  Cosmic Microwave Background experiments need to measure polarization
properties of the incoming radiation very accurately to achieve their
scientific goals. As a result of that, it is necessary to properly characterize
these instruments. However, there are not natural sources that can be used for
this purpose. For this reason, we developed the PROTOtype CALibrator for
Cosmology, PROTOCALC, which is a calibrator source designed for the 90 GHz band
of these telescopes. This source is purely polarized and the direction of the
polarization vector is known with an accuracy better than 0.1 deg. This source
flew for the first time in May 2022 showing promising result.
