Multi-GPS feeds to EFIS and AP - How would you configure?

Folks,
I'm trying to design the wiring connectivity for my GPS's, EFIS and AP. I would appreciate informed comment.

Here is the scenario: VFR airplane with AFS EFIS/EMS (4500EE & 4500MFD + ARINC module), AF-Pilot (Trutrak) AP, Garmin 695, Garmin 296, Trutrak ADI with built-in GPS.

OPERATING REQUIREMENTS
1) Have route defined in 695 with alternate/backup route in 296 with ability for the EFIS to direct the AP to follow either route.
2) Allow EFIS Hdg & ALt Bugs to direct AP
3) Allow AP to fly a heading without any input from EFIS.

Here is the conundrum.......
- For best performance (in turns and to avoid hunting) the AP wants the GPS's configured for FAST OUTPUT (1 second data interval and minimum NMEA sentence subset).
- The EFIS want (e.g. for Synthetic Vision) the GPS's configured for NORMAL OUTPUT (2+ second data interval and full NMEA sentence output).

WIRING CONNECTIONS
Here is one strategy I have considered:
- Connect ADI GPS Data Out to AP Serial In (4800 Baud GPRMC only.FAST OUTPUT?)
- Connect Data Out from 695 and from 296 to AF-4500's (NORMAL OUTPUT)
- Connect AF-4500 ARINC A&B to AP ARINC A&B (via AFS ARINC Module)

This configuration appears to give the AP the 1 second interval data it needs in Serial In Mode and gives the AF-4500 the NORMAL OUTPUT full GPS data it needs.

BUT... I'm not sure of the characteristics of the ARINC data traffic between the AF-4500 and the AP? As the EFIS underlying GPS source data is 2+ second interval I presume the AP ARINC input will still be 2+ second interval (meaning sub-optimal AP performance), OR does the AF-4500 do some clever extrapolation calculations to determine intermediate position data in between GPS data points?

I have been in email discussion with Trevor and Lucas at AFS and Trutrak but I am still not yet sure of the best way forward to get over the apparent FAST OUTPUT / NORMAL OUTPUT conflict.

How would you wire up this kit combination, and why?

I would be intrigued to know if the AF-4500 can take notice of two GPS feeds at the same time? E.g. If I were to connect the ADI GPS Data Out to the AF-4500's as well as the 695 & 296, could it make use of the 1 second interval position data from the ADI GPS while still following the navigation/route data in either the 695 or 296? (I'm pretty sure this is not the case though.)

Thanks in advance,

Steve Hutt

Don't forget about wiring your EFIS and GPS output to your transponder... My Dynon and Garmin GPS both provide inputs to my Garmin 327. No other encoder needed. The Dynon is the primary source, the GPS is the alternate.

Thanks Rob,

Don't think I need Serial connectivity to my transponder. I am using a Funkwerks TRT800. It has it's own static port so can work out the altitude for itself. It does have Serial In and Serial Out pins but these are used for TIS & ADS-B, neither of which are in use here in the UK/Europe.

Steve

Snowflake said:

Don't forget about wiring your EFIS and GPS output to your transponder... My Dynon and Garmin GPS both provide inputs to my Garmin 327. No other encoder needed. The Dynon is the primary source, the GPS is the alternate.

Click to expand...

Just an FYI, GPS does not provide altitude for transponder altitude reporting like an encoder does.

Walt, I think you're mostly right. I'd have to go back and check the manual, but I think the 327 uses the GPS input for automatically taking you in and out of standby mode. And I may be completely wrong here, but I thought you could tell the 327 to use the GPS input for altitude if you wanted to... For example, if your encoder failed? I realize that the GPS altitude accuracy is extremely low, but as a backup maybe it's better than nothing?

The various Garmin transponders can use GPS velocity data from a panel-mount GPS (via the Aviation RS-232 format or via ARINC 429) to assist with auto airborne/ground determination. They don't use GPS altitude for Mode C, and they can't receive NMEA 0183 formatted data such as that which is transmitted by a handheld GPS. So, there's no benefit in supplying data from a handheld GPS to a GTX 327. Hope that helps.

mcb

I'm going to have to go and check my setup again so I know what formats are going where, but my Garmin Aera handheld feeds my Dynon D10A, my iCom 210, and my Garmin 327 simultaneously. I thought they were all getting the same signal, the NMEA feed from the serial output. The Dynon further feeds its altitude (from the static port, not the NMEA feed) info to the Garmin 327 for altitude reporting.

Keep it simple! Connect one GPS to your system and build on.

