Soil moisture is an essential soil parameter affecting energy transfer and surface heat exchange. Accurately acquiring surface soil moisture is important for guiding agricultural production, understanding the global water cycle, and studying climate change. This paper conducted an inversion study of soil moisture in the Shandian River Basin based on the change detection method (CD) using Sentinel-1A and MODIS time series data. It was found that the traditional CD has a mismatch problem between the amount of change in soil moisture and the amount of change in radar backscattering coefficient, which reduces the accuracy of soil moisture inversion. This paper proposes an improved CD that corrects for soil moisture variations and radar backscattering coefficient variations using the normalized difference vegetation index (NDVI) piecewise function. Compared with the traditional CD and the Alpha model, the improved CD proposed in this paper has better applicability in the vegetation cover area and improves the accuracy of the soil moisture inversion, with a Pearson correlation coefficient of 0.813, a coefficient of determination of 0.645, and a root mean square error (RMSE) of 0.046 cm3 /cm3 . The improved CD does not require complex parameters and has the potential for large-scale application. Using the improved CD to invert soil moisture, the inversion of soil moisture value can capture the changing trend of measured soil moisture value in a time series.
In near-space fast-speed synthetic aperture radar (SAR), the azimuth multichannel technology is usually required to achieve high resolution wide swath (HRWS) imaging. Compared with traditional linear array SAR (LA-SAR), arc array SAR (AA-SAR) is with attractive omnidirectional and real-time dynamic imaging capacities. In the paper, the azimuth multichannel signal model of LA-SAR and AA-SAR are both established and compared, compared with the echo model of linear array SAR, the echo model of arc array SAR has an additional phase, which leads to the imbalance of multichannel. Directly using the linear array azimuth multi-channel reconstruction method, the imaging results will appear false targets. Finally, the inference is verified by point target simulation.
Arc array bistatic SAR with a satellite transmitter is a new imaging mode of array radar, which has a good application prospect in helicopter emergency rescue, military, environmental monitoring and other fields. However, due to the special structure of the receiving antenna, obtaining the two-dimensional spectrum directly from the fixed-phase principle poses a considerable challenge. In addition, when the satellite is moving at high speed, there is a serious range migration. To address the aforementioned challenges, this paper proposes an innovative imaging algorithm based on series inversion. Firstly, the range migration introduced by the satellite's high-speed motion is compensated in the range frequency domain. Then, the two-dimensional spectrum of echo data is obtained by the method of series inversion, the corresponding compensation factors are constructed to complete the migration correction and secondary range compression. Finally, simulation test is used to validate the suggested algorithm's efficacy.
Addressing the inefficiencies of conventional stripmap Synthetic Aperture Radar (SAR) imaging in scenarios where the target area does not align with the satellite ground track, this paper introduces a novel approach, the distance scanning off-axis stripmap imaging mode. This innovative mode dynamically adjusts the slant angle, Pulse Repetition Frequency (PRF), and start of sampling to adapt to complex imaging scenarios, thereby enhancing imaging efficiency and reducing imaging time. The paper also presents a comprehensive SAR system imaging process flowchart, which includes the computation of nadir satellite parameters, analysis of scene inclination angle variation, and definition of the sampling start rule. By employing a block variable PRF, the proposed mode effectively mitigates the issues associated with large range migration in conventional stripmap imaging. The effectiveness of the proposed mode is validated through imaging simulations. The results underscore its potential to enhance the performance of spaceborne SAR systems, particularly in scenarios where the target area does not align with the satellite ground track
The beam scanning range of frequency diversity arc array (FDAA) has all-round advantages. When it is equivalent to a linear array, it exhibits the characteristics of "The middle spacing is large, and the spacing between the two sides is gradually reduced", and there is an inverse density weighting phenomenon, which will lead to a high sidelobe of the FDAA beam. In order to further reduce the influence of sidelobe level and inverse density weighting, the amplitude weighting is carried out on the basis of the nonlinear frequency offset of the array element, but the amplitude weighting is realized by the attenuator in each channel, which will lead to the decrease of the antenna gain, which is generally used when the radar receives the signal. For the transmitter of antenna radar, this paper proposes a phase weighting method for nonlinear frequency offset. The effectiveness of this method for sidelobe suppression is proved by simulation.
In the bistatic arc array synthetic aperture radar (AA-BiSAR) with a moving transmitter, the moving transmitter is helpful to improve the flexibility of the imaging system and expand the imaging scene, which has important significance in the field of helicopter assisted landing, emergency rescue and so on. The motion state of the transmitter is closely related to the bistatic arc array SAR echo model and imaging quality. This article establishes the echo signal model under accelerated motion, analyzes the impact of acceleration on echo phase and imaging quality, studies the effect of different accelerations on the approximate expansion phase error of slant range, and provides relevant analysis results through point target simulation experiments. The analysis results have a certain guiding significance for the follow-up moving transmitter bistatic arc array SAR high-precision and accurate imaging and motion error compensation.
The spaceborne squint sliding spotlight mode provides the capacity to observe the Earth in high resolution with different angles. However, with the increased squint angle, the imaged swath is obviously reduced due to the large range cell migration. To extend the reduced swath, a new spaceborne wide swath sliding spotlight mode with a high squint angle is proposed in this paper. Besides azimuth beam steering to improve the azimuth resolution, antenna beam is also steered in elevation to improve the swath width. The imaging principle of the proposed imaging mode is described in detail, while its corresponding flowcharts of SAR system design and imaging processing are given. Furthermore, the beam steering law of the designed system example with azimuth resolution of 0.5m and swath width of 20km is given, while the imaging result of the designed scene with three targets is given. Both simulation results validate the proposed imaging mode in the high squint case.
Object detection in remote sensing images is a challenging task in field of computer vision since detection performance is negatively influenced by complicated background and various object size. However, most studies have only focused on object appearance, with only a few taking into account scene information, which is closely related to existence and category of objects. In this paper, we put forward a new method by integrating scene information into detection with aim of generating more powerful feature. Specifically, we made use of GRU cell, a special kind of RNN, in order to enhance object feature. The proposed method was verified through experiments on a challenging dataset, i.e., DOTA. Compared to the baseline model RoI-Transformer, the proposed method has achieved around 2.7% improvement in terms of mAP, which is initial attempt to integrate scene information into object detection.
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