Riometer (Relative ionospheric opacity meter) measures cosmic radio noise absorption (CNA) in the D-region of ionosphere. Frequencies used for the measurement are reserved for the military communication, so time to time local transmitters can saturate the receiver. One of the strongest radio sources on the sky is Cygnus α.
Receiver system consists three main parts: the receiver antenna, riometer radio and datalogging system.
SGO uses analogical La Jolla riometers. The riometer measures the signal received from the antenna by compasating the signal. Compasation voltage is measured. During the absorptin the signal level drops from the quiet time level. Active flares in the Sun can be seen as spikes in the signal. This is more common during the high solar activity.
Today SGO uses dual half-wavelength dipole antenna for the receiver. These are used to form 60 ° wide beam towards zenith of the sky.
Datalogger type used for the SGO riometers excluded Hornsund uses Linux operating system. 8 channel GPS-timed AD-converter unit is made in the house. Three of the AD- channels are used for pulsation magnetometer. Two 10 Hz sampled channels are used for the riometer. One for the compensation voltage and second for the temperature monitoring of the riometer box. In Finnish Lapland outdoor temperature varies from +30 °C to -40 °C.