PDA

View Full Version : Trio EZ Pilot and GPS approaches?


Garage Guy
12-02-2008, 09:27 PM
I'm considering a Trio EZ Pilot, and I would want to use it coupled to our GX60 for nonprecision GPS approaches. But I have heard that Trio recommends manually setting and keeping the CDI fullscale range of the GPS at 5nm, because the EZ Pilot does not track well at higher (smaller fullscale distance) sensitivities.

The problem with that is a nonprecision GPS approach requires automatically stepping down the fullscale distance from 5nm to 0.3nm during an approach. So, follow this recommendation and... no GPS approaches for you.

Anybody out there use a EZ Pilot for coupled approaches with a TSO C129a A1 GPS, like a Garmin 155XL or 300XL, King 89b or 90b, Apollo GX60 or GX50, etc., and can let me know how you deal with this?

Thanks --

--Paul

Brantel
12-03-2008, 06:23 AM
Not sure what CDI scaling has to do with an NMEA or Aviation format connected AP. ARINC 429 enabled AP's and GPS's have the ability to transmit CDI scaled information but I don't think NMEA or Aviation format does.

Ron Lee
12-03-2008, 09:35 AM
When I was taking instrument training I believe that I had my Trio coupled to the GNS 430 for approaches and it worked fine. That is just recollection. The Trio tracks to an accuracy far better than 0.3 nm.

If the weather is good this weekend I can fly down to Pueblo (PUB) and follow an approach.

http://airnav.com/airport/KPUB

Just let me know what you want me to check. Of course Sam B can probably answer the question.

Brian's comments may make my offer of no value since I think the GNS 430 outputs ARINC format (unverified).

Sam Buchanan
12-03-2008, 10:32 AM
When I was taking instrument training I believe that I had my Trio coupled to the GNS 430 for approaches and it worked fine. That is just recollection. The Trio tracks to an accuracy far better than 0.3 nm.

If the weather is good this weekend I can fly down to Pueblo (PUB) and follow an approach.

http://airnav.com/airport/KPUB

Just let me know what you want me to check. Of course Sam B can probably answer the question.

Sam B referred Paul to this forum. I don't have any TSO-ed GPS stuff in my plane so have no personal experience to offer Paul.

Kahuna
12-03-2008, 10:36 AM
the GX60 has NMEA only. Its data will interface with the Trio.
Your 430 install likely had the Garmin "aviation" stream selected on its 232.
Other models you just have to look at whats available.
Best

Garage Guy
12-03-2008, 10:57 AM
Not sure what CDI scaling has to do with a NMEA connected AP.
The GX series doesn't do NMEA. It has its own version of "Aviation" format.
When I was taking instrument training I believe that I had my Trio coupled to the GNS 430 for approaches and it worked fine...
Just let me know what you want me to check.
Thanks, Ron, but I'm realizing now that this is an issue specific to the Apollo GX series.

Here's what I know so far. The printed documentation for the GX digital navigation data stream says the cross track error field is in units of hundredths of a nm, so not a function of CDI scaling. Garmin AT technical support says the documentation is correct. On the other hand, Trio says their flight testing (with a GX55, which is enroute only) showed that CDI scaling affected the EZ Pilot's ability to track, and so they doubt that the documentation and Garmin AT tech support is correct.

Trio has emailed 5 customers with GX60's to ask specifically about behavior during EZ Pilot coupled approaches. I haven't heard anything back yet...

--Paul

Brantel
12-03-2008, 10:58 AM
If your 430 was connected to the Trio EZ pilot, it was over RS232 and most likely using the Aviation format. Similar setup to one using the GX60 and the "GXAVIATION" (they call it MovMap) data just a bit more data in the stream.

In either case, the CDI scaling value should not impact the way the AP tries to follow the desired course.

Old analog AP's that just follow the CDI needle would be impacted by this setting.


