  We present the detector performance and early science results from GRBAlpha,
a 1U CubeSat mission, which is a technological pathfinder to a future
constellation of nanosatellites monitoring gamma-ray bursts (GRBs). GRBAlpha
was launched in March 2021 and operates on a 550 km altitude sun-synchronous
orbit. The gamma-ray burst detector onboard GRBAlpha consists of a 75x75x5 mm
CsI(Tl) scintillator, read out by a dual-channel multi-pixel photon counter
(MPPC) setup. It is sensitive in the ~30-900 keV range. The main goal of
GRBAlpha is the in-orbit demonstration of the detector concept, verification of
the detector's lifetime, and measurement of the background level on low-Earth
orbit, including regions inside the outer Van Allen radiation belt and in the
South Atlantic Anomaly. GRBAlpha has already detected five, both long and
short, GRBs and two bursts were detected within a time-span of only 8 hours,
proving that nanosatellites can be used for routine detection of gamma-ray
transients. For one GRB, we were able to obtain a high resolution spectrum and
compare it with measurements from the Swift satellite. We find that, due to the
variable background, the time fraction of about 67 % of the low-Earth polar
orbit is suitable for gamma-ray burst detection. One year after launch, the
detector performance is good and the degradation of the MPPC photon counters
remains at an acceptable level. The same detector system, but double in size,
was launched in January 2022 on VZLUSAT-2 (3U CubeSat). It performs well and
already detected three GRBs and two solar flares. Here, we present early
results from this mission as well.
