The Story of Mode S

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

Introduction

Mode S Design

Aftermath

Conclusion

Introduction

Background

The Project History

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

Our Focus

AviationCommunityInfluences

Mode SDesign

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

Our Focus

AviationCommunityInfluences

Mode SDesign

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

Our Focus

AviationCommunityInfluences

Mode SDesign

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

Our Focus

AviationCommunityInfluences

Mode SDesign

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

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

Background

Mode S Design

Aftermath

Conclusion

Background

Introduction

Early Air Traffic Control

“The current choking of the federal airways and traffic control systems…[was] forecast in detail...during the past decade. But nobody really did anything about it. ” - Robert Hotz, editor, Aviation Week (1968)

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]...seek[s] new rules and regulations, different licensing procedures, and heaped-on layers of government control.” - Max Karant, AOPA Pilot founding editor

Meanwhile...

• Herb Weiss, head of Lincoln Laboratory’s 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 knock[ed] on the door of the FAA and kind of introduced myself.” - Herb Weiss, LL

Mounting Pressure

VietnamWar FAA

Budget CutsControllerOverwork

LL DefenseBudget Cuts

LL Interest inNon-Military

DoTForms

FAAReorganizes

Reassessmentof ATC

Developmentof New ATCTechnology

Combining Forces

Opportunity for Collaboration

LL Expertise in ATC(SAGE, Radar,

Communications)

ATCAC Researchand Recommendations

The LL ATC Group• Small group (5-6) recruited from different parts of

LL, led by Paul Drouilhet (1970)• Charter: prove that a new

system could be completelyinteroperable with existing ATC

• Initially, FAA provided littlefunding and a short timeframe

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

Super Beacon• FAA and LL started the

Discrete Address BeaconSystem (DABS) project,later renamed Mode S

• Enable two way ground-airdata transmission

• S = Select: Uses discreteaddressing to interrogatejust one aircraft

Mode S

Mode S DesignMode S Design

Aftermath

Conclusion

Background

Introduction

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

Overview

interrogation

reply

Mode S

Interoperability Issues• Transparency: Mode S must not break existing systems• Backwards-compatibility: Existing systems must still see

Mode S equipped planes

existinggroundstation

other aircraft

Mode Sgroundstationnew signal

existing signal

Mode S equipped

Frequency

• New frequency: difficult to allocate• Same frequency as old system (1030/1090

MHz): interoperable, but may cause interference

“The neatest technical solution would have been to put it on its own [frequency] band.” - Paul Drouilhet, LL

UHF300 MHz 3000 MHz1030 MHz 1090 MHz

VHF SHF

Sharing Frequencies

• Find an “invisible” signal– experiment with different signal characteristics

• Interoperability: both systems share the same channel without causing problems to each other

1030(interrogation)

1090(reply)

1000 1120

MHz

Transponders

• Flaw in FAA National Standard:doesn’t specify what ATCRBStransponders should not do:– 549 transponders on the market– Each had unique behavior

“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

The Hack

- Existing ATCRBS transponders used sidelobe suppression

side lobegroundstation

aircraft 1

P1 main

lobe

aircraft 3

aircraft 2

INTERFERENCE!!!

P2

The Hack- Existing ATCRBS

transponders used sidelobe suppression

P1 main

lobe

side lobegroundstation

aircraft 1

aircraft 2

aircraft 3

P1 P2A2

P2P1A1

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

Mode S Design

INTEROPERABILITY

FrequencyChoice

SignalDesign

Transponder &Sensor Design

Aftermath

Mode S Design

Aftermath

Conclusion

Background

Introduction

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 1975…they went to study it…we call [this] the handholding period, where a couple individuals stayed onboard [to advise the FAA]...” - Thomas Goblick, LL

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

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

ConclusionConclusion

Mode S Design

Aftermath

Background

Introduction

What We Learned

• It’s all about INTEROPERABILITY!• Aviation community is conservative

– Interoperability allows long transition periods– Interoperability allows a system that everyone can use,

since there won’t be 100% compliance• Interoperability had an effect on almost every

design decision

The Big Picture

Successful technologies are not developed in isolation.

AviationCommunityInfluences

Mode SDesign

Decisions

Key Concern:Interoperability

Comments?Questions?