Automatic Dependent Surveillance-Contract (ADS-C)

What is Automatic Dependent Surveillance - Contract?

ADS-C is a method of surveillance that relies on (is dependent on) downlink reports from an aircraft's avionics that occur automatically in accordance with contracts established between the ATC ground system and the aircraft's avionics. Reports can be sent whenever specific events occur, or specific time intervals are reached.

ADS-C provides accurate surveillance reports in remote and oceanic areas. The reports are converted by more advanced data link equipped ground stations into a track and presented on the controller's air situation display to provide enhanced situational awareness and the potential for reduced separation standards.

ADS Contracts

The establishing of ADS-C contracts is controlled by the ground station in all situations other than emergency contracts. Only the flight crew can declare and cancel ADS-C emergency reporting. Although the crew can initiate the emergency reporting mode, the aircraft cannot initiate a contract.

Contract establishment generally occurs following a successful logon and can occur at the same time that a CPDLC Connection Request message is being sent to the aircraft. Although, ADS-C and CPDLC are separate applications, they both use the same logon from the aircraft for their own purposes.

There are three basic contract types; the Periodic Contract, the Event Contract and the Demand Contract. There are also a number of "on-request" information groups within the FANS-1/A ADS functionality that can be added to the Basic Group for downlinking as part of an ADS-C report. However, these groups must be requested by the ground system when the contract is established.

The Periodic Contract specifies the reporting rate at which the avionics is required to assemble and downlink the requested information to the ground system.

Only one periodic contract can be established between an individual ground system and a specific aircraft at any one time, although it is possible for an aircraft to have independent contracts established with multiple ground systems at the same time (up to a maximum of four ground systems simultaneously for the B744 and a maximum of five for all other aircraft models).

Once a periodic contract is established, it remains in place until it is cancelled or replaced by another periodic contract.

Controllers in some systems have the ability to alter the periodic reporting rate to cater for situations, such as traffic density, where a higher or lower reporting rate is required.

The Event Contract specifies a requirement for a downlink ADS-C report to be assembled and sent whenever specified "events" occur. Multiple Event Contracts can be established between a ground system and a specific aircraft.

The Event Contracts available in the FANS-1/A system are:

• the Vertical Rate Change Event,
• the Lateral Deviation Event,
• the Altitude Range Change Event, and
• the Waypoint Change event.

The Vertical Rate Change Event is triggered when the aircraft's vertical rate (rate of climb or descent) is greater or less than the parameter defined in the contract request. This event does not have a great deal of relevance to ATC operations and is rarely implemented.

The Lateral Deviation Event is triggered when the aircraft's actual position exceeds a lateral distance parameter from the aircraft's expected position on the active (FMS) flight plan, as defined in the contract request. Note that in the case of an offset path created within the Flight Management Computer (FMC) that exceeds the distance parameter, a Lateral Deviation Report will be sent by the avionics at the moment of offset execution as the FMC “considers itself” to be already located on the offset path. However, in this situation, many ground systems will check the coordinates received in the Basic group of the report and determine that the aircraft is not currently outside of the deviation corridor.

The Altitude Range Change Event is triggered when the aircraft's altitude exceeds the altitude ceiling or floor defined in the contract request, which is set at a parameter above and below the cleared flight level (for example, +/- 200 feet).

Each of the event contracts discussed above has a life of only one event. When the specified event has occurred, the particular event contract must be re-initiated by the ground system.

The Waypoint Change Event is triggered by a change made to the Next, or the Next-plus-one waypoint. This change normally occurs due to normal waypoint sequencing by the FMS. The Next or Next-plus-one waypoint can be either an ATS waypoint or a pilot inserted waypoint (these waypoints are discussed later).

The Demand Contract is a one-off request made by a controller for an ADS-C report containing only the Basic Group. The Demand Contract is commonly known as a "one shot" report and is useful for updating ADS-C data and position information.

A situation where a Demand Contract report is useful is when an ADS-C aircraft is climbing or descending. The ADS-C level displayed for the flight data record does not update dynamically. That is, unlike a Mode C level readout, the ADS-C level displays the last level information reported and does not change until a new report is received.

ADS Data Groups

The Basic Group is sent with every downlink ADS-C report and contains the current latitude, longitude, and altitude, a time stamp and a Figure of Merit (FOM), which relates to the current level of navigational accuracy.

