The Story of Mode S Fall 2000 Emily 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 ( ), an air traffic control technology developed by Lincoln Labs

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

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

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

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

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 Vietnam War FAA Budget Cuts Controller Overwork LL Defense Budget Cuts LL Interest in Non-Military DoT Forms FAA Reorganizes Reassessment of ATC Development of New ATC Technology

Combining Forces Opportunity for Collaboration LL Expertise in ATC (SAGE, Radar, Communications) ATCAC Research and 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 completely interoperable with existing ATC Initially, FAA provided little funding 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 Beacon System (DABS) project, later renamed Mode S Enable two way ground-air data transmission S = Select: Uses discrete addressing to interrogate just one aircraft Mode S

Mode 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 existing ground station other aircraft Mode S ground station new 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 UHF 300 MHz3000 MHz 1030 MHz1090 MHz VHFSHF

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) MHz

Transponders Flaw in FAA National Standard: doesn’t specify what ATCRBS transponders 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 lobe ground station aircraft 1 P1 main lobe aircraft 3 aircraft 2 INTERFERENCE!!!

P2 The Hack -Existing ATCRBS transponders used sidelobe suppression P1 main lobe side lobe ground station aircraft 1 aircraft 2 aircraft 3 P1P2 A2 P2P1 A1

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 P1P2Mode S data block 35 microseconds

Mode S Design INTEROPERABILITY Frequency Choice Signal Design 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

Conclusion 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. Aviation Community Influences Mode S Design Decisions Key Concern: Interoperability

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