  Context. Cassiopeia A is one of the most extensively studied supernova
remnants (SNRs) in our Galaxy. The analysis of its spectral features with the
help of low frequency observations plays an important role for understanding
the evolution of the radio source through the propagation of synchrotron
emission to observers through the SNR environment and the interstellar medium.
Aims. In this paper we present measurements of the integrated spectrum of Cas A
to characterize the properties of free-free absorption towards this SNR. We
also add new measurements to track its slowly evolving and decreasing
integrated flux density. Methods. We use the Giant Ukrainian radio telescope
(GURT) for measuring the continuum spectrum of Cassiopeia A within the
frequency range of 16-72 MHz. The radio flux density of Cassiopeia A relative
to the reference source of the radio galaxy Cygnus A has been measured on
May-October, 2019 with two subarrays of the GURT, used as a two-element
correlation interferometer. Results. We determine magnitudes of emission
measure, electron temperature and an average number of charges of the ions for
both internal and external absorbing ionized gas towards in Cassiopeia A.
Generally, their values are close to the ones suggested by Arias et al. (2018),
although for some there are slight differences. In the absence of clumping we
find the unshocked ejecta of M = 2.61 solar mass at the electron density of
15.3 cm^-3 has a gas temperature of T=100 K. If the clumping factor is 0.67,
then the unshocked ejecta of 0.96 solar mass the electron density of 18.7
cm^-3. Conclusions. The integrated flux density spectrum of Cassiopeia A
obtained with the GURT interferometric observations is consistent with the
theoretical model within measurement errors and also reasonably consistent with
other recent results in the literature.
