IFF interrogator (along with its associated transponder set AN/APX-77A) generates and transmits pulse-coded radar challenge signals to interrogate surface and airborne targets, which automatically respond by transmitting an identification code. Interrogation is at a frequency of 1030 MHz and response at 1090 MHz.The IFF interrogator can challenge stations on the surface or in the air in modes 1, 2, 3/A, 4 or C. Mode 1 allows 32 possible code combinations, modes 2 and 3/A up to 4096, while mode 4 allows complex computer-coded identification signals. When a radar target is shown on the TDS, the operator can initiate an 8-second IFF interrogation and review the indication for a correctly coded response. These responses are decoded, converted to a video signal and superimposed on the radar video for identification, using numbered cues next to the target. The ID code may also contain supplementary data such as aircraft position status within a group, aircraft altitude and emergency status. Currently, this supplementary information is not decoded for display or processing.
Even when no return is received, some attribute information can be assigned to the target, i.e., relative beliefs as to whether the platform is friend (but cannot answer due to faulty equipment), foe or neutral. These assignments necessitate the study of heuristics along the lines of the MSDF implementation for the CPF and clearly depend on the type of mission and environmental or climatic information. Typical distribution of beliefs were tried in the AIFIE study (Unisys, 1993) but will have to be refined depending on the type of Aurora mission, since the expected relative number of friends, foes or neutrals varies tremendously upon mission elements and on available pre-flight intelligence reports.
This extract from the Aurora Aircraft Operating Instructions, provides additional information about the system:
"The Identification Friend or Foe (IFF) Transponder system with its Selective Identification Feature (SIF), identifies own aircraft to interrogating friendly units by transmitting coded SIF replies. The IFF/SIF Transponder system is controlled by the copilot for all operations except Mode 2 and 4 code settings which are set on their respective units in Rack 9. The Transponder replies to Mode 1, 2, 3/A (civilian code A), C (altitude reporting), and 4A or B interrogations. Special replies, Emergency (EMERG) and I/P (identification of position) (IDENT), are available to denote the aircraft to the interrogating unit in other-than-normal IFF/SIF identification condition. The Transponder system operates completely off-line and shares the use of its antenna with UHF-1 and 2 Radio Sets. All modes, except Mode C, operate independently of each other and are not dependent on inputs from other avionics systems. Mode C uses altitude information from the pilot barometric altimeter to transmit to Mode C monitoring facilities and requires a functional Mode 3/A. The Transponder system receives interrogations on 1030 MHz and after determining that the signal is valid for the IFF mode selected, generates a coded SIF reply which is transmitted out the antenna on 1090 MHz at 500 W. This is found in all Pre-Block 3 CP-140/A. In the Block 3 version many of the functions associated with the IFF are embedded in the AIR radar system".
Reference:1) Information Fusion Concepts For Airborne Maritime Surveillance and C2 Operations by P. Valin ,É. Bossé and A. Jouan dated May 2006. Document #P525407