The
Development of Airborne Collision Avoidance System (ACAS) and
Air Traffic Control Services in Kano Flight Information
Region
INTRODUCTION
Writing
a paper on the development of Airborne Collision Avoidance
System (ACAS) and Air Traffic Control Services in the Kano
Flight Information Region -having being an active participant
on both sides as an Air Traffic Controller by profession and a
member of ICAO Secondary Surveillance Radar Improvement and
Collision Avoidance System (SICAS) Panel (a body charged with
the responsibility of ACAS development) for over fifteen years
gives me a great pleasure.
Air
Traffic Services can be sub-divided into three main parts
namely:
1. Procedural
ATC services (Time distance and rules)
2. Radar ATC
services (Electronic visual display)
3. CNS/ATM
(satellite data and automation)
The
Kano Flight Information region has been basically operating
procedural air traffic control service although radar services
were introduced in 1980 into the following terminal
areas:
1.
Lagos
2.
Kano
3. Port
Harcourt
4.
Abuja
5.
Enugu
With
the assistance of ICAO Technical programme for training
Nigerian Radar ATCOS and technical expertise for maintenance
technicians and Radar Instructors. The performance of Radar
control in Nigeria has been poor due to lack of spares;
unreliable power supply and inadequate technical back up.
Since European airspace is adjacent to African Indian Ocean
(AAFI) airspace most of the data available on ACAS, results on
implementation and compliance will be sourced from countries
in Europe in general and EUROCONTROL in
particular.
BACKGROUND
The
Airborne Collision Avoidance System (ACAS) is a genetic term
coined by ICAO for an airborne collision avoidance system that
alerts and advises pilots about other aircraft (the intruder)
entering a restricted volume of airspace around own aircraft.
Three levels of ACAS have been described by
ICAO:
a.)
ACAS I measures only range and range rate and alerts the pilot
of intruders entering a certain airspace volume as a potential
threat. It presents only Traffic Advisories (TA) to the pilot.
b.)
ACAS II is based on Secondary Surveillance Radar (SSR)
technology and uses the Mode C reports of the transponders of
adjacent aircraft. Based on the horizontal and vertical dosing
rates, an ACASII calculates dynamic protective volumes of air
around the own aircraft. If the dosing intruder becomes a
threat, the system proposes a Resolution Advisory (RA) to the
pilot as a Vertical Avoidance Maneuver. The system coordinates
its RA with the intruder aircraft, if it is ACASII-equipped so
that the maneuvers are complimentary.
c.)
ACAS III, will in addition, provide the pilot a Horizontal
Maneuver Advisory capability. The Traffic Alert and Collision
Avoidance System (TCAS) and was developed in the US. TCAS I
and TCAS II are the only system commercially available which
correspond to the requirements for ACAS I and ACAS II. TCAS I
is used in the USA while its use in Europe is not anticipated.
TCAS II is mandatory in the US airspace and will become
mandatory in Europe on the 1st of January
2000.
For
TCAS III, it was intended to use the antenna bearing to the
intruder for generation of the horizontal flight path
supporting a horizontal RA. This proposed development met
technical difficulties and was abandoned. TCAS IV is still
under development and will use other technologies to meet the
ICAO requirements for ACAS III.
This
presentation reports the results of TCAS II operation in the
ECAC airspace. TCAS II is an independent airborne system based
on SSR technologies. It interrogates and receives via the
aircraft transponder Mode C and Mode S messages. The
interrogation rate is about 1Hz and the derived range and
range rate and the received Mode C reports are used to track
other aircraft in the vicinity and the logic test for a
potential threat based on this data.
Currently,
the bearing of the intruder is only in the pilot display. (In
the forthcoming change 7, the collision avoidance system (CAS)
will use the bearing information). Once a potential threat is
detected the logic presents it to the pilot as a Traffic
Advisory (TA). If the threat poses imminent danger, then TCAS
II proposes an avoidance manoeuvre to the pilot in the
vertical sense - this is a Resolution Advisory (RA). Tracked
distance and closure rate - in the slant range and vertical
sense - permits the logic to calculate the time TAU until the
Closest Point of Approach (CPA). The critical TAU thresholds
depend on altitude and vary for the TA between 20s and 48s and
for the RA between 15s and 350s. The defined TAU thresholds
provide variable protective volumes for each altitude
band.