Basic Group example from an aircraft with callsign APC108:

APC108, Periodic,    07:22:08 Basic_Group   Latitude: 37.787S   Longitude: 144.473E Alt: 35000   FOM: 6

The FOM ranges from zero, the worst case, to seven, the best case. The FOM will degrade over time if a navigational update does not occur, eg. if the GPS input fails. The FOM value is generally not presented to the controller directly, but some ground systems use the FOM when deciding whether to paint the ADS-C track as a high quality or low quality ADS-C symbol.

A number of other on-request data groups are available for inclusion in an ADS-C report in addition to the Basic Group.

These are the:

• Flight Identification Group,
• Earth Reference Group,
• Air Reference Group,
• Airframe Identification Group,
• Meteorological Group,
• Predicted Route Group,
• Intermediate Projected Intent Group, and the
• Fixed Projected Intent Group.

The last three groups are explained in greater detail later in this section.

FMS Waypoints

FMS waypoints fall into two categories - Inserted Waypoints, and Pseudo Waypoints.

Inserted waypoints include normal ATS waypoints, and non-ATS waypoints inserted into the FMS flight plan by the pilot or the company to assist in the management of various operational functions. For example, a pilot can insert a waypoint as a trigger for an AOC flight plan report. An inserted waypoint does not change or update dynamically.

Pseudo Waypoints are waypoints inserted into the flight plan by aircraft systems for the management of flight parameters. Pseudo waypoints include items such as Top of Climb (TOC) or Top of Descent (TOD) and, as they are calculated and updated by the FMS according to the actual profile being flown, they are generally updated dynamically.

Pseudo waypoints may appear at the same position as an inserted waypoint, but they are generally positions in the flight plan where a change in parameters, such as altitude or speed, is programmed to occur.

Intent Information

In the FANS-1/A system there are two types of intent information that the FMS can provide as part of an ADS-C contract: the Fixed Projected Intent Group and the Intermediate Projected Intent Group.

The Fixed Projected Intent Group is computed along the FMS flight plan to indicate the aircraft's predicted position at a time specified by the ground system in the contract (x minutes from the current time). The Fixed Projected Intent Group may be the sole source of intent information.

The Intermediate Projected Intent Group includes up to ten eligible waypoints (if present) between the current position and the time specified in the Fixed Intent. These ten waypoints may be a combination of ATS waypoints, inserted waypoints, and pseudo waypoints. Waypoints are only eligible to be reported as Intermediate Projected Intent points if a change in flight profile (e.g. a change in altitude, speed, or direction greater than one degree) is programmed to occur at that point.

The Predicted Route Group

The Predicted Route Group is another source of information computed by the FMS.

The Predicted Route Group contains the latitude, longitude, altitude and ETA for the next inserted waypoint (ATS or non-ATS waypoint) and the latitude, longitude and altitude for the subsequent inserted waypoint (next-plus-one waypoint). Pseudo waypoints, such as the Top of Descent, are not reported in the Predicted Route Group.

Example of the Predicted Route Group (red text):

APC108, Periodic,    07:22:08   Basic_Group   Latitude: 37.787S   Longitude: 144.473E    Alt: 06004
Next_Position       Latitude: 37.826S    Longitude: 144.849E    Alt: 03000    Time: 07:26:33
Next+1_Position   Latitude: 37.768S    Longitude: 144.847E    Alt: 01952

From an ATS perspective, the Predicted Route Group provides information that is the closest to a voice or a CPDLC position report.

The Predicted Route Group and the Intermediate Projected Intent Group may both reference the same waypoint, except that the Predicted Route Group provides absolute coordinates (latitude and longitude), whereas an Intermediate Projected Intent Group waypoint is defined as a time from the current position and a track and altitude (if relevant) from the preceding position.

Diagram Key

The diagram above represents the FMS flight path of an aircraft (although it is not representative of the FMS display). The symbols and colours have been chosen purely to aid the explanation.

• The first yellow triangle at 0415 represents the aircraft's current position.
• The pink square is the Top of Climb, a psuedo waypoint calculated by the FMS.
• NOVAR is an ATS reporting point on the route. The flight plan indicates that the pilot will change speed and request climb at NOVAR   (NOVAR/M083F330).
• The orange circle is a waypoint inserted into the FMS flight plan by the pilot to trigger a report via ACARS to the airline's operational control.
• LADRO is an ATS reporting point. No changes in speed or altitude are planned to occur at LADRO
• The yellow triangle at 0445 represents the aircraft's Fixed Projected Intent Group position requested by the ground system for thirty (30) minutes ahead of the current position.

