The

equivalent noise temperature [T.sub.R] of the receiver is determined by that of the front-end amplifier.

where [T.sub.A] is the antenna noise temperature, [T.sub.R] the receiver equivalent noise temperature, B the pre-detection bandwidth, G the receiver's gain, r the integration time, k the Boltzmann constant, [DELTA]G/G the gain stability of the radiometer, and [varies] a factor equal to 1 for total-power radiometers and equal to 2 for Dicke radiometers.

In this way the unknown receiver gain constant (kBGr) and its equivalent noise temperature ([T.sub.R]) can be determined and then used, in the time interval between two calibrations, to relate the output voltage to the input antenna noise temperature, as in Eq.

Meanwhile, the noise power can be characterized by an equivalent noise temperature in radiometry [8].

After a noise power to radiometric temperature transform of the square of Equation (15), the additional equivalent noise temperature arising from reverse radiation noise is given by:

Recall that the equivalent noise temperature of a network is the temperature of a noisy resistor that, when placed at the input to a noiseless version of the network, produces the same output noise power as the noisy network.[2] The noise power available from a simple resistor is kTB (k is Boltzmann's constant and B is the measurement bandwidth).[2] Therefore, assuming the first stage has an available gain [G.sub.a1] and an equivalent noise temperature [T.sub.e1], the noise power available from the first stage is

However, if the cascaded stages are treated as a composite with equivalent noise temperature [T.sub.ecomp] and available gain [G.sub.ac], then

This noise is present only when the component is processing a signal, that is, it is not measurable without a signal at the input unlike the kT noise described by the noise figure or

equivalent noise temperature. This article analyzes the magnitude of this effect.

Therefore, lightning does not contribute to background noise for systems in either the 800 to 1000 MHz or 1700 to 2000 MHz ranges.[3] Cosmic background radiation accounts for an

equivalent noise temperature of slightly less than 3 K.[3]

The

equivalent noise temperature at output can be expressed as

As a note of caution in regards to the previously developed concepts,[7,8] the simplified formulas are a special case of the previously derived formulas.[9] Also, in the previously described concepts,[7,8] the two basic assumptions, namely that the correlation coefficient can be set to zero and that the

equivalent noise temperature of the gate is near ambient temperature, are suspect and are contradicted by experimental data.