Aircraft transition algorithm for conflict prediction, with specific reference to the North Atlantic Air Traffic Control System

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Kluyverweg 1 - DaFT _{AIRCRAFT TRANSITION ALGORITHM} FOR CONFLICT PREDICTION, WITH SPECIFIC

REFERENCE TO THE NORTH ATLANTIC AIR TRAFFIC CONTROL SYSTEM

by

D. J. Mohr

"

APR.

1973

..

AIRCRAFT TRANSITION ALG0RITHM'

FOR CONFLICT PREDICTION, WITH SPECIFIC . REFERENCE TO THE 'NORTH ATLANTIC

AIR TWFIC CONTROL "SYSTEM

by D. J. Mehr

Submitted December, 1972.

Acknowledgement

The author wishes to express his appreciation to his supervisor, Dr. L. D. Reid, for his guidance throughout the project.

The author is indebted ~o the director and staff of UTIAS for providing the facilities necessary to accomplish this project. I am indebted to my colleagues,

Mr.

W. Graf and Mr. C. Kim for their suggestions and computer assistance. Thanks

are due to Mr. J. Graham and

Mr.

T. Paine, both of the Ministry of Transport's Air Traffic Control Branch, for their consultation on the Gander A~tomated Air Traffic

System.

Financial support for this project was provided in part by the University of Toronto under a

UTIAS

fellowship.

Summary

The purpose of this project was to extend presently available digital fast~ time Air Traffic Control conflict prediction simulation modeis. A sim~lation algo-rithm was des~gned for compatability with the Gander Automated Air Traffic System

(GAATS) in cooperation with the Air Traffic Control Branch of the Ministryof fransport.

The algorithm, intended for application to a multiple aircraft strate-gically controlled enviro~ent, functions to search, detect and resolve conflict situations during enroute aircraft transitioning. The simulation model includes meteorological data, route structure, flight trajectory construction, and a

con-flict resolution option.

This project was designed to fulfill the specific needs of the Gander Automated Air Traffic System, while retaining the characteristics re~uired for

',J Table of Contents I. INTRODUCTION lIl.

IV.

V.

VI.

1.1

Conflict Prediction

1.2 Types of Air Traffic CeRtrol

1.3

Simulation in ATC

NORTH ATLANT IC

2.1 Air Carriers

2.2 Type of Aircraft on North Atlantic Routes

2.3

Navigati0n Systems

2.4

ATC Organization

2.5

Future

SEPARATION STANDARDS

3.1

Definition

3.2

Reasons for ~stablishment of Standaràs

3.3

Observation Data

3.4

Mathematical Derivation

a.5

Theory Application anà the 'Blunàer'

3.6

ICAO North Atlantic Standaràs

CONFLICT FREDICTION 4.1 Intr0duction

4.2

~actical Control

4.3

strategie Control

404

Cornbineà Str.ategic/Tactica~ Approach

4.5

Specific N0rth Atlantic Froblems

GANDER AUTOMATED AIR TRAFFIC SYSTEM

5.1

Intreàuction to GAATS

5.2

Pre-Flight Planning

5.3

In-Flight C00ràination

5.4

Westbounà Traffic

5.5

GAATS: Conflict Preàiction Subroutines

TRANSITION ALGORITHM CONCEPrS

6.1

Introàuction

6.2

Relationship to GAATS

6.3

Basic Concepts

6.4

Approximations and Assumptions

6.5

Meteorological Data

6.6

Conflict Flight Flan Table

6.7

Block Airspace Concept

6.8

Sorting Technique

6.9

Transition Maneuver iv 1 1 1 2 2 3 3

6

7 7 7

8

9

9 10 10 I I I I I I 12 12 13

13

13

14

16

17

1,7

1

8

18

19 , 19 20 21 21 21

23

24

VII. VIII.

IX.

TRANS PROORAM 7.1 Introàuction 7.2 Input 7.3 Search Technique 7.4 Detection Technique

7.5 Potential - Non Potential 7.6 Resolution Routine

7.7 Wind Correction Technique 7.8 Transition Routine

7.9 Separation Standards Check 7.10 Update Routine

7.11 Comrnent EXAMPLE PROBLEM .8.1 Met Model

8.2 Conflict Flight Plan Tab1e 8.3 Initial Conditions

8.4 Search Subroutine 8.5 De'tection Subroutine 8.6 Resolution Subroutine

Conflict Situation

8.7 Transition Aircraft Wind Correction Sueroutine 8.8 Transition Maneuver

8.9 Separation Standards 8.10 Update Subroutine 8.11 Sumrnary

RESULTS AND DISCUSSION

9.1 Algorithm-Program Efficiency

9.2 Validity of Search and Detection Routine 9.3 Discussion on the Reso1ution Technique

9.4 Nalidity of Transition Maneuver

9.5 Suggested Improvements 9.6 Other Applications / REFERENCES APPENDIX A: B: C:

D:

TABLES FIGURES

Examp~e Prob1em Output

F1ight Trajectory Construction Program Li s ting VariabIe Listing Page 26 26 26 27 33 34 34 35 37 49 50 51 51 51 52 5'2 52 52 54 55 56 56 57 57 58 58 58 59 60 60 61 62 63

ACC

A/c

AC2AC AFTN ARTCC ATC ATD CAS CFPr CONFL CTS CY--ETA ETD FAA FIR FL F/F GAATS GMr IATA IBM ICAO IFATCA IFR IFL INS I/Ö LF LORAN MET MOT MrT NA

ars

ABBREVIATIONS

Area Control Center Aircraft

Aircraft to Aircraft Conflict Prediction S~broutine

Aeronautical Fixed Telecommunication Network Air Route Traffic Control Center

Air Traffic Control Actual Time of Departure Collision Avoidance System Conflict Elight Plan Table Conflict Frediction Subroutine Composite Track Structure Location Identifier

Estimated Time of Arrival Estimated Time of Departure Federal Aviation Administration Flight Information Region

Flight Level Flight Flan

Gander Automated Air Traffic Control System Greenwich Mean Time

International Air Transport Association International Business Machines

International Civil Aviation Organization

International Federation of Air Traffic Control Ass0ciations Instrument Flight Rules

Intermediate Flight Level Inertial Navigation System Input/Output

EowiJ Fre(;}.uency

Long Range Navigation Meteorological (Met) Ministry of Transport Minimum Time Track North Atlantic

Organized Track Structure

Abbreviations - continued RAS R.P. SFL SFN STS TOF XRANS TWA USNM:!

vr

IAS

VFR

VLF ZULU

Reserveà Airspace Conflict Prediction Subroutine

Rep€>I~ting Point

Standard Flight Level

Sy~tem Flight Number

Stanclard Track Structure Time Over Fix

Transition Program written for UTIAS 1130 Trans World Airlines

United States National Meteorological Center

University of Teronte, Institute for Aerospace Stuàies Visual Flight Rules

Very Low Frequency

~w B _v D~

D

Lt

DpER

5

.

