Browsing by Author "Morrison, Keith"

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    Airborne S-Band SAR for forest biophysical retrieval in temperate mixed forests of the UK
    (MDPI, 2016-07-20) Ningthoujam, Ramesh K.; Balzter, Heiko; Tansey, Kevin; Morrison, Keith; Johnson, Sarah C. M.; Gerard, France; George, Charles; Malhi, Yadvinder; Burbidge, Geoff; Doody, Sam; Veck, Nick; Llewellyn, Gary M.; Blythe, Thomas; Rodriguez-Vega, Pedro; van Beijma, Sybrand; Spies, Bernard; Barnes, Chloe; Padilla-Parellada, Mark; Wheeler, James E. M.; Louis, Valentin; Potter, Tom; Edwards-Smith, Alexander; Polo Bermejo, Jamie
    Radar backscatter from forest canopies is related to forest cover, canopy structure and aboveground biomass (AGB). The S-band frequency (3.1–3.3 GHz) lies between the longer L-band (1–2 GHz) and the shorter C-band (5–6 GHz) and has been insufficiently studied for forest applications due to limited data availability. In anticipation of the British built NovaSAR-S satellite mission, this study evaluates the benefits of polarimetric S-band SAR for forest biophysical properties. To understand the scattering mechanisms in forest canopies at S-band the Michigan Microwave Canopy Scattering (MIMICS-I) radiative transfer model was used. S-band backscatter was found to have high sensitivity to the forest canopy characteristics across all polarisations and incidence angles. This sensitivity originates from ground/trunk interaction as the dominant scattering mechanism related to broadleaved species for co-polarised mode and specific incidence angles. The study was carried out in the temperate mixed forest at Savernake Forest and Wytham Woods in southern England, where airborne S-band SAR imagery and field data are available from the recent AirSAR campaign. Field data from the test sites revealed wide ranges of forest parameters, including average canopy height (6–23 m), diameter at breast-height (7–42 cm), basal area (0.2–56 m2/ha), stem density (20–350 trees/ha) and woody biomass density (31–520 t/ha). S-band backscatter-biomass relationships suggest increasing backscatter sensitivity to forest AGB with least error between 90.63 and 99.39 t/ha and coefficient of determination (r2) between 0.42 and 0.47 for the co-polarised channel at 0.25 ha resolution. The conclusion is that S-band SAR data such as from NovaSAR-S is suitable for monitoring forest aboveground biomass less than 100 t/ha at 25 m resolution in low to medium incidence angle range.
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    High resolution PolInSAR with the ground-based SAR (GB-SAR) System: measurement and modelling
    (2006-05-08T09:13:21Z) Morrison, Keith; Williams, Mark L.
    Ground-based work is necessary for a comprehensive assessment of the operational potential and limitations of PolInSAR in airborne and satellite SAR applications. A study is made of the performance and usefulness of the UK’s Ground-Based SAR (GB-SAR) Outdoor System in high-resolution PolInSAR studies of vegetation using modeling results. The facility provides fully-polarimetric L- through X-band imagery down to a resolution of several wavelengths. However, the measurement process is slow in relation to pulsed systems as it requires the antenna head to be mechanically scanned across an aperture. The PolInSAR technique requires high coherence between interferometric image pairs, and the long data acquisition times raise the question of temporal decorrelation. We developed two models incorporating motion, a physics-based model and a signal processing model. The former incorporates a PolInSAR crop simulator employing the distorted Born approximation, applied to a simulated canopy of wheat plants based on field-collected physiological measurements. GB-SAR simulations of mature wheat canopies suffering a range of wind-blown disturbances are examined for coherence stability. These calculations permit the analysis of the behaviour of coherence with system and canopy descriptive parameters, such to quantify the suitability and performance of measurement environments for PolInSAR analysis. The models indicate that clutter motion will degrade interferometric performance both during aperture formation, and between repeat-pass observation. However, we conclude that the GB-SAR system will be robust to small amounts of clutter motion and will serve as a suitable tool for PolInSAR experimental studies.
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    High-resolution measurements of scattering in wheat canopies-implications for crop parameter retrieval
    (IEEE, 2006-05-04T00:00:00Z) Morrison, Keith; Brown, Sarah C. M.; Quegan, Shaun; Bennett, John C.; Cookmartin, Geoff
    Polarimetric X- and C-band measurements by the University of Sheffield ground-based synthetic aperture radar(GB-SAR)indoor system provide three-dimensional images of the scattering processes in wheat canopies, at resolutions of around a wavelength (3–6 cm). The scattering shows a pronounced layered structure, with strong returns from the soil and the flag leaves, and in some cases a second leaf layer. Differential attenuation at horizontal (H) and vertical (V) polarization, due to the predominantly vertical structure of the wheat stems, gives rise to marked effects. At both C and X bands, direct return from the canopy exceeds the soil return at large incidence angles for VV polarization, but is comparable to or less than the soil return in all other cases. At HV, the apparent ground return is probably due to a double-bounce mechanism, and volume scattering is never the dominant term. Direct sensing of the crop canopy is most effective at X band, VV, and large incidence angles, under which conditions the return is dominated by the flag leaf layer. Field measurements with the outdoor GB-SAR system suggest, however, that for sensitivity to biomass and reduced susceptibility to disturbances by rainfall, a two-channel C-band system operating at a medium range of incidence angles is preferred.
