Читать книгу Inverse Synthetic Aperture Radar Imaging With MATLAB Algorithms - Caner Ozdemir - Страница 64

Similar to all electronic devices and systems, radars must function in the presence of internal noise and external noise. The main source of internal noise is the agitation of electrons caused by heat. The heat inside the electronic equipment can also be caused by environmental sources such as the sun, the earth, and buildings. This type of noise is also known as thermal noise (Johnson 1928) in the electrical engineering community.

Let us investigate the signal‐to‐noise ratio (SNR) of a radar system: Similar to all electronic systems, the noise power spectral density of a radar system can be described as the following equation:

(2.36)

Here, k = 1.381 × 10⁻²³ W/K° is the well‐known Boltzman constant, and T_eff is the effective noise temperature of the radar in degrees Kelvin (K°). T_eff is not the actual temperature but is related to the reference temperature via the noise figure, F_n, of the radar as

(2.37)

where the reference temperature, T_o, is usually referred to as room temperature (T_o ≈ 290 K°). Therefore, noise power spectral density of the radar is then being equated to

(2.38)

To find the value of the noise power, P_n, of the radar, it is necessary to multiply N_o with the effective noise bandwidth, B_n, of the radar as shown below:

(2.39)

Here, B_n may not be the actual bandwidth of the radar pulse; it may extend to the bandwidth of the other electronic components such as the matched filter at the receiver. Provided that the noise power is determined, it is easy to define the SNR of radar by combining Eqs. 2.28 and 2.39 as below:

(2.40)

The above equation is derived for the bistatic radar operation. The equation can be simplified to the following for the monostatic radar setup:

(2.41)