.

' D_TS D~

D

Y2

Dn

11 SJ KK Lat A Lat F LatLima Lat t LatREL .À N À

D

À~H

À ITH Àli ~ À IT

)

À

&

NOTATION cra~ angle block vàlue

along track distance between Repnrting PointBLima and Mike along track distance between Reporti~ Point Limaand the transition point

percent of distance traversed (time) between principal meridians at the transition time

relative track angle

Displacement between t and t along time East

o Tl

Dist~ce to next Checking Meridian frem Point S

Northerly (Southerly) Distance te New Reporting Point Distance to next checking meridian from transition point vertical component of Block Value

lateral component of Block Value longitudinal component of Block Value latitude at aft checking meridian latitude at forward checking meridian Latitude at Reporting Point Lima Latitude of the transition point Relative Latitude LatREL

new track angle track heading change

instantaneous air~raft heading instantane0us track headin~

initial aircraft heading initial track heading

track angle of potential conflict air cr aft track angle of transition aircraft

tf t Zl t Ml t Tl t Z2 t M2 t T2 T FL t

ss

Time A Time F Timet" Time K TimeREL

?/I

?/I

_f

VA

~AS

Vert REL V

_w

V

Wl

V

W2

Mach Number of potential conflict aircraft Mach Number of transition aircraft

Flight Level of petential conflict aircraft relative wind angle

initiatien time of transition maneuver termination time of transition maneuver

time delay before initiating vertical maneuver time delay before initiating Mach Number change time delay before initiating a turning maneuver time for termination of vertical maneuver, time for termination of Mach Number change, time for termination of turning maneuver. Flight Level of transition aircraft

time for separations standard check (every minute on the minute)

time at aft checking meridian time at forward checking meridian transition time

time of crossing the principal meridian upon which point K is located

Relative Time Difference

instantaneous track angle change departure angle from turn

Speed of sound ft/sec true airspeed

Relative Height Vert REL wind speed (knots or ft/sec)

along track wind component (ft/sec) across track wind component (ft/sec)

w

x(tL

X(t)

J),AC

~W

x (t)

w y(t) y(t)

YNEW

Y

_pAC yw(t) z(t)

Z{t)

track ground speed (ft/sec) of transition Aircraft

track ground speed (ft/sec) of potential conflict aircraft turning rate (deg./sec.)

am.ount of Flight Level Change

longitudinal displacement in track fixed reference frame longitudinal displacement in Earth fixed referenced frame Initial Longitudinal displacement from the zero time boundary updated loUgitudinal displacement

same as V Wl

lateral displacement in track fixed reference frame lateral displacement in Earth fixed reference frame updated lateral di~placement

Initial Lateral displacement from La~t

same as V

W2

vertical displacement in track fixed reference frame vertical displacement in Earth fixed reference frame

I. INTRQDUCTION

1.1 Conflict Prediction

A flock of seagulls soaring above a fishing baat have no need of conflict prediction. God gave seagulls instinctive ability to avoid collision with one

another in flight. Man, however, operating an aircraft in the flight environment

requires compensation, usually from ground-based facili ties, to saf'ely avoid

con-flict with other aircraft.

One of the primary functions of Air Traffic Control is to expedite and

maintain an orderly flow of air traffic. By doing so, ArC reduces the possibility

of collision between aircraft. Conflict prediction provokes a more positive

attitude towards avoiding collision. In predict.ing a colll:sion situation, correc-tive measures can be made well in advance of its projected occurrence.

Predicting future events in a dynamic environment, influenced by many

variables, gives conf~ict prediction its complex nature. Conflict prediction '

involves the functions of search, detection and in so~ cases resolution of a

potential conflict si~uation. The potential conflict situation may come in the

precise form of an impending collision or a less precise form as in violation of

separation standards minima. Unrealistic assumptions and inaccurate approximations

could preclude theoretically valid conflict prediction methods. Improved search,

detection, and resolution techni~ues will benefit air safety. The major problem

of air safety ~ coping with the increase in air traffic density.

1.2 Types of Air Traffic Control

There are two extremes in air traffic control; one is pure tactical co~­

trol, the other is pure strategic control. Conflict prediction techqiques vary

within this spectrum to meet the demands of the controlling system.

Pure tactical control involves desultory manipulation of air traffic

environment. Visual or radar scope observations provide the controller with

essential information to mentally perform conflict prediction.

Pure strategie control involves predetermination of aircraft intent before

entering the traffic environment. The controller in this situation monitors the

traffic ~o ensure aircraft movement corresponds as closely as possible to aircraft

intent.

Present methods of air traffic control incorporate a tacticaljstrategic

combination to fulfill the re~uirements for a particular air traffic environment.

1.3 Simulation in A.T.C.

The tremendous growth of air traffic in the past twenty years has produced equally large problems in maintaining safe and expeditious air traffic movement.

Ideally, Air Traffic Control systems attempt to lead the de~d on facilities

and equipment, but more frequently the demands have led the system's capacity. It

is becoming increasingly evident that simulation can provide the information needed

to make improvements on the present ATC system, as well as providing a vehicle for

testing new systems. More specifically, digital computer simulation fulfi~ls the

large computation requirements while keeping operational costs low. Simulation must start with a problem; in ATC the conversion from manual to automated techniques

presents the type of problems well suited to digital simulation. Basic ATC func-tions, illustrated in Fig. 1, can be broken down into a series of instrucfunc-tions, calculations anä data exchange routes. Each èf these three processes are readily adaptable to automateä techniques. Systemàtic methoäs of analysis for simulation techniques readily fit ATC moäels. The integrity of ATC simulation m0äels is a

matter of selecting the ~roper parameters for variables, while approximating other

parameters assumeä to have a negligib~e effect on the moäe~.

ATC simulation can be broken down int0 four areas of study. Evaluation of present ATC systems (Refs. 1,2)

Evaluation of present ATC proceäures (Refs.

2,3,4,5)

Evaluation of new ATC systems (Refs.

6,7,8)

Evaluation of new ATC procedures (Refs. 9,100

A cencise introäuction to ATC simulation can be founä in Refs. 11 and 12. Much ATC simulation study involves the evaluation of incorporating automated techniques into present control systems. Gander Automated Air Traffic (Ref. 13) is a working example of sophisticated automation techniques applied to a specific ATC environment. Automated Radar display, on-board navigation

com-puters, and meteorological models are just a few samples of simulation applications.

~wo types of simulation techniques are of particular interest in this

work; Real-Time and Fast-Time simuiation. Real-Time simulation performs system

modelling. on anormal (real-world) time scale. Gander Automateä Air Traffic

System (GAATS, section

5),

as a complete system, is a real-time model of an ATC

system. GAATS requires a human operator, the air traffic controller, as part of its control loop.