If
both aircraft involved in an encounter are TCAS-equipped,
there is an exchange of messages in the Mode S band to ensure
that the RA are complimentary. RA issued to pilots are either
Preventive RA, instructing the pilot to maintain his vertical
rate or indicate which vertical rates he should avoid or
Corrective RA which propose to the pilot an avoidance
manoeuvre, based on a pilot model. The method assumes that the
pilot reacts within 5s after receiving the RA and accelerates
with 0.25g into either a climb or descent altitude until he
reaches a vertical speed of ±1500 ft/min or, if that is not
sufficient, ±2500ft/min, which should be maintained until the
RA, is altered or the Clear of Conflict is enunciated. In the
majority of cases, the deviation caused by such a manoeuvre is
less than 500ft. It should be pointed out that the logic aims
for a Vertical Miss Distance at the closest point of Approach
in the order of 400ft (extrapolated trajectories ± standard
pilot reactions). The pilot reaction should therefore be
prompt and precise to avoid reducing this safety margin.
However, an overreaction or disregarding a weakening RA does
not provide additional safety but may lead to an excessive
deviation.
THE
ACAS STANDARDISATION PROCESS
To-date, the urge for the development of
the TCAS stemmed mainly from the US. ICAO was involved from
the beginning and charged the SSR Improvement and Collision
Avoidance System Panel (SICASP) in May 1983 with the
responsibilities for ACAS and the development of the SARPS.
Draft SARPS were proposed and discussed by SICASP IV in 1989
and the panel recommends that they serve as interim SARPS for
the purpose of evaluation. Subsequently, the ICAO Air
Navigation Commission (ANC) issued the draft SARPs and
qualified and quantified the impact of AACASII operation in a
wide variety of environments, an international evaluation
programme was instituted in the USA, Japan and Europe. Enough
operation experience was thus acquired to enable the US to
complete the transition to mandatory TCAS II operation in the
US airspace after 1st January 1994. On the 11th of November
1993, ICAO endorsed ACAS and the definitions are laid down in
Annex 2. PAN-OPS (Procedure for Air Navigation Services -
Aircraft Operations) and PAN-RAC (Procedure for Air Navigation
Services - Rules of the Air and Air Traffic Services) have
been modified and contain the new procedures for pilots
(PAN-OPS) (DOC.8168) and phraseology to be used and procedures
to be observed by controllers (PAN-RAC, DOC4444). These
modifications as approved by ICAO require that the
introduction of ACAS shall be on a regional basis with a
lead-time of
5years.
ACAS
IN EUROPEAN AIRSPACE
At the beginning of 1991, EUROCONTROL was
charged with organising the operational evaluation of ACAS in
the European airspace and for European carriers. This
evaluation was conducted under the auspices of ICAO; in
parallel, the UK and France conducted their own evaluation for
their respective airspaces and these evaluations form an
integral part of the EUORCONTROL evaluation. A forum for
exchange of experience and coordination group -AECG - was
formed which controlled and guided the evaluation. A smaller
Event Analysis Group 'EAAG' was formed and tasked specifically
to evaluate and analyse TCAS events and report to AECG. Based
on the experiences gained, member states requested EUROCONTROL
to establish a common ACAS policy. Consequent upon the above,
the 'ACAS Policy Task Force' (APTF) was founded and charged at
the end of 1994 with developing a common policy for the
introduction of ACAS in Europe.
This
policy:
Endorsed
the mandatory carriage and operation of an airborne collision
avoidance system conforming to ICAO SARPs in the airspace of
ECAC member states;
Adopted,
in principle, an implementation schedule for mandatory
carriage and operation of ACAS II such
that
a.)
