Aperture Synthesis Radiometers Research Articles

Radio Frequency Interference Mitigation in Data and Image Bi-Domains for an Aperture Synthesis Radiometer

For synthetic aperture microwave radiometers, the problem of Radio Frequency Interference (RFI) is becoming more and more serious, which affects both the scientific retrieval of remote sensing data and the imaging quality of brightness temperature (BT) images. In the visibility data domain, the array factor synthesis algorithm is commonly employed to mitigate RFI sources and their Gibbs trailing. In the BT image domain, the CLEAN algorithm is typical applied to mitigate RFI sources and their Gibbs trailing. However, the array factor synthesis algorithm can result in anomalous BT points near the “zero trap” region, and the CLEAN algorithm will miss some BT points below a certain threshold. In this paper, a Bi-domain combined mitigation algorithm is proposed to mitigate RFI sources and their Gibbs trailing. Following initial mitigation in the visibility data domain, dual thresholds are applied to normalize anomalous BT points near the “zero trap” region, thereby enhancing imaging quality. The effectiveness of the Bi-domain combined mitigation algorithm is verified by using both measured data from SMOS L1A and simulated data. The experimental results demonstrate that the Bi-domain combined mitigation algorithm is superior to the array factor synthesis algorithm and the CLEAN algorithm in mitigating RFI sources and their Gibbs trailing.

A Near-Field Imaging Method Based on the Near-Field Distance for an Aperture Synthesis Radiometer

For an aperture synthesis radiometer (ASR), the visibility and the modified brightness temperature (BT) are related to the Fourier transform when the distance between the system and the source is in the far-field region. BT reconstruction can be achieved using G-matrix imaging. However, for ASRs with large array sizes, the far-field condition is not satisfied when performing performance tests in an anechoic chamber due to size limitations. Using far-field imaging methods in near-field conditions can introduce errors in the images and fail to correctly reconstruct the BT. Most of the existing methods deal with visibilities, converting near-field visibilities to far-field visibilities, which are suitable for point sources but not good for extended source correction. In this paper, two near-field imaging methods are proposed based on the near-field distance. These methods enable BT reconstruction in near-field conditions by generating improved resolving matrices: the near-field G-matrix and the F-matrix. These methods do not change the visibility measurements and can effectively image both the point source and the extended source in the near field. Simulations of point sources and extended sources in near-field conditions demonstrate the effectiveness of both methods, with F-matrix imaging outperforming near-field G-matrix imaging. The feasibility of both near-field imaging methods is further validated by carrying out experiments on a 10-element Y-array system.

Visibility Extension of 1-D Aperture Synthesis by a Residual CNN for Spatial Resolution Enhancement

In order to improve the spatial resolution of a one-dimensional aperture synthesis (1-D AS) radiometer without increasing the size of the antenna array, the method of visibility extension (VE) is proposed in this article. In the VE method, prior information about the visibility distribution of various scenes is learnt by a residual convolutional neural network (ResCNN). Specifically, the relationship between the distribution of low-frequency visibility and that of high-frequency visibility is learnt. Then, the ResCNN is used to estimate the high-frequency visibility samples from the low-frequency visibility samples obtained by the AS system. Furthermore, the low- and high-frequency visibility samples are combined to reconstruct the brightness temperature image of the scene, to enhance the spatial resolution of AS. The simulation and experiment both demonstrate that the VE method can enhance the spatial resolution of 1-D AS.

Deep Learning Imaging for 1-D Aperture Synthesis Radiometers

For one-dimensional aperture synthesis (1-D AS) radiometers, truncated sampling occurs in the frequency domain due to the system baseline limitation. Therefore, there is an obvious Gibbs oscillation in the reconstructed image. To solve this problem, an imaging method based on a one-dimensional convolutional neural network (1-D CNN) is proposed in this paper. Compared with deep learning methods based on two-dimensional convolutions, the 1-D convolution not only reduces the amount of computation but also produces further performance improvements. The input data of the network are the 1-D visibility function samples, and the output data are the 1-D brightness temperature (BT) samples. The network learns the mapping relationship from the training of the 1-D visibility function samples and 1-D BT samples to complete 1-D AS imaging without any prior knowledge. To verify the performance of this imaging method, simulations and experiments based on the airborne C-band one-dimensional microwave interferometric radiometer (ACMIR) system are implemented. The simulation and experimental results demonstrate that the proposed AS-CNN method achieves higher performance than the IFFT method in terms of image quality and Gibbs phenomenon suppression. In the case of an antenna failure and missing baseline, the AS-CNN method proposed in this paper can still obtain a BT image with high imaging quality, which shows that the robustness of the network is better than that of the IFFT method.