Fast-ti~e simulation is system modelling on a compressed time scale

frequently eliminating or approximating ~y human factors (controllers) involved.

An

example of a fast-time simulation model can be found in Section

5.5.

One

particular GAATS function is to simulate transatlantic flights to check f0r possible conflict with other aircraft. Modelling the transatlantic flight for conflict

prediction takes only seconds, whereas the actual flight may take

8

hours or more.

11. NORTH ATLANTIC 2.1 Air Carriers

The North Atlantic airspace, at times, is densely populated with air traffic. Much time and effort have gone into improving the aircraft, the naviga-tion techniques, and the ATC facilities to cope with the problems of the North Atlantic. Ensuring adequate levels of passenger safety has been one result of the

improvements thus faro

In 1972* alone, more than

8

mi~lion passengers crossed the At~antic via

one of the thirty air carriers operating North Atlantic r~utes. 'Economy' is the

goal of the air carrier followed closely by 'Safety'. Air carriers schedule their flights to meet passenger demands. This results in a predominently Eastbound flow of air traffic occurring during night hours, corresponding to a Westbound flow of

*

Base~ on ~rojected data from Aviation Week and Space Technology, 13 March,

1972.

air. traffic during daylight hours.

Transatlantic flights ca~ produce profits for the air carriers provided

specific rules on aircraft operation and optimum flight rou~ing are obeyed

religi-ously. With aircraft operation costs running anywhere from $2,5~O to

$5,qoo

per

hour, every minute of flight time is preeious. Accurate meteorological data,

co-herept ATC - aircraft coordination, and optimum transatlantic routing accurately

navigated are the ideal conditions the air carriers strive for.

2.2 Types of Aircraft on North Atlantic Routes

The North At~antic air traffic is composed of a variety of aircraft

coming from four major sources.

...

tions.

1) Commercial Air Carriers

2) Air Charter Organizatiops

3) Military

4) Privately owned business jets

The first two sources provide the majority of transatlantic air

opera-The following subsonic aircraft are (or will be) in use.

BOEING - 707, 720, 747 series.

McDONNELL DOUGLAS - DC-8,DC-10.

GENERAL DYNAMICS - CONVAIR 880,990.

LOCKHEED - L10ll.

AEROSPATIALE - CARAVELLES, A-300.

BRITISH AIRCRAFT CORPORATION - VC-l~.

By design, all these aircraft have similar performance characteristics;

cruise Mach Number, flight level cruise, fuel consumption/hour, etc. Piloting

techniques for each type of aircraft are designed to be quite similar, especially

for enroute maneuvering at cruising speeds.

2.3 Navigation Systems

Economy of operation and actual levels of safety hinge on the accuracy

of the navigational system. Since separation standards are, in part a function

of the accuracy of navigation systems and equipment, a brief overview of present

transatlantic navigational techniques follows.

~he Navigational systems of interest are:

Ground-Based Systems

LORAN-A LORAN-C

OMEGA (under development) Airborne Self-Contained Systems

DOPPLER-NAVIGATOR

INERTIAL NAVIGATION SYSTEM (INS) CELESTIAL

Signa~ pulses are emitted trom a master station which are received by

both the aircraft and another ground station cal led a slave station. The signal

pulse from the master station triggers the slave station to emit a sec0~dary pulse

called a slave pulse, also received by the aircraft.. The differential delay

be-tween recept ion of the master pulse and the slave pulse is measured onboard the

.aircraft. Position is calculated by measurement of this differential delay.

Out-put of position comes in two farms; direct digital readout or in a form to be

translated into position using hyperbolic position charts. Ground based system

errors depend primarilyon the aircraft's position relative to the stations.

Some problems associated with ground-based systems are:

'J;'YPE RANGE Airborne Set Cost Signal Frequency Typical Error Operational Costs Gover~ent Coordination Signal Distortion

Summary of Ground-Based Navigation Systems

LORAN-A LORAN-C OMEGA

500-800 st. mi. 1200 st. mi. up to 8000 st. mi.

up to $6,000 $20,000 to $50,00~ $15,000 to $5Q,OQO

2

MHz

100-KHz 10-14 KHz

1/4 st.mi. 1/5 st. mi. 1 st. mi.

Ref. 48

Airborne Self-Contained systems are the more recently developed type of

navigational sy~tems. Doppler navigation is performed by measuring the aircraft's

velocity using Doppler Radar. Incorporated with velocity measurements,

direc-tional sensors are used to continuously reference the true vertical and the

head-ing. Velocity measurements are translated into earth coordinates and integrated

to yield displacemento Accuracy of Doppler tends to degrade with the distance

travelled, necessitating periodic position updates on long rangeflightso

Al-though radar measurements are made ~hrough the atmosphere, only heavy rain degrades

the radar signalo For oceanic flights, Doppler Radar velocity measurements are

affected by the scattering properties of water, sea currents, and surface wind

effects.

The Inertial.. Navigation System (INS) operation is based on measuring

the aircraft's accelerations and translating this information into velocity and

displacement measurements. As a complete system, the outputs are: position,

ground speed, headin~, and vertica~. These four parameters accurate+y describe

horizontal and vertical aircraft guidance. INS has the advantage of complete

self-containment, whereas Doppler and celestial navigation require external references.

The problems associated with acceptance of INS are realistic, at least to the air

carriers. Initial cost per unit is quite high while maintenance and periodic

alignment are frequently needed, boosting the cost. The accuracy of an INS is

quite good, some units determine Longitude and Latitude to l/lOOth of a degree.

_ Celestial Navigation is by far the oldest form of ocean navigation. New technology in opties, electronics and computers has automated the celestial naviga-tion technique for accurate posinaviga-tion fix informanaviga-tion in high speed aircraft.

Suitable for day or night operations, star tracking in high speed aircraft re~uires_

sensi ti ve optical measurem,ent devices. Celestial navigation works weil except in

turbulence, although tracking errors exist due to acmospheric refraction.

TYPE RANGE Airborne

S~t Co st Typical Error

Summary of Airborne Self Contained Navigation Systems DOPPLER RADAR UNLIMITED $ç5,QOO Function of Distance INERTIAL UNLIMITED $lQO,OOO Function of Time CELESTIAL UNLIMITED $30P,000 (estimated) Due te Angular Measurement Ref.

48

Post transatlantic navigation errors can be found in references

28

and

49

for Doppler Radar and INS, respectively. Using Doppler Radar Navigation,

across track errors at the first landfall af ter a transatlantic flight run about

5 to 10 n.mi. when assisted by Loran A for position-fix information. INS error

accumulated for oceanic portions of a transatlantic flight are nominally within

5 n. mi. Westbound and within 10 n.mi. Eastbound. The greater error Eastbound

is due to the effects of the Earth's rotation.

