Position accuracy

[stewart_uk]

Hi, I am fairly new to RB, and am having great fun seeing what it can do.
This morning I was watching flights on the network in and out of LHR.

Whereas most seemed to be at the right place, speed and altitude when it came to the runway, there was one A/C a Continental flight that was shown landing about  1 mile to the North of the runways.

It made me wonder what the accuracy of the ADSB system is, and whether the crew of this A/C either got it wrong or couldn't be bothered to enter the right information.
I am sure that it is not my RB at fault.

T.I.A

Stewart

[EK01]

Stewart,

The RB is not wrong. I'm sure I read on a thread somewhere, that it is the actual aircraft itself which is giving out slightly incorrect position information. Is does happen fairly regularly.

Ian

[malc41]

Stewart

It is an on-going problem, if you search the forum you will see that it has been mentioned before.

Some suggest it is an Inertial Nav error, but some aircraft do it regularly, BAW 767, if I remember rightly are one of the worst culprits

[DaveReid]

Some suggest it is an Inertial Nav error, but some aircraft do it regularly, BAW 767, if I remember rightly are one of the worst culprits

Yes, the problem affects aircraft that derive their position data from inertial reference systems, which are subject to drift, whereas most ADS-B aircraft use GPS, which isn't.

Other notorious offenders here at Heathrow are DLH's 737 Classics.

All ADS-B aircraft send a parameter called NUCp, which we don't see on either RadarBox or SBS, indicating the accuracy of the ADS-B position data.  For BA 767s, this typically indicates that the position may be in error by 20nm or more(!), compared to, say, BA's 747-400s which normally report positions to within 0.1nm.

[stewart_uk]

Thanks for the replies Guys. Now makes sense. 20nm is some way off !
Just as well LHR is such a big target :-)

Stewart

[Fenris]

All ADS-B aircraft send a parameter called NUCp, which we don't see on either RadarBox or SBS,

Is this relatively easy to get at Dave? I was thinking that RB (or SBS) could then indicate this with a "position uncertainty" flag just so that you would then know that the map display was inaccurate for that aircraft.

[DaveReid]

Is this relatively easy to get at Dave? I was thinking that RB (or SBS) could then indicate this with a "position uncertainty" flag just so that you would then know that the map display was inaccurate for that aircraft.

It's no more complicated to decode than any of the other ADS-B fields so, yes, there's no reason why RadarBox or SBS/BaseStation couldn't be programmed to display the relevant data, and I agree it would be a good idea.