The Story of Mode S

2. The Story of Mode S 6.933 - Fall 2000 Emily Chang, Roger Hu, Danny Lai, Richard Li, Quincy Scott, Tina Tyan

3. Introduction Introduction Background Mode S Design Aftermath Conclusion

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

5. Our Focus • Theme: Successful technologies are not developed in isolation • Key example: Interoperability with the existing system drove the design of Mode S Aviation Community Influences Key Concern: Interoperability Mode S Design Decisions

6. Our Focus Aviation Community Influences Key Concern: Interoperability Mode S Design Decisions • Theme: Successful technologies are not developed in isolation • Key example: Interoperability with the existing system drove the design of Mode S

7. Our Focus Aviation Community Influences Key Concern: Interoperability Mode S Design Decisions • Theme: Successful technologies are not developed in isolation • Key example: Interoperability with the existing system drove the design of Mode S

8. Our Focus Aviation Community Influences Key Concern: Interoperability Mode S Design Decisions • Theme: Successful technologies are not developed in isolation • Key example: Interoperability with the existing system drove the design of Mode S

9. 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

10. Background Introduction Background Mode S Design Aftermath Conclusion

11. 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)

12. 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

13. 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

14. 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

15. Combining Forces LL Expertise in ATC (SAGE, Radar, Communications) ATCAC Research and Recommendations Opportunity for Collaboration

16. 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

17. 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

18. Mode S 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

19. Mode S Design Introduction Background Mode S Design Aftermath Conclusion

20. 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

21. reply interrogation Overview

22. Mode S

23. existing signal Mode S ground station Mode S equipped new signal 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

24. Frequency • New frequency: difficult to allocate • Same frequency as old system (1030/1090 MHz): interoperable, but may cause interference 300 MHz 1030 MHz 1090 MHz 3000 MHz VHF UHF SHF “The neatest technical solution would have been to put it on its own [frequency] band.” - Paul Drouilhet, LL

25. 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 1000 1030 (interrogation) 1090 (reply) 1120

26. 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 Standard:doesn’t specify what ATCRBStransponders should not do: • 549 transponders on the market • Each had unique behavior

27. aircraft 1 P1 main lobe side lobe aircraft 3 aircraft 2 The Hack • Existing ATCRBS transponders used sidelobe suppression INTERFERENCE!!! ground station

28. aircraft 1 A1 P1 P2 aircraft 2 aircraft 3 P2 A2 P1 P2 The Hack • Existing ATCRBS transponders used sidelobe suppression P1 main lobe ground station side lobe

29. 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

30. Mode S Design Frequency Choice INTEROPERABILITY Signal Design Transponder & Sensor Design

31. Aftermath Introduction Background Mode S Design Aftermath Conclusion

32. 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

33. 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

34. 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

35. Conclusion Introduction Background Mode S Design Aftermath Conclusion

36. 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

37. Aviation Community Influences Key Concern: Interoperability Mode S Design Decisions The Big Picture Successful technologies are not developed in isolation.

38. Comments? Questions?