A wide range of ion species and velocity distributions are expected to be found as the Giotto spacecraft traverses the coma of Halley's Comet. The outer coma is characterized by the interaction between solar wind and cometary plasmas, the inner coma by the outflow of cometary neutrals and their ionization products. The resultant demands on instrument dynamic range preclude use of a single sensor for measurements of ion composition. The Giotto Ion Mass Spectrometer (IMS) therefore consists of two sensors: one optimized for the outer and the other for the inner coma, with each obtaining complementary information in the region for which it is not optimized. Both sensors feature mass imaging characteristics, thereby permitting simultaneous measurements of several ion species by means of multi-detector arrays. The prime objective of the High-Energy Range Spectrometer (HERS) is to measure the ion abundances and three-dimensional velocity distributions outside the cometary contact surface.