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airplanecrazy
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Joined: Thu May 11, 2006 8:09 am

ADS-B NACv

Thu Mar 11, 2021 9:24 pm

I have been observing the ADS-B NACv (velocity figure of merit) for a number of aircraft on https://globe.adsbexchange.com/. Most commercial aircraft show a value of "< 10 m/s", but some aircraft show values of "< 0.3 m/s" (which is more what I expect, to be honest). The aircraft showing 10 m/sec seem correlated to AAL, DAL, UAL, JBU, and those that show 0.3 m/sec seem correlated to be A32X NKS and FFT. Does anyone know if NKS and FFT A32X aircraft have different avionics, or if Airbus configured them differently?
 
GalaxyFlyer
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Re: ADS-B NACv

Thu Mar 11, 2021 11:45 pm

What is NACv?
 
airplanecrazy
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Posts: 86
Joined: Thu May 11, 2006 8:09 am

Re: ADS-B NACv

Fri Mar 12, 2021 12:17 am

GalaxyFlyer wrote:
What is NACv?


It is a value that tells how accurate the ADS-B velocity data is. My understanding is that a NACv of 0.3 m/sec says that 95% of the time the velocity will be 0.3 m/sec of the published velocity.
 
airplanecrazy
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Re: ADS-B NACv

Fri Mar 12, 2021 1:21 am

What's even more surprising to me is that most GA aircraft are < 3m/sec, which is better than the majority of commercial aircraft!
 
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dennypayne
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Re: ADS-B NACv

Fri Mar 12, 2021 2:35 am

airplanecrazy wrote:
What's even more surprising to me is that most GA aircraft are < 3m/sec, which is better than the majority of commercial aircraft!


I'm speculating here, but if the system does not receive velocity data directly, but rather computes it from positional data, and given that the faster something moves, the harder it is to determine accurate position, it would make sense that slower moving objects would result in being able to obtain more accurate data.

I have no explanation for the difference in the jets though.
 
airplanecrazy
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Posts: 86
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Re: ADS-B NACv

Fri Mar 12, 2021 4:20 am

dennypayne wrote:
airplanecrazy wrote:
What's even more surprising to me is that most GA aircraft are < 3m/sec, which is better than the majority of commercial aircraft!


I'm speculating here, but if the system does not receive velocity data directly, but rather computes it from positional data, and given that the faster something moves, the harder it is to determine accurate position, it would make sense that slower moving objects would result in being able to obtain more accurate data.

I have no explanation for the difference in the jets though.


My understanding is that for most implementation the velocity is computed from the doppler on the GNSS signals, and not from positional data. https://insidegnss.com/wp-content/uploads/2018/01/marapr15-SOLUTIONS.pdf The velocity of the aircraft is insignificant compared to the velocity of the satellites: https://sciencing.com/how-fast-do-gps-satellites-travel-12213923.html

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