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- Frölind, Per-Olov, 1959, et al.
(author)
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Analysis of a ground target deployment in an airborne passive SAR experiment
- 2017
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In: 2017 IEEE Radar Conference, RadarConf 2017, 8-12 May. - 9781467388238 ; , s. 0273-0278
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Conference paper (peer-reviewed)abstract
- Air-to-ground surveillance requires using synthetic-aperture radar (SAR) technology for clutter reduction and for fulfilling resolution requirements. By using ultra-wideband VHF- and UHF-band SAR, detection of targets concealed by forests is enhanced, compared to using SAR operating at higher frequency bands. The need for fine resolution and hence large bandwidth, however, limits the use of the former technology due to lack of allocated frequency bands. Passive SAR using illuminators of opportunity is not limited in this respect and is an alternative concept to be evaluated. An experiment was performed in June 2016 in order to assess the potential of using passive UHF-band SAR. The airborne system LORA was used in a configuration with six 10-MHz receive channels, specifically with three antenna channels covering two 8-MHz digital TV (DVB-T) frequency bands centered on 530 and 594 MHz, respectively. The trial included two different target deployments on two different days for repeat pass analysis. Resulting passive SAR images shown are from the first day of deployment and based on one TV channel with three antenna elements providing beamforming. From these images, the target response of 13 objects consisting of corner reflectors and vehicles were evaluated.
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| 5. |
- Frölind, Per-Olov, 1959
(author)
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Ultra-wideband synthetic aperture radar in the VHF-band: Image formation algorithms and interferometric applications
- 2005
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Doctoral thesis (other academic/artistic)abstract
- Synthetic aperture radar (SAR) is a technology to generate high-resolution radar images for ground surveillance and mapping. By exploiting the lower VHF band, the attenuation reduces dramatically in forest areas compared to microwaves and new applications become feasible. Examples of these are detection of targets obscured by vegetation, stem volume estimation of forest stands, and generation of bald earth digital elevation models in forested areas. This thesis presents research results of importance for ultra-wideband VHF-band SAR,including interferometric technique.The processing steps and the results for stem volume retrieval are described in an early stage study of VHF-band SAR data. Low attenuation and backscattering levels were observed and thus a potential for forest inventory applications was recognized.Novel results of generating interferometric fringe images using VHF-band SAR data in a forest area are treated, and parts of the processing steps are discussed. The effects of the motion compensation for the interferometric coherence are investigated. The conclusion is that the coordinate system of individual SAR images should be selected as close as possible to the actual track heading.Further investigation of motion compensation effects in SAR processing is presented for quantitative geometrical error estimates of targets elevated from a nominal focusing height.An extensive investigation of bald earth height estimation in a boreal coniferous forest area is carried out and the choice of baseline is discussed. A wide range of baselines are applicable and give a similar height estimate quality. For a large area of 1 x 2 km2 an rms deviation of about 2 meters from ground truth data was obtained. Especially good results are obtained for small areas consisting of dense forest with strong backscattering and continuous high coherence, i.e. with ground truth rms deviation of less than 1 meter.As a final issue, angular interpolation is evaluated in a time domain SAR algorithm, based on fast factorised backprojection for generating high quality images. The results show that an image quality in parity with a global backprojection (GBP) reference image can be reached, using an adequate sampling rate and interpolation kernel size, while still gaining the benefit of a large reduction in the number of operations required. Using a cubic interpolator and a sampling rate of two times the maximum angular bandwidth, 2Bθ, and applying a Hamming window, the ISLR and PSLR along the one-dimensional azimuth direction is measured to -58 dB and -42 dB, respectively. Increasing the angular sampling rate to 8Bθ, results in an ISLR and PSLR of -60 dB and -48 dB, respectively, which equals the result obtained by the GBP reference algorithm.
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| 6. |
- Haglund, Anders, 1965, et al.