With effect from 1 January 2000, all civil fixed-wing
turbine-engined aircraft having a maximum approved passenger
seating 15,000Kg or maximum approved passenger seating
configuration of more than 30 will be required to be equipped
with ACAS II and:
b.)
With effect from 1 January 2005, all civil fixed-wing
turbine-engined aircraft having a maximum take-off mass
exceeding 5,700Kg or maximum approved passenger seating
configuration of more than 19 will be required to be equipped
with ACAS II.
The policy proposal was accepted by the
EUROCONTROL Committee of Management and further proposed for
adoption by ECAC (European Civil Aviation Conference). At the
same time, ACAS was established in the European Air Traffic
Control Harmonisation and Integration Programme (EATCHIP) as
an independent domain and the ACAS Implementation Group (AIG)
were created to control and coordinate the implementation
activities.
AIR
TRAFFIC CONTROL SERVICES
Air traffic control is enhanced with the
development of Secondary Surveillance Radar (SSR) services
using ground and airborne equipment.
DEVELOPMENT
OF SSR SERVICES
Radar
Equipment Development Air traffic Services (ATS) in Europe
continue to rely heavily on Secondary Surveillance Radar (SSR)
and most states require the carriage and operation of a
serviceable transponder capable of operating in Mode A with
4096 codes together with Mode C altitude reporting for
aircraft flying in controlled airspace in accordance with
Instrument Flight Rules (IFR).
In
specified areas it has been possible to dispense from carriage
and operation of a transponder with the use of primary radar.
SSR provides data on the position, identification and altitude
of the aircraft permitting the tacking of individual flight in
three dimensions. Azimuth accuracy is improved by the
utilization of monopulse techniques.
The
main problems associated with the Mode A/C systems
are:
Garbling
i.e. interference due to overlapping replies from two or more
aircraft in close proximity in azimuth and
distance.
Fruit
i.e. interference at one interrogator caused by the replies
from a transponder in response to interrogations from another
interrogator.
Availability
of only 4096 codes in Mode A for identification of aircraft.
Shielding of the antenna caused by the altitude of the
aircraft.
To
overcome the limitations of Mode C/A, ICAO has published
Standards and Recommended Practices (SARPS) for Mode S, a mode
selective system and which is interoperable with the Mode A/c
system and which has been designed to replace it Mode S
offers, inter alia, selective surveillance of individual
flight, each aircraft being assigned a unique address from
almost 17 million aircraft address available. Antenna
shielding can be reduced by antenna
diversity.
Airborne Equipment in order to improve the
effectiveness of ATS in areas where such services are provided
on the basis of SSR information, it is essential for aircraft
in the airspace concerned to be detectable by carriage and
operation of an appropriate transponder. Furthermore, many
aircraft have been, and will continue to be equipped with Mode
S transponders (at least level 2 capability) to support
Airborne Collision Avoidance Systems (ACAS). The operations of
these systems depend upon the SSR replies provided by adjacent
aircraft, regardless of the SSR coverage available for ATS
purposes.
Interoperability
of Ground and Airborne Equipment.
It
must be ensured that equipment used on the ground and on board
aircraft are compatible and remain so during their respective
development and full operational life. Accordingly, it is
necessary to define on a common basis the capabilities of the
airborne SSR equipment required for the various categories of
airspace users. On the basis of the work carried out under the
aegis of EATCHIP, the European Air Navigation Planning Group
(EANPG) determined on its 34th meeting in 1992, that a
proposed amendment to ICAO DOC 7030 requiring the mandatory
carriage of Mode S for IFR flights in controlled airspace
should be circulated to states and international
organisations. A similar proposal was adopted in 1993
concerning VFR flights. Both these proposals mandated the
carriage of Mode S transponders with effect from 1st January
1999.
In the light of up-dated scheduling for
the introduction of Mode S-based surveillance services,
revised time scales and capability requirements for the
carriage and operation of airborne equipment have been
defined. These have been brought to the of the EANPG so that
action can be taken to amend ICAO DOC 7030
accordingly.
