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D2: function without GPS ? (Read 7457 times)
Oct 30th, 2014 at 3:01pm

JohanMinne   Offline
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Why does the D2 refuse to work without a GPS fix?
Is the exact location on earth required to make MEMS technology work?
I' m just wondering what will happen underneath a CB, when thermaling at cloudbase, with the risk of cloudsuck, and poor GPS reception due to a mountain of droplets above me.  Will the D2 stop working when I need it most?

Johan
 
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Reply #1 - Oct 30th, 2014 at 5:25pm

Dynon Avionics   Offline
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It does require a GPS fix. Anywhere on earth is fine Smiley.

Once you have a lock, it can cope with loss of GPS for up to a minute, but after that attitude will go away. Good GPSes don't usually suffer under clouds/rain. Basically, the D2 should suit you well (I might be biased Smiley )
 

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Reply #2 - Nov 27th, 2015 at 4:22am

MikeS   Offline
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If GPS goes down then we no longer have an attitude indicator.  Why advertise you have a "true artificial horizon" when you don't?  A "true artificial horizon" is independent of outside data sources.  A "true artificial horizon" uses internal gyros, whether electronic or vacuum driven.  This appears to be intentional dissemination of misleading information by Dynon.  It is NOT a "true artificial horizon" any more than the other portable low-cost GPS-driven "panel displays" available from other vendors.      

It's a nice little instrument for what it is - but why is Dynon so desperate for sales that you're willing to mislead people?  This is my first Dynon product and will likely be my last.  I'm not happy to discover that my "true artificial horizon" is nothing but a paperweight when GPS goes down - which it sometimes does.  Oops forgot . . . I can stay right side up for a minute after GPS signals are lost.  Does it give any warning during that minute that GPS signal has been lost?  I didn't see anything in the book about that but I'll look again.  I can't imagine that it wouldn't. 

As a back up instrument, this device is only as trustworthy as the GPS system it depends upon.  A true back up instrument would be internally self sufficient.       
 
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Reply #3 - Nov 28th, 2015 at 11:09pm

hotrod180   Offline
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Per my posts in another thread, I'd like to see an attitude-only portable device which doesn't have or need GPS input. I don't esp need the speed/altitude/ directional information which I already get from my Garmin 196 nav -- just the artificial horizon display for maintaining a safe attitude.
Maybe something for Dynon to consider for a future product.
 
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Reply #4 - Nov 29th, 2015 at 8:39am

Joe Dubner   Offline
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Independence, OR

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I was also looking for an attitude indicator (although the search was made more complicated and expensive by my need for a 2-1/4 inch instrument).  I decided the D2 wasn't for me due to its reliance on GPS (and other factors) but I bought a TruTrak ADI that I've grown to like very much.  I don't think the ADI is sold any longer but has been replaced with other instruments.

...

The ADI has provisions for a GPS connection but GPS is not needed for its "attitude indicator" functions.  I quoted "attitude indicator" because it is a rate-based instrument, not a true attitude indicator.  But as I mentioned, I like it a lot and find it superior in some instances (such as holding straight and level or climbing or turning at a fixed rate).

--
Joe
 

RV-8A
Independence, OR
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Reply #5 - Nov 29th, 2015 at 3:09pm

JohanMinne   Offline
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Hello,
I have been looking for an explanation on how the AHRS works with Mems. I found a decent description on the Kanardia website. They have the Horis AHRS, witch needs no GPS fix to work, but they need Ptot to work properly. Please reed the topic of 3 febr 2013 on their Misc page: AHRS working principle. The Kalman-filter in their software needs the true airspeed to calculate attitude, based on apparent gravity. I suppose that Dynon uses the GPS-speed for this calculation. So, no GPS fix means, long term attitude with errors (Dynon decide to stop after 60 seconds). This is IMHO due to the StandAlone option that was chosen for the D2. I use the D2 for 2 years now and had no GPS problems. The D2 even saved my life when I entered Instrumental conditions and kept me level in a white-out at 150 ft AGL. The D2 reaches its limits when thermaling at low speed with frequent uncoordinated turns to center in the core of the rising air. I suppose this is because the software of the D2 is also calculating attitude based on assumed coordinated flight.
I keep the D2 because it is a valid backup (own battery), it needs no connection to any tube, and the WiFi connection to Air Nav Pro for 3D AHRS with terrain is a plus.

