| 1. |
- Ivanenko, Yevhen, et al.
(författare)
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Interpolation methods for SAR backprojection at THz frequencies
- 2021
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Ingår i: 2021 4th International Workshop on Mobile Terahertz Systems, IWMTS 2021. - : Institute of Electrical and Electronics Engineers Inc.. - 9781728184395
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Konferensbidrag (refereegranskat)abstract
- The paper proposes the extensions of the available linear and cubic interpolation methods for backprojecting complex SAR data into an image plane. Due to the fact that the phase of complex SAR data is very sensitive to the shift in time, the proposed interpolations include the phase control of the interpolated complex values. The proposed methods are examined with the global backprojection algorithm that is used to process SAR data at THz frequencies. In numerical examples, a two-dimensional indoor THz SAR imaging for a point target is considered, where the developed interpolation methods are compared with the nearest neighbor approach. © 2021 IEEE.
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| 2. |
- Ludwig Barbosa, Vinícius, 1990-, et al.
(författare)
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Supervised Detection of Ionospheric Scintillation in Low-Latitude Radio Occultation Measurements
- 2021
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Ingår i: Remote Sensing. - : MDPI. - 2072-4292. ; 13:9
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Tidskriftsartikel (refereegranskat)abstract
- Global Navigation Satellite System (GNSS) Radio Occultation (RO) has provided high- quality atmospheric data assimilated in Numerical Weather Prediction (NWP) models and climatol- ogy studies for more than 20 years. In the satellite–satellite GNSS-RO geometry, the measurements are susceptible to ionospheric scintillation depending on the solar and geomagnetic activity, seasons, geographical location and local time. This study investigates the application of the Support Vector Machine (SVM) algorithm in developing an automatic detection model of F-layer scintillation in GNSS-RO measurements using power spectral density (PSD). The model is intended for future analyses on the influence of space weather and solar activity on RO data products over long time periods. A novel data set of occultations is used to train the SVM algorithm. The data set is composed of events at low latitudes on 15–20 March 2015 (St. Patrick’s Day geomagnetic storm, high solar flux) and 14–19 May 2018 (quiet period, low solar flux). A few conditional criteria were first applied to a total of 5340 occultations to define a set of 858 scintillation candidates. Models were trained with scintillation indices and PSDs as training features and were either linear or Gaussian kernel. The investigations also show that besides the intensity PSD, the (excess) phase PSD has a positive contribution in increasing the detection of true positives.
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| 3. |
- Ramos, Lucas P., et al.
(författare)
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A wavelength-resolution sar change detection method based on image stack through robust principal component analysis
- 2021
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Ingår i: Remote Sensing. - : MDPI AG. - 2072-4292. ; 13:5, s. 1-16
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Tidskriftsartikel (refereegranskat)abstract
- Recently, it was demonstrated that low-frequency wavelength-resolution synthetic aperture radar (SAR) images could be considered to follow an additive mixing model due to their backscatter characteristics. This simplification allows for the use of source separation methods, such as robust principal component analysis (RPCA) via principal component pursuit (PCP), for detecting changes in those images. In this manuscript, a change detection method for wavelength-resolution SAR images based on image stack through RPCA is proposed. The method aims to explore both the temporal and flight heading diversity of a set of wavelength-resolution multitemporal SAR images in order to detect concealed targets in forestry areas. A heuristic based on three rules for better exploring the RPCA results is introduced, and a new configurable parameter for false alarm reduction based on the analysis of image windows is proposed. The method is evaluated using real data obtained from measurements of the ultrawideband (UWB) very high-frequency (VHF) SAR system CARABAS-II. Experiments for stacks of four and seven reference images are conducted, and the use of reference images acquired with different flight headings is explored. The results indicate that a gain in performance can be achieved by using large image stacks containing, at least, one image of each possible flight heading of the data set, which can result in a probability of detection (PD) above 99% for a false alarm rate (FAR) as low as one false alarm per three square kilometers. Furthermore, it is demonstrated that high PD and low FAR can be achieved, also considering images from similar flight headings as reference images. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
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| 4. |
- Sievert, Thomas, et al.
(författare)
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Using A Sliding Window Phase Matching Method for Imaging of GNSS Radio Occultation Signals
- 2021
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Ingår i: Remote Sensing. - : MDPI. - 2072-4292. ; 13:5
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Tidskriftsartikel (refereegranskat)abstract
- Global Navigation Satellite System Radio Occultation (GNSS-RO) is a technique used to sound the atmosphere and derive vertical profiles of refractivity. Signals from GNSS satellites are received in a low-Earth orbit, and they are then processed to produce bending angle profiles, from which meteorological parameters can be retrieved. Generating two-dimensional images in the form of spectrograms from GNSS-RO signals is commonly done to, for instance, investigate reflections or estimate signal quality in the lower troposphere. This is typically implemented using, e.g., the Short-Time Fourier Transform (STFT) to produce a time-frequency representation that is subsequently transformed to bending angle (BA) and impact height (IH) coordinates by non-linear mapping. In this paper, we propose an alternative method based on a straightforward extension of the Phase Matching (PM) operator to produce two-dimensional spectral images in the BA-IH domain by applying a sliding window. This Sliding Window Phase Matching (SWPM) method generates the spectral amplitude on an arbitrary grid in BA and IH, e.g., along the coordinate axes. To illustrate, we show both SWPM and STFT methods applied to operational MetOp-A data. For SWPM we use a constant window in the BA-dimension, whereas for STFT we use a conventional constant time window. We show that the SWPM method produces the same result as STFT when the same window length is used for both methods. The sample points in impact parameter and bending angle are those generated by and the main advantage is that SWPM offers the user a convenient way to freely sample the BA-IH space. The cost for this is processing time that is somewhat longer than implementations based on the Fast Fourier Transform, such as the STFT method.
