  The characteristic frequencies of a system provide important information on
the phenomena that govern its physical properties. In this framework, there has
recently been renewed interest in cryogenic microwave characterization for
condensed matter systems since it allows to probe energy scales of the order of
a few $\mu$eV. However, broadband measurements of the absolute value of a
sample response in this frequency range are extremely sensitive to its
environment and require a careful calibration. In this paper, we present an
\textit{in situ} calibration method for cryogenic broadband microwave
reflectometry experiments that is both simple to implement and through which
the effect of the sample electromagnetic environment can be minimized. The
calibration references are here provided by the sample itself, at three
reference temperatures where its impedance is assumed or measured, and not by
external standards as is usual. We compare the frequency-dependent complex
impedance (0.1--2 GHz) of an a-Nb$_{15}$Si$_{85}$ superconducting thin film
obtained through this Sample-Based Calibration (SBC) and through an
Open-Short-Load Standard Calibration (SC) when working at very low temperature
(0.02--4 K) and show that the SBC allows us to obtain the absolute response of
the sample. This method brings the calibration planes as close as possible to
the sample, so that the environment electrodynamic response does not affect the
measurement, provided it is temperature independent. This results in a
heightened sensitivity, for a given experimental set--up.