Mode
S Transponder
According
to ICAO Annex 10, Mode S transponders shall conform to one of
the following five levels of capability.
Level
1: This is the basic transponder. It permits surveillance
based on Mode A/C as well as on Mode S. With a Mode S aircraft
address it comprises the minimum features for compatible
operation with Mode S interrogators. Level 1 has no data link
capability and will be used by international air traffic (this
transponder will not be permitted for use in the ICAO EUR
regions).
Level
2: This has the same capability as level one and permits
standard length data link communication from ground to air and
air to ground. It included automatic aircraft identification
reporting. This is the minimum level permitted for
international flights.
Level
3 has the same capabilities as level 2 but permits extended
data link communications from the ground to the
aircraft.
Level
4 has same capabilities as level 3 but allows extended data
link communications from the aircraft to the
ground.
Level
5 permits Comm-B and extended length data link communications
with multiple interrogators without requiring the use of multi
site reservations. This level of transponder has a higher
minimum data link capability than the other transponder
levels.
Multisite
acquisition of Mode S transponders is carried out by using the
Mode S-only all-call interrogation. The interrogator code of
the interrogating site is contained in the interrogation and
two types of interrogator codes are defined in ICAO Annex 10,
which shall be available in the mode S
transponder:
The
interrogator identifier Code(ll-code) is for multisite
surveillance and data link coordination. II codes between 9
and 15 are valid (II code 0 is interpreted as non-selective).
The surveillance Identifier (Si-code) is used for multisite
surveillance only SI codes between land 63 are valid. SI code
0 is not used.
Mode S extended squitter is a technique
that combines the capabilities of the SSR Mode S system with
those of the Automatic Dependent Surveillance (ADS). This sis
accomplished by using an extended squitter as the broadcast
data link for transferring the aircraft-derived ADS report
from the aircraft to airborne or ground users. This type of
operation is known as ADS-Broad (ADS-B). The formats and data
sources for the squitter messages are defined in the Manual on
Mode S Specific Service (DOC. 9698). New Mode S transponders
are expected to be able to use this technique for future
applications.
DOWNLINK
AIRCRAFT PARAMETERS
Detailed
technical definitions of all the parameters are contained in
the ICAO Annex 10, Vol. IV, Chapter 3 and the ICAO manual on
Mode S Specific Services (ICAO DOC
9688).
The
specific requirements for down link aircraft parameters (DAPs)
related to the Basic
Functionality Elementary Surveillance are classified as
follows: 24-bit aircraft address
• SSR Mode
3/A Flight Identity (callsign used in flight), Ref. ICAO Doc
9688 (BDS 2.0). Transponder Capability Report, Ref. ICAO Doc
9688 (BDS 1.0 and BDS 1.7) Altitude reporting in 25ft
intervals
• Flight
Status (Airborne/on the ground) Ref. ICAO Annex 10 Vol. IV
Chapter 3 Para. 3.1.2.8.6.7 The specific requirements for DAPS
related to the Functionality Enhanced Surveillance are
currently defined as follows; but are still under
investigation: Enhanced Surveillance Functionality Ref. ICAO
Doc 9688 (BDS 1.0 and BDS 1.7 and BDS 2.0 and BDS 5.0 and BDS
6.0) are currently defined. Basic Functionality Elementary
Surveillance with the addition of: Magnetic Heading Speed
(lASAAS/Mach No) Roll Angle Track Angle Rate Vertical Rate
(Barometric Rate of climb/descents or preferably baro-inertia)
True Track Angle Ground Speed
•
Selected Altitude Selected Altitude Rate
•
Selected Magnetic Course/Magnetic
Heading
• Selected Air Speed/Mach No.
SURVEILLANCE
REQUIREMENTS, AS A FUNCTION OF AIRSPACE
The
performance of a surveillance system necessary for its use in
given area is to be defined by the responsible authorities.
Their decisions will be based on operational requirements and
the type of airspace to be covered by this system. In general,
there are three types of airspace:
a.