When I was taking instrument training I believe that I had my Trio coupled to the GNS 430 for approaches and it worked fine. That is just recollection. The Trio tracks to an accuracy far better than 0.3 nm.

If the weather is good this weekend I can fly down to Pueblo (PUB) and follow an approach.

http://airnav.com/airport/KPUB

Just let me know what you want me to check. Of course Sam B can probably answer the question.

Brian's comments may make my offer of no value since I think the GNS 430 outputs ARINC format (unverified).

Brantel
12-03-2008, 11:09 AM
Sorry, I knew they used their own version called MovMap but could not remember if it was a variant of NMEA or Aviation.

Either case, the value the AP uses is the cross track error and every Aviation data spec I have ever seen has this as a fixed multiplier of hundredths of a NM off course. Never seen one documented to change its weight with a change of CDI scaling.

The GX series doesn't do NMEA. It has its own version of "Aviation" format.

Garage Guy
12-03-2008, 11:31 AM
every Aviation data spec I have ever seen has this as a fixed multiplier of hundredths of a NM off course.
Still, Trio says their flight testing showed that GX CDI scaling affected the EZ Pilot's ability to track, and they say several complaints from customers were fixed by changing CDI scaling. Interesting. (If you're interested in that kind of thing :).)

Here's another bit of info... Garmin tech support tells me that though the digital cross track error scale does not change, the maximum cross track error does change. (For example if the scaling is 1 nm full scale, the cross track field will encode all errors greater than 1 nm as 1 nm.)

Conceivably this would affect course intercept... If you are really 5 nm off track but the error field is saying you are 1 nm off track, it might not work so well. But Trio says the problem shows up while tracking, with less than fullscale CDI deflection...

--Paul

Brantel
12-03-2008, 12:29 PM
Jamie Painter had this setup at one time with a GX50 so maybe he could tell you how it worked in his situation. He pulled the Trio but I am not sure why.

Hey, I just thought of something... The later firmware versions in the GX series supported GPSS over RS232 (weird but true), whats the chances of getting Trio to support this?

Garage Guy
12-03-2008, 01:00 PM
Hey, I just thought of something... The later firmware versions in the GX series supported GPSS over RS232 (weird but true), whats the chances of getting Trio to support this?

I had a conversation about this with Trio a while back (let me pause for a moment and say that the reputation that the Trio guys have for being helpful is definitely deserved in my experience, they are very generous with their time in discussing their products) and if I remember correctly, the turn anticipation that the GX's do for flyby waypoints doesn't require any particular interpretation on the part of the autopilot... the data stream is massaged by the GX so the autopilot can just follow the stream as it usually would and this pseudo-GPSS happens. Anyway, that is something I am looking forward to trying out, if I ever get to that point! :(

Trio has said that real GPSS over ARINC will be a future option in the Pro Pilot.

--Paul

Brantel
12-03-2008, 01:17 PM
Yea but having true bank angle comands from the GPS instead of the AP having to calculate it based off of track error is tons better even if the GPS does factor turn anticipation into it's cross track error output.

This would also most likely solve the CDI scaling issue if it really exist.

Not sure Trio would see enough demand for this on the GX series to go in there add these commands to their decoder but hey would'nt hurt to ask!;)

As far as I know, the GX is the only one that outputs GPSS on RS232. Shame this did not catch hold because it is much cheaper than the ARINC 429 interface! To bad more AP's and EFIS's don't support it because the GX series is a good unit and more and more are available for great prices!

Garage Guy
12-03-2008, 01:50 PM
Not sure Trio would see enough demand for this on the GX series to go in there add these commands to their decoder but hey would'nt hurt to ask!;)
Interesting... I thought it was done just by fudging the regular Moving Map data stream.

Where are the GPSS sentences documented? I can bring this up with Trio.

--Paul

Brantel
12-03-2008, 01:58 PM
Sept 2003 Install Manual

http://i33.tinypic.com/15s692t.jpg

http://i36.tinypic.com/2a851dl.jpg

Brantel
12-03-2008, 02:02 PM
Hey Dynon...Beat Trio to the punch by adding this protocol to your decoder and you might gain some sales on your AP for people who want GPSS and have GX series GPS's......

