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  #1  
Old 11-29-2022, 04:14 PM
NYTOM's Avatar
NYTOM NYTOM is offline
 
Join Date: Jun 2005
Location: Raleigh, North Carolina
Posts: 1,216
Angry ADS-B altitude reporting

Have a FreeFlight (new) ADS-B working with a GTX 327 (used) and a Trans-Cal encoder(new). Noticed today when I checked todays flight on FlightAware that my altitude was a LOT lower than what I was flying. Like a thousand feet!
I installed all this equipment myself and it has been fine for three years and gone thru two VFR checks at the local avionics shop. Just did a performance check on the FAA site now and came back fine. Any advice or tips on what I can check before I make a appointment at the shop, have to wait over a month and then possibly have to give up my first born to get this corrected?
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  #2  
Old 11-30-2022, 05:51 AM
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NYTOM NYTOM is offline
 
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Thumbs up Might help someone else.

Since no one had any suggestions I did some searching and on the Mooney site found that a common problem with erroneous readings is poor pin connections due to corrosion/ dirt & vibration. Takes 10 separate connections just to transfer altitude reading. Suggest contact cleaner, especially on slide in transponder connections and encoder plugs. Other common problem is connection to static is incomplete or has leaks. Thought this info might at least help someone and give them something to try before a $$$ trip to the avionics shop.
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  #3  
Old 11-30-2022, 06:50 AM
BH1166 BH1166 is offline
 
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Location: Eatonton Georgia
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Default Flightaware

I would not put so much faith in what they report. Before spending a dime, go fly, ask approach, center to verify what they see. Not sure about your adsb device, but if it is UAT, then accurate reporting isnt always available via Flightaware. 1090s systems are generally accurate as seen by Flightaware.

Flight aware reports altitude at 29.92 barometer. Recent flight @ 5500 reported by Flightaware 5125
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  #4  
Old 11-30-2022, 07:17 AM
moosepileit moosepileit is offline
 
Join Date: Dec 2010
Location: Louisville, KY
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Asking ATC is the easiest way, it's the only way to know their local altimetry from QNE FL.

Grounds are every bit as important, hopefully all involved are common to a forest of tabs.

You can check your own datastream with free Fltplan Go by long touching your aircraft symbol if you have an ADSB-In device.

You can buy a 978 MHZ SDR dongle and antenna if you have a device that supports it. Android has good freeware, not all Androids support USB dongle/On the Go/OTG.

$40, used to be $20. Easier to ask ATC

NESDR Nano 3 OTG - Premium Tiny RTL-SDR Bundle for Android and Other USB OTG (microUSB) Host Devices. RTL2832U
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  #5  
Old 11-30-2022, 07:17 AM
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GalinHdz GalinHdz is offline
 
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NEVER use FlightAware, SkyRadar and other INTERNET sites to verify your ADS-B reported altitude. FlightAware, and ALL ADS-B based receive sites, show GPS altitude not Barometric Pressure altitude and they can be quite different. As BH1166 posted:
Quote:
Originally Posted by BH1166 View Post
Flight aware reports altitude at 29.92 barometer. Recent flight @ 5500’ reported by Flightaware 5125’
Ask ATC what they are displaying. THAT is Barometric Pressure altitude and what must be accurate. Also, you ran the FAA ADS-B compliance check and it came back fine which tells me you are chasing a problem that does not exist.

FWIW, GPS altitude is best for not colliding with things on the ground. Barometric altitude is best for not colliding with other aircraft.

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Last edited by GalinHdz : 11-30-2022 at 07:25 AM.
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  #6  
Old 11-30-2022, 09:44 AM
lr172 lr172 is offline
 
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I believe ADSB data includes both GPS altitude and baro altitude. I know foreflight reports GPS altitude for me and for traffic and think that each app can choose which altitude to show on the screen.

It is not uncommon for GPS altitude to be 1000' from baro altitude. If a source is being used for traffic, it is critical to know which altitude is being used. Most integrated systems will be using the same source for ownship and other traffic targets.

Also important to note that baro altitude is sent from your plane at 29.92 and ATC does the correction before displaying it or re-transmitting it to others.

As you can see, there is all different kinds of altitude floating around in the ether, so need to fully understand it before using it.

Larry
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Last edited by lr172 : 11-30-2022 at 09:56 AM.
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  #7  
Old 11-30-2022, 10:30 AM
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jliltd jliltd is offline
 
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First of all. I completely agree with not relying on FlightAware or it's competitors for valuable information. The FAA PAPR report is truly the only thing that matters. And understand the PAPR completely relies on the encoded pressure altitude and cannot correlate this with what you altimeter reads.

I have two FreeFlight ADS-B systems installed and they have been rock solid. And, if you have the control head (recommended) it has a digital encoder built in if you want to use it. It's use is optional.

You don't mention what Trans-Cal model you have. I would assume since it is new is has digital RS-232 altitude data to use (one wire connection). Which is the preferred source as it is more precise than the old grey code bundle of wires. Most Trans-Cals have both type of outputs but I think the latest compact model provides digital signal only.

If an aircraft has a G5 I like to use that as an encoder since it can send altitude directly to your transponder/ADS-B system via digital data (one wire). No correspondence trouble when the altimeter itself is the encoder.

The best way to truly check out your altitude is to fly under flight following or IFR flight plan or talk and squawk with an approach/departure controller. If you truly are off 1000' you don't have to ask. They will immediately bring it up to you. ATC does not mind reading back to you your reported altitude.
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  #8  
Old 11-30-2022, 10:37 AM
moosepileit moosepileit is offline
 
Join Date: Dec 2010
Location: Louisville, KY
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You can have clean PAPRs and useless ATC ADSB!

Until I ran a dedicated ground from firewall FOT to the Skybeacon wingtip- that was my case.

PAPRS read what you put out, but not if it is accurate PA/encoder/Mode 3C!

The PAPR helps with a lot, but not accurate baro alt. Check the max level on top of the PAPR and see if it matches your flight max height in coverage- that's about all it has for height accuracy.

If the OP has the right transcal issue or connection or static error- an IFR pitot static might help.

My plane passed those and was good to FL180, maybe it would help troubleshoot.
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Last edited by moosepileit : 11-30-2022 at 10:39 AM.
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  #9  
Old 11-30-2022, 01:44 PM
msires msires is offline
 
Join Date: Nov 2022
Location: Florida
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globe.adsbexchange.com gives all the ADS-B data reported by an aircraft, including baro altitude and GPS (WGS84) altitude.
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  #10  
Old 11-30-2022, 04:44 PM
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GalinHdz GalinHdz is offline
 
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Quote:
Originally Posted by lr172 View Post
I believe ADSB data includes both GPS altitude and baro altitude.
Yes, but INTERNET based sites show GPS altitude not Barometric. Those few that report Barometric altitude report UN-ADJUSTED Barometric altitude which is based on 29.92" not on your local altimeter setting. Neither of them will be the same as your flying altitude, unless the Altimeter setting where you are flying happens to be 29.92".

This is a separate issue from any static port problem. We have to "assume" the OP has a good static system since it works correctly at times.

Quote:
Originally Posted by jliltd View Post
The best way to truly check out your altitude is to fly under flight following or IFR flight plan or talk and squawk with an approach/departure controller. If you truly are off 1000' you don't have to ask. They will immediately bring it up to you. ATC does not mind reading back to you your reported altitude.
^^^^^THIS^^^^^

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Last edited by GalinHdz : 11-30-2022 at 04:52 PM.
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