All this is . . . In My Humble Opinion.  Wink
Regards,
Johan

Ps.: French ULM = VFR (entering IMC is stupid and will be avoided in the future)
« Last Edit: Nov 30th, 2015 at 9:59am by JohanMinne »  
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Reply #6 - Nov 30th, 2015 at 6:29pm

Dynon Avionics   Offline
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MikeS wrote on Nov 27th, 2015 at 4:22am:
If GPS goes down then we no longer have an attitude indicator.  Why advertise you have a "true artificial horizon" when you don't?  A "true artificial horizon" is independent of outside data sources.  A "true artificial horizon" uses internal gyros, whether electronic or vacuum driven.  This appears to be intentional dissemination of misleading information by Dynon.  It is NOT a "true artificial horizon" any more than the other portable low-cost GPS-driven "panel displays" available from other vendors.      

It's a nice little instrument for what it is - but why is Dynon so desperate for sales that you're willing to mislead people?  This is my first Dynon product and will likely be my last.  I'm not happy to discover that my "true artificial horizon" is nothing but a paperweight when GPS goes down - which it sometimes does.  Oops forgot . . . I can stay right side up for a minute after GPS signals are lost.  Does it give any warning during that minute that GPS signal has been lost?  I didn't see anything in the book about that but I'll look again.  I can't imagine that it wouldn't. 

As a back up instrument, this device is only as trustworthy as the GPS system it depends upon.  A true back up instrument would be internally self sufficient.       


We're not trying to mislead here. The D2 DOES need an external aiding source (GPS), as do ALL robust and reliable MEMS-based attitude indicators that actually work (including every ADAHRS in every Cirrus/Cessna/G1000/etc). SkyView and our other EFIS systems use airspeed primarily with GPS as a backup, but the D2 uses GPS since it does not have airspeed information. The importance of GPS is why we include the external antenna for installations where a clear view of the sky is a challenge from lower mounting locations in the panel. Between the internal sensors or the external puck, most people should be able to keep a rock solid GPS lock all of the time once acquired. Please touch base with our support department at 425-402-0433 or support at dynonavionics dot com if this is not the case.

The D2 does in fact let you know when it loses GPS with a big GPS LOST: CROSS-CHECK HORIZON banner that's hard to miss.

The D2 does show actual attitude. To be perfectly clear - when you have a GPS fix, you're seeing attitude, not GPS-derived flight path. The D2 shows the actual artificial horizon, which is NOT the same thing as you see with GPS-BASED depictions of attitude which are really just flight path. For example, if you do a slow mushing descent with the nose-up with a D2 - as you might if you're disoriented and way behind the power curve - you'll see a nose-up attitude on the D2, as you should. If you did the same thing with a GPS-driven panel display, you'll see a descent, because all it knows is GPS-based flight path.

Finally, these requirements are documented in the PERFORMANCE NOTES section of the D2's manual. The relevant portions are excerpted here:


Attitude Performance

Optimal attitude performance depends on a number of environmental factors:

GPS REQUIRED

The attitude indication provided by the D2 is primarily created by combining information from
solid state MEMS-type accelerometers and rotation rate sensors, and supplemented by GPS
ground speed. In order to display as reliable an indication as possible, a GPS fix is required. The D2
will not display attitude until it has obtained a GPS fix. Once a GPS fix is obtained, it is important
that the GPS fix be maintained by either mounting the D2 in a position that affords it a good view
of the sky, or, if that is not possible (such as when using the pinch mount to panel-mount the D2),
using the included external GPS antenna.

