Interrogation Modes
Several different RF communication protocols have been standardized for aviation transponders:
| Military Mode |
Civilian Mode |
Description |
|---|---|---|
| 1 | provides 2-digit 5-bit mission code (cockpit selectable). | |
| 2 | provides 4-digit octal unit code (set on ground for fighters, can be changed in flight by transport aircraft). | |
| 3 | A | provides a 4-digit octal identification code for the aircraft, assigned by the air traffic controller. Mode 3/A is often combined with Mode C to provide altitude information as well. |
| C | provides the aircraft's pressure altitude and is usually combined with Mode 3/A to provide a combination of a 4-digit octal code and altitude as Mode 3A/C, often referred to as Mode A and C. | |
| 4 | provides a 3-pulse reply to crypto coded challenge. | |
| 5 | provides a cryptographically secured version of Mode S and ADS-B GPS position. | |
| S | provides multiple information formats to a selective interrogation. Each aircraft is assigned a fixed 24-bit address. | |
Read more about this topic: Aviation Transponder Interrogation Modes
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“Men and women are brothers and sisters; they are not of different species; and what need be obtained to know both, but to allow for different modes of education, for situation and constitution, or perhaps I should rather say, for habits, whether good or bad.”
—Samuel Richardson (1689–1761)