VLBI observations of the Crab pulsar with the 64-m radio telescope at Kalyazin ( Russia ) and the 46-m radio telescope of the Algonquin Radio Observatory ( Canada ) at 2.2 GHz and single-dish observations of the millisecond pulsar B1937+21 with the GBT ( USA ) at 2.1 GHz were conducted to probe the interstellar medium and study the properties of giant pulses .
The VLBI data were processed with a dedicated software correlator , which allowed us to obtain the visibility of single giant pulses .
Two frequency scales of 50 and 450 kHz were found in the diffraction spectra of giant pulses from the Crab pulsar .
The location of the scattering region was estimated to be close to the outer edge of the nebula .
No correlation was found between the power spectra of giant pulses at left- and right-hand circular polarization .
We explain this lack of correlation through the influence of the strong magnetic field on circularly polarized emission in the region close to the Crab pulsar .

Combining the measurement of the decorrelation bandwidth with that of the scattering time of giant pulses for B1937+21 , we found three frequency scales of 1.7 , 3.8 , and 16.5 MHz .
The scattering time of giant pulses of B1937+21 at 2.1 GHz was found to be 40 \pm 4 ns .
We obtained an upper limit of the intrinsic width of giant pulses from B1937+21 of less than 8 ns .
The frequency dependences of the scattering times for the Crab pulsar and PSR B1937+21 were found to be different .
They are characterized by exponents of -3.5 and -4.2 , respectively .
We attribute the difference to the large influence of scattering in the Crab nebula .