the possibility of ambient flight deck noise interfering with  transmissions to the controller. Headphones deliver the  received signal directly to the ears; therefore, ambient noise  does not interfere with the pilot’s ability to understand the  transmission.  [Figure 2-3] ATC radars have a limited ability to display primary returns,  which is energy reflected from an aircraft’s metallic structure.  Their ability to display secondary returns (transponder replies  to ground interrogation signals) makes possible the many  advantages of automation. A transponder is a radar beacon transmitter/receiver installed  in the instrument panel. ATC beacon transmitters send out  interrogation signals continuously as the radar antenna  rotates. When an interrogation is received by a transponder, a  coded reply is sent to the ground station where it is displayed  on the controller’s scope. A reply light on the transponder  panel flickers every time it receives and replies to a radar  interrogation. Transponder codes are assigned by ATC. When a controller asks a pilot to “ident” and the ident button  is pushed, the return on the controller’s scope is intensified for  precise identification of a flight. When requested, briefly push  the ident button to activate this feature. It is good practice  for pilots to verbally confirm that they have changed codes  or pushed the ident button. Primary radar returns indicate only range and bearing from  the radar antenna to the target; secondary radar returns can  display altitude, Mode C, on the control scope if the aircraft  is equipped with an encoding altimeter or blind encoder. In  either case, when the transponder’s function switch is in the  ALT position, the aircraft’s pressure altitude is sent to the  controller. Adjusting the altimeter’s Kollsman window has  no effect on the altitude read by the controller. Transponders, when installed, must be ON at all times when  operating in controlled airspace; altitude reporting is required  by regulation in Class B and Class C airspace and inside a  30-mile circle surrounding the primary airport in Class B  airspace. Altitude reporting should also be ON at all times. Switching the transmitter selector between COM1 and  COM2 changes both transmitter and receiver frequencies.  It is necessary only when a pilot wants to monitor one  frequency while transmitting on another. One example is  listening to Automatic Terminal Information Service (ATIS)  on one receiver while communicating with ATC on the  other. Monitoring a navigation receiver to check for proper  identification is another reason to use the switch panel. Most audio switch panels also include a marker beacon  receiver. All marker beacons transmit on 75 MHz, so there  is no frequency selector. Figure 2-4 illustrates an increasingly popular form of  navigation/communication radio; it contains a global  positioning system (GPS) receiver and a communications  transceiver. Using its navigational capability, this unit can  determine when a flight crosses an airspace boundary or fix  and can automatically select the appropriate communications  frequency for that location in the communications radio. Mode C (Altitude Reporting) Radar and Transponders Figure 2-4.  Combination GPS-com unit. Figure 2-3.  Boom microphone, headset, and push-to-talk switch. Headset Push-to-talk switch Boom microphone 2-3