Automatic Dependent Surveillance(8)

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Complex paths may be created when an aircraft’s trajectory is generated with both MCP/FCU and FMS targets. Such a situation can occur when the lateral and vertical modes correspond to different control states, when FMS-based modes are armed prior to activation, or when an autopilot target value affects an FMS planned trajectory. The latter case is most common when the MCP/FCU selected altitude lies between the aircraft’s current altitude and the programmed FMS altitude, i.e. cruise altitude or altitude constraint. In this case, the aircraft will level out at the selected value, i.e. selected altitude acts as a limit value on the planned climb or descent.
Both short (TS report) and long-term (TC report) intent information offer a potential benefit to airborne conflict management, separation assurance, surveillance, flight plan consistency, and conformance monitoring applications. Short-term intent is available in almost all flight modes, while 4D TCPs are only available when equipped aircraft are using sophisticated FMS and RNAV systems. The newly defined TS report enables aircraft with less complex automation systems and aircraft operating with target state control to exchange available intent with nearby aircraft and ground stations. This capability should help facilitate the NASA DAG-TM goal of providing benefits to National Airspace System users having a wide variety of aircraft equipage.3

4 Target State (TS) Reports
Short-term intent parameters are assembled in the TS report shown in Table 1. The first three elements of the TS report: Participant Address, Address Qualifier and Time of Applicability are common to all ADS-B reports. Each aircraft has a participant address that is unique from other air vehicles in the same operational domain. This address enables the receiving system to correlate messages received from transmitting air vehicles. The address qualifier denotes the type of address used to identify the transmitting air vehicle (24-bit ICAO address or other). Receiving systems update the time of applicability as new ADS-B messages are received. The time of applicability represents the time in which the reported values are valid.
The principal elements of the TS report are the Target Altitude and Target Heading or Track Angle. These parameters represent the transmitting aircraft’s vertical and horizontal target states and will also be included in the TC report if they are part of a TCP. In order to provide a target state value, aircraft must be equipped with an autopilot or flight director that controls the axis consistent with the target value. The flight director must be on or the autopilot engaged while target state values are broadcast.
Target Heading is provided if the aircraft is actively being controlled to an air-referenced heading angle (such as when operating in a Heading Select or Heading Hold mode). Target Track Angle is used if the aircraft is controlled to a ground or inertial-referenced track angle, such as when flying between waypoints on a flight plan. The Target Heading/Track Indicator specifies whether the aircraft is controlled to a heading or track angle. A bit is reserved for Target Heading/Track Capability. This field will indicate whether or not the transmitting aircraft has the capability to provide the horizontal guidance target. If implemented, it will allow aircraft unable to determine target heading or track angle as defined above to provide appropriate substitutes.
Target Altitude is the aircraft’s intended level-off altitude if in a climb or descent, or the aircraft’s current intended altitude if it is being commanded to hold altitude. This definition is consistent with that adopted by the European Downlink of Airborne Parameters (DAP) program. 12 When determining target altitude, lower capability aircraft may not be able to consider all aircraft systems supporting vertical guidance. These aircraft may broadcast autopilot selected altitude or holding altitude as a substitute for target altitude. The Target Altitude Capability field indicates the transmitting aircraft’s ability to determine target altitude. The Target Altitude Capability can be used by the receiving ADS-B sub-system to assess the target altitude integrity. Receiving systems should consider that the target altitude field, for aircraft having less than full target altitude capability, may not contain the target altitude as defined above.
　
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