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| 5. |
- Voigt, Gustavo H. M., et al.
(författare)
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Change detection method for intensity VHF wavelength-resolution SAR images
- 2021
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 9781510645684
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Konferensbidrag (refereegranskat)abstract
- This paper presents the proposal of a new change detection method for intensity VHF wavelength-resolution images. High-Amplitude pixels are related to the presence of strong scatterers, resulting in high detection probability performance. However, the number of false alarms tends to be high too. In this initial study, difference images are considered to reduce the influence of the strong scatterers that are not related to targets, i.e., present in both surveillance and reference images. The proposed change detection method is based on a likelihood-ratio test, where the tested hypothesis is the bivariate exponential distribution. The derivation of the proposed likelihood test is presented. Finally, the proposed change detection method is assessed considering data measured with the CARABAS II VHF UWB SAR system. Preliminary results show that the proposed method is efficient in detecting positive changes. © 2021 SPIE.
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| 6. |
- Vu, Viet Thuy, 1977-
(författare)
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Area Resolution for Bistatic Ultrawideband Ultrawidebeam SAR
- 2021
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Ingår i: IEEE Transactions on Aerospace and Electronic Systems. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9251 .- 1557-9603. ; 57:2, s. 1371-1377
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Tidskriftsartikel (refereegranskat)abstract
- Conventional SAR resolution equations cannot be applied to ultrawideband ultrawidebeam systems, particularly, bistatic ultrawideband ultrawidebeam systems. This is because the dependence of system frequency response in the range and across range is not considered. In this case, area resolution, i.e., area of the resolution cell, will be an alternative. This article introduces a new area resolution formula for bistatic ultrawideband ultrawidebeam SAR systems based on the relationship between radar signal frequency, range, and cross-range wavenumbers for bistatic synthetic aperture radar.
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| 7. |
- Vu, Viet Thuy, 1977-, et al.
(författare)
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Changing flight heading during pass to enhance SAR change detection performance
- 2021
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Ingår i: IET radar, sonar & navigation. - : John Wiley and Sons Inc. - 1751-8784 .- 1751-8792. ; 15:8, s. 817-826
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Tidskriftsartikel (refereegranskat)abstract
- Due to the wavelength-resolution characteristics, wavelength-resolution synthetic aperture radar (SAR) is considered for the change detection application aiming to achieve high detection probability and low false alarm rate. However, wavelength-resolution SAR change detection suffers from the problems caused by specular reflection of the elongated structure in the SAR scene, the dependency of radar cross-section on incident angle of electromagnetic wave, the interference due to antenna backlobe and so forth. Hardware and software solutions are therefore desired to deal with these problems. The authors propose to change flight heading during passes as a natural solution. The proposed solution is tested with the CARABAS data using a typical SAR change detection method. The achieved change detection results show the good performance of the proposed solution, for example, the detection probability is up to 100% with the false alarm rate of 0.5 per square kilometre. © 2021 The Authors. IET Radar, Sonar & Navigation published by John Wiley & Sons Ltd on behalf of The Institution of Engineering and Technology.
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| 8. |
- Vu, Viet Thuy, 1977-, et al.
(författare)
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New Forms of Likelihood Ratio Test for SAR Change Detection
- 2021
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Ingår i: IEEE Access. - : Institute of Electrical and Electronics Engineers Inc.. - 2169-3536. ; 9, s. 127906-127916
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Tidskriftsartikel (refereegranskat)abstract
- The Neyman-Pearson lemma, i.e., the likelihood ratio test and its generalized version, have been used for the development of the synthetic aperture radar (SAR) change detection methods. For detecting changes caused by targets on the ground such as vehicles, a target model, or at least certain assumptions concerning the targets, are always required for deriving a statistical hypothesis test. Without the prior knowledge on targets, it is difficult to make any assumption. An inappropriate assumption can degrade change detection performance significantly. To avoid this technical issue, the new forms of likelihood ratio test for SAR change detection are introduced in this paper. The proposed forms are shown to be very flexible. They can be utilized to develop change detection methods for different types of data, e.g., data in scalar form, data in vector form, data represented in complex number, and data represented in real number. The flexibility of the proposed forms is also shown by the the capability to implement change detection methods in the iterative and non-iterative ways. For the illustration purpose, a new change detection method is developed on one of the introduced forms and tested using TanDEM-X data measured in Karlshamn, Sweden in 2016. Author
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| 9. |
- Vu, Viet Thuy, 1977-, et al.
(författare)
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Terahertz three-dimensional SAR imaging using time- And frequency-domain algorithms
- 2021
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Ingår i: 2021 4th International Workshop on Mobile Terahertz Systems, IWMTS 2021. - : Institute of Electrical and Electronics Engineers Inc.. - 9781728184395
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Konferensbidrag (refereegranskat)abstract
- Terahertz (THz) SAR opens opportunity for some new applications, e.g., sub-mm localization based on 3D SAR imaging, that cannot be based on the conventional SAR operating at frequency ranges below THz. For 3D THz SAR imaging, the image formation algorithms play an important role. Two 3D SAR image formation algorithms, Range Migration (RM) operating in frequency domain and Backprojection (BP) operating in time domain are considered in this comparative study. The algorithms are first used to form 3D images from simulated THz SAR data. The comparison is then based on image quality measurements such as range and cross-range resolutions, and peak sidelobe ratio (PSLR). © 2021 IEEE.
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