Loss of GPS

Lapses in GPS coverage may degrade attitude accuracy, depending on the length of the outage.
During short lapses, the message GPS LOST: CROSS-CHECK HORIZON will appear at the bottom of
the display to indicate that the D2’s attitude indication may be degraded and should be
interpreted cautiously. Additionally, the indications that rely solely on GPS will be replaced with
red Xs.
 

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Reply #7 - Dec 2nd, 2015 at 7:53am

gtae07   Offline
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I think the thing to remember is that the D1 and D2 are intended as (portable) backup/standby instruments.  You wouldn't use one as your primary or only attitude display in an IFR aircraft. 

From that standpoint, a reliance on GPS is not as critical as it would be in a primary instrument.  The chances of you losing your primary attitude reference (whether due to an electrical/vacuum source failure, a hardware/software fault, mechanical problem, etc) and losing GPS reception at the same time are quite remote. 

If you don't feel like that level of redundancy is sufficient for your needs, there are other options that don't rely on GPS.  But I don't think you're going to find any attitude indicator completely independent of all inputs (air data, GPS) that isn't hard-mounted and/or very expensive.
 
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Reply #8 - Dec 9th, 2015 at 4:34pm

MikeS   Offline
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As Joe Dubner mentions in post #4, the TruTrak ADI works extremely well as an attitude indicator and does not depend upon GPS.  I've had the 3 inch version as my primary attitude instrument sincde 2008 and have used it in instrument conditions - a lot.  It has never failed.  My aircraft responds instantly to control input and the TruTrak ADI responds instantly to attitude changes so they've been a perfect match.  I love TruTrak products. 

I bought this GPS-dependent back-up instrument (Dynon D2) for ferrying an aircraft across the country that has old vacuum driven instruments that do not work.  The D2 is going to be used just for that trip and then sold.  Selling an emergency attitude instrument that is dependent on GPS is disappointing.  I didn't do my research, obviously.  At least it comes up with a warning if GPS is lost.  If that works then Dynon at least did that right. 

I should have bought a used TruTrak ADI for my ferry trip instead of the D2.  It would have been cheaper and more reliable. 
 
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Reply #9 - Dec 9th, 2015 at 7:50pm

Dynon Avionics   Offline
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The Trutrak ADI has limitations and dependencies of its own. It's definitely an interesting and useful instrument, but it also has an external dependency on your static and pitot ports. It also isn't an attitude indicator.

"Pitch" is really vertical speed, with a short term reaction to pitch rate. So it agrees roughly with attitude as long as you have as your attitude is going in the same direction as your climb rate (not a given if you've ended up disoriented and in an unusual attitude).

To prove this to yourself, try holding altitude or descending while in slow flight with a pitch-up attitude. You'll see a "pitch down" attitude on the ADI even though your nose is pointed at the sky. Which means it shows you nose down when you're in a stall and very nose up.

Also, since it measures vertical speed from static, it will read incorrectly if your static source is blocked. It also requires pitot to function properly or the bank angle will be totally wrong.
« Last Edit: Dec 10th, 2015 at 12:28pm by Dynon Avionics »  

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Reply #10 - Dec 11th, 2015 at 3:22pm

MikeS   Offline
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"To prove this to yourself, try holding altitude or descending while in slow flight with a pitch-up attitude. You'll see a "pitch down" attitude on the ADI even though your nose is pointed at the sky. Which means it shows you nose down when you're in a stall and very nose up."

No, it doesn't do this.  It responds exactly like a vacuum driven AI.  I've had mine since 2008 and am familiar with its behavior in all flight modes - slow flight nose-high close-to-the-stall included, which is the plane's configuration when approaching the runway for landing.  In that configuration it shows my nose above the horizon.  I understand there is some differences between it and a conventional AI in the way it arrives at the information it displays.  In reality, it displays horizon information exactly as do the vacuum driving AI instruments I also fly.  If it didn't I would have gotten rid of it a long time ago.

