For radar, the procedure of pointing a radar at the sun to receive and measure the energy radiated by the sun at the radio frequency to which the radar receiver is sensitive. Using the sun as a signal source, it is possible to check the azimuth and elevation alignments of the antenna and to estimate the antenna gain. Antenna gain measurements require knowledge of the solar irradiance in the frequency band of the receiver. This information is available from several observatories around the world that observe the solar spectrum at local noon each day. See solar signal.