(author)
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Recovery of missing samples in Orthogonal Frequency Division Multiplexing signals with optimisation using data carriers
- 2024
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In: IET Radar, Sonar and Navigation. - 1751-8792 .- 1751-8784. ; In Press
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Journal article (peer-reviewed)abstract
- A method is proposed for reconstructing an Orthogonal Frequency Division Multiplexing (OFDM) signal that contains data gaps, with the aim to improve demodulation. The main objective is to use the method in a passive radar application with missing data samples and to improve target detection. The OFDM signal is assumed to comply with the Digital Video Broadcasting Terrestrial standard. The proposed recovery method is based on optimisation of a novel objective function, which consists of two parts. The first part is a function of the energy in the out-of-band frequencies, whereas the second, and novel part, uses the location of data carriers in the constellation diagram. The method is evaluated using both simulations and real data. The authors show that the proposed method significantly improves the OFDM signal in just a few iteration steps. The proposed method improved the condition number more than a factor ten thousand millions compared to using the least square method on the out-of-band frequencies only. The authors also decode the symbols with the Viterbi decoding algorithm and show how the required number of iterations with the proposed algorithm depends on the amount of missing samples and on the Signal-to-Noise Ratio in order to achieve a Bit Error Rate of less than one in one hundred thousand millions.
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| 7. |
- Rasmusson, J.R., et al.
(author)
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Bistatic VHF and UHF SAR for urban environments
- 2007
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In: Proceedings of SPIE -- Volume 6547, Radar Sensor Technology XI, James L. Kurtz, Robert J. Tan, Editors, 654705, May 2, 2007. ; 6547
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Conference paper (other academic/artistic)
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| 8. |
- Sjögren, Thomas, et al.
(author)
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Simultaneous passive SAR imaging and detection of airborne targets
- 2017
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In: IET Conference Publications. ; 2017:CP728
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Conference paper (peer-reviewed)abstract
- Passive radar has been given much attention for many years. It has several advantages in comparison to active radar, such as possibility of lower cost sensors, ability to operate without disturbing the environment as well as allowing the radar covert operation. As such, passive radar exploits signals of opportunity e.g. FM radio and DVB-T. Ground based passive radars is well proved and is usually for simplicity on a stationary platform, using an array of antennas and a dedicated antenna for reference channel. Most commonly these radars aim at detecting airborne objects. However, recently results have been presented with passive radar on airborne platforms, in which synthetic aperture radar (SAR) images using DVB-T signals has been performed. Only limited research results are published on the topic of air to air passive radar and even less in combination with SAR. In this paper, we present a method that combines passive SAR with airborne moving target detection.
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| 9. |
- Ulander, Lars, 1962, et al.
(author)
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Airborne passive SAR imaging based on DVB-T signals
- 2017
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In: IEEE International Geoscience and Remote Sensing Symposium (IGARSS). - 2153-7003. ; , s. 2408-2411
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Conference paper (peer-reviewed)abstract
- Passive SAR data have been registered using the airborne low frequency SAR system LORA. For this purpose, the radar sensor was reconfigured to not transmit any signals but only gather data along the pre-defined synthetic aperture. The receiver unit was tuned to fully accommodate the DVB-T signal centred at 634 MHz (channel #41), acting as the illuminator of opportunity within the ground scene mapped. SAR images have successfully been formed for areas located in the vicinity of the DVB-T transmitter mast but also at distances 40 km away. In the data set processed, the direct and the ground-reflected signals have been registered with the same antenna element.
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| 10. |
- Ulander, Lars, 1962, et al.
(author)
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Airborne SAR for Calibration of P-Band Tower Radar
- 2019
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In: International Geoscience and Remote Sensing Symposium (IGARSS). ; , s. 541-544
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Conference paper (peer-reviewed)abstract
- BorealScat is a tower-based radar which measures multipolarization and tomographic backscatter time series data at a forest site in Southern Sweden. In this paper, we describe a methodology for absolute radiometric calibration of BorealScat and apply it to a set of backscatter measurements at P-band (420–450 MHz). We present radiometric calibration corrections which take into account antenna gain and range variations over the tomographic resolution cell. We also use data acquired by the LORA airborne UHF-band SAR over the BorealScat site to determine the absolute radiometric calibration factor for P-band and HH- polarization. It is shown that the methodology produces consistent results for multiple SAR images.
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