The philosophy behind ~ood navigation is to never rely on one system as

a sole source of guidance information. Four of the systems mentioned above are classified as 'position-fix' type of navigation system, the other two have

'continuous' type output. An aireraft's navigation system for oceanic flight

usually contains at least 1 position-fix type output and at least 1 continuous

type output, each operating separately from the other. Common practice is to monitor a continuous type output with periodic update of position using a pesi-tion-fix type output.

Two other types of navigation systems are worth mentioning due to their

potential in long-range navigatio~. A synchronous orbit Satellite Navigation

System with data link would be quite feasible, at least on paper, in the high-density traffic environment. Of course, the drawback to a satellite system is primarily cost, for the satellite, airborne and ground equipment. State-of-the-art navigation for intercontinental aircraft has demanded the use of at

least o~e on-board computer. For aircraft carrying mere than one type of

naviga-tion system, the hybridizanaviga-tion of two or more independent systems has recently

been studied (Ref.

14).

Hybridization involves the use of two or more navigation

systems sharing the same on-board navigation computer facility.

Common practice, especially for North Atlantic air traffic, is to

employ one of the following combinations of navigation systems. LORAN-A } or LORAN-C with { DOPPER or INERTIAL

2.4 Arc Organization

All North Atlantic air traffic is subject to the International Civil Aviation Organization (ICAO) rules and regulations g0verning air traffic control.

(Refs. 15,16,17,18,19). ICAO provides for Domestic and Oceanic air traffic control through designated ATC Orga~ization, specifically GANDER ACC located in Gander,

Newfoundlan~ and SHANWIGK ACC located in ~reland. The North Atlantic airspace is divided ipto domestic and oceanic airspace regions.

Jurisdiction over air traffic in each region is the responsibility of the ICAO member country, (or state). Airspace within and in some cases adjacent to ICAO member countries, is the responsibility of that country. Figure 2 is a map of the airspace organization of the North Atlantic. Canada, in addition to her own airspace, is responsible for Gander nomestic and Gander Oceanic airspace r~gions.

Approximately 95% of all North Atlantic air traffic traverses the Gander Domestic - Gander Oceanic - Shanwick Oceanic airspace regions. This traffic consists

of mostly turbo-jet aircraft owned by the many air carriers flying the North Atlantic. Other traffic consists of military and privately owned aircraft.

Due to the high traffic density and other problems related specifically to the North Atlantic, ICAO has adopted the Organized Track Structure to expedite safe, yet dense, traffic flow. The Organized Track Structure is composed of a number of specified routes, (Eastbound up to 10 routes), that air traffic is expec -ted to follow (Ref.

2p).

The track structure is flexible to allow for dynamic meteorological conditions and to provide increased capacity of prime airspace when needed.

Under normal traffic conditions, the standard Track System is employed pröviding parallel Eastbound tracks spaced

90

nautical miles apart. The peak traffic conditions oceurring once daily for Eastbound traffic, requires track reorganization to aceommodate more aircraft in the same airspace volume. This re-organization to concentrate air traffic is termed Composite Track structuring. Composite Tracks are spaced

qo

nautieal miles apart, staggering the aircraft vertically to maintain legal separation. A summary of ICAO separation standards for the North Atlantie Region is given in Section

3.6.

An Eastbound aircraft adhering to the Organized Track Structure leaves Gander Domestic airspace, which uses tactical control, and enters Gander O~eanic

airspace, which is under strategie Control. The prime airspace is generally limited to a corridor approximately 250 nautical miles in width by 6000 ft. in height. At peak flow periods, night hours for Eastbound and daylight hours for Westbound, air traffie flow may reaeh a rate of

40

flights per hour. Due to the prevailing Easterly winds and the jet stream, Eastbound and Westbound corridors are separated by as

much as 1000 nautieal miles center to center.

Reference 15 defines the separation standards used by all air traffic in North Atlantic airspace. Air traffic is classified according to whether or not aircraft adhere to the Organized Track Structure.

,-Air Traffic Classification.

a) Free Track (all ftight levels) b) Organized Track (above FL 290)

1) Standard Track

2) Composite Track

Each classification has a different set of separation standards to govern a particular situatio~. The Organized Track air traffic runs only East-West, where-as the Free Track air traffic is not confined to any direction. Oftentimes situa-tions arise where Free Track aircraft cross the Organized Track structure presenting controllers with the demanding task of maintaining a non-éonflict condition.

2.5 Future

The 1970's saw the introduction of a new generation of subsonic turbojet-turbofan transport aircraft, the airbus, and jumbo-jets. Th~se new aircraft will help offset the increasing traffic, a primary problem in the North Atlantic.

In 1972 for example, only about 60% of the available passenger-seat capacity was filled. In other words, air traffic cauld be reduced if air carriers

were willing to double up on some flights to eliminate others. However, the use of

prime departure and arrival times to please the passengers will prevent air carriers fröill reducing flights.

Likely to offset the potential reduction in aircraft movement using the jumbo-jet, 'is the recent movement toward low cost one-way transatlantic air fares tending to increaseGthe number of air travellers. This will tend to fill the empty sea~ and perhaps increase air traffic at off peak hours.

One must consider the immense responsibility of the air traffic con-trollers and flight crews working in the North Atlantic environment. Any one

mistake which goes undetected may contribute to the loss of hundreds of lives and

millions of dollars. No matter what level of safety is achieved with sophisticated equipment and controlling techniques, the probability of collision over the North Atlantic exists, although it is remote.

111. SEPARATION STANDARDS

3.A

Definition

Separation Standards are accepted rules in governing aircraft movement in a multiple aircraft environment. Separation Standards provide the guidelines ~y which the controller administers air traffic under his jurisdiction. In

return, these stanrlards provide the pilot with reasonable assurance that no other aircraft will penetrate a defined volume surroundinghis aircraft. This airspace volume is sized according to the appropriate separation standards governing the airspace environment. Factors which influence the choice of separation standards minima are listed below.

1) Traffic Density 2) Common Sense

3)

Type of Air Traffic Control

5) Communication Exchange Density

6)

Geography

7) Weather

An assured level of safety exists for each set of separation standards applied to a specific situation. Due to the differences between pure Tactical control and pure Strategic control, different separation criteria are appropriate to each type. As air traffic density increases for a particular airspace envir on-ment, re~ulations are usually modified to cope with the demands on the ATC system. These modifications have taken the form of reduced separation standards, in some

c~ses justified by theory, in other cases justified by speculation.

3.2 Reasons for Establishment of Standards

An aircraft intending te navigate on a given course at a prescribed altitude (or Flight Level) will deviate to some extent from course and altitu4e due to guidance errors. These errors can be broken down into three components:

a) Vertical - Height Keeping Errors b) Lateral - Across Track Errors c) Longitudinal - Along Track Errors

These errors are discussed in the following subsections.

