DR20-A1 is a high-accuracy direct (normal incidence) solar radiation sensor, or pyrheliometer. DR20 pyrheliometer complies with the Class A specifications of the ISO 9060 standard. It has a better temperature response than DR15-A1, which is a spectrally flat Class B pyrheliometer. These instruments are used in tracker-mounted operation, and offer analogue millivolt outputs. DR-series pyrheliometers have superior window heating that leads to high data availability.