TABLE OF CONTENTS The sensitivity of an intercept receiver is defined as the minimum signal power required at the receiver input. Required is a tricky word here, for it is necessary to describe the purposes to be fulfilled before the minimum signal level can be determined. At the outset, it should be stated that the reason for the existence of a sensitivity floor (below which the interceptor s purposes cannot be fulfilled) is noise. Generally, it is random, thermal noise. Man-made noise or interference can also be a problem, but it must be considered on a case-by-case basis.
Nearly all noise at radar frequencies is generated in the receiver itself (internal receiver noise). Noise also arrives via the antenna, and is basically a consequence of the thermal radiation reaching the antenna from either terrestrial sources or space. The level of this radiation is a function of the temperature in the direction the antenna is pointing. (The dark night sky has a lower noise level than if the antenna were pointed toward the Earth or the Sun.) This is the principle used in radio astronomy to determine maps of the radiation intensity across the sky at various wavelengths. [3.] If the antenna is directed toward a quiet region of the night sky, it might observe an equivalent temperature of only a few degrees Kelvin or, if looking at the Earth s horizon, the observed temperature might be closer to 290 Kelvin (290K is used as the standard or nominal noise temperature expected in...
