The interstellar medium scatters radio waves which causes pulsars to scintillate .
For intrinsically short bursts of emission , the observed signal should be a direct measurement of the impulse response function .
We show that this is indeed the case for giant pulses from \object PSR B1957+20 : from baseband observations at 327 MHz , we demonstrate that the observed voltages of a bright pulse allow one to coherently descatter nearby ones .
We find that while the scattering timescale is 12.3 \mu s , the power in the descattered pulses is concentrated within a span almost two orders of magnitude shorter , of \lesssim 200 ns .
This sets an upper limit to the intrinsic duration of the giant pulses .
We verify that the response inferred from the giant pulses is consistent with the scintillation pattern obtained by folding the regular pulsed emission , and that it decorrelates on the same timescale , of 84 s. In principle , with large sets of giant pulses , it should be possible to constrain the structure of the scattering screen much more directly than with other current techniques , such as holography on the dynamic spectrum and cyclic spectroscopy .