POLRAD is a step-frequency analyser (SFA) aimed at measuring intensity of the AKR and its polarization parameters in the frequency range selected between 4 kHz and 2 MHz with a frequency resolution of 4.096 kHz. Three orthogonal antennae are used to detect wave electric fields: two 22 m long (tip-to-tip) dipoles perpendicular to the spin axis of the spacecraft (Y and Z antennae) and one monopole 11 m long (center of the s/c to tip) deployed along it in antisolar direction (X antenna). The spin axis was kept in the direction of the Sun with accuracy of < 4° from the start of Interball-2 mission till May 1998 (later it was uncontrolled).

Two basic modes of instrument operation are used:

• single antenna mode:Y antenna operation,

• 3-antenna mode: simultaneous operation of X, Y, Z antennae.

In the single antenna mode the analyser provides a power density of the Y antenna component of the wave electric field. In the 3-antenna mode the instrument provides 9 channel outputs. 3 of them are measuring power densities of the X, Y, Z components of the wave electric field. Next 6 outputs are used for determination of wave polarisation parameters and are not available for the public use since each case needs individual and careful data processing.

All channels are swept simultaneously, step by step, over a pre-selected frequency range with a pre-selected step duration of 25, 50 or 100 ms and integration time of 6 ms. The sweep period depends on a pre-selected range of a frequency sweep and an available telemetry bit-rate, which determines the step duration. The sweep range is controlled by the number of frequency steps. In most cases the sweep period is 6, 12 or 24 seconds and the frequency range is 4 - 500 kHz, 4 - 1000 kHz. At the beginning stage of the flight the frequency range from 4 to 2000 kHz, and sweep periods of 48 s were also used.