  Accurate estimates of the surface solar radiation (SSR) are a prerequisite
for intra-day forecasts of solar resources and photovoltaic power generation.
Intra-day SSR forecasts are of interest to power traders and to operators of
solar plants and power grids who seek to optimize their revenues and maintain
the grid stability by matching power supply and demand. Our study analyzes
systematic biases and the uncertainty of SSR estimates derived from Meteosat
with the SARAH-2 and HelioMont algorithms at intra-hour and intra-day time
scales. The satellite SSR estimates are analyzed based on 136 ground stations
across altitudes from 200 m to 3570 m Switzerland in 2018. We find major biases
and uncertainties in the instantaneous, hourly and daily-mean SSR. In peak
daytime periods, the instantaneous satellite SSR deviates from the
ground-measured SSR by a mean absolute deviation (MAD) of 110.4 and 99.6 W/m2
for SARAH-2 and HelioMont, respectively. For the daytime SSR, the
instantaneous, hourly and daily-mean MADs amount to 91.7, 81.1, 50.8 and 82.5,
66.7, 42.9 W/m2 for SARAH-2 and HelioMont, respectively. Further, the SARAH-2
instantaneous SSR drastically underestimates the solar resources at altitudes
above 1000 m in the winter half year. A possible explanation in line with the
seasonality of the bias is that snow cover may be misinterpreted as clouds at
higher altitudes.
