mode-s.ppt
DESCRIPTION
sTRANSCRIPT
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 newsystem 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 theDiscrete 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 Sgroundstation
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
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?