3.2.1 Height Keeping Errors

For enroute navigation, the barometric a~timeter is used universally to determine altitude above a Reference Datum. Reference 21 describes the standards and calibration requirements for legal use of a barometric altimeter. Being a

m~chanical device, the altimeter is subject to errors. For simplicity, errors

associated with height keeping are:

1) Instrument Errors (Mechanical

&

Calibration) 2) Static System Error

3)

Flight TechRical Error

References 22 and 23 discuss height-keeping errors related to turbojet transports. Flight Technical Error is a term encompassing a variety of errors associated with altimeter-pilot interaction. Years of operational experience have proven the barometric altimeter reliable but too frequently misread by the pilotes).

3.2.2 Along and Across Track Errors

Along and Across Track Errors are associated with the type of horizonta~

navigatio~ techniques used (Section 2.3). Horizontal guidance is the responsibility

of both the controller and the pilot. The degree of responsibility depends on the type of ATC (tactical or strategic) employed in the airspace environment. Longi-tudinal separation is basically a controller function, provided the pilot ean adhere to the controller's instructions. Láteral separation is more the pilot's respon-sibility in keeping Across Track Errors minimale Factors governing the establish-ment of horizontal separation minima are:

Instrumentàtion Error EstiÏ.mation Error Time-Keeping Error Elight-Technical Error

8

( Ref 0 13 )

Errors in horizontal navigation are not uncommon, ànd dep end on the type of navigation system{s) used. For continuous position output, error increases with time or distance travelled. Using posi~ion-fix type navigation, errors usually accumulate between updated positions. The accuracy in position-fix type navigation is afunction of the winds and how weil the pilot can compensate for winds to keep on track (see Appendix B).

3.3 Observation Data

Separation standards were initially established on the basis of common sense estimates. If these estimates worked, meaning no colli sion occurred over aperiod of time, they were considered safe. With increased traffic density and faster aircraft cruise speeds, justification to reduce separation standards needed a substantial basis. Observational data provided a foundation for a mathematical approach to separation standards reduction. References 23,24 and 25 are reports on observations of height keeping errors. References 26,27 and 28 report on track-keeping errors accumulated for North Atlantic air traffic approaching landfall from the Oceanic Regions. The problem associated with obtaining oIDservational data is simply high cost. A large sample of data is required for statisticians to work with; somewhere on the order of 5,000 observations.

3.4 Mathematical Derivation

With an accumulation of data on the one hand, and information about navigation system accuracy on the other, mathematicians attempt to piece together theories by which separation standards can be ·derived. A survey of the literature wil 1 produce a variety of approaches to the problem. Most of the differences lie in interpreting the geometry of airspace surrounding the aircraft, termed the Çonflict Volume. Unfortunately, another problem exists for the mathematicians

(statisticians). To apply a reliable mathematical theory, the analysis of the observational data should reasonafu1y fit the theory. Interpretation of ~he

observational data allows statisticians to estimate the probability of an aircraft deviating from its intended flight path. The statistician is interested in select-ing a probability of the position error sufficient to ensure an adequate level of safety.

For example, in North Atlantic airspace, under current separation standards, the probability of two aircraft occupying the same airspace is 1 in 107 (Ref. 29). This one in ten million chance is based on each aircraft deviating from its

intended course and/er Flight Level, by an amount e~ual te 1/2 the standard used to separate each in the appropriate direction. Figures 3a and 3b illustrate the probability of positional errors for each dimension. Note that the aircraft sil-houettes are not to scale. Figure 4 is an enlarged view of the tails of two ad-jacent probability curves.

Statisticians have been unable to fit a probability curve to the obser-vational data in the tail region of the curve. Thus the tail region of the Qrob-abiaity curve is usually approximated to produce a conservative estimate of the error. ~he value of the standards used in the North Atlantic are based on assuming the aircraft will stay within its legal separation limit 99.95% of the time. .

A detailed explanation of the mathematical derivation of separation standards can be found in Reference 29 through 36.

3.5 Application of Theory and the 'Blunder'

In estimating the probability curve for the distribution of errors~

assum-ptions and approximations were made in the process. No matter how sophisticated

the mathematics involveà in developing the theoretical probability curve~ some

errors cannoV be accounteà for. The fact that a human is an integral part of the

control loop (Fig. l)~ as a pilot and controller~ leaves an unpredictable margin

fer error.

The Blunder is that one chance that an aircraft will not be where int en

-àed~ due to some error by the pilot, controller or navigator. Mathematical theory

breaks down when human error is introàuced into the problem. Reference 37 discusses

the effect of the Blunder on collision risk ca!culations. Ristery has proven~

through numerous mid-air collisions, that the Blunàer exists and always wille

306 ICAO North Atlantic Separation Standards

In Section 2.4 air traffic was classified as Free Track or Organized

-Track. Although each have their own set of separation criteria, the difference

between them is only in the magnitude of the separation. There are six parameters from which North Atlantic separation stanàaràs are àerived.

a) Time b) Longitudinal Displacement c) Lateral Displacement d) Vertical Displacement e) Track Reading f) Mach-Differential

Longitudinal separation is defined in terms of Time difference and Mach

Number d~fferential. For two aircraft at the same Mach Number and on the same

track, the time between the passage of the aircraft over the same point is the time difference. lf a condition exists where the forwarà aircraft is cruising

at a higher Mach Number, the ~nimum time difference is legally reduced. Using

both Mach Number and time difference to define a separation standarà is known as applying Mach Number Techniquef. The Mach Number Technique only applies to air traffic on Organizeà Tracks. Turbojet aircraft are required to maintain a

cruise Mach Number ~thin a to+erance of + 0.01 when the Mach Number techni~ue

is applied.

Lateral separation is defined in terms of distance (nautical miles) with exceptions made when the tracks of two aircraft diverge according to certain initial conditions.

For vertical separation ICAO has assigned cruH~ng flight rules. Eastbound

traffic are assigneà flight levels 290,330~370,4l0,etc. Westbounà traffic are

assigned flight levels 3l0,350,390,43~,etc. These stanàard direction-of-flight

cruising flight levels are usually supercedeà by the controller when traffic

conditions warrant a more concentrated flow. The process of overruling ICAO

regulations requires the approval of both Ganàer ACC and Shanwick ACC. Only when

the North Atlantic COIDposite Track Structure is in use, are intermediate flight levels 300,320,340,360,etc., legally available to carry air traffico lntermediate

Flight Levels ~ay be used for either eastbound"or westhouncl traffic.

In North Atlantic airspace, East-West traffic is required by ICAO regula-tions to adhere to flight tracks (Free or Organized) orientated to intersect half

or whole degrees of latitude at principal meridians. Principal meriàians in North

Atlantic airs"pace are West to East, 600W,50OW,400W,3~oW,20oW, (15 0W), 10oW.

