Mode S Technology
Interview with George Colby - 11/19/00
Summary: Had to design a waveform that was compatible with ATCRBS.
Discrete addressing would cut down on traffic. Knew that Lincoln Lab
had to build a cheap transponder for the general aviation community.
In 1970, Lincoln Laboratory decided to pursue research work with the
FAA. They took one of the divisions run by Herb Weiss. Under him was
Paul Drouilhet, who was told to build a group. George Colby was the 8th
person to join that group. Unless most people, he came from industry
rather than from Lincoln Lab.
His first effort was to do a characterization of ATCRBS transponders.
They built an airborne measurement facility (AMF) that one could fly
around and map signals. They visited different local airports and
issued a report. Then the development turned to collision avoidance
One of the considerations was the design of an appropriate
waveform, since it had to be compatable with the ATCRBS waveforms.
"At that time the projection for air traffic was growing exponentially
and there was a fear that the air traffic control system would just
become simply overloaded." They felt that discrete addressing would
"cut down on the traffic."
His specialty was in hardware design. He designed and built the
transponder. His group knew that any system had to be affordable to
general aviation. There were already quite inexpensive conventional
ATCRBS transponders, but "there semeed to be a very strong correlation
between cost and consistency of the transponder....the cheaper were all
over the place in terms of frequency and stability."
"As I recall, personal concern of mine was to keep power consumption
down...just a matter of efficient circuit design so that we didn't get a
huge battery drain." The problem didn't quite matter nearly much to
commercial airlines, but battery drain was a big deal for general