Improved CLEAN Algorithm for RFI Mitigation of Aperture Synthesis Radiometer Images

For synthetic aperture radiometer, radio frequency interference (RFI) seriously contaminates the observed data and affects the quality of the brightness temperature (BT) image. CLEAN algorithm is usually used to mitigate the Gibbs effect generated by RFI. However, the closely distributed RFI sources cannot be well mitigated by the original CLEAN algorithm. This letter proposes an improved adaptive CLEAN algorithm to enhance the performance of RFI mitigation, especially the closely distributed RFI sources. The loop factor is forced to synchronize with residual BT of RFI for every iteration to avoid over mitigation. Over mitigation can result in a BT vacancy area or an extra BT point generated at the position of RFI. Soil Moisture and Ocean Salinity (SMOS) L1B real data are applied to verify the effectiveness of the improved CLEAN algorithm. The simulation shows that the improved CLEAN algorithm can be competent in eliminating isolated and closely distributed RFIs better than the classic CLEAN algorithm, and the over mitigation problem is solved by the proposed method.

Orbit Design for a Satellite Swarm-Based Motion Induced Synthetic Aperture Radiometer (MISAR) in Low-Earth Orbit for Earth Observation Applications

Soil Moisture and Ocean Salinity mapping by Earth observation satellites has contributed significantly toward a better understanding of the Earth system, such as its hydrosphere or climate. Nevertheless, an increased spatial resolution below 10 km with a radiometric resolution in the range of 2 K–3 K of radiometric data could yield a more complete picture of global hydrological processes and climate change. Operational radiometers, such as SMOS, have already approached prohibitive sizes for spacecraft due to the required large antenna apertures. Therefore, radiometer concepts based on a large number of satellites flying in close proximity (swarms) have been proposed as a possible solution. This article investigates the orbit mechanics of placing a satellite swarm-based motion induced synthetic aperture radiometer (MISAR) in low Earth orbit for Earth observation applications. The aperture synthesis antenna array is formed by a large number of individual antennas on autonomously controlled nanosatellites (deputies) and a correlator antenna in the Y-configuration carried by a chief satellite. The proposed design methodology is based on the optimization of satellite positions within a plane and the subsequent translation of coordinates into initial conditions for general circular orbits (GCOs). This enables a more computationally efficient orbit optimization and ensures the time invariance of the antenna array response. Based on this methodology, simulations have been performed with swarms consisting of up to 96 satellites. Simulations show that the spatial resolution of an aperture synthesis radiometer can be increased to less than 10 km for applications where the requirements on radiometric sensitivity are more relaxed ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta T\sim 3$ </tex-math></inline-formula> K).

System Reliability Optimization of Thinned Linear Arrays in Interferometric Aperture Synthesis Radiometers

This letter deals with linear array design for satisfactory performance of system reliability in interferometric aperture synthesis radiometer (IASR), which is important to the instrument robustness against failures. We address the reliability performance in the case of one-element wreckage, as well as two other systematic metrics of IASR, i.e., spatial resolution and radiometric sensitivity. First, an array design model concerning the above three metrics is established within the framework of multiobjective optimization. Then, a state-of-the-art multiobjective method based on nondominated neighbor immune algorithm is applied. Finally, simulation results are presented to verify the effectiveness of designed array configurations.

Initial Results of H-Matrix Reconstruction Method for 1-D Mirrored Aperture Synthesis Radiometers

The mirrored aperture synthesis radiometer (MASR) has been proposed for high-resolution observation in recent years. Since the MASR is a kind of linear system, the matrix reconstruction method should be an alternative for image reconstruction of a practical MASR. However, the matrix reconstruction method is neither theoretically analyzed nor experimentally validated at present, and the performance of the matrix reconstruction method for a MASR in practical applications is still unknown. In this letter, the H-matrix reconstruction method including the truncated singular value decomposition (SVD) and Tikhonov regularization, not yet applied to a MASR, is introduced. Then, the numerical simulations and imaging experiments are performed to validate the presented method, and the performance of the presented method is also shown. Finally, the comparisons between the presented method and the cosine-transform-based iterative method are made in the view of imaging experiments. The H-matrix reconstruction method is more convenient in the situation of a small MASR with a small alias-free field of view (FOV).

