Mode S Technology
Interview with Paul Drouilhet, 10/29/00
The "biggest effect that [GPS Squitter] will have is the ability for aircraft
to know where they are, who's around them, and who's out there...." Right
now aircraft are "totally dependent on ground stations to keep them
separately." It "opens up a whole new way of possibilities" for managing
aircraft in the sky.
There are other Europeans experimenting with ADS-B, but using three
different alternatives: GPS, UAT (Universal Access Transponder), and VDL
Mode (Very High Data Link, developed by the Swedish). GPS and UAT use
frequencies above 1000 MHz, but VDL uses the VHF frequency bands.
In DABS, selective interrogations can be used (hence the term Mode
S). Each aircraft has a unique address and broadcasts it every so often,
sort of "like a flashing light....every once in a while, it sends out
saying I am here, here is my address code. "If another aircraft hears
that, then it will send that aircraft...an interrogation with that
The GPS squitter "is an expansion of the signal." "What
has changed is that GPS Squitter "adds to the broadcast, GPS
info...the previous Mode S squitter [just] broadcasts altitude." "The
GPS Squitter has taken [Mode S], added more bits and in those bits,
transmit information as derived from GPS."
"It's a complicated question where [the original idea] can
be traced...you see in the cartoons as suddenly the light goes off."
"There was a lot going on in ways to improve aircraft safety, so I
can't honestly say..." "I was very involved with Mode S...and quite
familiar with Mode S, so I was the first to say that we have a signal
that is ready-made but when and where it was invented is a mystery."