ADS B (Pressure altitude vs. geometric)

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ADS B (Pressure altitude vs. geometric)

Postby davidpatton111 » Fri Aug 04, 2017 10:32 am

I thought ADS-B used an inter-connected system of basically GPS reporting data which gives what I think is called geometric altitude data, etc.

Why then are we still required to have an altitude encoder which also reports barometric altitude.

I started getting very interested in this when with my 1090ES ADS-B it is still required to have an altitude encoding source attached to the unit, meaning it is taking in barometric altitude data.

I think the purpose is for ATC to see not only geometric altitude data (which is GPS triangulation) but also barometric (pressure data) for the purpose of double checking,etc?

Are we ever going to have a pure GPS system with ATC where data is highly accurate triangulation?

I was confused when finding out that I'll have to use either my existing altitude source or at least have an altitude source installed when I thought that ADS B was additionally using the technology of triangulation and accurate GPS data to now separate aircraft.

Help me please....thanks.

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Re: ADS B (Pressure altitude vs. geometric)

Postby AirportDude » Tue Sep 05, 2017 9:23 am


The primary reason is that GPS height data can still be 10% inaccurate, as opposed the baro-based height (if the correct pressure is dialed into the altimeter)

Here is a pretty decent explanation of the difference between the two.

Now... as to why you need an encoder showing both heights, I can only suspect that its for flight centers to verify the true height of an aircraft in their airspace. Also remember that they're looking at a radar height indication in addition to the other two readings. Perhaps its for some future algorithm in the all equipment if/when ATC decides to do away with ground-based radar? Otherwise I'm guessing its for redundancy.

Switching to advisory, squawking VFR. Good day!
- AirportDude

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Re: ADS B (Pressure altitude vs. geometric)

Postby jackhodgson » Sun Oct 01, 2017 7:56 am

I concur with airportdude that GPS is not terribly accurate in height.

I've always speculated that this is due to the complex algorithm of calculating elevation above an irregular shaped sphere.

// Jack

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