  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+-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.
