This is known as comm C capability of a transponder. A transponder must be a level 3 to accept a UELM message. UELM operates over UF24, which allows for long data messages to be sent from the ground station to the airframe. UF24 works similar to any other interrogation with one exception. The UELM message is capable of up to 16-bit to 80-bit message segments for this extended message. The working component of UELM is the comm C message (MC) field. This MC field works in conjunction with the number of C (NC) or segments to allow for the extended message.

This is known as comm D capability of a transponder. A transponder must be a level 4 to transmit a DELM message. DELM operates over DF24, which allows for long data messages to be sent from the airframe to the ground station. DF24 works similar to other interrogations with one exception. The DELM message is capable of up to 16-bit to 80-bit message segments for this extended message. The working component of DELM is the comm D message (MD) field. This MD field works in conjunction with the number of D (ND) segments to allow for the extended message.

Flight ID has its origins in the International Civil Aeronautics Organization (ICAO). The flight ID is an eight-character identification that is entered by the pilot usually via a flight deck CDU. The flight ID may contain the company-assigned number for that particular flight. If the company-assigned number is not available or not used, the flight ID then becomes the aircraft tail number. The flight ID supplements the unique 24-bit aircraft discrete address and is used by ATC for monitoring purposes.

The level of a transponder is an important point when testing an installation. The level reports the capabilities of the transponder. A level 1 transponder is one with basic surveillance capabilities. This is UF0, UF4, UF5 and UF11 reporting. A level 1 transponder will have no provision for datalink capabilities or extended length messaging. A level 2 transponder will have all the features of a level 1 with capabilities for UF16, UF20 and UF21 or comm A/B protocol. Most transponders installed today are level 2 capable. The level 3 transponder adds uplink extended length messaging (UELM) to the level 2 transponder. The level 3 transponder is capable of receiving a UELM from the ground interrogation using comm C (UF24) format, but does not need to reply directly. The UELM reply will include a summary of the interrogation. A level 4 transponder is the most advanced in its capabilities. This transponder adds downlink extended length messaging (DELM) to the level 3 transponder. The DELM is used similar to the UELM except the DELM is air to ground transmissions.

These parameters make up an important part of elementary and enhanced surveillance. Comm B is the integral part of DF20 and DF21 that has the message B (MB) field within its reply. The MB message field may contain downlink data requested from the transponders' BDS registers. These BDS registers or comm B designated subfield registers contain information about the status, intent and location of the airframe.

Elementary surveillance is as easy as 1, 2, 3 or BDS 1, 2 and 3. This concept is driven largely in part by EuroControl, that takes basic surveillance (UF0, 4, 5, 11) a step further in adding the features of DF20 comm B communications for datalink and identification of the airframe. Elementary surveillance is made up of several new components. These are 25-foot resolution altitude decode, interrogator identification (II) and surveillance identification (SI) verification, flight status, data link capability report (BDS 1,0), GICB report (BDS 1,7), aircraft identification (BDS 2,0), and ACAS active resolution advisor report (BDS 3,0). A EuroControl mandate stated that elementary surveillance installations started March 31, 2003 with a transitional period up to March 31, 2005.

Enhanced surveillance, as in elementary surveillance, is as easy as 4, 5, 6 — that is BDS 4, 5 and 6. Take the concept of elementary surveillance and add new aircraft intent reporting fields. These fields are selected vertical intent (BDS 4,0), track and turn report (BDS 5,0) and heading and speed report (BDS 6,0). The selected vertical intent report (BDS 4,0) is used to indicate such items as the barometric altitude of the airframe, the selected altitude of the mode control panel or flight control unit (MCP/FCU) and the flight management system (FMS). The target altitude of the aircraft also reports what altitude the aircraft is intending to use. The track and turn report (BDS 5,0) will indicate the roll angle, true track angle and rate, ground speed and true air speed of the aircraft. The heading and speed report will reflect the indicated air speed and mach, the barometric altitude rate, the magnetic heading and inertial vertical velocity.

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