(d)  
An identification switch (IDENT) which causes the active transponder to transmit a special pulse identifier with the next ground station interrogation.

 (e)  
An altitude reporting switch (ALT RPTG OFF/ON) which disables or sets the altitude reporting function on the active transponder.

 3.  ATC Transponder (Fig. 1)_______________
 A.  The ATC transponders are located on the E2 electronic equipment rack. The No. 1 transponder is located on shelf E2-2. The No. 2 transponder is located on shelf E2-4.
 B.  The active ATC transponder transmits a coded reply when it is
 interrogated by an ATC ground station.

 (1)  Both transponders are identical to each other. Each transponder has the following switches and indicators:
 (a)  
A green PASS indicator light which comes on when a transponder self-test is successful.

 (b)  
A red FAIL indicator light which comes on when a transponder self-test detects a failure.

 (c)  
A red UPPER ANT indicator light which comes on during a self-test when the upper antenna impedance is incorrect.

 (d)  
A red LOWER ANT indicator light which comes on during a self-test when the lower antenna impedance is incorrect.

 (e)  
A red ALT indicator light which comes on when the altitude input source indicates a barometric altitude failure.

 (f)  
A red CTL indicator light which comes on when there is a failure of the control input.

 (g)  
A TEST switch which starts a transponder self-test.