  The phenomenon of subpulse drifting offers unique insights into the emission
geometry of pulsars, and is commonly interpreted in terms of a rotating
carousel of "spark" events near the stellar surface. We develop a detailed
geometric model for the emission columns above a carousel of sparks that is
entirely calculated in the observer's inertial frame, and which is consistent
with the well-understood rotational effects of aberration and retardation. We
explore the observational consequences of the model, including (1) the
appearance of the reconstructed beam pattern via the cartographic transform and
(2) the morphology of drift bands and how they might evolve as a function of
frequency. The model, which is implemented in the software package PSRGEOM, is
applicable to a wide range of viewing geometries, and we illustrate its
implications using PSRs B0809+74 and B2034+19 as examples. Some specific
predictions are made with respect to the difference between subpulse evolution
and microstructure evolution, which provides a way to further test our model.