Spaceborne High Precision Sea Surface Salinity Remote Sensing by Interferometric Radiometry

The global sea surface salinity (SSS) can be accomplished by means of the L-band microwave radiometers from space, such as European Space Agency's SMOS (soil moisture and ocean salinity) and NASA's Aquarius/SAC-D. However, the microwave imaging radiometer using aperture synthesis in SMOS mission suffers from various errors and unexpected in-orbit phenomena, which degrade SSS accuracy. To further improve SSS accuracy, a full polarization aperture synthesis microwave radiometer (FPASMR) has been proposed for China's next-generation Haiyang satellite program with two important improvements, which is composed by an L- and X-band aperture synthesis radiometer (ASR) with two dimensions. One important improvement is that the sea surface roughness and sea surface temperature are simultaneously measured by the full polarization X-band ASR, and another important improvement is that the microwave-optical receiving technology and in-orbit thermal control subsystem are employed to improve the stability of the FPASMR for reducing visibility phase errors. The system of the FPASMR is introduced in terms of L-band subsystem, X-band subsystem, and the calibration subsystem. The main specifications of the FPASMR are analyzed in terms of field of view, angular resolution, and radiometric sensitivity. In addition, the FPASMR calibration is also introduced. The FPASMR demonstrator has also been developed and a series of experiments is performed to assess the performance of the FPASMR demonstrator and the improvements in the FPASMR.

Image Reconstruction for Mirrored Aperture Synthesis Radiometers

The mirrored aperture synthesis radiometer (MASR) has been proposed as a promising technique for Earth observation. However, considering the system imperfections, the cosine visibility function and the brightness temperature distribution do not constitute a cosine transform pair. The method to improve the quality of reconstructed brightness temperature images for a practical MASR has not yet been developed. In this article, the complete formulation of cosine visibility functions in a practical MASR is derived to better understand the measurement of a MASR with non-negligible effects of the system imperfections. The array factor of a practical MASR is analyzed. Then, the alias-free field of view (FOV) for a MASR is presented, and an iterative method is proposed to reconstruct the brightness temperature image in the alias-free FOV to reduce the computational burden. The numerical simulation and experimental results demonstrate that the proposed method can be applied to practical MASRs and that the reconstruction quality is significantly improved by the proposed method. Finally, the H-matrix inversion method is discussed as an alternative reconstruction method for a practical MASR. Compared with the H-matrix inversion method, the proposed method is more convenient for practical applications.

Combinatorial Design of Low-Degradation Linear Arrays in Interferometric Aperture Synthesis Radiometers

Radiometric sensitivity is a crucially important performance metric of interferometric aperture synthesis radiometers (IASRs). When synthesizing array geometries for optimum sensitivity performance, the minimum-degradation or low-degradation arrays (MDAs or LDAs) are employed to fulfill requirements of baseline coverage and redundancy distribution. This paper presents a study on combinatorial design of low-degradation linear arrays (LDLAs), based on a concept of multiple difference basis (MDB). Such a deterministic method can attain a satisfactory radiometric sensitivity performance in short computation time and can be easily applied in large array synthesis, compared with stochastic design methods (e.g., simulated annealing). Furthermore, the resultant LDLAs also have advantages of angular resolution and system reliability to element wreckage. Representative experiments are carried out to demonstrate the significance of the proposed MDB-based technique for designing LDLAs.

Imaging With 3-D 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.

On Baseline Weighting for Radiometric Sensitivity Analysis in Microwave Interferometric Radiometry

This letter presents a study on the effects of baseline weighting on the performance of radiometric sensitivity in interferometric aperture synthesis radiometers (IASRs). After a brief review on the computation of radiometric sensitivity, its theoretical lower bound, associated with the concept of coherent gain , is derived. We also draw attention to the performance tradeoffs between radiometric sensitivity and several spatial-domain properties of IASRs from which different weightings can be compared. Numerical examples are given to demonstrate the significance of the comparative analysis.

Expansion Design of Interferometric Aperture Synthesis Arrays Based on Multi-Objective Optimization

This paper presents a study on expansion design of thinned arrays in interferometric aperture synthesis radiometers. The design problem deals with how to properly employ a limited number of antenna elements around a fixed array to fulfill performance requirements of interest, such as angular resolution, peak sidelobe level, and array redundancy. These factors of merit are grouped and discussed within the framework of multi-objective design. Three state-of-the-art optimizers based on multi-objective particle swarm optimization, non-dominated sorting genetic algorithm II, and non-dominated neighbor immune algorithm are implemented for specific design examples. A comparative analysis is performed to point out the features and performances of the above-mentioned search methods.

