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provides all the same information the ATCRBS, but will do so more rapidly and with significantly more accuracy. By broadcasting aircraft position information to a ground station, ADS–B can also provide coverage in areas that do not have radar coverage. In addition, ADS–B provides trajectory information that includes speed and direction of motion. 

## Transponder 

The transponder is the airborne portion of the secondary surveillance radar system and a system with which a pilot should be familiar. The ATCRBS cannot display the secondary 

information unless an aircraft is equipped with a transponder. A transponder is also required to operate in certain controlled airspace as discussed in Chapter 15, “Airspace.” 

A transponder code consists of four numbers from 0 to 7 (4,096 possible codes). There are some standard codes or ATC may issue a four-digit code to an aircraft. When a controller requests a code or function on the transponder, the word “squawk” may be used. Figure 14-43  lists some standard transponder phraseology. Additional information concerning transponder operation can be found in the AIM, Chapter 4. 

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