Aircraft landing off centre - Barbados (TBPB)

[bailey_uk]

Tracking a fair few aircraft into TBPB today, but can only pick them up about 5 mile final due to the position of the aerial that I cant really better at the moment.

Ive had 2 aircraft land on the correct path, but most are showing a mile to the side, (both sides too)

Is this common, and a fault with the aircraft equipment?

[bratters]

I track landings at East Midlands Airport. The majority are straight down the white line but the odd one or two appear to touch down in adjacent fields runnning parallel to the runway.

Obviously it's just a glitch of some sort although the first time it happened I had a long hard look at it.

[DaveReid]

Is this common, and a fault with the aircraft equipment?

INS drift, more a feature than a bug, it's relatively common.

[Levery]

Hi, Dave do you live on Barbados? I live on Curacao and am planning on purchasing the RB, how has it worked out for you there? and did you do your own out lines for TBPB. Does it work ok without the online features. it is my intention to use it mobile. and 1 more ??. Does the RB recieve Mode S out of the box or do i have to decifire it my self, we have alot of traffic coming i from svmi over here. Thanks in advance!!!

P.S If possible can you send me a sreen shot of the tncc area so that i can decide what i will do.

[jgrloit]

The low-level positioning 'error' has been discussed in other threads on the Forum.
It is due to a different level of accuracy, including a basic position when the Wheels are down.
If you set your home position to that of the airfield, you should see all the aircraft land in the correct positions, and taxi around the airfield correctly.

[DaveReid]

The low-level positioning 'error' has been discussed in other threads on the Forum.
It is due to a different level of accuracy, including a basic position when the Wheels are down.
If you set your home position to that of the airfield, you should see all the aircraft land in the correct positions, and taxi around the airfield correctly.

You are confusing two different things.

The precision of the lat/lon coordinates is greater (by a factor of 4) for aircraft on the ground, for obvious reasons.  However the decoding algorithm, and the fact that the coordinates are dependent on the aircraft knowing where it is, work the same way whether an aircraft is in the air or on the ground.

Gross position errors are normally a result of INS drift (for aircraft that don't use GPS) and so if an aircraft looks like it's offset from the runway on approach it isn't going to suddenly appear in line with the runway once it has touched down - the INS drift error will remain until the system is re-aligned (which is normally done at the gate, where the coordinates are known).

[jgrloit]

DAVE - I Hate so ay sthis, but if the position is NOT within 96km, and the gear is shown as DOWN then the incorrect position will be shown.

This was mentioned in another thread, on this forum, and included the mentioning that the highorder section of the position  was LOST when on the ground. - Ground being GEAR DOWN.

[DaveReid]

DAVE - I Hate so ay sthis, but if the position is NOT within 96km, and the gear is shown as DOWN then the incorrect position will be shown.

I'm afraid you have lost me now.

"but if the position is NOT within 96km"   ... within 96km of what ?

Anyway, the characteristics of the decoding algorithm have nothing to do with the phenomenon that's being discussed, i.e. aircraft appearing offset from the runway.  The reason that happens is simply that the aircraft doesn't know its own position with sufficient accuracy.  That's why it is particularly noticeable with aircraft like BAW 767s and DLH 737 Classics (which have INS, but not GPS).

[ACW367]

Drift happens in all inertial navigation systems. Aircraft that are older and not equipped with GPS can be seen reporting bad ADS-B position information, this will be to ATC ADS-B devices as well as to us.  Of course ATC primarily use radars to confirm aircraft position.  INS drift is one of the major reasons that ADS-B has not yet been cleared for use by ATC as a primary source of control.  I suggest that you look at the Wikipedia entry for Inertial Navigation Systems for a quick laymans guide.  It says INS drift can be as much as 0.6 miles for every hour that it is in operation.  The aircraft themselves update thier position and fly acurately by use of radio navigation aids and ILS systems, however the information is not fed into the INS which is connected to the ADS-B broadcast information during flight, as an exact lat/long need to be physically programmed in. This is only practically done in a known position whilst stationary on the ground. ie at the parking gate.

Regards
ACW

[DaveReid]

Drift happens in all inertial navigation systems. Aircraft that are older and not equipped with GPS can be seen reporting bad ADS-B position information, this will be to ATC ADS-B devices as well as to us.  Of course ATC primarily use radars to confirm aircraft position.  INS drift is one of the major reasons that ADS-B has not yet been cleared for use by ATC as a primary source of control.  I suggest that you look at the Wikipedia entry for Inertial Navigation Systems for a quick laymans guide.  It says INS drift can be as much as 0.6 miles for every hour that it is in operation.  The aircraft themselves update thier position and fly acurately by use of radio navigation aids and ILS systems, however the information is not fed into the INS which is connected to the ADS-B broadcast information during flight, as an exact lat/long need to be physically programmed in. This is only practically done in a known position whilst stationary on the ground. ie at the parking gate.

Excellent - a very clear explanation of something that causes a lot of confusion !

[viking9]

I thought the whole idea of ADS-B was superior position information through GNSS. If an aircraft does not have a GPS system then it cannot have full ADS-B. Surely the GNSS is built into the aircraft package?

[malc41]

That is why light aircraft do not give positional info on RB

[viking9]

That is why light aircraft do not give positional info on RB

Those that don't do not have ADS-B transponders, just modeS.

[malc41]

And i don't think many will until the CAA/JAA make it compulsory.

Shame that.

[DaveReid]

I thought the whole idea of ADS-B was superior position information through GNSS. If an aircraft does not have a GPS system then it cannot have full ADS-B. Surely the GNSS is built into the aircraft package?

ADS-B in general does not have rules regarding the source of positional data, so either GNSS or INS will satisfy the requirements, bearing in mind that the ADS-B transmission from the aircraft includes a component that indicates the precision/accuracy. 

Having said that, some specific ADS-B airspace implementations (like the Canadian Hudson Bay) have navigational accuracy requirements which effectively rule out INS.