What you are stating is more misinformation.  Dynon seems to have a penchant for doing this. 
   

 
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Reply #11 - Dec 11th, 2015 at 4:24pm

Joe Dubner   Offline
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Dynon Avionics wrote on Dec 9th, 2015 at 7:50pm:
. . . since it measures vertical speed from static, it will read incorrectly if your static source is blocked.

From my TruTrak ADI Flight Instrument Installation ManualQuote:
Unless the aircraft is pressurized, there is no need to connect the static line.
 

RV-8A
Independence, OR
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Reply #12 - Dec 11th, 2015 at 4:36pm

mmarien   Offline
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MikeS wrote on Dec 11th, 2015 at 3:22pm:
No, it doesn't do this.  It responds exactly like a vacuum driven AI.
From the manual:

Quote:
The pitch display is perhaps the most beneficial feature of the ADI when compared with conventional horizon
indicators.  For short term or immediate movement of the nose up or down, this instrument responds and is flown like
any other gyroscopic pitch indicator.  The difference is that after the display has moved in response to an attitude change
the deflection is sustained by the resulting vertical speed.  As compared to a pitch attitude display, the vertical speed display makes it easier to hold altitude and to maintain stable climbs and descents.

Seems to me this would indicate that if I pitch up, and hold altitude, the display would indicate the altitude hold and not the pitch up would it not? or as Dynon stated above, pitching up and sinking would indicate the sink and not the pitch up.  I don't think this is conventional attitude and the manual clearly indicates "as compared to" that it isn't Huh
 

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Reply #13 - Dec 12th, 2015 at 10:04am

Dynon Support   Offline
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MikeS wrote on Dec 11th, 2015 at 3:22pm:
In reality, it displays horizon information exactly as do the vacuum driving AI instruments I also fly.  If it didn't I would have gotten rid of it a long time ago.

What you are stating is more misinformation.  Dynon seems to have a penchant for doing this. 


Mike,
Here is a detailed discussion on Van's Air Force from 10 years ago on how the TruTrak ADI works, and it is absolutely based on vertical speed and turn rate, not pitch and bank angles. Hold one in your hand and it will show level after a few seconds no matter what angle it's at.

http://www.vansairforce.com/community/showthread.php?t=3598

It's a perfectly reasonable instrument, but it isn't an attitude indicator, and it doesn't operate without pitot and static information.

 

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Reply #14 - Dec 14th, 2015 at 9:55pm

MikeS   Offline
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RE:  TruTrak ADI

> "It's a perfectly reasonable instrument, but it isn't an attitude indicator"

Sure could have fooled me.  It and I have been through a lot of bad weather together. 

Since that VAF discussion was written before I bought my unit in 2008 I'm sure I read whatever's there.  As usual on these forums, Van's in particular, there's always one or two self-styled experts in love with their interpretation of what they think they know.  Whatever is there is really irrelevant in comparison with my experiences in the ensuing years in using the ADI through thick and thin. 

It doesn't pay, in the case of the ADI, to get too caught up in discussing how the ADI arrives at the information it displays.  There's a lot of room for confusion when going there.  "How" it works is really irrelevant.  Does it perform at all times and in all ways like an attitude indicator is supposed to perform?  In my experience the answer is unequivocally yes.     

My ADI uses cockpit static pressure - the instrument is vented to my cockpit. 

The pitot connection, if I recall correctly, does not affect the instrument's ability to display my aircraft in relation to the horizon correctly.  What it affects, when connected, is the rate at which it does so.  Its response time.  Honestly, it's been so long since I hooked it up I can't remember just why it works better when hooked up to pitot pressure.  It worked fine without it as I recall but when talking to TruTrak about the behavior of the instrument I was advised to connect the pitot connection for better results. 

If the issue here is the instrument being "dependent", I think we can safely say that the ADI with performance slightly degraded with loss of pitot pressure against the D2 being totally dead with loss of GPS, the dependency issue favors the ADI I think you'd agree, especially if you were in the clouds. 

