Radar can see an aircraft, but it cannot always identify it. The transponder solves that. When a ground radar interrogates, the transponder replies with a code and often altitude information, letting controllers match the blip to a flight.

Modes matter. Mode A provides a four-digit squawk code. Mode C adds pressure altitude. Mode S includes a unique aircraft address and supports data link. Modern ADS-B systems broadcast position directly, supplementing radar coverage.

Common squawk codes

• 1200, visual flight rules in the United States
• 7500, hijacking
• 7600, radio failure
• 7700, general emergency

Controllers assign codes to identify specific flights. Pilots must set them correctly, and a wrong code can confuse the entire traffic picture during busy periods.