Inverse Synthetic Aperture Radar Imaging With MATLAB Algorithms. Caner Ozdemir

Inverse Synthetic Aperture Radar Imaging With MATLAB Algorithms - Caner Ozdemir


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continuous wave (FMCW),

      3 stepped‐frequency continuous wave (SFCW),

      4 short pulse, and

      5 chirp (linear frequency modulated [LFM]) pulse.

      Next, these waveforms will be investigated while their time and frequency characteristics are demonstrated and their common usages and applications are addressed.

      2.6.1 Continuous Wave

      A CW radar system transmits radio wave signals at a particular frequency. If both the radar and the target are stationary, then the frequency of the received CW signal is the same as the transmitted signal. On the other hand, the returned signal's frequency components are shifted from the transmitted frequency if the target is in motion with respect to the radar. This type of shift in the frequency spectrum is called Doppler frequency shift and plays an important role in finding the velocity of the target in most radar applications. The concept of Doppler frequency shift is also important for ISAR imaging. We shall see the use of Doppler frequency shift concept in Range‐Doppler ISAR imaging applications in Chapter 6.

      The time‐domain signal of the CW radar is as simple as

      (2.43)equation

Graphs depict example of CW radar waveform in (a) time domain, (b) frequency domain.

      (2.44)equation

Schematic illustrations of police radar: (a) returned wave has the same frequency as the transmitted signal for the stationary target, (b) returned wave's frequency is increased for the approaching target; (c) returned wave's frequency is decreased for the target going away.

      The use of CW waveform in various radar applications provides the following advantages. First of all, the radars that use CW waveforms are easy to manufacture, thanks to their simple waveform shapes. Second, they can detect any target on the range as far as the power level permits. Therefore, there is no range constraint for detection. In addition, they can be used in both very low‐frequency band (e.g. radio altimeters) and very high‐frequency band (e.g. early warning radars).

      CW radars have the following disadvantages. They cannot estimate the range of a possible target. Range is normally measured by the time delay between different pulses created by the radar. In CW radars, however, the waveform is continuous and not pulsed. Furthermore, they can only detect moving targets. Reflected energy from stationary targets is filtered out since their basic operation is based on measuring the Doppler shift in the frequency.

      Another disadvantage comes from the fact that they maximize the power consumption since they continuously broadcast the outgoing signal.

      2.6.2 Frequency‐Modulated Continuous Wave

      While the CW radar can only estimate the Doppler shift created by the movement of the target with respect to radar, FMCW radar can be used to determine the range of a possible target. The common way to modulate the frequency is done by simply increasing the frequency as the time passes. This type of modulation is also known as linear frequency modulation or chirp modulation.

      The waveform of an LFM CW signal is simply given by

      (2.45)equation

      where A is the signal amplitude, fo is the starting frequency, and K is the chirp rate (or frequency increase/decrease rate). In the above equation, the “+” sign indicates an upchirp signal and the “−” sign is for a downchirp signal. The instantaneous frequency of this signal can be easily found by taking the time derivative of the phase as

      (2.46)equation


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