  We report on an analysis of the drifting subpulses of PSR B0320+39 that
indicates a sudden step of ~180 degrees in subpulse phase near the centre of
the pulse profile. The phase step, in combination with the attenuation of the
periodic subpulse modulation at pulse longitudes near the step, suggests that
the patterns arise from the addition of two superposed components of nearly
opposite drift phase and differing longitudinal dependence. We argue that since
there cannot be physical overlap of spark patterns on the polar cap, the drift
components must be associated with a kind of ``multiple imaging'' of a single
polar cap ``carousel'' spark pattern. One possibility is that the two
components correspond to refracted rays originating from opposite sides of the
polar cap. A second option associates the components with emission from two
altitudes in the magnetosphere.
