This instrument (WHISPER: Waves of HIgh frequency and Sounder for Probing of the Electron density by Relaxation) is part of the Wave Experiment Consortium (WEC) on Cluster. Its objectives are to: (1) measure the electron density via active sounding of plasma resonances; and, (2) record via passive wave analysis the natural wave emissions in the high-frequency range, from 4--80 KHz. An active sounder can provide the total local electron density, including the cold component, which is the dominant one in many of the regions to be explored. Most of the time, this cold component cannot be obtained from particle experiments, owing to the satellite's electric potential and photoelectron sheath disturbance. The sounder consists of three main parts: (1) a sensitive double-sphere antenna, measuring the AC electric field, provided as part of the EFW experiment; (2) a transmitter, a receiver, a digital spectrum analyzer, and a controller unit, which form the WHISPER instrument itself; and, (3) command and data processing systems, which are part of the DWP experiment, which also takes care of command and telemetry for the entire WEC unit. During the active part of WHISPER operation, in each frequency step a short (1 ms in normal mode; 0.5 ms also possible) wave train at the fixed frequency is transmitted. The receiver is then activated to receive until the end of the frequency step. This is repeated for a series of frequencies, sweeping a designated range of frequencies. The frequency step duration is a multiple of 13.33 ms to allow good synchronization with the EFW instrument's sampling period, and ranges up to ~250 ms. The sweep duration is a number of steps, and ranges from 0.5 s to ~10 s. The total frequency range available for the active transmitter is from 4--80 KHz. Different signal strength levels can be selected, e.g. 50, 100, 200 V peak to peak. During passive operations, there is no short pulse transmitted. The receiver signal is measured each 13.33 ms, and after A/D conversion it is analyzed in frequency by the vector signal processor (VSP), which performs a fast Fourier transform and delivers 64 to 512 useful bins in the range 0--80 KHz. The 512-bin option is always used in sounding mode. The operational details of the instrument are managed by the microcontroller, which also handles the interface to the DWP. Extensive data compression is used, either in the DWP (normal) or within WHISPER itself as backup. There is a considerable number of WHISPER modes, with a corresponding range of frequency resolutions and final data rates. In the standard WEC modes, the telemetry rate is about 980 bps in routine and 5800 bps in high rate. Plasma densities from 0.2 to 80 cm**-3 are measured, with a time resolution of 4 s routine and 1 s at best. Natural wave electric fields are measured with a sensitivity of 0.05 microV/(m sqrt(Hz)). Time and frequency resolutions are 3 s and 640 Hz routine, and 100 ms and 320 Hz at best. In any given mode, the dynamic range is about 100 dB, but 124 dB absolute range can be reached by commanded gain change. For more details of the Cluster mission, the spacecraft, and its instruments, see the report Cluster: mission, payload and supporting activities,'' March 1993, ESA SP-1159, and the included articleWHISPER, a Sounder and High-Frequency Wave Analyser Experiment,'' by P. M. E. Decreau et al., from which this information was obtained.