This is the typical setting for a transponder. The pressure altitude is then corrected to true altitude by the radar system, removing any errors that might be caused by one aircraft having set the incorrect altimeter setting. In ALT mode, a Mode-C transponder transmits the four-digit discrete code, as well as the aircraft’s pressure altitude. This setting should only be used if your Mode-C altitude report is inaccurate, and the pilot is requested to “stop altitude squawk.”
If the transponder is set to “On,” then it is only transmitting Mode-A information. Note, the individual digits on the transponder are only programmable between zero and seven. These Mode-A transponders are sometimes called 4096 transponders since they can send 4,096 different numerical codes. The most basic type of transponder only transmits the programmed four-digit code. This extra information, when connected to a primary return, is known as a secondary return on the controller’s scope.įor a good idea of what the radarscope looks like from the controller’s point of view, check out Chapter 4 of the FAA’s Aeronautical Information Manual. They send out the four-digit code that the pilot has set, and if they are a Mode-C unit, the aircraft’s pressure altitude. They transmit on a special frequency that is picked up by all air traffic radar facilities within range. Transponders solve both of these problems. If left to primary radar alone, a controller would not know whether two converging aircraft are at the same height and in danger of collision, or if they are separated by thousands of feet vertically and perfectly safe. But the radar provides no way to tell two aircraft apart from one another, and it gives no way of telling what altitude those planes are flying. Meggar Transponderīy watching the primary return on the radar scope move, a controller can gauge how fast it is moving and in what direction. These are objects that are not necessarily identified, and they could be any solid object. It then presents a blip on the controller’s radar screen, which is known as a primary return.
Since the beam is directional, it can figure out the bearing to an object and its distance from the radar station. The transmission tower is capable of “listening” for radio waves that are bounced off of solid objects and returned to the station. Like all radar systems, the controller’s radar sends out a beam of electromagnetic energy in a specific direction. To understand the importance of transponders and how they work, first, one should know a little about air traffic control radar. The results appear on the controller’s radar screens.
Once a setting is made on the transponder, there is no visible result inside the cockpit.
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