Why is the position of the plane sometimes wrong?

[bur]

I noticed that planes landing sometimes appear to be located 100 m or more away from the runway. Is that a problem with google maps or is the transmitted position inaccurate?

[jmhayes]

I noticed that planes landing sometimes appear to be located 100 m or more away from the runway. Is that a problem with google maps or is the transmitted position inaccurate?

I've always thought that this is because the GPS that is in use is out of spec. I believe that in-spec GPS equipment will be required for any airspace that requires ADS-B (just like other kinds of avionics must be within spec to be used for precision approaches, etc.), but until then I think we'll see lots of cases where stuff isn't working well and No One Cares.

[Oblivian]

Bit of both.

You have to remember google maps does not take into account curvature of the earth, so altitude and position can sometimes be represented incorrectly in a visual sense.

Likewise, if the server is sending you 'guessed' data position based on last known position and speed received (but is actually decreasing speed), it may appear to over-shoot if there is data lost at a point of touching down. It may also pickup again and go backwards if another one takes over and has the ground data giving correct location from an earlier point.

It can also get a bit bung when the pilots forget to localise the barometric pressure to the airfield http://en.wikipedia.org/wiki/QNH

[Mike]

There can be several issues behind incorrect positions.

* Map calibration error
* Transponder error (sending wrong or random data)
* Receiver error (some kind of error in receiving so numbers get incorrect)
* Database error (some kind of mix up in database)
* Old transponder (some aircraft like RJ100 don't have GPS position but calculated position. It works OK on take off from Gothenburg, but once they reach Stockholm they land up to 30 km off from Bromma airport).

[Kemistry]

My experiences watching "real" real-time positions in PP and observing actual AC in the sky lead me to believe the vast majority of positional errors are due to badly calibrated AC transponders.
I have seen many AC flying and then landing that are sending data that is up to a km away from where they really are. Sometimes just an incorrect Easting or Northerning and sometimes both.
Since we receive different message types for ground positions and flying positions, one has to conclude it is the GPS decoder within the AC that is not calibrated properly.
I just hope they never use this positional data to land in bad weather!
Having said this, most reputable company's AC appear (from my ground observation) to be sending very accurate data - as you should expect.
I guess I will select who I fly with in future according to the quality of their positional data - maybe it says something about their overall company integrity?
Would "shame and name" help improve this?

[Mike]

There are several aircraft in Europe, including one A340 from SAS, that has been flying with incorrect heading and speed for several years, so it looks like airlines don't care at all.

I think that the reason behind the bad data quality is that ADS-B is not approved for ATC use. I think we need to wait another 2-8 years before ADS-B becomes mandatory and used by ATC and we can expect better quality of the data.

[jmhayes]

I think we need to wait another 2-8 years before ADS-B becomes mandatory and used by ATC and we can expect better quality of the data.

Aren't there a few places in the world where ADS-B is mandatory? I thought I had heard about a program over Hudson Bay? Ah right, here it is ... it specifies the standards on page 6.