Pilot Courses of Instruction

Air Traffic Control Procedures

Transponder Modew

The following transponder modes are used in the National Airspace System. Pilots typically turn the transponder, typically nowadays, to the ALT setting, and that is that: enter a code, 1200 or as advised by ATC, the light flashes, IDENT when requested, and that is it; however, the transponder has a rich history with vaious "modes" that convey a variety of information. These are the transponder modes in use today.

• Mode 1 is a military mode, a two-digit octal code transmitting a 5-bit mission code, the original (IFF) system.
• Mode 2 is a military mode, a four-digit octal code used to identify military aircraft missions.
• Mode 3/A is a joint military/civilian code, (i.e., military mode 3/civilian mode A), a four-digit octal code used to identify aircraft generally
• Mode C is a civilian mode that reports an aircraft's pressure altitude: operating along with Mode A, ATC (centers, approach controls, towers with radar repeaters) can receive an aircraft’s unique code giving not only position (and track) but also pressure altitude.
• Mode S (which is supposed to be short for "Mode Select") is a joint military/civilian code, a discrete selective interrogation with more advanced communication capabilities than modes A/3: each aircraft is assigned a fixed, unique 24-bit address. Mode S is the basis of TCAS and ADS-B.
• Mode 4 is a military mode, which is cryptographically protected, based on pulses from the "interrogator" (e.g., the ATC radar) with specific pulse timing: what the radar screen shows is based on the aircraft's transponder having or not having matching cryptographic setting as to whether or not it will send proper response pulses, which determines whether or not the aircraft shows, or how it shows, on the radar scope (at least as far as I understand it.
• Mode 5 is a military mode, which is a cryptographically secured version of Mode S and ADS-B GPS position.

Thus, the numbered modes, 1, 2, 3, 4, and 5, are military modes, while the lettered modes, A, C, and S are civilian modes; military mode 3 and civilian mode C are identical. Technically, there are additional modes, civilian modes B and D, that are not in use.

References

• [ Stack Exchange, Aviation, What does "mode-four check" mean? ]
• [ Wikipedia, Transponnder Interrogation Modes ]

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