Modes
See also: Aviation transponder interrogation modesMode 1 – provides 2-digit 5-bit mission code. (military only – cockpit selectable)
Mode 2 – provides 4-digit octal unit code. (military only – set on ground for fighters, can be changed in flight by transport aircraft)
Mode 3/A – provides a 4-digit octal identification code for the aircraft, assigned by the air traffic controller. (military and civilian)
Mode 4 – provides a 3-pulse reply to crypto coded challenge. (military only)
Mode 5 – provides a cryptographically secured version of Mode S and ADS-B GPS position. (military only) Mode 5 is divided into two levels. Both are crypto-secure with Enhanced encryption, Spread Spectrum Modulation, and Time of Day Authentication. Level 1 is similar to Mode 4 information but enhanced with an Aircraft Unique PIN. Level 2 is the same as Mode 5 level one but includes additional information such as Aircraft Position and Other Attributes.
Mode C – provides 4-digit octal code for aircraft's pressure altitude. (military and civilian)
Mode S – provides multiple information formats to a selective interrogation. Typically aircraft are assigned a unique 24-bit Mode S address. The Mode S address is partitioned and a group of address ranges are allocated to each country. Some countries change the assigned address for security reasons, and thus it might not be a unique address. (military and civilian)
- Notes
Modes 4 and 5 are designated for use by US ARMY, USAF, USN, and NATO forces.
Read more about this topic: Identification Friend Or Foe
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