Nonuniform aperture synthesis radiometers (NASRs) have the advantage of flexibility in practical applications. However, the main limitation of NASRs for Earth observation is the poor reconstruction quality. In this letter, the array factor of 1-D NASR is analyzed. Array factor forming (AFF) is introduced to improve the reconstruction quality of 1-D NASRs. The limitations of the AFF on array configurations are given. The NASR that meets the given limitations is called the optimizable NASR (ONASR). The numerical results and experimental results demonstrate that the AFF is able to improve the reconstruction quality of 1-D ONASRs. To assess the performance of the AFF thoroughly, the comparisons are made against the linear interpolation and the regularized G-matrix method. Meanwhile, the comparisons between the AFF-based 1-D ONASRs and the 1-D uniform ASRs are also made. The comparison results demonstrate that the AFF can obtain better reconstruction quality than the linear interpolation and the regularized G-matrix method. Additionally, the comparison results also demonstrate that the reconstruction quality of AFF-based 1-D ONASRs can approach that of 1-D uniform ASRs.

Array Factor Forming for Image Reconstruction of One-Dimensional Nonuniform Aperture Synthesis Radiometers

The spatial resolution is still a problem in passive microwave remote sensing, especially in low frequency. In recent years, the satellite formation flying has been proposed. Based on this technique, a large array is able to be synthesized in orbit to achieve higher spatial resolution. However, it is a big challenge for the control system to constrain all the satellites in a coplane in orbit. The 3-D array configuration is a good choice for a synthesized array based on satellite formation flying. In this paper, the complete formulation of visibility functions, including system imperfections, in a 3-D aperture synthesis radiometer (3-D ASR) is derived. The array factor of a 3-D ASR is defined. The reconstructed modified brightness temperature (BT) is a 3-D linear convolution of the modified BT and the array factor. Based on this relationship, the reconstruction method for a practical 3-D ASR is studied. The numerical results demonstrate that the reconstruction method is correct and stable. Finally, a discussion is given to analyze several existing methods that were proposed to reconstruct BT image for 3-D arrays in radio astronomy and earth observation. Compared with these existing methods, our imaging method is more suitable for earth observation based on the technique of satellites formation flying in low earth orbit. In addition, according to the derivations, some mature techniques that were developed for 2-D ASRs may be applied to 3-D ASRs.

Imaging With 3-D Aperture Synthesis Radiometers

When observing the Earth from space, the synthetic aperture radiometer antenna array is sometimes set as a non-uniform array. In non-uniform synthetic aperture radiometer image reconstruction, the existing brightness temperature image reconstruction methods include the grid method and array factor forming (AFF) method. However, when using traditional methods for imaging, errors are usually introduced or some prior information is required. In this article, we propose a new IASR imaging method with deep convolution neural network (CNN). The frequency domain information is extracted through multiple convolutional layers, global pooling layers, and fully connected layers to achieve non-uniform synthetic aperture radiometer imaging. Through extensive numerical experiments, we demonstrate the performance of the proposed imaging method. Compared to traditional imaging methods such as the grid method and AFF method, the proposed method has advantages in image quality, computational efficiency, and noise suppression. 

/pdf/non-uniform-synthetic-aperture-radiometer-image-142n2mjc.pdf

Non-Uniform Synthetic Aperture Radiometer Image Reconstruction Based on Deep Convolutional Neural Network

