[Milsurplus] old IFF systems control panels

[rertman at ix.netcom.com]

Ray's correct in that the APX-6 code was preset before mission takeoff.

As for modern IFF and civilian radar transponders, it's a different story.

Military:  They have modes the civilian transponders don't have and can use
encryption both ways.  The interrogation from the ground must have the proper
crypto key to get the IFF to respond, which it will do with an encrypted code.

Civilian radar transponder:  No encryption capability, but will report altitude
if that feature is turned on and the altimeter is reporting to the transponder.
Codes are changed as the aircraft moves from one air traffic control (ATC) sector
to another, depending on the ATC facility's plan and needs.  "Squawking" code 1200
is the standard one for aircraft flying under visual flight rules (VFR) below
18,000 feet above mean sea level (MSL).  That altitude is the "floor" of positive 
control airspace and all air traffic above 18,000 must be on an instrument flight 
rules (IFR) flight plan and have a transponder.  Some areas with very heavy local 
VFR traffic may assign a code other than 1200 to transiting VFR aircraft and,
upon the aircraft's leaving the area, ATC will instruct the pilot to resume the 
1200 VFR squawk.  Some transponder codes are reserved for special circumstances.  
For instance, 7700 is a general emergency squawk.  Other codes are reserved for 
the aircraft to indicate loss of radio transmitter capability, radio receiver 
capability, hijack, etc.

Hope this helps,

Dick
Retired electronics engineer/engineering test pilot

Ray Fantini wrote:
> 
> My limited understanding of the APX-6 and similiar  generation of WWII
> radar  identification transmitters  is they were not remote operated by
> the
> crew  and the type of radar target generation symbol  that would be
> transmitted by their aircraft would be  be preset by the aircrew or by
> maintenance.  Nothing  like todays "Transponders" which require the
> flight
> crew to change the transmitter IDER setting many times during a flight
> so
> that it could be "interrogated" by the ground. I think that  since
> 911
> it's not uncommon for even VFR aircraft which normally would have to
> squawk at standard 1200 code   to have to set special settings in
> designated air space- but the old APX was not that user friendly, and
> when the master switch was turned on it sent the signal continiously
> which cluttered up the radar screen so it was either ON or OFF. But the
> good news, the APX-6 got many folks started in 1296
> operations using the modified sets.
> Ray Fantini KA3EKH
> 
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