Electromagnetic transients have been recorded at the Massachusetts Institute of Technology field station in West Greenwich, Rhode Island, coinciding with sprite-producing lightning flashes in northern Australia, at a distance of 16.6 Mm. Single-station Schumann resonance methods have been used to locate the parent lightning flashes and to evaluate their vertical charge moments. The charge moment thresholds for sprite production are consistent with similar measurements with identical methods made at considerably closer range (~2 Mm). The use of a uniform model for the Earth-ionosphere waveguide can produce systematic errors in the source-receiver distance, of the order of 1 Mm. Further analysis of the observations has shown an important role for the day-night asymmetry of the waveguide in causing the systematic error, by lending appreciable asymmetry to the short and long paths of propagation round the world.