.  ADS-B IN
From the own aircraft perspective, ADS-B IN refers to the capability to acquire and to process surveillance data from other aircraft capable of ADS-B OUT. ADS-B IN is part of a TCAS capable of ATSAW function, Air Traffic Situational Awareness. Refer to 3.6 – Description of ATSAW.
2.1.3.8. EXTENDED SQUITTER
A squitter is an unsolicited transmission. Distance Measuring Equipment (DME) was the first equipment to use squitters. The ground DME station broadcast squitters. When the aircraft is in the range of the ground DME station, the airborne DME unit receives the squitter. Then, the airborne DME unit interrogates the ground DME station to get the range information. The use of squitters prevents unnecessary transmissions.
In ADS-B operations, a squitter designates the set of surveillance data broadcast by a Mode S transponder capable of ADS-B OUT. Squitters from Mode S transponders are of two types: short (56 bits) or extended (112 bits). The short squitter contains the aircraft 24-bit address amongst other pieces of information (communication protocol information on 32 bits). The extended squitter contains in addition, but not limited to:
- Longitude, latitude, barometric altitude, GPS height, surveillance status, etc.
- Movement, ground track, etc.
- Aircraft identification, flight number, aircraft category, etc.
- GPS velocity, vertical rate, etc.
Notes:
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Refer to 2.2.4 – Generic Emergency Indicator for details on the surveillance status.

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The aircraft category identifies several types of category: From “no reporting” to surface vehicle or space vehicle.

TCAS also uses the short squitter (also called acquisition squitter) from Mode S transponder. The own aircraft TCAS listens to short squitters from surrounding aircraft Mode S transponders to detect surrounding aircraft. Surrounding aircraft are identified with their 24-bit address. When TCAS detects a new surrounding aircraft (i.e. a new 24-bit address via the acquisition squitters), TCAS starts selective interrogations and tracking of this new surrounding aircraft. Refer to 3.1.2
– TCAS Principle.

2.1.3.9. 1090 EXTENDED SQUITTER
The 1090 MHz Extended Squitter (1090 ES) is the medium used to transmit ADS-B data. The reader may find other media to transmit ADS-B data:
- Universal Access Transceiver (UAT): FAA supports it for general aviation.
- VHF Data Link Mode 4 (VDL 4): The Swedish CAA supports VDL 4. However, as VDL 4 causes interferences with other onboard radio transmitters, AIRBUS does not support this medium.

2.1.4. TRANSPONDER CONTROLS
The transponder operating modes are the following: STBY, AUTO, ON. In addition, the flight crew may set the altitude reporting function to OFF or ON.
　
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