SPN1 measures global (total) and diffuse radiation and sunshine state. It uses an array of 7 miniature thermopile sensors and a computer-generated shading pattern to measure the global and diffuse components of incident solar radiation. The shading pattern and thermopiles are arranged so that at least one thermopile is always fully exposed to the solar beam, and at least one is fully shaded from it, regardless of the position of the sun in the sky. All 7 thermopiles receive an equal amount of diffuse light. From the individual thermopile readings, a microprocessor calculates the global and diffuse horizontal irradiance and from these values an estimate of sunshine state is made.
SPN1 has an internal heater that keeps the dome clear of dew, ice, and snow down to −20 °C (−4 °F) (in still air conditions), ensuring reliable readings in difficult climatic conditions.