Top panel: pulses of PSR B0809+74 before (left) and after dedispersion with the frequency dependence ∝ f⁻² (right). On the bottom panel, a terrestrial signal with a linear frequency dependence (∝ f ⁻¹) after dedispersion at f⁻² is shown. It radically differs from the pulses of a cosmic source after the dedispersion procedure: even in the uppermost frequency band (0.1 of the total operating range, 31.5–33 MHz), the difference in dispersion delay is tenths of a second, which is much larger than the signal width (ten milliseconds). Such interference signals are easily identified and eliminated.