The

15

0W meridian is used as a principal meridian when 10~ is situated over a

land-fall. Principal meridians constitute mandatory position Reporting P0~nts for

East-West traffic. Track segments between principal meridians

(60

o

w

to

50

W, etc.) .

follow a great circle route.

N~erous special separation standard conditions exist for entry into or exit from the Organized~rack Structure along its IDoundaries. Figures

5, 6

and

7

illustrate the various separation stan~ards for the different Track classifications.

References

13,15

and

17

give a detailed explanation of special North Atlantic separation standards.

IV. CONFLICT PREDICTION

4.1

Introduction

Conflict Prediction is a term for the gamut of methods used to prevent aircraft collision. This range of conflict prediction extends from pre-flight

clearance for the entire flight plan, to last minute diversion commands provided

by such equipment as Colli sion Avoidance Systems (CAS). The airspace enviropment and the type of ATC dictate the requirements to employ a particular conflict predic-tion methode The need for conflict predicpredic-tion is the result of increased air traffic density, higher cruising speeds, regu~ation complexity, and controller work load. What makes conflict prediction a difficult problem te tackle is the large numIDer of variables influencing the situation.

below.

The parameters which influence the conflict prediction method are listed

Type of ATC (TacticaljStrategic) Type of Aircraft

Type of Flight RUles (IFRjVFR) Traffic Density

Ground-Based/Airberne Facilities Communication Delays

Controller Werkload Separations Re-quirement

The methods of theoretical conflict prediction are about as numerous as the number of authors who devise them. The literature contains many differen~ approaches to the problems involved. References

7,10,38-41

are typical of the various approaches made to conflict prediction development. Defining the conflict volume (Section

3.1)

leads to just one of the various differences in approaching conflict prediction problems. Other differences are found in the parameters used to sort out conflict situations. For the most part, Sections

4.2

and

4.3

will cover

the scope of conflict prediction methodology used by ATC.

4.2

Tactical Control

From the previous discus sion in Section

+.2,

conflict predict.ion in a tactically controlled environment represents the most complex form.

The system is dynamic with events occurring in rapid succession. The control loop is highly depe~dent on pilot-controller coordination giving rise to potential human errors. Take for example Terminal Control. The approach and tower controllers attempt to vector inbound air traffic using manuab conflict prediction

techniques. Manually predicting conflict involves visual contact by radar display and mental computation of predicted positions on a second by second basis. The controller's intuition and experience are key factors in attempting to vector in-bound aircraft in combined VFR/IFR traffic. This situation is further complicated by the (usually) saturated communication frequencies, delay time for response to changes, human variability, etc. Present technological advancements may soon aid the radar controller through digital fast-time simulation techniques to give the controfler a preview of potential conflict situations (Refs.

10,42

and

43).

4.3

Strategical Control

At the other end of ~he control spectrum is conflict prediction in a strategically controlled environment. Although the system is still dynamic, strategic control requires strict adherence to a predetermined flight plan.

Essentially the predicted flight of an aircraft from leaving departure control to entering terminal control at the destination is cleared of conflict situations long before the aircraft takes-off. This forces the aircraft to conduct its flight according to defined prócedures with no expected deviations from the flight plan. Specifically for Eastbound NorthAtlantic flights, the conflict prediction func-tion is performed by Gander ACC. Conflict prediction in this instance amounts to comparing all new flight plans entering the system with all other flight plans al-ready in the system. A~y detected violation of separation standards along proposed routes requires rerouting a~d reclearance prior to entering strategically controlled airspace. The controller work load arises from determining a conflict-free flight plan by hand computation or automated techniques and from monitoring flight progress.

4.4

Combined Strategic!Tactical Approach to Conflict Prediction

The following table \ists the advantages and disadvantages of tactical and strategic control in terms of conflict prediction criteria.

'rACTICAL IAdvantages:

1) Constant knowledge of Traffic Position.

2) Small Separation requirements mean dense air traffic flow.

J)isadvantages:

1) No preview of events limits pre-diction time.

2) Limited coverage in terms of aircraft and airspace.

3) Human factor dominant in the control loop~ (decision making).

S TRA'MG IC Advantages:

1) Aircraft int ent is kBown.

2) Decision making given adequate time.

3)

Large airspace coverage.

4) Large speed range of aircraft. Disadvantages:

1) Only the predicted aircraft position is known,not actual position.

2) Changes ~f flight plan enroute are generally not allowed.

3)

Large separation requirement.

4)

Extensive preflight calculations are required.

To provide efficient air traffic control, the proper cornbination of both types of control should be uti~ized. For the most part, the degree of tactical control can be measured by the amount of influence the controller has over conflict prediction. In pure-tactical control, the controller maintains alt the responsi-bility for applying conflict prediction logic in vectoring aircraft to avoid

conflict. In pure-strategic control, the system itself has determined

conflict-free conditions allowing ~he controller to monitor flight progress.

Examples of combined tacticaljstrategic control are prevalent in various ATC environments. Air traffic flying in high density domestic enroute airspace is

required to adhere to regulatiens governing flight planning and clearances. At

the same time, the traffic is monitored by an Air Traffic Control Center te provide

radar coverage and assist in manual conflict prediction when deemed necessary by

the controller. This author would call the situation a 'strategic-gone tactical'

control. In terminal areas the opposite has occurr~d. Pure Tactical control is

acceptable up to a limited volume of air traffic. When this limit is exceeded,

restrictions are imposed to ferm the random flow of traffic into an orderly flow.

This could be termed 'tactica~-gone strategic' control.

4.5

Specific North Atlantic Pr0blems

The North Atlantic Oceanic Eegions employ pure-strategic control through Gander and Shanwick Air Control Centers. As long as aircraft can adhere to their

cleared flight plans in terms of routing and schedule (time over Reporting Points), the prese~t form of control and conflict prediction are adequate. The situation,

however, is complicated by the followingllist of factors, which strategic contrel

I cannot adequately account for.

a) Airport Delay Times

b) Domestic Enroute Delay Times

c) Dynamic Meteorological Conditions

d) Flight P~an Changes Enroute

e~ Emergències

The present conflict prediction methods are unable to previde enough

fle~ibility to allowenroute flight plan changes. This results in full centroller

respensibility for manual conflict prediction in the event of a flight plan

change. The controller, based on predicted position-time information, must manually

determine what effect one flight plan change has on the rest of the traffic under

I

his jurisdiction. A large amount of air traffic increases with difficulty in manually predicting conflict.

