| Mode S In London
Terminal Control Airspace
Bill Casey & Adrian Price - NATS (UK)
Note: This article has been updated in June 2006
1. INTRODUCTION AND BACKGROUND
NATS provides Air Traffic Control (ATC) services to aircraft flying in
UK airspace, and over the eastern part of the North Atlantic. The
London Terminal Control Centre (LTCC) is based at West Drayton and
is responsible for traffic below 24,500 feet arriving at, or
departing from, the five main airports close to London. For a 30
Year period NATS, along with every other ATC service provider,
relied on Secondary Surveillance Radar (SSR) to provide aircraft
information. During that time, traffic levels have increased
dramatically and it became vital to exploit new technology, which
addressed the limitations of SSR. In December 2005 NATS introduced
the next generation of SSR technology called Mode Select (Mode S)
into the LTCC (References 1 and 2). The purpose of this article is
to give a short insight into Mode S, its development, functions, and
its operational introduction.
2. TRADITIONAL SSR
The previous Mode A/C SSR technology worked on a general ‘broadcast
and receive’ principal that has recognised problems and limitations.
The integrity of Mode A/C surveillance can be adversely affected by
garbling of replies from aircraft close together (especially in
stack airspace). In addition, Mode C replies have no encoded
identity and rely on the association to the correct target by the
ground system processing. These acknowledged limitations can affect
the integrity of the information displayed to ATC.
3. MODE S TECHNOLOGY
There are two levels of Mode S, Elementary and Enhanced. Elementary
allows selective interrogation of aircraft providing the potential
to eliminate, amongst other things, the synchronous garbling of
replies. Enhanced Mode S enables the selective interrogation of
aircraft plus the facility to down-link specific airborne parameters
from the cockpit, such as Indicated Airspeed and Selected Altitude.
4. BENEFITS OF MODE S TECHNOLOGY
Improved integrity of radar surveillance and the availability of Mode
S Down-Linked Airborne Parameters (DAPs) have enabled NATS to
develop controller support tools that have provided benefits to the
safety and efficiency of ATC operations. A three year development
programme has culminated in a system where the display of aircraft
information is easily assimilated and understood. The Mode S
tool-set consists of two main elements: the Vertical Stack List (VSL)
and the display of DAPs in the Target Label.
5. SAFETY BENEFITS
For Enhanced Mode S aircraft the Selected Altitude entered by the crew
into the Mode Control Panel (MCP) or Flight Control Unit (FCU), that
feeds into the aircraft’s auto pilot system, can now be down-linked
and displayed to ATC. Mode S Selected Altitude is potentially one of
the most useful pieces of data to prevent Level Busts as it provides
the opportunity of alerting ATC if there has been any
misinterpretation of the altitude/level clearance. Aircrew should be
aware that a controller may challenge that reading if there is any
doubt over the cleared level.
6. DISPLAY OF SELECTED ALTITUDE
Operational experience to date indicates that the facility to display
Selected Altitude has helped controllers to intervene in situations,
which might have otherwise led to the erosion of standard
separation.
It is accepted that whilst the display of selected altitude is an
obvious safety enhancement, aircraft label overlap and human
workload limitation will affect the controllers’ ability to check
the Selected Altitude every time a vertical clearance is issued.
Time delays incurred whilst information is input into the MCP/FCU
reduce the controllers’ ability even further. Therefore, the
requirement for aircrew to read-back all clearances and for
controllers to check the read-back will not be affected by
introduction of Mode S and the display of Selected Altitude.
7. TARGET LABEL
Mode S information has been displayed in the aircraft Target Labels of
suitably equipped aircraft, on all LTCC sectors since December 2005.
Fig 1 shows the Target Label of BMA3XF. The Mode C readout and
intention (or destination) code shown in line 2 is unchanged from
the previous format. (LL represents a flight inbound to Heathrow.)
As well as traditional information, the Mode S Target Label also
displays aircraft DAPs.
Fig 1: Mode S information in the Target Label -
click for larger image
The MCP/FCU Selected Altitude is displayed in line two in the dark
orange colour to distinguish it from the Actual Flight Level (Mode
C). In Fig 1 BMA3XF has selected 15000 feet and is passing Flight
Level 165.
Many pilots will select the missed approach altitude once the aircraft
is established on final approach. The Selected Altitude is
automatically removed from the target label before this occurs to
avoid any confusion. If the system detects that the aircraft has
broken off the approach and is climbing then the Selected Altitude
will automatically be displayed again providing the controller with
the aircraft’s revised intention.
