The signal analyzer and sampler (SAS) experiment was placed on the Active spacecraft as a collaborative effort between the E¿tv¿s University (Budapest), the Technical University of Budapest, and IZMIRAN (Moscow). The scientific objective of the experiment was to study whistler/VLF ducted propagation, VLF duct structure, and the hyperfine structure of whistlers. Digitally sampled waveforms of several field components were transmitted in real time by the SAS telemetry system at 460.4 MHz. For the transmission 900-Hz wide bands were selected between 0.5 and 21.5 kHz or a single 5 kHz wide band was transmitted. Data were received in Budapest, Hungary and at Wallops Island, Virginia. Although the obtained data were processed for various purposes, here some results concerning the hyperfine structure of whistlers are presented. The studied whistlers, recorded as several pairs of closely spaced traces (doublets), were interpreted as ducted whistlers escaping at a high altitude from a single or two closely space narrow ducts and reaching the satellite directly from above or after reflection from below. The whistler traces were processes by a sophisticated matched filter technique which enabled us to obtain very high resolution dynamic (frequency-time-amplitude) spectra. The hyperfine structure of traces revealed by this technique demonstrates the complexity of whistler propagation. The observed splitting of traces may be explained, for example, in terms of a number of guided modes (waveguide mode splitting) or by the superposition of closely spaced ducting structures (duct splitting). ¿ American Geophysical Union 1991