Mode S Technology
Interview with Steve Bussolari - 11/3/00
In 1984, the group began conducting experiments at oil platforms off
the coast of the Gulf of Mexico. They had gone down to Baton Rouge to
one of the Helicopter Safety Air Controllers (HSAC) quarterly meetings
and "briefed them on the technology." HSAC agreed to help conduct a
helicopter-to-helicopter test and provided one of their helicopters,
while the LL researchers leased another helicopter to use.
Bussolari noted that the principal challenge in air traffic control
design is "to work in the existing system." In the case of the GPS
Squitter, the challenge was to share the channel with the ATCRBS
system. The "principal strength is that you don't use spectrum",
since it uses the same frequency band for transmitting and receiving
signals. Since TCAS already has a 1090 MHz receiver, the design for
GPS Squitter is intended to be "seamless."
"Cost for delay is high (5%), so if you can save a little bit of
delay, that's enormous savings." "Where the info will be useful is for
aircraft to be able to see each other, [so] you can preserve your
operations even in bad weather." Right now pilots have to rely on
"1090 [MHz] is the most loaded channel." "ATCRBS does 15-20 planes
in a beam sweep...it's the replies...these are called fruit (false
replies uncorrelated in time)....it's that high fruit environment"
which causes such heavy traffic. "Frankfurt has a lot of military
aircraft...before the Berlin Wall came down, all the Allied and Russia
aircraft were [constantly] interrogating their borders."
Many years ago, the FAA did try to mandate that Mode S transponders be
installed in brand new transponders. "What they failed to do was
deploy Mode S radar stations...so they were on very weak ground to be
forcing the general aviation community" to along with their plan. "It
polarized the community" and many people said "we don't want Mode S"
and became very afraid of Mode S.
Currently, Mode S transponders are still more expensive than ATCRBS
transponders. "The waveforms of Mode S transponders require much
tighter control," and the cavity oscillators in ATCRBS "tend to droop"
so they cannot be used for Mode S. Mode S, in addition, is a
"solid-state front end" which has both processing and software. The
development costs for the processor and software are quite high, so
Bussolari believes the FAA "needs to adopt a common chipset with the
same processor and a common chipset and the same software" to help
lower the cost. "It's a perfect function of government, [but] not
welcome by manufacturers [such as Honeywell and Allied Signal] because
[they] don't want to be undercut."
"We're doing air traffic control in a very manual process. San
Francisco is an example where PRM will help. You have to look at
where what am I trying to accomplish here? What paradigm do I need to
shift to? What technology do I need to get there?" To think in terms
of the technology is a "dangerous trap that we as technologists fall
"At the end of the war, the focus was Soviet bombers coming
over the pole...what was needed was a system of radars on the system
and radar interceptors...in order to do that, it was the digital
computer. Lincoln built an air defense system...and was asked by the
Department of Defense as an outgrowth. Lincoln Laboratory focus in
1951 sole job was Project Lincoln called the SAGE system." The notion
that "radars [are] linked by a digital computer" is "no different than
ATC." Lincoln's strength was "building and showing that it worked."
A company was spunoff MITRE to do "contract monitoring" because MIT
did not want to be responsible for maintaining the air traffic control