TRIS. I. ABSOLUTE MEASUREMENTS OF THE SKY BRIGHTNESS TEMPERATURE AT 0.6, 0.82, AND 2.5 GHz

At frequencies close to 1 GHz the sky diffuse radiation is a superposition of radiation of Galactic origin, the 3 K relic or cosmic microwave background radiation, and the signal produced by unresolved extragalactic sources. Because of their different origin and space distribution, the relative importance of the three components varies with frequency and depends on the direction of observation. With the aim of disentangling the components we built TRIS, a system of three radiometers, and studied the temperature of the sky at nu = 0.6, 0.82, and 2.5 GHz using geometrically scaled antennas with identical beams (HPBW 18 degrees x 23 degrees). Observations included drift scans along a circle at constant declination delta = +42 degrees, which provided the dependence of the sky signal on the right ascension, and absolute measurement of the sky temperature at selected points along the same scan circle. TRIS was installed at Campo Imperatore (latitude 42 degrees 26' north, longitude 13 degrees 33', elevation 2000 m a.s.l.) in central Italy, close to the Gran Sasso Laboratory.