What does steering angle mean?
Steering angle, according to the text you bolded:


  pointing at perpendicular to the flight heading (90 degrees counterclockwise)


means the steering angle 090 is the angle, relative to the flight heading counterclockwise, that the data sample was taken.

If the flight heading was 345 degrees, 90 degrees counter clockwise (CCW) is 255 degrees.  This steering angle has no aviation context and is just a relative offset from the aircraft heading for data acquisition (which in this case sounds like a radar sample).