Millimeter-wave radar imaging has potential uses in applications such as material
analysis and medical in-vivo scanning of tissue. This thesis is about measuring
objects using a millimeter wave radar, implement radar imaging algorithms and
evaluate the resulting images. The measured objects are plates and rods made of
metal and a rod made of acrylic glass (PMMA). The objects are measured using
the inverse synthetic aperture radar technique, in which the radar is stationary
and and the scene is either moving in a line or rotating in front of the radar, in our
case at a few decimeters distance. A pulse generating circuit is used as transmitter
and the receiver is a sampling oscilloscope. Multiple imaging algorithms, including
backprojection and gridding, are implemented in Matlab. In order to test the
algorithms, scripts are implemented to create simulated ideal data from line and
rotation measurements. Data for multiple point scatterers is simulated and used
in the imaging algorithms to identify if the scatterers are correctly resolved. Using
this approach the implemented methods are shown to work as intended. Using
strip-map measurement data for two separated metal plates it is shown that an
image can be produced where the objects are resolved with correct width, spacing
and down-range position. Both the backprojection and gridding algorithms are
shown to produce higher quality images with a resolution of less than one centimeter
when used in spotlight mode. For gridding it is found that objects with
a small distance-to-radius ratio are distorted, which limits the object size for a
specific distance.

Short Range Millimeter-Wave Inverse Synthetic Aperture Radar Imaging

The present invention discloses a three-dimensional base line integration aperture imaging method based on array factor integration. The method comprises: calculating a base line vector coordinate sequence and removing redundant base lines wherein to form integrated aperture array non-redundancy uvw distribution; filling a visibility function corresponding to non-redundancy uvw according to the multiple correlation output through system output, obtaining a non-redundancy visibility function vector, and performing discretization of scene brightness temperature distribution; establishing an integration aperture forward model according to the redundancy average visibility function, the non-redundancy uvw distribution, the scene discretization distribution and an extinction fringe function, and establishing an optimal weight solution model according to the inversion image noise minimization and the array directional diagram major lobe pointing observation direction; and traversing object scene observation points, calculating the brightness temperature of each observation point according to an optimal weight, and obtaining an inversion image. Compared to a traditional inversion method based on a G matrix, the three-dimensional base line integration aperture imaging method based on array factor integration is equivalent in prevision, but is better in inversion efficiency and storage space occupation.

Three-dimensional base line integration aperture imaging method based on array factor integration

Nonuniform aperture synthesis radiometers (NASRs) are emerging in Earth remote sensing. The main disadvantage of NASRs is the complexity of the image reconstruction. In this letter, the gridding method is introduced as an efficient method to reconstruct brightness temperature images for NASRs. In the gridding method, a Voronoi diagram is applied to dividing the sampling region in the spatial frequency domain into a number of small cells, and the area of each cell is regarded as the density compensation factor. Meanwhile, a method is proposed to produce the outermost Voronoi cells for the outermost visibility samples. The errors induced by using the gridding method are analyzed. The nonuniformity factor (NF) is defined to indicate the nonuniformity of a sampling pattern. The numerical results verify that the gridding method can be applied to image reconstruction for NASRs and that the accuracy of reconstructed images is dependent on NF. The comparison between the gridding method and the nonuniform fast Fourier transform algorithm shows that the gridding method has lower computational complexity.

The Gridding Method for Image Reconstruction of Nonuniform Aperture Synthesis Radiometers

The invention discloses an array factor molding method of a synthesis aperture microwave radiometer. The array factor molding method comprises the steps of calculating the positions of sampling points in a space frequency domain corresponding to an antenna unit according to the position coordinates of the antenna unit; multiplying each sampling point by a coefficient corresponding to the sampling point to obtain amended array factors, selecting a desired array factor, and calculating the error function of the amended array factors and the desired array factor; calculating a coefficient under the condition of enabling the norm or mean square error of the error function to be minimal; multiplying the calculated coefficient by the corresponding sampling points to obtain amended array factors. Each amended array factor has a smaller sidelobe or narrower main wave beam, and the inversion precision of a non-uniform sampling synthesis aperture radiometer is improved; the array factor molding method is suitable for both the non-uniformity sampling synthesis aperture radiometer and a uniform sampling synthesis aperture radiometer, and has a better effect for improving the inversion precision and the space resolution of the synthesis aperture radiometers.

Array factor molding method of synthesis aperture microwave radiometer

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The Gridding Method for Image Reconstruction of Nonuniform Aperture Synthesis Radiometers

Array factor molding method of synthesis aperture microwave radiometer