In article ,
Robert Ehrlich wrote:
Eric Greenwell wrote:
...
In any case, the bearing info appears to be determined by using
two or more antennas.
...

More than 2 are necessary. With 2 antennas, you can only measure
the time difference between the 2 received signals. This time
difference can be translated into a distance difference. The points from
where a given difference is observed are on an hyperbola, which
can be considered as the same as its 2 asymptotes, as the distance
to the antennas is high compared to their mutual distance. But
to determine which of the both asymptotes is the correct one, you
need some more information, i.e. another antenna.

Well, the mode C height information makes this 2 points, and
if you turn the aircraft a few degrees, and assume the points are
(relatively) stationary, you can distinguish between the two points,
right?

The mode C is surely there, but the second part seems
complex. They must do it some other way...