  The cosmic microwave background (CMB), a remnant of the Big Bang, provides
unparalleled insights into the primordial universe, its energy content, and the
origin of cosmic structures. The success of forthcoming terrestrial and space
experiments hinges on meticulously calibrated data. Specifically, the ability
to achieve an absolute calibration of the polarization angles with a precision
of < 0.1 deg is crucial to identify the signatures of primordial gravitational
waves and cosmic birefringence within the CMB polarization. We introduce the
COSMOCal project, designed to deploy a polarized source in space for
calibrating microwave frequency observations. The project aims to integrate
microwave polarization observations from small and large telescopes,
ground-based and in space, into a unified scale, enhancing the effectiveness of
each observatory and allowing robust combination of data. To demonstrate the
feasibility and confirm the observational approach of our project, we developed
a prototype instrument that operates in the atmospheric window centered at 260
GHz, specifically tailored for use with the NIKA2 camera at the IRAM 30 m
telescope. We present the instrument components and their laboratory
characterization. The results of tests performed with the fully assembled
prototype using a KIDs-based instrument, similar concept of NIKA2, are also
reported. This study paves the way for an observing campaign using the IRAM 30m
telescope and contributes to the development of a space-based instrument.
