How can you get the state of an aircraft that is receive only?

To determine the state of an aircraft that is receive only, Mode 1 squawks are particularly useful. Mode 1 refers to the transponder codes that an aircraft can transmit, which provides information about its identification, altitude, and other flight data. This information is crucial for air traffic controllers to track and manage aircraft, especially those not equipped to send or receive data directly.

Even though radar data can help track aircraft, receive-only aircraft may not show up clearly or provide altitude information without active transponder responses. ACARS (Aircraft Communications Addressing and Reporting System) is a data link system for transmitting messages and information but may require more comprehensive communication systems than a receive-only setup has. Voice communications could facilitate interaction with the aircraft but would not provide a continuous or automatic status update like the squawk codes do.

Therefore, Mode 1 squawks provide a clear and standardized way to ascertain the state of an aircraft that has limited capability, confirming its altitude and identity even in a receive-only context.