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  #1  
Old 03-18-2023, 08:04 AM
buz buz is offline
 
Join Date: Jan 2006
Location: Ft. Pierce, Florida (FD88)
Posts: 164
Default ADSB speed?

When viewing my speed from a trip on flightaware, the speed was very erratic. Like not reality. Indicated airspeed on the AFS was stable.

Question is what provides speed data for ADSB?

Can view on Flightaware N442PM

Thanks
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  #2  
Old 03-18-2023, 08:40 AM
blaplante blaplante is offline
 
Join Date: Jan 2010
Location: Southern California
Posts: 607
Default Part of ADS-B out packets

Velocity is transmitted by your ADS-B out:
https://mode-s.org/decode/content/ad...-velocity.html

In my case my transponder only has altitude and GPS inputs, so I'm making an educated guess that the speed is coming from your onboard GPS.
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  #3  
Old 03-18-2023, 09:14 AM
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Snowflake Snowflake is offline
 
Join Date: Oct 2009
Location: Sidney, BC, Canada
Posts: 4,451
Default

It's groundspeed based on GPS.
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  #4  
Old 03-18-2023, 12:02 PM
Avanza Avanza is offline
 
Join Date: Jul 2020
Location: Vastervik Sweden
Posts: 315
Smile

Yes, GS is based on GPS and ALT is based on 29.92".
Take a look at Flightradar.

https://www.flightradar24.com/42.68,-95.99/7

Good luck
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  #5  
Old 03-18-2023, 05:17 PM
Richard Connell Richard Connell is offline
 
Join Date: Aug 2006
Location: Sydney Australia
Posts: 494
Default

To your question about it being erratic,
FR relies on the public sniffing adsb out as much as it relies on other sources. (MLAT etc)
Quite often the data sniffed by public receivers is junk or intermittent for whatever reason. FR has all sorts of smoothing algorithms to mask this but quite often the info you see is really only an approximation or has been augmented.
Quite often here even in a major city my flights drop off or have wild speed fluctuations because of this.
Itís just a FR limitation. Doesnít affect ADSB for its real use.
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  #6  
Old 03-18-2023, 08:16 PM
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N546RV N546RV is offline
 
Join Date: Aug 2010
Location: Brookshire, TX
Posts: 1,227
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Is your aircraft equipped with ADS-B and is it working properly? Looking at all recent flights for your aircraft, I don't see any ADS-B positions; they're all either MLAT (multilaterated) or radar (ie ATC). We have good ground station coverage for both 1090MHz and UAT in that area, so I don't think that's the problem. It might be worth requesting a PAPR to ensure everything is broadcasting correctly.

Anyway, back to MLAT - this is a fallback position source when there's no radar/ADS-B position data available. MLAT relies on vanilla Mode S transmissions, and uses time-of-receipt differences between numerous ground stations to determine position data (sort of like triangulation). MLAT is better than nothing, but tends to be pretty inaccurate, which is why there's so much apparent variation in reported speeds.
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  #7  
Old 03-18-2023, 09:12 PM
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BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 7,008
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BTW Plane finder also apparently uses some averaging and projecting to provide location of a specific aircraft. I was watching a commercial flight come into the local airport, my friend was at the airport reporting on the flight as it came into sight. PF show it continuing along the runway well beyond the airport at a fixed (low) altitude and landing speed, but it had actually landed on the runway and turned off.

Lesson - be careful of what one believes from the provided position reporting.

Pretty disappointing.
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  #8  
Old 03-19-2023, 08:48 AM
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airguy airguy is offline
 
Join Date: Oct 2006
Location: Garden City, Tx
Posts: 6,176
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I haven't noticed it on the speed, but I see my altitude varying sometimes by several hundred feet on a flight where I know it was stable, smooth, and on-altitude for the entire cruise. I've never had ATC question my altitude, so I can only guess it's an artifact of the receivers or network on the ground.
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