Once your plane takes off, your pilot activates a transponder device inside the aircraft. The transponder detects incoming radar signals and broadcasts an amplified, encoded radio signal in the direction of the detected radar wave. The transponder signal provides the controller with your aircraft's flight number, altitude, airspeed and destination. A blip representing the airplane appears on the controller's radar screen with this information beside it. The controller can now follow your plane.
An airplane's transponder transmits flight data to incoming radar signals.
The departure controller is located in the TRACON facility, which may have several airports within its airspace (50-mile/80-km radius). He or she uses radar to monitor the aircraft and must maintain safe distances between ascending aircraft. The departure controller gives instructions to your pilot (heading, speed, rate of ascent) to follow regular ascent corridors through the TRACON airspace.
Photo courtesy Federal Aviation Administration (FAA) Departure and approach corridors for westward air traffic to and from airports in the San Francisco Bay Area TRACON airspace. (Click on the image for a larger version.)
The departure controller monitors your flight during ascent to the en route portion. When your plane leaves TRACON airspace, the departure controller passes your plane off to the center controller (ARTCC controller). Every time your plane gets passed between controllers, an updated flight progress slip gets printed and distributed to the new controller.