Garage Guy
12-03-2008, 07:57 PM
Well that is interesting. My antique 1997 install manual does not have that GPSS format shown... but I see my recently-upgraded GX60 does have it as a data output option.

But it looks like the GPSS stream doesn't just add an "h" field for horizontal command to the regular MovMap stream, and it is leaving out info like present lat/long, waypoint identifier, etc. If so, my guess is that with only one RS232 input on the EX Pilot, that Trio wouldn't be interested in giving up that other data. This may be what they were trying to tell me when I was talking to them months ago, but I didn't get it. I'll ask when I talk to them again.

--Paul

Brantel
12-03-2008, 08:39 PM
I don't think they actually use that other info anyway so you might be able to talk them into adding the "h" sentence to the decoder.

Garage Guy
12-03-2008, 08:54 PM
They use waypoint ID, for sure...
http://kokoro.ucsd.edu/images/EZPIL.jpg

Brantel
12-04-2008, 05:06 AM
Oh, well, hmmm ... Who needs that? :p

Jamie
12-04-2008, 05:56 AM
I have a GX-50 GPS but it does not have GPSS as an ouptut option. I suppose I need a firmware upgrade.

Can someone tell me how much I can expect to pay for my GX-50 firmware to be updated? Also, I called our local avionics shop yesterday and asked them about updating the firmware on my GX-50 and they had no clue what I was talking about.

Me: "Hi, I have an Apollo GX-50 GPS and I would like to have the firmware upgraded to the latest version"
Him: "You want to upgrade the database?"
Me: "No, I would like to upgrade the firmware"
Him: "Uuuhh...well...we don't really do that".

So anyone know how to do this?

Dynon says they will fly whatever course is being fed to it, including GPSS so I would imagine it would work. It would be fun to try it anyway.

Brantel
12-04-2008, 06:56 AM
Jamie,

Make sure Dynon understands what your asking them before you spend some money to get a feature that not many support....

GPSS over RS232 using a custom version of the Aviation format is not common and the only other AP that I know that supports this is the more advanced TT units and some certified units with an adaptor box.

The GX when set to GPSS outputs a custom subset of the Aviation format which includes:

Ground Track
Ground Speed
Distance to Active Waypoint
Bearing to Active Waypoint
Horizontal command in the form of bank angle (GPSS)

Everything is normal Aviation format except for the GPSS command and it uses a "h" ID for its sentence and I betcha that the Dynon does not decode that sentence when set to accept aviation format.

There may also be some issues with the fact that the Dynon might need more data than what is provided when you set the GX to output their version of GPSS.

I know they could get this to work, the problem will be convincing them that they need to add support for this. Might be a tough sell since the GX is now considered old. It is a shame because it is my understanding that the GX is a great unit for non-precision and enroute work and RS232 is way cheaper than ARINC 429.

Garage Guy
12-04-2008, 09:15 PM
Can someone tell me how much I can expect to pay for my GX-50 firmware to be updated?

The only way I know how to do it is via Garmin AT flat rate out-of-warranty repair, which is $400 for a GX-50. Whether you think that is a good deal might depend on how much else needs doing at the same time... See http://www8.garmin.com/support/outofwarranty.html (http://http://www8.garmin.com/support/outofwarranty.html).

--Paul

Ron Lee
12-05-2008, 10:30 AM
Can you get database updates for these units? If not, it would seem like their usefulness is questionable.

tommylewis
12-05-2008, 10:33 AM
yes you can get database updates for the GX series of GPS boxes from Jepp

Brantel
12-05-2008, 10:58 AM
Yes you can... There are thousands of these still in use on certified planes and experimentals.

Can you get database updates for these units? If not, it would seem like their usefulness is questionable.