> "Hold one in your hand and it will show level after a few seconds no matter what angle it's at." 

I've never held it in my hand and done that so can't say yay or nay as to whether it does what you say or not.  I can say that I have done many approaches to landings in a nose high configuration, sometimes in instrument conditions, and it has never acted abnormally.  I have done a lot of slow flight, especially in the pattern just above stall, hanging on the prop and very nose high, and never had my ADI "return to level."    My aircraft is a taildragger so it spends a lot of its life nose high  Cool.  As I mentioned earlier, if it had ever done anything in the slightest to cause me to distrust it . . . if it had acted in any way other than the way an artificial horizon is supposed to act, I would have gotten rid of it.  I do a lot of weather flying in my plane - I enjoy bad weather odd as that may sound - and the instantaneous response of the ADI compliments the very touchy control response of my plane very nicely.  They are a perfect match.  I especially like the way it presents attitude and heading - just those two things - in such a clear and uncomplicated (and instantaneous) fashion.  If I ever come across a used 3-inch ADI in like-new condition I will buy it against the day that mine ever fails.  TruTrak charges $400 basic shop charge to repair an ADI (if it's long out of warranty) and I'm pretty sure a used but like-new ADI wouldn't cost much more than $400 these days.  TruTrak has gone on to produce newer and fancier versions of this instrument - none of which make me want to replace what I've got.  I think they still make the 2-inch model but I like the bigger one.

With the ADI, the way the background moves against the horizon bar is more comfortable, to me, than having background and horizon bar electronically generated - especially with the very poor resolution graphics used on the D2.  It reminds me of the old CGA graphic standard we had before VGA and the higher resolutions came along.  I'm sure it's something I'll have no trouble using on the ferry flight coming up.  I'm sure the D2 will work just fine for what I bought it for.  I'm just sayin' . . . the jerky graphics this thing has is, well, jerky.  You can almost count the pixels. 

There are some really expensive little electric attitude indicators out there I'm sure I'd like just as well as I like my TruTrak, but until I win the lottery I can't imagine anything better than the horizon instrument I've got.  The rest of the information displayed on the D2 (other than G meter) I have either on my GPS or on other dedicated instruments.  As I think I said in my first post, paraphrasing here . . . "for what it is, it's a nice little instrument."  I still think it is (even though I've not used it yet).  What I was complaining about is the emphasis in your advertising, in several different places and in several different ways, stating how the D2 is a "true" attitude indicator.  It's not until you get into the fine print of things that one realizes that it only works as long as it has a GPS signal.  I think Dynon's advertising department, in emphasizing the distinction between having a dedicated AI as opposed to a smartphone app or GPS "panel mode", (I could quote numerous examples as I went through and collected them) conveniently leaves out the information that the D2 doesn't work any better than the "pseudo panel" displays if the GPS signal is lost.  If the GPS signal isn't  lost, I've no doubt the D2 is a heck of a lot more responsive than the "panel mode" that certain GPS makers offer. 

I'm sure it's a great little instrument.  I just think the advertising department got a little carried away.  They may not have crossed it but they certainly got close to the line that defines mis-representation of a product.  I certainly could have done a better job of doing my research as well - although even if I'd known it was GPS dependent I would have bought it anyway since when I bought it I thought I had an immediate situation that didn't allow time for used ADI shopping.  Since then there have been some delays so I wasn't as pressed for time as I thought I was when I bought it.  But no matter . . . .  I'm not unhappy.  It's a great "extra" for the flight bag and Marv Golden gave me a great price.  I may or may not sell it once the plane is delivered. 

*****************

I'm wondering, since we're on the subject, does the D2 or other Dynon products take advantage of the European Galileo GPS satellites?  How about the Russian GLONASS system? 

If it did, that would go a long way towards decreasing the possibility of losing function of the AI.  Just curious.   

Mike
 
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