  Aims. Since launched on 2021 March 22, the 1U-sized CubeSat GRBAlpha operates
and collects scientific data on high-energy transients, making it the smallest
astrophysical space observatory to date. GRBAlpha is an in-obit demonstration
of a gamma-ray burst (GRB) detector concept suitably small to fit into a
standard 1U volume. As it was demonstrated in a companion paper, GRBAlpha adds
significant value to the scientific community with accurate characterization of
bright GRBs, including the recent outstanding event of GRB 221009A. Methods.
The GRB detector is a 75x75x5 mm CsI(Tl) scintillator wrapped in a reflective
foil (ESR) read out by an array of SiPM detectors, multi-pixel photon counters
by Hamamatsu, driven by two separate, redundant units. To further protect the
scintillator block from sunlight and protect the SiPM detectors from particle
radiation, we apply a multi-layer structure of Tedlar wrapping, anodized
aluminium casing and a lead-alloy shielding on one edge of the assembly. The
setup allows observations of gamma radiation within the energy range of 70-890
keV with an energy resolution of ~30%. Results. Here, we summarize the system
design of the GRBAlpha mission, including the electronics and software
components of the detector, some aspects of the platform as well as the current
way of semi-autonomous operations. In addition, details are given about the raw
data products and telemetry in order to encourage the community for expansion
of the receiver network for our initiatives with GRBAlpha and related
experiments.
