Mode S Technology
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
[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
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