A desirable improvement over strict strategic contrel would be to allow some flexibility by permitting enroute flight plan changes. To do this, an alge-rithm is needed to construct a conflict prediction technique by which enroute

flight plan changes can easily be checked. The algorithm presented in Section

7

is designed specifically for enroute flight plan changes in a generalized

strate-gically controlled airspace environment. Section

5

discusses the automated

tech-nique presently used in the North Atlantic. The application of the algorithm,

called 'Transition Algorithm' ,to the ~orth Atlantic ATC System is presented in

Section

8.

y!

GANDER ATJrOMATED AIR TRAFFIC SYSTEM

5.1 Introduction to GAATS

Due to the factors listed in Section

4.5,

the work load ef traffic

con-trollers covering the North Atlantic airspace is quite demanding. Their work load

is divided into two distinct functions; monitering air traffic and computation of flight plan information. The first function is specifically a human task, whereas

the second function lends itself well to automated techniques. To assist the controller in necessary computational functions, the Gander Automated Air Traffic System (GAATS) was introduced. GAATS was designed as the first step in the

appli-cation of modern computer technQ~Qgy and hardware to Canadian Air Traffic Control.

GAATS began operational service in 1968, intended for use into the late 1970's. As operationalexperience accumulates and system requirements evolve,

modifica-tions to the present system are expected. The purpose of this author I s work is

to provide one such modification in the area of conflict prediction for transition-ing aircraft.

GAATSis a semi-automatic digital computing faci l i ty designed to aid the

controller on request with specific ATC functions. Using real-time data-acquisition

and data processing techniques, an IBM 18QQ_process control computer (32K) serves

the. controller in computation and back-up decision making functions. System Inputs are:

.1) Meteorological Informatio~

2) Flight Plans

3) Fixed Track Structures System Outputs are:

1) Flight Data Strips

2) Up to Date Fix Estimates

3) Conflict Prediction

4) Error Messages

5) Controller Notification Messages

A brief description of the System is required in preparation for the

~resentation of the 'Transition Algorithm' (Section 7). The size and complexity

of GAAîS prohibits any detailed discussibn of its operation. Of interest here is

the background material required to prov~de pertinent information related to

con-flict prediction. Sections 5.2, 5.3, 5.4, are introductory material. Section 5.5 discusses the conflict prediction technQque.

5!2 Pre-Flight Planning

The ATC facility at Gander, Newfoundland (known as Gander ACC) is

res-ponsible for controlling, all air traffic in Gander Domestic and Gander Oceanic

Airspace Regions. This includes omnidirectional traffic, but more specifically

it ~s responsible for legally separating all Eastbound Transatlantic·Flights.

Shanwick ACC (contraction of Shannon

&

Preswick) is correspondingly responsible

for_Shanwick Oceanic Airspace plus legally separating all Westbound Transatlantic

Fli~hts •

Obviously, every Air earrier strives for flight plans which will provide the minimum in fuel consumption and flight time. For a given set of aircraft opera-ting characteristics (Section 2.2) and meteorological conditions, there exists one flight plan which will provide the minimum transatlantic flight time, called the

-hMinimum Time Track (MrT). Since most turbojet and turbofan aircraft cruise most

efficiently between Flight levels 30g and 410, this restricts optimum airspace in

the vertical dimension. The path between New York and London along a Great Circle'is.

used as the reference for North Atlantic Eastbound

MTT

calculations. fO find the

one optim~ flight plan NY-London Route, FL 300 and Mach Cruise of 0.82 are used as

standards of reference. Thus, the 'Prime Airspace' for Eastbound flights is located in the vicinity of the MrT and FL 300. One preflight function of GAATS is te compute the current MrT from meteorological winds-aloft forecast,

5.2.1 Meteorological Data

The meteoralogical data for the North Atlantic is most frequently obtained by Data Link from the U.S. National Meteorological Center, locateà at Suitland,

Maryland. USNMC issues daily two 300 mb Prognostic Meteorological Charts at times

OO:ROZ and 12:00Z, each valid fer the following 24 hours. The Met Data gives the

temperature anà winds at five pressure levels (40C\mb, 300 mb, 250 mb, 200 mb anà 150 mb), along with the estimated tropopause heigbt. Each data point covers an area 2-1/20 of Latituàe by 50 of Longitude which is 1/8th the area of a Marsden

Square. ~igures 8a and 8b illustrate the Met Data grid and the pressure level-flight

level conversion chart. GAATS interpolates this Met Data to a finer grià of 10 of

Lati tuàe by 50 _{of Longitude illustrated in Fig. Sc. Further interpolation is performed}

to obtain winds and temperature at each Flight Level (290 through 410). Froffi. this Met Data, a MrT is found fer a

cru~s~ng at FL 300 anà Mach number of 0.82. Using constructs a series of tracks parallel to the MTT.

is a fu~ction of the track structure in use.

fictitious NY-London flight, this MTT as reference, GAATS The distance between tracks

Af ter significant meteorological data has been accumulateà to àictate a

change in the routing of the MrT~ a new MT~ is constructed for new flights entering

the system.

Reference 44 discusses the algorithm used for calculation ef the MTT.

Figure

9,

from Ref. 45, illustrates a typical track structure with the

MTT

desig-nated by track 'X'.

5.2.2 Air Carrier· Functions

Hours before an Eastbound transatlantic flight departs, the crew is

briefed on aircraft conditiens, weather, changes in ATC procedures, and the current North Atlantic Route structure in use. Air carriers are given the option of select-ing an Organized Track which best corresponds to their origin-destination intent,

or they may select not to adhere to the Organizeà ~rack structure. This second

option is usually undesirable in terms ef flight time and fuel costs. Each flight is required by lCAO regulations to file a flight plan with the air traffic control

centers involved. A sample Flight Plan is illustrated in Fig. 10. The Flight

~l~_information contains:

Aircraft ldentity

lntended Flight Level(s) Mach Number Cruise

Domestic Route and Estimated Ttmes Oceanic Track and Estimated Times Destination

Flight Plans are relayed byeach air carrier to Montreal, then Gander ACC via the Aeronautical Fixed Telecommunications Network, (AFTN Computer Center, Montreal).

5.2.3

Gander ACC Function

Gander ACC receives flight plans from Montreal and manually edits each

for entry into GAATS. This input is done on a standard IBM

1816

Keyboard/Type-writer. Each flight plan entering GAATS is issûed a System Flight Number (SFN)

to coordinate identification of a particular aircraft durin~ Controller-System

interaction. Flight plans are stored until notification is received that the aircraft has departed.

5.3

In-Flight Coordination (Ref.