Understanding the Data

The easiest way to understand Intent Groups and the Predicted Route Group is to consider how the extrapolation of an ADS-C track will occur.

Fixed Projected Intent Group

Based on the diagram key above,  the Fixed Projected Intent Group has been requested by the ground system from an aircraft currently at the position represented by the first yellow triangle. The current time is 0415. The projected time requested in the Group by the ground system is 30 minutes ahead of the current position - this correlates to the second yellow triangle at 0445.The projected time interval (30 minutes) is calculated along the route of the FMS flight plan.

Fixed Projected Intent and Intermediate Projected Intent Groups

As there are waypoints in the FMS flight plan that qualify as Intermediate Intent waypoints (psuedo waypoints and inserted waypoints at which a change of flight profile will occur), both the Fixed and Intermediate Intent groups are reported. The yellow triangle at 0445 is calculated by the FMS as the Fixed Projected Intent point and then a series of waypoints are reported in the Intermediate Projected Intent Group between the current position and the Fixed Projected Intent point.

In this case, the Top of Climb psuedo waypoint, and NOVAR are the only waypoints to be reported in the Intermediate Intent. NOVAR is included because the flight plan indicated an intention to change speed and to climb at this waypoint, and, in any case, there is a turn in track of greater than one degree. The other two inserted waypoints are excluded as one (the orange circle) was inserted only to trigger an ACARS report to AOC,  and there are no planned changes to the flight profile (eg. speed, altitude, or a change in direction) at LADRO.

The two intent groups combined report the current position (0415), the TOP of Climb (TOC), NOVAR and the Fixed Projected Intent point at 0445. The extrapolation of the track symbol by the ground system between actual reports will be along the flight plan route held by the ground system.

The ground systems that request the Fixed Projected and Intermediate Projected Intent Groups (they are both optional, on-request groups)  generally use the information for automatic route conformance monitoring by comparing the intent information received in the report with the flight plan held in the ground system. If there is a discrepancy between the intent coming from the aircraft and the ground system's flight plan, the controller will be alerted.

Example of an ADS-C report containing the Basic Group, Predicted Route Group, the Meteorological Group, the Fixed Projected Intent Group, and ten Intermediate Projected Intent points:

16:43:26 - Periodic Rpt
Basic_Group Lat: 27.003S Long: 121.961E Alt: 39004

Next_Pos Lat: 28.000S Long: 123.999E Alt: 39000 ETA: 16:57:32
Next+1_Pos Lat: 30.870S Long: 130.000E Alt: 39000

Met_Group Wind: 267.188deg / 76kt Temp: -54.75

Fixed_Intent Lat: 37.685S Long: 144.843E Alt: 00420 ETA: 19:27:56

Int_Pt1 Alt: 39000 ETA: 16:57:31

Int_Pt2 Alt: 39000 ETA: 18:10:28

Int_Pt3 Alt: 39000 ETA: 18:33:37

Int_Pt4 Alt: 41000 ETA: 18:35:34

Int_Pt5 Alt: 41000 ETA: 18:39:41

Int_Pt6 Alt: 41000 ETA: 18:58:51

Int_Pt7 Alt: 16124 ETA: 19:11:24

Int_Pt8 Alt: 11192 ETA: 19:14:12

Int_Pt9 Alt: 10660 ETA: 19:14:53

Int_Pt10 Alt: 09000 ETA: 19:16:45

Predicted Route Group

The Predicted Route Group provides information on the next and next plus one inserted waypoints only (pilot inserted or ATS). The Predicted Route Group in this case would ignore the pseudo waypoint (TOC) and provide the intended latitude, longitude, altitude and estimate for NOVAR, and the longitude, latitude and intended altitude for the waypoint inserted by the pilot to trigger the ACARS report.

A Predicted Route Group waypoint and an Intermediate Projected Intent waypoint can occur at the same point on the flight plan, but they are determined differently. A Predicted Route Group waypoint is designated by absolute coordinates (latitude and longitude), while an Intermediate Intent waypoint is calculated relative to the previous waypoint, ie. as a time, track and altitude (if different) from the previous qualifying waypoint.

In the diagram above the turning point NOVAR would be reported as both a Predicted waypoint and an Intermediate Intent waypoint.

