Separation Assurance in the Future Air Traffic System
Presented at
ENRI International Workshop on ATM/CNS
byHeinz Erzberger
Adjunct Professor of Electrical Engineering, U.C. Santa Cruz
Consulting Professor of Aeronautics and Astronautics, Stanford UniversityMarch 2009

Outline of Presentation
 Architectures and Operational Concepts for NextGenATM System Ground Centered Separation Assurance Aircraft-Centered Separation Assurance

Algorithms for Conflict Resolution
 Short range, tactical conflicts
 Strategic conflicts and arrival management
 Evolutionary Steps toward AutomatedSeparation AssuranceArchitecture for Ground-Based System
Paradigm shift: Ground-based systems connected to flight deck by data link areresponsible for separation assurance
Controllers responsible for traffic management and for handling special situations
TCAS

Unequipped

TCAS

ADS-B

ADS-B

Voice Link

Data Link
Automated Conflict
Resolution(2-20 min time horizon)

Safety Assurance
Controller
Interface(0-2 min time horizon)

Conflict-Free Trajectories for all Aircraft
Data Link

Analytical Model for Short Range Conflict Resolution•••

Modeling of turn dynamics required for resolution of short range conflicts
Resolution trajectories consist of heading changes followed by a straight line
segment
Objective is to achieve separation equal to or greater than required minimum
separation

VA2
RA =
where φA is the bank angleg ⋅ tan φAConflict Resolution Analysis in Separation-Turn Angle
Coordinates with Bank Angle as a Parameter
Aircraft B at
480 knLeft turns
A/C A 400 kn

Separation during turn fora range of bank angles
Minimum sep. in straightline segment after end ofturn
Time to min. sep. at 30
deg. bank angle

Right turns

Resolutions for Eight Maneuver Types
A turns left/right, B flies straight

B turns left/right, A flies straightA turns left/right, B turns right

A turns left/right, B turns left

TSAFE Resolution Decision and Event Time LineTime to loss of separation
2 min.

TSAFE
Action Time*~1 min.

0 min.

TSAFE-Res.
Window for autonomous uplink ofadvisory displayed
TSAFE-Resolution advisory;
to controller;
controller retains
control
Pilot initiates maneuver expeditiously whenclearance is received
If no action was taken by controller
and conflict remains unresolved, or
if conflict is first detected after thistime, TSAFE Res. is uplinked toaircraft autonomously

Controller resumes
responsibilityRequired separation
established;*Definition of TSAFE Action Time:
The smallest time to first loss for which
resolution trajectory exists that avoids loss ofseparation

Application of Mode S Specific Services Data Link for TSAFE
Level 2 Transponder

GICB
Stored Resolution
Message Reply
GICB
Readback Request

Handshake Reply
Com A
Resolution Message•

Mode S Specific Services– Mature technology, ICAO
approved• Com A Protocol for uplinking
TSAFE Messages– 48 bits available for specifying
resolution maneuver– Handshake reply by a/c verifies
24 bit parity check• Ground Initiated Com B (GICB)
Protocol for Readback of TSAFE
message– Sends resolution maneuver
received by the a/c back toground system for verification• Combination of handshake andreadback with parity checks gives
extremely low error rate

TSAFE Conflict Detection & Resolution

Strategic Conflict Resolution Algorithm••••

Resolves three types of conflicts:– Loss of separation conflicts– Weather conflicts– Arrival sequencing conflicts
An aircraft may be involved with all three types at the sametime.
Uses flight plans and aircraft performance models
Order of resolution:
1.
2.
3.

Weather conflictsSequencing conflicts
Loss of separation conflicts

Resolution Algorithm• Multiple resolutions are generated for each aircraft in a conflict– Left turns, right turns– Altitude: Temporary step-up or step-down in cruise,
temporary level off during climb or descent– Speed change in cruise or speed profile change during descent– Direct-To’s• From available successful resolutions, the resolution producing
the least delay is selected (with certain exceptions)– Selectable delay handicap parameter gives delay advantage tohorizontal resolutions

Vertical Resolution Maneuvers

Temporary altitude,Early descentTemporary altitude

Step climb in cruise

Step descent in cruisePath Stretch Vector Resolution Envelope
A/C position

1 min. delay ellipse
Symmetryline

2 min. delay ellipse

A specified delay generates a family of path stretches, whose
locus forms an ellipse
The algorithm starts generating resolutions with a path stretch
on the symmetry axis
It increases the vector angle if needed to avoid secondary
conflicts

Weather Cell Avoidance Algorithm
Weather cell probe provides entry and exit
locations of cell for any flight plan.
Shape of cell is not known.

Weather avoidance
flight plan

Current flight path

Algorithm
Increment heading in small intervals.
For each increment locate auxiliary waypoint at
midrange between entry and exit of a cell.
When vector path is first clear of cell, check return
path for cell penetration.
Increase range of auxiliary waypoint along vector
direction in small increments until return path isclear of cell.

Flow Chart for Algorithm
Input Conflict Pair

Resolution Maneuver Generator

Formulasand logic for
calculating
simplified
resolution traj.

Trial Planner
4D Trajectory Synthesizer
No

Heavy duty
numericalcalculations

Traj. completed?
YesCheck traj. for conflicts

YesConflicts detected ?

NoRes. Traj.
acceptedConflict-Free Arrival Sequencing
Automation of descent and arrival management requires thesimultaneous solution of two types of conflicts to ensure thatdescent trajectories are conflict free along the entire descent profileand meet in-trail time separation constraints at the arrival fix
1. Loss of Separation Conflict•Location can be anywhere along flight path•

Separation criterion is specified as a min. distance and min. altitude

2. Sequencing Conflict• Location of conflict at merge point, usually at the arrival fix• Separation criterion is specified as min. time interval between two
consecutive aircraft crossing the fix

Sequencing and Loss of Separation Conflicts
A: 1stC: 3rd

B: 2nd∆T minD: 4thFreeze Horizon ~ 20minutes from arrival fix

Arrival Fix:
Cross at10,000 ft and250 KCAS

A loss-of-separation conflicts isNew arrival, D, has crossed the projected between aircraft D and C
freeze horizon and has become at merge point, and sequencingconflict (not shown graphically) witheligible for sequencing andC is projected at the arrival fix
deconfliction

Finding a Slot for a PopupDTW SE Arrival Gate for Jets at 2x Traffic Level

New arrivals

New (red) arrivals are merged,
added to list
scheduled and deconflictedin the current
into frozen stream

epoch

OETA

STAprev.

STAupdate
Frozen (blue) aircraft scheduled in previous epochs

FreezeHorizon

Evolutionary Steps toward Automated Separation
Assurance
Level of Automation

Benefits

Controller Role1.

Automate long range(strategic) enrouteconflictsIncreased efficiency,
reduced controller
workload

Controller retains
responsibility forseparation assurance

2.

Automate arrival
sequencing

Reduced fuelconsumption

Controller retains
responsibility forseparation assurance

3.

Automate short range
conflicts

Increased safety,
increased capacity

Controller has
conditional and limited
responsibility for SA

4.

Integrate short and longrange SAGreatly increased safety,
efficiency and capacity

Controller handles
special situations