Question

[Jeremy]

Another question from me - I am learning fast, honest -  when tracking an aircraft the little aeroplane label is there. Sometimes I see a circle with a cross in it. What is this?
Beta V 3.0
Jeremy

[tarbat]

Circle/cross symbol means that Radarbox doesn't know the heading of the aircraft.  Either it's not being transmitted, or the aircraft has timed-out in Radarbox.

[Jeremy]

Thanks.
Sometimes it tracks across the screen with the circle/cross so it must know the direction.
Thanks for your help.
J.

[tarbat]

Even though it can track the change in position, that doesn't mean the aircraft is transmitting it's heading.  Depending on wind direction, heading and direction of travel can be different.

[DaveReid]

Sometimes it tracks across the screen with the circle/cross so it must know the direction.

The majority of ADS-B-equipped aircraft transmit "velocity over ground", from which both groundspeed and track can be calculated.
 
However some (for example UAL B772s) transmit airspeed and heading instead - I assume it's these that don't show up with the aircraft symbol.