The Predicted Route Group waypoint would be reported as a specific latitude and longitude along the FMS route, but for Intermediate Intent, NOVAR would be defined as a time and track from the Top of Climb (TOC) waypoint.

Altitude in the Predicted Route Group

The Predicted Route Group provides the altitude at the next and the next + 1 waypoints and this may cause confusion for controllers in some ground systems if they do not fully understand the information being presented.

In flight the FMS continually calculates the optimal level (based on weight) in conjunction with steps inserted by the crew. The default step is 4000 feet, but steps of 1000 feet and 2000 feet can be manually inserted. These steps are used to calculate the optimal levels for the next and next + 1 waypoints and the optimal levels are reported in the Predicted Route Group information. The B747-400 calculates optimal levels above the current level only, but the B777, B757, B767 also calculate optimal levels below the current level (wind trade steps).

This level information is a projection and the flight crew may not actually wish to climb (or descend) to these levels due to environmental factors, such as strong head winds or turbulence, or certification issues.

As an example of the altitude information provided in the Predicted Route Group, an aircraft may be cruising at FL 330 with no intention of climbing any further, but the Predicted Route Group could be projecting a level of FL 350 at the next waypoint and FL 370 at the next + 1 waypoint.

The crew can insert steps of 1000, 2000, or 4000 feet into the FMS.  In the example represented by the diagram above, the crew may request to climb 2000 ft when the points on the flight plan associated with the optimum altitudes are reached.

If the crew wish to maintain FL 350, the Predicted Route Group would project the continued climb at the next and next + 1 waypoints based on the optimal level steps .

If the crew overwrites the FMS by inserting a step of  zero,  the Predicted Route Group for the next and next + 1 waypoints will project the level currently being maintained by the aircraft.

Example of vertical profile plotted from altitude values extracted from various groups contained within ADS reports for the same aircraft:

The graph is plotted over a period of 48 minutes and shows predicted altitudes from the Predicted Route Group (Nxt Pos and Nxt+1 Pos) and Intermediate Intent points, in addition to actual altitude values taken from the Basic group (Periodic reports) and the Waypoint Event and Altitude Range Change Event reports.

Time and Altitude Predictions from ADS-C Intent

Due to the binary nature of the FANS-1/A system there some standard intervals provided in ADS-C output. For example, for the B744 the difference between the time stamp of the ADS-C report and an Estimated Time of Arrival (ETA) provided in that report for an Intermediate Projected Intent waypoint or for a Predicted Route Group waypoint, when converted to seconds, is always a multiple of four.

Although this is not the case for the A345 or B777, as both of these aircraft will provide ETAs to an odd second.

However, the predicted altitudes provided by Intermediate Projected Intent and the Predicted Route Group, in addition to "current" altitudes provided in the ADS-C Basic Group for all three models (B744, B777, A345), are also always multiples of four.

Offsets Executed via the FMS

Boeing and Airbus aircraft (except for a B747-400 that does not have the Load 15 or higher software update) perform in exactly the same manner with respect to FMS Offsets. The intent information is projected along the offset path as represented by the pink lines in the diagram below. For Boeing 747-400 without the Load 15 update, the intent path for an offset projects back towards the route being referenced by the offset, as per the blue lines in the diagram.

However, the Predicted Route Group is the same for both Boeing and Airbus aircraft. When only the Predicted Route Group is requested, both the Airbus and the Boeing would be extrapolated along the offset path.

The intent path of the B747-400 has been updated in Load 15 to project along the offset path. In the meantime, many ATS ground systems already use the Predicted Route Group in preference to the Intent groups for the presentation of operational ADS information to the controller.

A final word on offsets executed via the FMS. Assuming that the offset in the diagram was executed at the position represented by the yellow triangle with the intention of joining the offset path at the abeam NOVAR position, the FMS considers itself to be on the offset path the moment that the pilot executes the offset, i.e. it does not take the transition path into account.

Sequencing Waypoints

When an aircraft is on an offset from the FMS route that exceeds a parameter distance from the route (B-A), any relevant waypoints on the route (A) will not be sequenced by the FMS.

When a waypoint is not sequenced by the FMS, the intent information (red arrows) will continue to project to that waypoint until manually corrected by the pilot.

The relevant parameter distances from the FMS route are:

• greater than 21 NM for Boeing aircraft, and
• greater than 7 NM for Airbus aircraft.