332 Radio Engineering for Wireless Communication and Sensor Applications
Figure 12.16 MTI based on a delay line.
successive pulses are compared. For a fixed target, the echoes are similar, giving no output. Moving targets can be detected because the distance of a moving target changes from one pulse to the next and there is a phase difference between successive received pulses. However, if the distance of a target changes by a multiple of the wavelength during the delay 1/f p , the phase difference is zero and the target cannot be detected. Such blind speeds can be avoided by using two or more different pulse repetition frequencies. The delay circuit may be an analog filter or a digital shift register.
Example 12.2
The properties of an air surveillance radar are: transmitted power P t = 250 kW, antenna gain G = 40 dB, pulse length t = 1 ms, system noise temperature TS = 500K, wavelength l = 0.1m. The radar cross section of the target is s = 1 m2 and the S /N required for detection is S /N = 13 dB = 20. Find the maximum operating range.
Solution
The noise bandwidth is about 1/t = 1 MHz. From (12.7) we obtain the
minimum power Pr , min = 1.38 × 10−23 × 500 × 106 × 20W = 1.38 ×
10−13 W. Substituting this in (12.6) gives R max = [250 × 103 × 108 × 0.12 × 1/(1.38 × 10−13 × 43p 3 )]1/4m = 174 km.
12.6.2 Doppler Radar
The block diagram of a simple Doppler radar, called continuous wave (CW) radar, is shown in Figure 12.17. The radar transmits a continuous and unmodulated wave at a frequency of f 0 . If the radial velocity of the target is vr , the frequency of the reflected wave is f 0 + f D where the Doppler frequency is
f D = ± |
2vr |
(12.8) |
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l |
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where Df is the maximum change of the transmitted frequency and f m is the modulation frequency. Equation (12.9) is valid only if the reflecting object is stationary. Other modulating waveforms can also be used.
Example 12.3
The frequency of FM-CW radar, as shown in Figure 12.19, is modulated with a triangular wave between 3.0 and 3.2 GHz. The modulation frequency is 50 Hz. The output frequency is 1,200 Hz. Find the distance of the target.
Solution
Now D f = 3.2 GHz − 3.0 GHz = 200 MHz, f m = 50 Hz, and f d = 1,200 Hz. From (12.9) we solve R = cf d /(4Dff m ) = 3 × 108 × 1,200/(4 × 200 × 106 × 50)m = 9m. This distance is too short to be measured with pulse radar.
12.6.4 Surveillance and Tracking Radars
Surveillance radar covers for example an air space surrounding an airport, whereas tracking radar follows a target continuously. Surveillance and tracking radar are usually pulse radar, and they differ from each other mainly by their beam shape and scanning techniques.
The beam of surveillance radar is usually scanned in the horizontal plane mechanically by rotating the antenna or electronically by using a phased array. In the circular scanning shown in Figure 12.20(a) the beam is fan-shaped, that is, narrow in the horizontal plane and broader in the vertical plane. If the beam is cosec2-shaped in the vertical plane, a target flying at a constant height produces an echo having a constant strength. A simple conical scanning reveals only the azimuth angle of the target. Stepped circular scanning [Figure 12.20(b)], and nodding circular scanning [Figure 12.20(c)], also give information on the elevation angle. Now the antenna may have a symmetrical pencil beam.
Tracking radar is used to track the paths of airplanes, missiles, rockets, and so on. Often tracking radar has a surveillance mode in which the radar seeks targets for tracking. As the target moves, the direction of the antenna has to be changed. In a conical scanning [Figure 12.20(d)], the axis of the beam makes a cone. If the target is not on the axis of the cone, the amplitude of the received pulses is modulated at the scanning rate. An error signal is generated from this modulation to correct the direction of the cone axis.
Monopulse radar has four beams, as shown in Figure 12.20(e). Now an error signal can be derived from a single pulse by comparing the amplitudes