Great information from CT Flier Forum

Source: http://ctflier.com/index.php?/topic/1733-ads-b-question/

Jr. Crew Member (Dan) writes:

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.

Senior Crew Member (Dave) responds:

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.avionicsw…PS Altitude.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.

**

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.

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