Mode S Technology
Interview with Loren Wood - 11/7/00
Mode S normally is a data link that encodes a 56-bit message composed of a
5-bit message type, 3 bits of the characteristic of the Mode S link, and
the 24 bits of plane id, and 24 bits of parity. The idea is that you
could put latitude and longitude into the 112-bit extended message. He
cites Paul Drouilhet as the one who came up with the idea and says he was
"sort of a genius."
His involvement was to determine what kind of Mode S signals would
propagate on an airplane on the ground. If you "put an antenna on the
ground station, the signal comes from four different directions." They
did a lot of experiments to evaluate the effects of multipath
reflection, his background being in assessing interference, tracking,
and multilateration (receiving three different signals and calculating
distance of arrival).
Compact Position Reporting (CPR): The idea is that "if you want a
certain accuracy and you want to cover the entire globe", you soon
find out that 34 bits to describe latitude and longtitude (17 bits
each) would not be enough. So the way to do is "partition the world"
with this limitation in mind while "throw[ing] away the high order
bits [and] retaining the low order bits." "Now there's an
ambiguity...we know where we are but we don't know where the other
guys is...but we have some idea where he is..." and to do some
algorithms to pinpoint his location.
MITRE had handed all the code over to him. "MITRE sent all this code and
data structures and subroutines...it was a compiling program..." He had
about 6 months to work on it and had to understand 25,000 lines of
code. He spent most of the time trying to decipher it and managed to
figure out what they were doing wrong. "I could see it was
trackable...then reduced it in my mind" to simple algorithms. "Most
people said I wasn't getting anywhere..." because it was only the last two
weeks when he finished in time.
Anyhow, he mentioned that the "technical is about 5%....all the other
factors are litigated as if they were technical..."