RS-232 NMEA protocol. Sends the position data every second.

But: Is it that the Trutrak might need a vector in which the airplane is headed instead of singel GPS points? Do we need a track information or GPS position points for the Digitrak? The manual is unclear in that regard.

Unfortuantely we don't have space in our panel for a garmin type GPS, we navigate with our phones and use the space for a EFIS.
 
Hi Bob, we did exactly what you suggested last week. We used a microcontroller to get the data the GPS receiver puts out: It is a RS-232 NMEA protocol. Sends the position data every second.

But: Is it that the Trutrak might need a vector in which the airplane is headed instead of singel GPS points? Do we need a track information or GPS position points for the Digitrak? The manual is unclear in that regard.

Unfortuantely we don't have space in our panel for a garmin type GPS, we navigate with our phones and use the space for a EFIS.

You just identified your issue.

Typically, the serial interface has just lat/long data. All the Trutrak APs that I’m familiar with expect to see flight plan data coming in via the ARINC interface. That’s missing with your portable gps.

If you also have an EFIS, most are capable of using an internal map and using the NMEA feed. Then your EFIS can send flight plan data to the AP via ARINC.
 
Hi all,

unfortunately our winglever won't accept the NMEA GPS Signal from out Holux GPS reciever. We got a 12V NMEA Signal but the autopilot head won't use the signal. In track hold mode the autopilot will wander away in no time and can't be used for real flying. Is this normal?

What kind of protocol is needed for the Trutrak devices?

Thanks ahead guys.
Simon,

Your Digitrak supports Track and NAV lateral modes, both of which are supported by providing the proper NMEA data over RS-232.

When first engaged, it defaults to Track mode by capturing the current GPS ground track provided by the NMEA GPRMC message. You can use the buttons to change the selected GPS track to other values to steer the aircraft left or right of present track. If the Digitrak is not displaying OFF before engaging the autopilot, and a digital value for GPS ground track when engaged, it is not receiving a proper GPRMC message at the correct baud rate.

When selecting and using NAV lateral mode, the connected GPS navigator must provide course line cross track error for the current Direct To or flight plan selected active leg using the NMEA GPRMB message.

To answer your question, no it is not normal for the aircraft to deviate from the selected GPS ground track when operating with the autopilot engaged in Track mode.

Steve
 
Thanks guys, this really helps.

@Steve: The aircraft won't deviate with GPS engaged, but in heading hold mode - we were not able yet to get a valid GPS signal into the TruTrak Digitrak.

BUT: We found that our MGL Xtreme EFIS can output a RS232 NMEA GPRMC signal and has a heading bug an autopilot feature. We will try to use that signal to work with the autopilot - this should hopefully do the job.

Will keep you posted. Thanks again all for your valuable input!
 
Got it fixed! The GPS Signal from our MGL Efis works with the autopilot.

Now we need to get the direction of the motion right.
 
Back
Top