Remote Areas: This type of airspace today is not covered by
any surveillance system. Since there would be an attempt to
use any available surveillance data, provisions have to be
made to ensure a reliable correlation, data integrity, etc.
The performance necessary to support surveillance applications
(e.g. airborne separation assurance) has to be investigated.
Nevertheless, it must be stated that some form of validation
will likely be necessary if a dependent system is used for
surveillance purposes.
b.
Transition Area This type of airspace today is partially
covered by a means (e.g. single radar coverage) to provide
surveillance, e.g. offshore to oceanic areas. Operational and
technical procedures are necessary to allow and support the
transition of aircraft from one airspace to another. Even if
this might be considered a local issue, guidelines are
necessary to allow and improve operation of adjacent systems.
Validation and, at least, fall back surveillance will be
required in this airspace; and
c. High Density Airspace This type of
airspace is usually controlled using multiple radar coverage
i.e terminal and enroute. Redundancy is provided to prevent
the system from reduced performance in case of failure. This
guarantees the same level of safety in these areas, the future
surveillance system will need to provide the same level of
performance, validation and back up.
GENERAL
Africa
is not covered adequately by Radar ground Stations, therefore
ACAS encounters are not monitored or recorded. Rather our data
are based on researches conducted in the USA, Europe and
Japan. All these reports on ACAS encounters in Africa are from
Pilots flying international routes with modern wide-bodied
aircraft. These Pilots have been trained on the
following:
1.
Theoretical concept of ACAS
2.
Operational Concept of ACAS
3.
Practical Use of ACAS In most cased of ACAS encounters
report, the ATCOS receiving such reports do not understand the
implications of infringements being referred to by the Pilots.
On some other occasions, especially in the holding pattern -
Pilots reporting vacating an assigned level for a lower one
usually descend at a rate uncoordinated with those below her
thereby creating proximity hazards unknown to the procedural
ATCOS who has no radar to verify what is happening but would
rather like to expeditious but unsafe.
SIDS
AND STARS
The SIDS and STARS commissioned for Lagos
Airspace in 1977 should be completed to reduce the stress and
number of near collisions and Airmisses. Around 1986, Lagos
ACC was established yet till date it has not been fully
functional since it lacks the manpower, airways, communication
and other essential facilities. The workload on the Approach
Controller in Lagos has been very high in the past twenty
years. A situation where management cannot objectively defined
the core professional in Air Traffic Services will not
guarantee growth and success.
RECOMMENDATIONS
1.
Training of ATCOS on ACAS system and operations should
commence immediately with staff at busy terminals in Lagos,
Abuja, Kano, Port Harcourt and
Maiduguri.
2.
Radar stations in Lagos, Kano, Abuja and Port Harcourt should
be reactivated as a short-term measure for monitoring close
encounters between aircraft.
3.
NAMA management should commence installation of modern radar
system with Mode S capabilities at busy ATC terminals without
further delay.
4.
ATS Headquarters Directorate should as a matter of priority
arranged cockpit flights for ATCOS on international carriers
fitted with ACAS II to enable them experience the
contributions of ACAS to safety by observing the cockpit
Visual Display Unit, Pilot response and aircraft
reaction.
5. All ATCOS should be programmed for
exchange visit to modern ATC facilities such as West Drayton
in UK, Eurocontrol Centre in Brussels and ACC in Atlanta to
enable them have knowledge of ATC development
worldwide.
CONCLUSION
1.
Discussions on ACAS are generally technical especially in
airspace like the Kano FIR, which lacks radar service,
automation, computerisation and modem
communication.
2.
The presentation is essentially an introductory part of
training ATCOS as recommended by ICAO during the introduction
of ACAS. NAMA management should be applauded for having
provided an opportunity within the first six months of its
existent to exposing ATCOS to discourse on ACAS. However, a
more detailed programme in the simulator or civil aviation
school should be organised on regular
basis.
3.
The purposes of this presentation would have been achieved if
the recommendations mentioned above were
implemented.
