- 
The aircraft altitude, -The RA report, etc.

2.1.3.3. ENHANCED SURVEILLANCE (EHS)
The Enhanced surveillance refers to the data link message set of the Elementary surveillance plus an additional set of data link messages. These additional messages convey parameters such as:
- 
The selected altitude, -The ground speed,

- 
The barometric pressure setting, -The true air speed,

- 
The roll angle, -The magnetic heading,

- 
The track angle rate, -The indicated airspeed,

- 
The true track angle, -The Mach number, etc.

3 Mode S stands for Selective.

2.1.3.4. GROUND INITIATED COMM B (GCIB)
The Mode S ELS/EHS is sometimes called GCIB. It indicates that data are transmitted following an interrogation from the ground. GCIB is different from Extended Squitters (refer to 2.1.3.8 – Extended Squitter) that transmit data without solicitation.

2.1.3.5. COMM A AND COMM B
Comm A is communication protocol for an interrogation of 56 or 112 bits on 1030
MHz from the ground to the aircraft.
Comm B is a communication protocol for a reply of 56 or 112 bits on 1090 MHz
(following a Comm A interrogation) from the aircraft to the ground.

2.1.3.6. 24-BIT ADDRESS OR MODE S ADDRESS
The 24-bit address is also called Mode S address of aircraft ICAO code. The 24-bit address format permits 16 777 216 different addresses. Therefore, each aircraft has its own 24-bit address, and a selective interrogation is possible.
The State of Registry delivers the 24-bit address with the aircraft registration documents. The 24-bit address is usually given on 6 digits (hexadecimal format) or on 8 digits (octal format).

2.1.3.7. AUTOMATIC DEPENDENT SURVEILLANCE – BROADCAST (ADS-B)
ADS-B is a application to transmit surveillance data from aircraft to ATC ground stations and between aircraft themselves, and to receive surveillance data from other aircraft. Automatic: No action is required from the flight crew. Dependent: The aircraft provides the surveillance data to the ATC ground station. The GPS sensor provides aircraft position and speed for ADS-B transmission. Surveillance: The ATC controller or flight crews from other aircraft use data broadcast from own aircraft to have a picture of the traffic. Broadcast: In opposition to Modes A, C, and S operations, ADS-B periodically transmits surveillance data to the ATC ground station without preliminary interrogation from a ground station. Compared to ADS-Contract (ADS-C, refer to
2.4 – Aircraft identification and Position Reporting with FANS), ADS-B transmits
surveillance data to the ATC ground station, without specific connection between the aircraft and the ATC ground station.
. ADS-B OUT
From the own aircraft perspective, ADS-B OUT refers to the capability to broadcast
(i.e. to transmit without preliminary interrogation) surveillance data. This capability is part of Mode S transponders installed on AIRBUS aircraft. Refer to 2.2 – Aircraft
　
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