GPS altitude is NOT valid for mode C. Mode C output is based on barometric altitude. That is what air traffic uses for separation.

Actually its pressure altitude. The ATC computers correct it for the local conditions.

Dan,

Dan B said:

Keep it simple!

Click to expand...

Sound advice, I know.

Dan B said:

Connect one GPS to your system and build on.

Click to expand...

My fundamental question is not actually about multiple GPS's. It relates to the apparent conflicting requirements of AP and EFIS that seem to demand two different configurations (FAST vs. NORMAL) in a single GPS at one and the same time. Clearly this is not possible and results in a need for at least two GPS's, each configured appropriately for it's audience. The AP Serial In GPS feed is only needed when the AP is flying the airplane with no EFIS input (either deliberately (AP in AP Mode) or in a potential EFIS or ARINC Module failure scenario).

My supplementary question relates to the frequency of the ARINC Data received by the AP from the EFIS when the EFIS is fed with NORMAL 2+ second interval GPS data. On a different forum I was advised it is highly likely that the EFIS does fill in the 'gaps' and supply the AP with calculated position infomation (apparently Kalman Filters come into play here) that enable the AP to maintain smooth flight.

Steve

Normal output will be fine for the AFS Pilot. Since 99% of the time you will NOT be using the autopilot in Track Selector mode the serial input speed won't matter as it's only used for the DG display while you're in EFIS mode.

Hi Lucas,

Thanks for responding here. I appreciate what you say and that is good to know. I am though interested in that other 1% of the time as well, i.e. AP Track Selector Mode.

The primary reason I went for the AFS EFIS with AF-Pilot kit combination was because of the ability to run the AP with the EFIS AND without the EFIS.

With the AP in Track Selector Mode my understanding is that I am effectively dealing with a stand-alone AP with GPS guidance and that everything in the TT manual concerning Track Selector Mode applies. The TT GPS Configuration Guide ( http://www.trutrakap.com/documents/GPSConfigurationGuide.pdf ) is very clear about the required GPS configuration for best AP performance. I do understand that executing turns may not be not such a key issue when in Track Selector Mode but maintaining a straight line with corrections every second is bound to be better than corrections every two seconds.

I believe I have the kitlist with which I can hopefully establish optimum configurations for the AP in all possible operating modes. That is what I am seeking to achieve.

Regards,
Steve

shuttle said:

Folks,
Here is the conundrum.......
- For best performance (in turns and to avoid hunting) the AP wants the GPS's configured for FAST OUTPUT (1 second data interval and minimum NMEA sentence subset).
- The EFIS want (e.g. for Synthetic Vision) the GPS's configured for NORMAL OUTPUT (2+ second data interval and full NMEA sentence output).

Click to expand...

Steve, I believe that you will find that the EFIS prefers a faster data rate. There have been a few threads on AFS' forum about this, I would recommend searching there for clarification. The AFS GPS puck is in reality a Garmin GPS 18X-5Hz, meaning it provides updates 5 times per second. This rate is preferred to minimize the jumpiness of the synthetic vision.

Hi Noah, Thanks,

I'll check-in there again. I did do a search there but didn't see what I was hoping for. I was searching on Serial inputs. I'll search again on GPS rates (or equivalent wording).

It was direct from AFS that I was told that AFS SV needs (Garmin) NORMAL OUTPUT (more data but 2+ second intervals).

I must admit that I feel this area of the documentaion could do with a little more detail about pro's and con's of various configuration options. At the moment one docs basically says "Do X" and the other says "Do Y" and there is a mismatch.

I hope I'm not boring people with this but it seems worth the effort to clearly understand it and setup my plane right for all opearating scenarios.

Thanks,
Steve

The normal Garmin mode updates faster than 2 seconds if you turn all that extra Garmin proprietary stuff off. That stuff is not needed anyway by the AP.

Normal mode will work just fine.

A 5hz GPS does not necessarily output it's data at 5hz.

And if in doubt, you can use the GPS out from the ADI. It runs at 4800 baud and has a 1Hz update.

Steve, the FAST output configuration is only a recommendation from TruTrak. Your autopilot will work and will work very well with the NORMAL output rate which the EFIS requires. We do plan on implementing a software change so you can use FAST output with the EFIS. It has to do with the sentence we look at that contains GPS altitude. They are different sentences in FAST and NORMAL modes.

Configure the GPS units that are connected to the autopilot and EFIS for 9600bps NORMAL output and you will be golden my friend!

Thanks Trevor, Lucas and all,

Greatly appreciate all your assistance.

Trevor,
That future FAST OUTPUT software change would seem to be helpful.

Steve