Abstract
With the rapid development of the multiple-input multiple-output synthetic aperture radar (MIMO SAR) system, the demands for miniaturization and high gain of antenna are increasing. The digital array-fed reflector antenna has such virtues so that it can play an important role in such system. However, the geometric models and signal models based on a reflector antenna are considerably different from the directly radiating planar antenna. The signal processing for the reflector antenna is more complex and difficult. As a result, the applications of the reflector antenna in SAR system are not as mature as those of the planar antenna. A combination of multidimensional waveform encoding (MWE) technique and digital beamforming (DBF) technology at the receiving end can greatly improve the MIMO SAR system performance, especially ambiguity suppression and waveform separation. This configuration can realize different radar functions and meet multidimensional observation requirements, such as the polarized SAR. Thus, this study combines digital array-fed reflector antenna and the DBF technique in the elevation direction for MWE SAR waveform separation. The echo models for the array-fed reflector antenna and the planar antenna are established based on short-time shift-orthogonal waveforms. In the models, a mismatch in steering vectors is inevitable if DBF processing is continuously performed traditionally in the azimuth-elevation two-dimensional time domain. This mismatch will worsen the waveform separation effect and the image quality. Therefore, we propose a DBF method which is processed in range-Doppler domain. The method enables waveform separation without ambiguity at the receiver. Then, the conventional SAR imaging methods are enabled, and we acquire an ideal SAR image. The simulation results for both point targets and distributed targets prove the effect and feasibility of the proposed DBF method.
Highlights
Synthetic aperture radar (SAR) has been widely used in military and civil fields because of all-weather and all-day capabilities, as well as its high resolution
The results show that the digital beamforming (DBF) in R-D domain can accurately obtain the targets’ position and the performance parameters are kept at an ideal level
Before the simulation results are given, the RCM of the steering vector should be compensated. This is the inverse of the range cell migration correction (RCMC) in the imaging algorithm, as shown in Figure 16 below
Summary
Synthetic aperture radar (SAR) has been widely used in military and civil fields because of all-weather and all-day capabilities, as well as its high resolution. This signal form has a high sidelobe level after pulse compression, resulting in image quality degradation [12]. To ensure the orthogonality of signals, this method strictly requires the time-invariant characteristics of a radar channel; otherwise, it will greatly weaken the waveform separation performance. A transmitting signal can be freely coded in the time domain, frequency domain, space domain, and so on, which may greatly enhance its flexibility and the operation ability in multiple modes of a radar system [14] In this mode, the number of multi-phase centers and baseline length are increased, and the waveforms can be distinguished by combining two-dimensional array antenna and digital beamforming (DBF) in elevation, which can greatly improve the imaging capability. The further signal models and difference between hk,VV,i(τ), hk,VH,i(τ) and hk,VV(τ), hk,VH(τ) will be discussed for the planar antenna and reflector antenna, respectively
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