GPS Offset

mimtarsden · July 28, 2008, 07:48:23 PM

Hi All
I live about 2 miles away from Manchester Airport and have noticed, as I'm sure has everybody has, when watching lots of the old charter 767's landing and taking off that they appear to be taking off miles off the runway because the GPS is that far off.

I was thinking would it be an idea for Airnav to add a facility where an optional GPS 'Offset' could be added to 'mylog' so that a user can correct an eronious aircraft on screen, say by dragging it. If you watch the taxiing and taking off from Manchester on the radar box I think it would be possible to correct the error to within a meter or so just by listning to the tower and watching the taxiing aircraft line up.

People could then share the error correction factor and eventually everyone could have nicely lined up aircraft.

Probably difficult to do but good idea or not?

Tim.

AirNav Support · July 28, 2008, 07:56:24 PM

Its a good idea however implementing that isn't great and the offsets could vary and you may not ever get them accurate. Also allowing dragging of any aircraft may not be right.

What is probally better is that the aircraft get an upgrade and fix there onboard systems :)

RodBearden · July 28, 2008, 08:02:06 PM

As I said on another thread, it is planned that aircraft will use ADS-B for collision avoidance in the future - "ADS-B-In".

It could be disastrous if these offsets are allowed to continue.

Rod

mimtarsden · July 28, 2008, 08:17:49 PM

Yes I can see it could be difficult to implement. And of course you could never get the plane pin point accurate but at the moment some appear to land/Takeoff literally 'miles' off where they should if you could improve this to a meter or so accuracy I'm sure it would be better!

I agree that the Aircraft themselves should be calibrated... I have a suspicion that some are intentionally off...

Just an idea anyway.

flightchecker · July 29, 2008, 07:14:05 AM

Tim,
it is not the "GPS" that causes the offset. Some aircraft still feed "INS" data to their tranponders, those data beeing more or less incorrect, as the INS suffers (random) "DRIFT" (as a function of time) and requires (manual) updating from time to time by the flight crew, commonly performed if the aircraft "rests" at a stand between flights. Drift so NOT to be a "CONSTANT" that might be compensated for on your RB's display.
GPS data instead, if fed to the transponder (s), is what you experience if an aircraft is in coincidene with what you expect it to be regarding its position.

Rod,
ADS-B is not yet in use officially until today. Should it become whenever, and even be part of collision avoidance, it will of course rely on accurate GPS data without any doubt.

Kind regards

Karl

RodBearden · July 29, 2008, 11:09:10 AM

Hi Karl - it's great to have someone around here who knows stuff!

I didn't realise that something as high-tech as ADS-B ouuld be fed by something as low-tech as INS - that explains a lot - for example why some B733's will often show an error, but B738's don't.

Thanks!

Rod

mimtarsden · July 29, 2008, 02:51:32 PM

Quote from: flightchecker on July 29, 2008, 07:14:05 AM
Tim,
it is not the "GPS" that causes the offset. Some aircraft still feed "INS" data to their tranponders, those data beeing more or less incorrect, as the INS suffers (random) "DRIFT" (as a function of time) and requires (manual) updating from time to time by the flight crew, commonly performed if the aircraft "rests" at a stand between flights. Drift so NOT to be a "CONSTANT" that might be compensated for on your RB's display.
GPS data instead, if fed to the transponder (s), is what you experience if an aircraft is in coincidene with what you expect it to be regarding its position.

Rod,
ADS-B is not yet in use officially until today. Should it become whenever, and even be part of collision avoidance, it will of course rely on accurate GPS data without any doubt.

Kind regards

Karl

Every days a school day!

I'm intrested to know then, if older aircraft use INS feeding in to their instrumentation then does this have any inflance on TCAS or does this use a different positional system?

Thanks

Tim.

Allocator · July 29, 2008, 05:37:42 PM

The Wiki article on TCAS is very informative:

http://en.wikipedia.org/wiki/TCAS

mimtarsden · July 29, 2008, 08:16:26 PM

Quote from: Allocator on July 29, 2008, 05:37:42 PM
The Wiki article on TCAS is very informative:

http://en.wikipedia.org/wiki/TCAS

Thanks for that,

It was actually the wiki article that got me wondering if the TCAS system used the same positional information that is being broadcast over ADS-B?

Tim

dudbaker · July 30, 2008, 02:58:26 PM

Hi

I did not see the wiki article mention the directioal antenna fitted to the top of TCAS aircraft. The lower antenna is non directional.

Dudley