ADS-B question

[Dan Kent]

I own a Sting S4 with dual Dynon's and Dynon's transponder. I'm intending to get Dynon's ADS-B receiver next month.

 

If I understand how ADS-B works then I can forsee a possible incursion into class C and B airspace. My understanding is that ADS-B out transmits your GPS altitude as well as other data. GPS altitude can be quite different from pressure altitude. On a trip back in August I was flying at 6500 feet (pressure altitude) and the GPS altitude was different by a couple hundred feet.

 

So my question is someone could be flying under the shelf of class B or C airspace relying on their pressure altitude, but their ADS-B transmitter could be sending out their GPS altitude as higher and thus in the airspace.

 

Any idea how the government would handle this?

 

Am I missing something?

 

Thanks in advance as I'm a new sport pilot (9 months) with about 125 hours.

[S3flyer]

Hi Dan,

 

There are two issues involved -- the altitude transmitted and the altitude displayed. For ADS-B, the altitude transmitted to the ground stations is the GPS altitude above the model ellipsoid (model WGS-84) of the earth (also called HAE). This is analogous to the Mode C altitude being transmitted without correction for barometric pressure. Any correction is provided by the ATC systems so all position data is starting from a known base and known correction algorithm. You have to go through a lot of documents to figure this out but AC 20-165 is decent starting point.

 

The HAE can be off by several hundred feet depending on the distance from the poles with the equator having the worst error which is why a more refined geoid model that very closely models a specific region is typically added. The model for the US is GEOID03. There are mapping points between WGS-84 and GEOID03 so given an accurate GPS position with WAAS, your true altitude can be determined within a couple meters.

 

There's a decent laymen's explanation of the theory at: http://www.avionicswest.com/Articles/GPS%20Altitude.htm

 

Many aviation GPS receivers have GEOID03 correction built in so the altitude displayed on your device should be much more accurate than a Mode C altitude. Keep in mind Mode C has an allowable error of +-200ft. My Garmin 795 matched measured airport elevations while my altimeter was 30-40 ft off.

 

Center is going to bust you on the raw altimeter data corrected for local conditions (geoid or barometric pressure).

 

You might want to check with Dynon to see if they correct (display) for the geoid.

[Dan Kent]

Thanks Dave. I've read the link and think I understand it. In any even, if I understand you correctly, the raw GPS altitude is corrected twice to be relatively close to the pressure altitude. One of the corrections prior to transmission and one done on the ground by ATS.

 

I'll give Dynon a call once they open back up.

[S3flyer]

Also, an altimeter does not take into account temperature. It's a small error but there is a 4% error between displayed altitude for each 10 degree C delta from ISA conditions (15 degrees C). If the temperature is lower than standard, the altimeter will read higher than true altitude when corrected to the local pressure.

[BMcCand - N248CT]

Does the Mode-S Dynon transponder offer ADS-B out ? I already have traffic with a Zaon XRX, XM weather and don't plan on spending money for ADS-B in. I'll wait until I have to do ADS-B out (yeah, in 2020 or however long I last....). I'm leaning toward the Navworx system, but don't see an LOA

for CTSW. Yet.

I picked Navworx because it has its own WAAS GPS receiver, and the GPS396 I currently have doesn't meet the accuracy spec for

another ADS-B out UAT.

One lovely thing about the 2020 deadline is that it eliminates the ability to fly above Class B unlike now, where we can with Mode-C.

Has anyone installed ADS-B out in a CT ?

[paul m]

Bill, as I understand it, it does but it's not 2020 compliant. You need a certified GPS signal to make it 2020 compliant which apparently doesn't exist yet. However because it does send out an ADS-B signal (1090ES) until 2020 that works to get back for you the ADS-B feed (TIS-B really) you need to see all traffic. The Dynon has a flag that transmits that its an uncertfied GPS signal which the FAA will accept until 2020. After that its not ABS-B out compliant. Dynon expects a solution for the GPS issue by then. I ordered my CTLSi with the 261 transponder and the new 470 ADS-B receiver. I asked the same question and the above was Dynon's response.

[S3flyer]

Yes -- the Skyview integrated transponder offers ADS-B out. You will need a TSO'ed WAAS GPS to be compliant in 2020 but can use a non-certified GPS in the interim with the transponder telling ATC that your position is 'low integrity'.

[Doug G.]

I have the Skyview system on my LS. I understand that the GTX 330 transponder can be upgraded to provide ADS-B out. Does anyone know if Garmin has made this available yet?

That upgrade and the GL ADS-B in setup looks like the most affordable option for me. And, I believe it meets the 2020 standards.

[S3flyer]

You'd need to upgrade to the 'ES' model for around $1200 and attach a position source to the serial port.

[Doug G.]

Yeah, the question is if the upgrade is available yet? Is it an exchange, or is it an actual upgrade to the my current GTX 330? Looking at $1200 plus $799 (GDL - 39) is a lot better than the 330/ES at $4489 or G3X at about $6k. Has anyone done the 330 upgrade? If the Skyview GPS panel wasn't all the way to the right, or if I could configure the displays like Dynon intended - I would consider that route. It is simpy a pain to keep looking at the right half of the right display and to use the buttons that are so far away that I actually have to lean right to use them. (Doesn't make sense FD - this wasn't true of the pre-Skyview system.)

 

[paul m]

Doug, pardon my ignorance, where do you see ADS-B In information (if you want it) like traffic in the GTX330/GDL39 configuration? Does that integrate into Skyview?

[Doug G.]

I don't know that I am completely up to speed either. But I have been looking into the options. I do know that the incoming info is shown on my 696. Garmin has said that the GTX 330 is/will be upgradablge to provide the ADS-B out portion of the system. It won't be required until 2020. You can use the GDL-39 for ADS-B in which will soon cover the entire country. It provides NEXRAD weather and traffic info on a Garmin 696 (and a few others). As far as I know it does not have any affect on the Skyview system.

My issue with the current Skyview system is that if I go with their setup, the weather and traffic is going to show up on the far right of my panel. With the 696 that same info will be in the direct center.

(That is the reason I use the Garmin for my primary GPS - it is easier to see, and easier to use because of its positioning. This could be changed if I the Skyview were not fixed in its layout by FD. If it were like the previous version of Dynon I would be able to rearrange the layout and bring the GPS, or the engine info in front of me. I am not sure of the reasoning to make the 10" Skyview less flexible than the 7" D-100 was.)

Sorry for the soapbox Paul. A minor frustration given the overall quality of the CTLS.

[paul m]

Ok I see what you're saying now. Traffic especially would be nice displayed right in front of you. I'll ask Tom about it next time I talk to him.