13)

~he Actual Time of Departure (ATD) is relayed to Gander ACC from the

appropriate departure control. Upon receiving the ATD, the appropriate flight

plan is activated in GAATS. There are four sections which coordinate a flight

_once the flight plan has been activated. These sections are manned by air traffic

controllers performiDg specific functions. These control positions are: 1) Domestic Coordinator

2) Domestic High Level Controller

3)

Oceanic P~anner

4)

Oceanic Controller

Once the flight plan has been activated, GAATS generated a Flight

Progress strip to the Domestic Coordinator and Oceanic Planner controllers. While the aircraft is enroute to Gander Domestic airspace, the Domestic coordinator updates any changes in the Flight Plan data using a special Programmed Keyboard

(Fig.ll). An updated Flight Progress Strip is issued to the Domestic High Level

controller r.esponsible for the control of aircraft up to the Oceanic Entry Points.

In the meantime the Oceanic Planner, using the special programmed Keyboard, has

programmed the oceanic portion of the intended flight plan allowing GAATS to

perform conflict prediction checks.

The intended flight plan is run through the computer to check for any

violation of separation standards along the oceanic portion of the flight. If

at any point in the flight plan, a violation occurs, the flight plan is rejected. This requires the Oceanic Planner to select another Flight level, or Track or

Oceanic Entry Time. Once clearance has been achieved, GAATS accepts the flight

plan and a Flight Data Strip is generated for the Oceanic Controller. Had any changes been made to the original flight plan requiring the aircraft to enter Oceanic airspace at another entry point, time or Flight Level, the Domestic High Level Controller would be notified. It is the responsibility of the Domestic High Level Controller to vector the aircraft to the new Oceanic Entry Point. During this time, if the pilot requests a flight plan change, the change musii be cleared before the last domestic reporting point. Once the flight enters Oceanic airspace, strategie control is employed by the Oceanic controller. The compulsory reporting points (events) for the flight are listed below.

1) Dompulsory Domestic Reporting Points 2) Crossing Coast - Out and Landfall Fixes

3) Crossing Principal Meridians in Oceanic Regions

(60OW, 50OW, 40OW, 30OW, '20

OW ,

15OW, lOOW)

4)

Within

60

Nautical Miles of an Ocean Region Boundary

5) Any pecessary Changes in Mach Number, Flight Level,

Heading.

Aircraft flying en an Organized Track are required to adhere strictly to their cleared Flight Plan. Any alteration of Flight Plan that is requested while enroute in the O~eanic region is dealt with manually by the Oceanic Controller. Normally once a flight plan has been cleared fer Oceanic airspace, the pilot is xequired to maintain the assigned flight level throughout the oceanic portion of

the flight. When initially submitti~g a flight plan, the pilot may request to fly

a porti on of the flight at one flight level and another portion at a different flight level. To check this flight plan for conflicts with the present computer algorithm, GAATS must consider this two level flight plan as two separate flight plans. Each must be cleared for the entire flight.

Gander Oceanic hands off control to Shanwick Oceanic when the eastbound aircraft crosses the 300W longitude.

5.4 Westbound Traffic

Shanwick ACC is responsible for organ~z~ng the track structure fer

westbound traffic. The identical methods and procedures are performed at both

Gander and Shanwick. Direct telephone communication links Gander ACC and Shanwick

ACC for coerdination of track structures, meteorological developments, emergencies, etc. Organized eastbound tracks seldom conflict with organized westbound tracks due to displaced MTT's and peak-slump traffic cycles.

5.5 GAATS: Conflict Prediction Subroutines (Refs.13 and 46)

GAATS processes iryitial and updated flight plan information through the use of various data tables. Depending upon the request by the controller, data tables are called up by subroutines whd.ch extract and manipulate the data to per-form specific tasks.

The Conflict Flight Plan Tab+e (CFPT) is a core resident table which contains the latest available informatton about assigned flight plans for conflict prediction purposes.

Table Content:

1) Systems Flight Number (SFN)

2) Mach Number at Cruise 3) Oceanic Routing

4) Reporting Point Times 5) Track Status

6) Flight Level(s) of Cruise

This 5,22e word table (20 by 260) is capable of holding information for up te 260 flight plans (single level), or 196 flight plans (single level) piLus 32

two level flight p~~s. Flight plans are arranged in the CFPT according to flight

levels. From this table, new flight plans and updated flight plans are checked for conflict against all other flight plans in the table.

The checking is performed by the GAATS Conflict Prediction Subroutine

(CONFL). CONFL supervises input/output,logs conflicts, and calls the two ot her

checking subroutines: Aircraft to Aircraft (AC2AC) and Reserved Airspace (RAS)

subroutines. (AC2AC) checks for aircraft to aircraft conflicts between eastbound

assigned flight plans. RAS checks for conflicts between eastbound aircraft and

Eastbound Track Structure has reserved for it a colume ef airspace accompanying

its flight path. The relationship between these subroutines and the mainline

pro-gram is illustrated in Fig.12.

~he AC2AC subroutine utilizes the informatien in the Conflict Flight

Plan Table to systematically check new flight plans with all other flight ~lans

in the table. Checks are performed by sorting out the flight plans, then checking

each aircraft one track segment at a time. Sorting is done by eliminating the

flight levels greater than 1000 ft. above and below the assigned flight level of the new flight plan. For each level that is searched, all the flight plans are checked one at a time. Each flight plan pair is checked one track segment. (e.g:

400

w

to 300W) at a time. Each track segment is checked first for lateral

separa-tion, then for longitudinal separasepara-tion, and finally for composite separation (when the composite track structure is in use).

All conflict prediction is performed before the aircraft is actually

cleared to enter Oceanic airspace. O~ce the aircraft enters Oceanic airspace,

no further automatèd conflict prediction can be performed.

If a pilot requests a change in his aircraft's heading, flight level, or Mach Number from his clearance directives, the controller has two alternatives. First, he could use intuition, experience and manual calculations to check for conflicts.

Second, he could choose to program through GAATS a fictitious flight plan adjusting for the change requested. This method is somewhat crude because it does not check for a conflict during the transition maneuver.

An enroute flight plan change, instigated by the pilot or controller,

is not uncommon. Weather, emergencies, and loss of navigational integrity are

the most frequent causes for change. The primary source of pote~tial conflict

aircraft is traffic having the same general direction-of-flight, in this case eastbound.

Flight plan change requests are likely to contain more than just a new

flight level for cruise. The performance characteristics of the aircraft m~y

warrant a new Mach Number for cruise at different altitudes. VI. TRANSLTION ALGORITHM CONCEPTS

6.1 Introduction

It was pointed out in Section

5.5

that enroute flight plan changes

re-quire controller intervention and decision-making without an automated back up. Froviding the controller with an autornated method of conflict prediction for

en-route transitioning wou~d give the present Orga~ized Track Structure substantial

flexibility. This would be one step toward a tactically controlled environment from pure-strategic control.

The concepts involved te develop a conflict prediction algorithm for transitioning are simple at first glance. The efficiency, validity, and accuracy required for a workable conflict prediction method tends to complicate matters.

This problem was approached from a practical standpoint, the author being more familiar with pilot and controller techniques than conflict prediction