Mode S Technology





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Second Interview with Paul Drouilhet - 11/8/00

Summary: Talked about how ATCRBS has a problem with interference. TCAS can see ATCRBS planes, but can't communicate with them. Lincoln Lab was a technical advisor for the FAA. Squitter has a easier transition path but UAT has greater ultimate capability. ADS-B will be the center of ATC in the future.

  • Talked about the early stages of air traffic control, starting with ATCRBS, and mentioned that ATCRBS is not a bad system when used with directional antennae because interference remains low, but because aircraft use omnidirectional antennae, ATCRBS was not suited to air-to-air communication. Following was the development of the superbeacon, which became known as DABS, and then mode-S (S for selective). The British had been developing a similar technology, and mode-S was a result of their combined work. Mode S became a worldwide standard for airliners if not for general aviation.

  • In parallel, some researchers were working on "air to air collision avoidance" with the idea of using the ATCRBS modes together with new technology rather than a new system with a new frequency. [This part he said as a response to a question I asked later but it fits] According to Drouilhet, collision avoidance was a goal developed very early on, predating any free flight ideas. The goal of a collision avoidance system was to take care of mistakes in air traffic control, if radar fails, etc. and prevent collisions between radar-controlled aircraft and non-radar-controlled aircraft, generally airliners and small private planes. It was important that a new technology for collision avoidance be able to see aircraft which did not use radar, but were transponder equipped.

    [Back to the history of ATC...] TCAS was developed in conjunction with MITRE, and TCAS can see ATCRBS aircraft but can't communicate with them. Problems with mode S: The FAA was slow to implement ground stations for mode S. Also, if you didn't know an airplane was there to begin with, as well as its call sign, you didn't know how to interrogate it. This led to the development of the "squitter" technology. [Drouilhet doesn't remember how the name squitter came about.] With squitter, aircraft spontaneously transmit signals that look like mode-S replies but actually aren't replying to anything. The next step was ADS, about ten years ago GPS technology became available at an affordable price, and the idea of using it in air traffic control occured to several people simultaneously/separately: if an aircraft would broadcast location in addition to identification information it could be extremely useful. Lincoln Labs had the idea of putting the signal into existing equipment - the mode-S transponder. This required switching from 64 bits to 128 bits, but this was mostly a firmware change.

  • There are several ways to use GPS information that are available: Mode S squitter, UAT - similar but using a higher frequency and a new dedicated system which was developed at MITRE, and SD TMA developed in Sweden which uses 130 MHz TDMA, resulting in a lower data rate. Also, aircraft need to be assigned time slots by some central authority.

    [Question about requirements from FAA]

  • Requirements from the FAA were loose, just that they develop a system that allow one aircraft to "see" other aircraft. Instead of being dictated, they are "agreed to" between LL and the FAA. Originally it wasn't clear that the 1090 *could* be used and that interference could be dealt with, but once that was clear it was pursued.

    [Question about Drouilhet's role at LL]

  • Drouilhet was with the air traffic control program when it started at LL. At Lincoln, there are different divisons, usually with 4-8 groups each. A group has about 15-30 people usually. Originally the ATC division had only six people, including Herb Weiss the division leader. In the early days he was involved in technical design (Mode S and TCAS) such as waveform selection. Around 1985 he became assistant director, still in charge of ATC group. The division at that point included all ATC work as well as some DoD stuff.

    [Question about FAA's relationship with LL]

  • The FAA considered LL to be a technical resource. LL had no input into what happened to the results of their development efforts, they just developed things and made recommendations. The FAA is responsible for setting standards and those kinds of things. At LL the engineers are "solving a problem, not building equipment" so requirements are not strict. Often LL understands what needs to be done better than their sponsors. LL doesn't just do a job to requirements, that's a job for industry or manufacturing. There's a tradeoff in doing design between cost/complexity and functionality.

    [Question about competing labs]

  • LL and MITRE are similar in some respects, some overlapping functions. LL deals more with radar and beacons, communication, while MITRE deals with automation, playing a central role in collision resolution logic.

    [Question about issues that prevent technology from being implemented]

  • An additional complication is there is a difference between handheld GPS and the certified equipment. The handheld unit is ~$700-$1000, is very capable, but only "quasi-legal" and has no official status, but pilots have them and use them a lot. However it doesn't have the same standards. Certified equipment costs ~$4000-$10,000 and is required for GPS for instrument navigation, for official use. It has lots of extra stuff; RAIM (receiver autonomous integrity monitoring) where the receiver receives redundant solutions and tests them, only draws conclusions if there are redundant solutions. "Built so they work" is not the same is "built to certification standards"

    [Question about the future of GPS and ATC]

  • It's hard to project the use of ADS-B and the evolution of its use. Parts of the aviation community are excited, cargo airlines (using GPS squitter) and capstone (UAT).

    [Question about whether needs of different users affected development]

  • During development, LL wanted to build a system that everyone *could* use, which is why they took into account economic issues and capacity - so they could accommodate high traffic if need be. For example in the LA Basin, there is a high amount of traffic, this area is considered challenging in the US. LL built a system to handle LA level use, simulated there and the system operated well. However these simulations were questioned by opponents in the aviation community.

  • LL is very integrated with the aviation community, in addition to working with the FAA as a technical advisor, LL interacts with GA and airlines informally and in committees. For example, by participating in RTCA (who sets standards for equipment) committees/studies. Interaction with the aviation community led to changes in technologggy, to make sure technology is acceptable to the community.

    [Question about an example of how interaction with the aviation community produced changes in technology]

  • One example, during mode-S development, the original idea for a superbeacon was on a separate band, but LL felt it was extremely important to build in the same band as ATCRBS for cross compatibility; they were considering GA and airlines.

  • This was repeated in the development of TCAS; a collision avoidance system should also operate on the same band. "The lab has been very cognizant of the techno-political aspects" and "what has a chance of being implemented versus what's the coolest technical solution"

    [Question about predicting the future of GPS, UAT]

  • It's hard to predict - both are being tested, both work. Squitter has a easier transition path but UAT has greater ultimate capability.

  • ADS-B the concept - aircraft broadcasting a precisely defined position - will become a "centerpiece" of ATC. "For what on the surface looks like a highly technical field, aviation is very conservative and very slow moving." Airlines resist if they don't see a direct financial payoff.

    [Question about dangers of ADS-B because of spoofing]

  • Spoofing is not an important issue, but there is one concern and perhaps the solution to both questions is the same. Aviation has always had independent surveillance and navigation systems. If one fails, you have the other; there is some redundancy. ADS-B takes this away; it determines where it is and keeps track of where it is, transmitting this to the ground. This creates the possibility of a single point of failure. For a long time, ATC will maintain some form of radar surveillance, along with ADS-B, even though ADS-B has more precise information.

  • The Story of Mode S: An Air Traffic Control Data Link Technology last modified: 12.06.2000