Re: Transponder: Secondary Return "Coast" From Primary Return

[Badmachine]

What exactly is being described here and why would this occur?:

"Assigned transponder code ... changed ... the secondary radar return (transponder) indicating aircraft speed, altitude, and flight information began to coast and was no longer associated with the primary radar return.” (Page 11)

http://www.gwu.edu/~nsarchiv/NSAEBB/NSAEBB165/faa7.pdf

How do ATC secondary returns remain associated with ATC primary returns?

Thanx! :ok:

[aterpster]

The host computer that processes and combines the secondary (transponder) radar data (data block) tags that information to the primary return. If the transponder stops responding to the center's interrogator the data block will coast for a period of time along a presumed track calculated by the host computer.

Whether the primary return remains depends upon whether the aircraft is actually still in a position to be "painted" by the center's primary radar system.

In effect the primary and secondary radar systems are two independent systems.

Data blocks often go into coast for a short period of time without any sinister cause.

[Badmachine]

Thanks Aterpster. :ok:

How do ATC computers reunite a separated secondary return with its primary return? In the case of the noted flight, the returns did not reunite. Would even the entry of an emergency code cause a substantial primary/secondary return separation.

[aterpster]

Badmachine:

How do ATC computers reunite a separated secondary return with its primary return? In the case of the noted flight, the returns did not reunite. Would even the entry of an emergency code cause a substantial primary/secondary return separation.

If the airplane crashes it is all over. If not, the host computer will reestablish the link in due course.