Down-linked Mode S parameters such as Ground Speed, Indicated Air
Speed, and Magnetic Heading can also be displayed in line 3 of the
Target Label either individually, or in combination using a
‘quick-set display’ facility. In Fig 1 Magnetic Heading has been
selected showing 135 degrees (prefixed with an ‘H’ to denote
Heading).
8. VERTICAL STACK LISTS
The most innovative function in the Mode S Tool Set is the VSL. It has
been designed to compliment and support existing operations by
providing controllers with enhanced vertical situational awareness
in busy stack airspace.
Fig 2: Vertical Stack List -
click for larger image
Fig. 2 shows a normal radar picture of the Bovingdon Hold on the left
together with an enhanced view of the same hold as depicted in the
Vertical Stack List on the controller’s display. By utilising the
integrity of Mode S derived altitude reports, the tool provides a
vertical representation of aircraft in stack airspace. So, by
monitoring the VSL, the controller can continuously see call-signs
and occupied levels in the stack, even when SSR labels overlap on
the radar display.
In the VSL, the white numbers in the column on the left-hand side of
the VSL are the Flight Levels that are selected for display by the
controller. The call-sign and Mode C (height derived via Mode S) of
the holding aircraft are shown in green in the middle two columns
and the final column shows the aircraft’s Selected Altitude.
The system tracks arrivals to each of the four Heathrow stacks.
Population to and exit from the vertical Stack Lists is automatic
with the aircraft descending in the VSL as the controller issues
descent instructions. As the aircraft continues its descent, its
progress is automatically updated in the list and the aircraft is
removed when it meets defined criteria. The system applies standard
ATC rules regarding level allocation and departure. Through the use
of the VSL, controllers no longer need to confirm via RT that levels
have been vacated. Thus, RT congestion is reduced and stack
management is enhanced.
9. COCKPIT PROCEDURES
The cockpit procedures for setting the MCP/FCU are vital in realising
the potential safety benefits of introducing Mode S. Fig 3 shows an
example of a typical Mode Control Panel.
Fig 3: Typical Mode Control Panel showing selected altitude of 23000
The following cockpit techniques will enhance safety in the London
Terminal Control airspace (Ref: 3):
• Upon receipt of an altitude clearance from ATC, immediately set
the assigned/cleared altitude in the altitude select window (except
when established on the ILS).
• Positively confirm the altitude clearance via R/T read-back.
Please note that many operators require the Selected Altitude to be
cross-checked by both pilots; an action that is endorsed by the
Flight Safety Foundation (Ref: 4).
Adherence to the above procedure will maximise the controllers’
opportunity to check the altitude selected in the cockpit.
Controllers have been advised that there is no absolute guarantee
that an aircraft will actually level off at or maintain the Selected
Altitude. For example an aircraft may not adhere to the Selected
Altitude when the autopilot is being used with an incorrect mode or
has suffered a technical problem. An incorrect pressure setting on
the altimeters’ subscale will also invalidate the Selected Altitude.
9. CONCLUSION
The display of Selected Altitude to ATC along with R/T read-back will
confirm the aircrew’s interpretation of the assigned/cleared level.
The ability of a controller to check the Selected Altitude in the
target label and the VSL is dependant upon workload the elapsed time
between the vertical clearance being issued and that value being
displayed back to the controller and the aircraft label being
clearly displayed. Aircrew can assist ATC by promptly entering the
new altitude in the altitude select window every time a vertical
clearance is issued. For Enhanced Mode S aircraft the display on the
controllers’ radar displays of Selected Altitude, together with VSLs,
are potentially a great step forward in safety and the campaign to
reduce level busts.
10. REFERENCES
1. AIC105/2004 (Yellow 155) promulgated by the CAA 11 Nov 2004.
2. AIC49/2005 (Yellow 171) promulgated by the CAA 23 Jun 2005
3. AIC4/2006 (Yellow 187) promulgated by the CAA 5 Jan 2006
4. Briefing note 3.2 issued Year 2000 by the Flight Safety Foundation
Approach and Landing Accident Reduction (ALAR), “Altitude
Deviations”. For more information on the Mode S in the LTCC please
contact:
Mr William Casey
Operational Support & Development
NATS
Porters Way
West Drayton
UB7 9AX,
United Kingdom
Telephone: +44 (0)1895 426356
eMail:
william.casey@nats.co.uk Web:
www.nats.co.uk
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