FlightAware Discussions

Flight Track Log

Hi, Can someone please explain the dip in this chart.

flightaware.com/live/flight/JST8 … N/tracklog

Cheers :smiley:

Nothing unusual.

Take a look at the data below the graph at about 02:23. The plane possibly lost contact with the ground stations so didn’t receive information for a couple of minutes. This happens all the time as the amateurs who have the ground stations don’t yet cover the whole world and sometimes they turn their computers off.

It appears that FlightAware’s servers switched over to a data packet from a different ADS-B feed that showed updated GPS coordinates, but had incorrect altitude information . This was either fixed in the next packet from the same feed source, or the third packet shown in the screenshot below came from a different feed provided that is also using ‘YSSY / SYD’ as their location. There is no way for us to tell if this was corrected information from the same feed or data from another feed that simply shows the same location as we have no way of telling one YSSY / SYD feed provided from another (unfortunately).

https://dl.dropboxusercontent.com/u/8750282/LiveATC/Track_error.png

There are around 30 feed providers in that area that would have been sending a stream of data to FlightAware about that flight (especially at that altitude), so one of them falling off all of the sudden wouldn’t have affected what we see in the track log.

https://dl.dropboxusercontent.com/u/8750282/LiveATC/Track_error2.png

Matt

This can happen if there’s a single-bit error in the CRC part of a DF4/DF20 message. Because the aircraft address is also contained in the CRC part of these messages, the receiver can’t distinguish this sort of damage from a real message from an aircraft with a one-bit-different address. If that other aircraft is actually in the air nearby what can happen is that the altitude from the message gets attributed to the wrong aircraft.

(it’s a hazard of being a passive receiver - secondary radar doesn’t have this issue because it knows what address it interrogated for and can detect/discard damaged replies)