Automatic Dependent Surveillance-Contract (ADS-C)
is Automatic Dependent Surveillance - Contract?
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
The following link provides a high-level overview of ADS-C. Note - when the presentation loads, click on the word "More" to the right of the start button to select full-screen mode.
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
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.
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.
Periodic Contract specifies the reporting rate at which the avionics is required
to assemble and downlink the requested information to the ground system.
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).
a periodic contract is established, it remains in place until it is cancelled
or replaced by another periodic contract.
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.
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.
Event Contracts available in the FANS-1/A system are:
Vertical Rate Change Event,
Lateral Deviation Event,
Altitude Range Change Event, and
Waypoint Change event.
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 was recommended by ICAO for implementation by all ANSPs following the crash of Air France 447. The recommendation is for all aircraft to have a contract set to a value of -5000 (an increase of descent rate to 5,000fpm) in an attempt to recognise a possible uncontrolled descent with a minimal number of false alerts.
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.
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).
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.
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).
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
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
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.
Group example from an aircraft with callsign APC108:
Periodic, 07:22:08 Basic_Group Latitude: 37.787S
Longitude: 144.473E Alt: 35000 FOM: 6
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.
number of other on-request data groups are available for inclusion in an ADS-C
report in addition to the Basic Group.
Projected Intent Group, and the
Projected Intent Group.
last three groups are explained in greater detail later in this section.
waypoints fall into two categories - Inserted Waypoints, and Pseudo Waypoints.
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.
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
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.
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.
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.
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
Predicted Route Group is another source of information computed by the FMS.
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
of the Predicted Route Group (red text):
Periodic, 07:22:08 Basic_Group Latitude:
37.787S Longitude: 144.473E Alt: 06004
37.826S Longitude: 144.849E Alt: 03000
Next+1_Position Latitude: 37.768S Longitude:
144.847E Alt: 01952
an ATS perspective, the Predicted Route Group provides information that is
the closest to a voice or a CPDLC position report.
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 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.
first yellow triangle at 0415 represents the aircraft's current position.
pink square is the Top of Climb, a psuedo waypoint calculated by the FMS.
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).
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.
is an ATS reporting point. No changes in speed or altitude are planned to
occur at LADRO
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
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
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.
Projected Intent and Intermediate Projected Intent Groups
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
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
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.
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.
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 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
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
In the diagram above the turning point NOVAR would be reported as both a Predicted waypoint and an Intermediate
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.
in the Predicted Route Group
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
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
level information (and time estimates based on the level) 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.
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.
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
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 .
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.
An example of the impact on time estimates arising from this issue comes from the Chilean volcano eruption of mid-2011. As a result of the ash cloud, large commercial aircraft (A380, B777...) were crossing the Tasman Sea between Australia and New Zealand at maximum levels of around FL170. Due to the FMS steps not being modiifed by flight crew, the estimates for waypoints were being calculated by these aircraft based on the projected optimal cruising levels (e.g. around FL370) and ATC was detecting errors in some estimates of between 8 and 12 minutes.
The graph below provides an example
of vertical profile plotted from altitude values extracted from various groups
contained within ADS reports for the same aircraft:
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.
and Altitude Predictions from ADS-C Intent
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
Although this is not the case for the A345 or B777, as both of these aircraft will
provide ETAs to an odd second.
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
via the FMS
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.
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.
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.
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.
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.
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
relevant parameter distances from the FMS route are:
than 21 NM for Boeing aircraft, and
than 7 NM for Airbus aircraft.