The function of the transponder is to transmit a responding
signal to a secondary ground radar which provides ATC with additional
information such as aircraft identification, altitude, etc. The
theory of radar operation is beyond the scope of this lesson, but,
since the radar environment is key to simulated ATC, the use of
the transponder is an important topic.
The transponders in most civil aircraft are 4096 code transponders.
When a code is selected and the transponder is on, it is referred
to as "squawking" and the code is called the "squawk code". The
highest number that may be selected is 7777.
The international emergency code is 7700. Two other codes have special
emergency meanings. Code 7600 is used for communication radio failure
and code 7500 is the international hijack code. In actual flight,
these three codes set off automatic alarms in ATC facilities and
should therefore never be selected, even momentarily, such as when
changing codes. The code 1200 is used for VFR flights in the United
States and 7000 is used for VFR in Europe. IFR flights use a code
specified by ATC in their clearance.
As to flying on IVAO and VATSIM networks: since the SB/PC environment
is entirely a radar environment, transponder use is of benefit to
both controllers and pilots.
Radio terminology used in reference to transponders is also important
in SB/PC so that pilots and controllers will be working on the "same
page". For example:
ATC: "...(callsign) squawk 4021"
Pilot response: "...(callsign) squawk 4021", and select 4021 on
A similar transmission would be to "squawk code 4021 and ident".
This means to select the proper code and press the IDENT button
on the transponder.