The Story of Mode S: An Air Traffic Control Data-Link Technology

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The Story of Mode S

• 6.933 - Fall 2000
• Emily Chang, Roger Hu, Danny Lai, Richard Li,
  Quincy Scott, Tina Tyan

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Introduction
Introduction
Background
Mode S Design
Aftermath
Conclusion
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The Project History

• Traces the history of Mode S (1968-1975), an air
  traffic control technology developed by Lincoln
  Labs

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Our Focus
Aviation Community Influences
Mode S Design Decisions
Key Concern Interoperability

• Theme Successful technologies are not developed
  in isolation
• Key example Interoperability with the existing
  system drove the design of Mode S

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Our Focus
Aviation Community Influences
Mode S Design Decisions
Key Concern Interoperability

• Theme Successful technologies are not developed
  in isolation
• Key example Interoperability with the existing
  system drove the design of Mode S

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Our Focus
Aviation Community Influences
Mode S Design Decisions
Key Concern Interoperability

• Theme Successful technologies are not developed
  in isolation
• Key example Interoperability with the existing
  system drove the design of Mode S

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Our Focus
Aviation Community Influences
Mode S Design Decisions
Key Concern Interoperability

• Theme Successful technologies are not developed
  in isolation
• Key example Interoperability with the existing
  system drove the design of Mode S

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Scope of Research

• Lincoln Labs - interviewed researchers and
  project leaders, read over 40 technical reports
• FAA - interviewed current and past administrators
• General Aviation - contacted AOPA communications
  dept. and other spokespeople
• Read Air Traffic Control history books, magazine
  articles, and web sites

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Background
Introduction
Background
Mode S Design
Aftermath
Conclusion
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Early Air Traffic Control
The current choking of the federal airways and
traffic control systemswas forecast in
detail...during the past decade. But nobody
really did anything about it. - Robert Hotz,
editor, Aviation Week (1968)
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Addressing the Problem

• Newly-formed Department of Transportation (1967)
  wanted reassessment of Air Traffic Control
• Formed the Air Traffic Control Advisory Committee
  (1968)
• decided old system, the Air Traffic Control Radar
  Beacon System (ATCRBS) was inadequate
• made several recommendations for a new system

When new blood takes over, the FAA...seeks
new rules and regulations, different licensing
procedures, and heaped-on layers of government
control. - Max Karant, AOPA Pilot founding editor
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Meanwhile...

• Herb Weiss, head of Lincoln Laboratorys Radar
  Division, flew regularly between Boston and D.C.
• Flights were often delayed, especially in bad
  weather
• He pushed for funding to examine ways to improve
  air traffic control (1968)

I knocked on the door of the FAA and kind of
introduced myself. - Herb Weiss, LL
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Mounting Pressure
DoT Forms
FAA Reorganizes
Reassessment of ATC
Development of New ATC Technology
LL Defense Budget Cuts
LL Interest in Non-Military
Vietnam War
FAA Budget Cuts
Controller Overwork
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Combining Forces
LL Expertise in ATC (SAGE, Radar, Communications)
ATCAC Research and Recommendations
Opportunity for Collaboration
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The LL ATC Group

• Small group (5-6) recruited from different parts
  of LL, led by Paul Drouilhet (1970)
• Charter prove that a newsystem could be
  completelyinteroperable with existing ATC
• Initially, FAA provided littlefunding and a
  short timeframe

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Why Interoperability?

• Hard to achieve 100 penetration at once
• Ground stations also take time to deploy
• Every aircraft in an airspace needs to be tracked
• Have to make sure that a hybrid system will allow
  this to happen

With air traffic control technology, there is no
instantaneous reset. - Jonathan Bernays, LL
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Super Beacon
Mode S

• FAA and LL started theDiscrete Address
  BeaconSystem (DABS) project,later renamed Mode
  S
• Enable two way ground-airdata transmission
• S Select Uses discreteaddressing to
  interrogatejust one aircraft

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Mode S Design
Introduction
Background
Mode S Design
Aftermath
Conclusion
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The Players

• MIT Lincoln Laboratory (Lincoln Labs)
• Federal Aviation Administration (FAA)
• General Aviation community
• Aircraft Owners and Pilots Association (AOPA)
• Other parties commercial and cargo airliners,
  military, transponder companies

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Overview
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Mode S
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Interoperability Issues

• Transparency Mode S must not break existing
  systems
• Backwards-compatibility Existing systems must
  still see Mode S equipped planes

other aircraft
existing ground station
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Frequency

• New frequency difficult to allocate
• Same frequency as old system (1030/1090 MHz)
  interoperable, but may cause interference

300 MHz
3000 MHz
1030 MHz
1090 MHz
UHF
VHF
SHF
The neatest technical solution would have been
to put it on its own frequency band. - Paul
Drouilhet, LL
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Sharing Frequencies

• Find an invisible signal
• experiment with different signal characteristics
• Interoperability both systems share the same
  channel without causing problems to each other

MHz
1030 (interrogation)
1090 (reply)
1000
1120
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Transponders
There seemed to be a very strong correlation
between cost and consistency of the
transponder....the cheaper ones were all over
the place....'' - George Colby, LL

• Flaw in FAA National Standarddoesnt specify
  what ATCRBStransponders should not do
• 549 transponders on the market
• Each had unique behavior

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The Hack

• Existing ATCRBS transponders used sidelobe
  suppression

INTERFERENCE!!!
ground station
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The Hack

• Existing ATCRBS transponders used sidelobe
  suppression

P1 main lobe
side lobe
ground station
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Hacking the Hack

• Purposely send a small P1 and large P2
• Disables ATCRBS transponders
• Use the time to cram in Mode S data blocks
• Limited number of bits can be sent in this window

P1
P2
Mode S data block
35 microseconds
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Mode S Design
INTEROPERABILITY
Frequency Choice
Signal Design
Transponder Sensor Design
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Aftermath
Introduction
Background
Mode S Design
Aftermath
Conclusion
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Slow Adoption

• Lincoln Labs spec delivered to FAA in 1975, first
  commercial transponder manufactured in 1980
• FAA slow to install Mode S ground stations, but
  still tries to mandate it being used

The spec we wrote went to the FAA in 1975they
went to study itwe call this the handholding
period, where a couple individuals stayed onboard
to advise the FAA... - Thomas Goblick, LL
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What Changed Things

• Mid-air collision in 1986
• Congress passes a law mandating that all
  commercial aircraft be equipped with a Traffic
  Collision and Avoidance System (TCAS) by 1993
• TCAS uses Mode S
• TCAS is now an international standard
• Mode S technology is now commercially available

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Mode S Today

• 108 of the U.S.s busiest airports have Mode S
  ground stations
• Majority of aircraft landing at these airports
  have Mode S transponders
• Without Mode S, the 1030/1090 Mhz band would be
  completely overloaded
• Mode S used in TCAS and many other applications

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Conclusion
Introduction
Background
Mode S Design
Aftermath
Conclusion
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What We Learned

• Its all about INTEROPERABILITY!
• Aviation community is conservative
• Interoperability allows long transition periods
• Interoperability allows a system that everyone
  can use, since there wont be 100 compliance
• Interoperability had an effect on almost every
  design decision

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The Big Picture
Successful technologies are not developed in
isolation.
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Comments?
Questions?
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