Garage Guy
01-03-2009, 01:02 AM
Just an update on where we are with this GX-to-Trio issue.

1. The documentation for the GX series says that the "G" sentence in the MovingMap serial output stream gives "distance off course, hundredths of nm". We've recorded the serial stream and confirmed that this is not true. In fact, the distance is in units of anything between .01 nm and .0006 nm, depending on the current CDI scaling the GX is using. However, what CDI scaling the GX is using is not represented in the serial stream. This presents a problem for any autopilot that depends on this "G" sentence to track a GPS flight plan; the control gain effectively can change by a factor of 15 without the autopilot knowing it.

2. We've looked at some workarounds for this. For example, by looking at the distance to waypoint, bearing to waypoint, and desired track, it should be possible for the autopilot itself to compute cross-track error. This will be a low-precision estimate (especially when close to a waypoint, but even in general, in the GX stream, BTW and DTK are only in whole degrees -- though the documentation says in tenths of a degree, the tenths are always zero), but the idea is that it could be used, together with the "G" sentence, to estimate the GX's current CDI scaling. Unfortunately, this idea breaks down badly when the GX is doing turn anticipation (because then its reported DTK and the XTK in the "G" sentence are relative to a tangent to the anticipation curve connecting to the next waypoint, while BTW and distance to waypoint are relative to the current waypoint).

3. GX's with recent software have the option of Moving Map with Extended Data format, which includes information about the state of the GX's external annunciators. The annunciators encode whether the GX is in approach enabled or approach active mode, and these are the modes that trigger CDI scaling changes. However, the rate of scaling change with mode change is not documented; not all GX's have the Extended Data option; and the user can change the scaling manually, which wouldn't show up in the annunciator state.

4. Garmin AT isn't interested in fixing this problem. Ideally, they should then open the source, and publish the toolchain permitting modifying the software of discontinued avionics, if only for the experimental community. But that's another topic...

5. Recent software version GX's provide a GPSS data stream. The roll commands in this stream are sent at only a 1Hz rate, so it remains to be seen how well it can be made to work. But it seems at this point that it is probably the best option to get good flightplan tracking performance from the approach-capable GX's. Trio wants to do development of this on their new Pro Pilot platform, and then backport to the EZ Pilot if possible. I'll be working with them on that, and I'll let you all know how it goes.

--Paul

Garage Guy
01-29-2009, 10:52 PM
5. Recent software version GX's provide a GPSS data stream... it is probably the best option to get good flightplan tracking performance from the approach-capable GX's.

OK, another update. For testing, we installed a Trio Pro Pilot development unit (basically the same as a production unit but with extra connectors for uploading software and downloading diagnostics) in our RV-6, with the "UPSAT GPSS" pin on the Pro Pilot driven by TxD2 pin on our GX-60, configured as GPSS. This version of the Pro Pilot software drives the roll servo using the GPSS stream when in TRK mode, if it is available; if not available or in CRS mode, is uses data coming in on its Avlink/NMEA pin.

Anyway, we've now flown it for several hours, tracking and intercepting flight plans including approaches, and it works very well. If I saw anything to complain about, it is that the GPSS roll commands the GX emits maybe seem a bit overdamped. This might be mostly a matter of taste though. It certainly can fly a LNAV GPS approach to the MAP well within the FAA instrument practical test standards. Looking at our tracks on the 496 afterward, the anticipation turns at fly-by waypoints are really pretty; fly-over waypoints are nailed just as they should be; track intercepts are quite good, maybe a bit overdamped like I said.

As a result, the Trio brain trust tells me this feature is now about ready to ship with production Pro Pilots. As for backporting to the EZ Pilot, which is how I got into all this: it could work, if the GX has the GPSS signal included in its Extended Moving Map output stream (the EZ pilot has only one RS-232 port). Some GX's are known to have this, but it isn't documented, and so the Trio folks are reluctant to release software that uses it. I guess you could contact them if you are interested in the availability of any of this.

--Paul