Radiometric Sensitivity and Angular Resolution Optimization of Thinned Linear Arrays in Microwave Interferometric Radiometry

This communication addresses the thinned linear array design for the optimization of radiometric sensitivity and angular resolution, both of which are crucial characteristics of interferometric aperture synthesis radiometers (IASRs). First, we establish a multiobjective optimization model concerning their tradeoff. Then, three state-of-the-art search algorithms, namely, multiobjective particle swarm optimization (MOPSO), nondominated sorting genetic algorithm II (NSGA-II), and nondominated neighbor immune algorithm (NNIA), are implemented for the design of linear interferometric arrays. Furthermore, a constraint principle of interferometric arrays is proposed to reveal the inherent relation among array parameters closely related to IASR performances, which would be useful for the multiobjective optimization of IASR arrays.

Analysis of Reflector Sizes in Rotating Mirrored Aperture Synthesis Radiometers

Rotating mirrored aperture synthesis (RMAS) has been proposed as a new method for high spatial resolution earth observation from the geostationary orbit. However, in previous researches, the reflector sizes are assumed to be infinite so that all antennas in the antenna array can receive the reflection signals. This assumption is obviously unacceptable in practical applications. How to estimate the reflector sizes is still a problem. In this letter, the principle of RMAS is briefly reviewed. Then, the reflector sizes of an RMAS radiometer are analyzed based on solid geometry, and detailed calculation formulae are derived. A numerical example is given for calculating the reflector sizes of an RMAS radiometer applied in the geostationary orbit.

Reduction of the Instrument Model Error in Aperture Synthesis Radiometers

Aperture synthesis radiometers measure the visibilities of the scene under observation by an antenna array, and based on these visibility measurements, inverse algorithm is employed to reconstruct the brightness temperature of the observed scene. The instrument model plays a key role in the inverse problem and its error will degrade the reconstruction accuracy. This work provides an analysis of the impact of the instrument model error on the reconstruction accuracy and a method regarding how to reduce its impact. It will be demonstrated in this work that the point spread functions (PSFs) of the imaging system at different spatial locations are highly correlated with each other, but the PSF errors are not. Benefit from this fact, the PSF can be represented as a sum of basis functions obtained from principal component analysis of PSF. It can be observed that most of the energy of the true PSF is concentrated on the first $L$ principal components, and the last $N$ – $L$ principal components can be neglected due to that they are dominated by the noise. Numerical studies are carried out using the data obtained from the Soil Moisture and Ocean Salinity (SMOS) official website, and the corresponding results validate that the proposed method is effective in reducing the instrument model error.

Relationship Between Mirrored Aperture Synthesis Radiometers and Aperture Synthesis Radiometers

The mirrored aperture synthesis radiometer (MASR) has been proposed as a new technique for high-resolution observation. Compared with ASRs, MASRs have the advantage of lower system complexity. However, the relationship between MASRs and ASRs has not been studied. In order to thoroughly study MASRs, it is necessary to establish the relationship between MASRs and ASRs. In this letter, the array factor of 1-D MASRs in the discrete cosine transform domain (DCT-domain) is defined. The reconstructed image for a 1-D MASR is a symmetric convolution of the observed brightness temperature distribution and the defined array factor in the DCT domain. Since a symmetric convolution can be turned into a linear convolution, the relationship between 1-D MASRs and 1-D ASRs can be established. A 1-D MASR is equivalent to a 1-D ASR that has a mirrored window. Additionally, for the equivalent 1-D ASR, its observed scene is a symmetrically extended real scene. This established relationship is validated by the simulation results. In practical applications, a MASR can be understood as an ASR.

Notice of Removal: Circular Hexagonal Lattice Array and Its Optimization in Aperture Synthesis Radiometers

An innovative array configuration, circular hexagonal lattice array (CHLA), is proposed to overcome nonuniform sampling drawback of a circular array for aperture synthesis radiometers. A differential evolution algorithm is applied to search optimal CHLA configurations taking into account the radiometric sensitivity degradation as well as the sample number. Representative numerical results are illustrated to evaluate performances of the proposed array in comparison to the previously proposed arrays. The performances of obtained optimal CHLAs are reported for 15 – 32 elements.

RFI Mitigation in Aperture Synthesis Radiometers Using a Modified CLEAN Algorithm

For aperture synthesis radiometers, sparse samplings on the $u$ - $v$ frequency plane cause undesirable sidelobes in the synthesized beam. Through these sidelobes, artificial sources emitting in the protected 1400–1427 MHz band contaminate the retrievals of the soil moisture and ocean salinity (SMOS) from MIRAS measurements. One effective way to correct the artificial interferences is to create a synthetic signal to compensate for the interference’s impact. Based on the similar idea, in this letter, we describe an algorithm to compensate for the interference’s impact by constructing an artificial signal as close as possible to the Gaussian beam. Numerical studies using synthetic and real SMOS data have been carried out to demonstrate that the proposed algorithm outperforms the classical CLEAN algorithm in correcting the impact of the extended radio frequency interference source.