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    Mapping forest cover and forest cover change with airborne S-band radar
    (2016-07-08) Ningthoujam, Ramesh K.; Tansey, Kevin; Balzter, Heiko; Morrison, Keith; Johnson, Sarah C. M.; Gerard, France; George, Charles; Burbidge, Geoff; Doody, Sam; Veck, Nick; Llewellyn, Gary M.; Blythe, Thomas
    Assessments of forest cover, forest carbon stocks and carbon emissions from deforestation and degradation are increasingly important components of sustainable resource management, for combating biodiversity loss and in climate mitigation policies. Satellite remote sensing provides the only means for mapping global forest cover regularly. However, forest classification with optical data is limited by its insensitivity to three-dimensional canopy structure and cloud cover obscuring many forest regions. Synthetic Aperture Radar (SAR) sensors are increasingly being used to mitigate these problems, mainly in the L-, C- and X-band domains of the electromagnetic spectrum. S-band has not been systematically studied for this purpose. In anticipation of the British built NovaSAR-S satellite mission, this study evaluates the benefits of polarimetric S-band SAR for forest characterisation. The Michigan Microwave Canopy Scattering (MIMICS-I) radiative transfer model is utilised to understand the scattering mechanisms in forest canopies at S-band. The MIMICS-I model reveals strong S-band backscatter sensitivity to the forest canopy in comparison to soil characteristics across all polarisations and incidence angles. Airborne S-band SAR imagery over the temperate mixed forest of Savernake Forest in southern England is analysed for its information content. Based on the modelling results, S-band HH- and VV-polarisation radar backscatter and the Radar Forest Degradation Index (RFDI) are used in a forest/non-forest Maximum Likelihood classification at a spatial resolution of 6 m (70% overall accuracy, κ = 0.41) and 20 m (63% overall accuracy, κ = 0.27). The conclusion is that S-band SAR such as from NovaSAR-S is likely to be suitable for monitoring forest cover and its changes.
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    Polarimetric calibration strategy for long-duration imaging with a ground-based SAR
    (2006-05-04T15:02:52Z) Morrison, Keith; Cookmartin, Geoff; Bennett, J. C.; Quegan, Shaun; Race, A.
    The Ground-Based Synthetic Aperture Radar (GB-SAR) facility in the UK provides high-resolution, fully polarimetrically calibrated L- through X-band SAR imagery, principally of targets of remote sensing interest such as soils and vegetation. The facility consists of an indoor laboratory and a portable outdoor imaging system. Details of the polarimetric calibrations of both systems are discussed, with consideration given to the special requirements of field operation. Because of the need to mechanically scan the real antenna to build up a synthetic aperture, the SAR imaging process is significantly longer than its airborne and satellite counterparts. Some of the extended imaging schemes, such as those used in three-dimensional tomographic imaging and diurnal monitoring campaigns, can last from hours to days. However, calibration is normally only possible just prior to, and just after, imaging, leaving the data susceptible to nonlinear system sensitivity fluctuations during the imaging process itself. To address this problem, a novel scheme is discussed that utilizes the signal that arises from the imperfection in the rf isolation of the antenna head as a diagnostic to account for sensitivity fluctuations. Variations of several decibels were seen on a time scale of hours over an extended 2 day measurement. Excellent agreement was found with radar cross section (RCS) fluctuations retrieved from contemporaneous SAR imagery of reference trihedrals placed in the scene.
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    Subsurface radar imaging from space
    (Cranfield University, 2018) Edwards-Smith, A. J.; Andre, Daniel; Morrison, Keith
    Ground Penetrating Radar (GPR) and Synthetic Aperture Radar (SAR) are two widely used techniques for acquiring radar images. GPR, as its name suggests, produces radar images of the below ground environment. SAR is a remote sensing technique which allows moving radar systems to produce radar images with dramatically improved resolutions over conventional radar systems. Despite their benefits, both GPR and SAR suffer from certain limitations. In the case of GPR, the radar system has to be in close proximity with the subsurface volume being surveyed, which limits the process to relatively small areas that are easily accessible. SAR allows large areas to be surveyed rapidly from large distances, but cannot distinguish buried objects from surface objects. This thesis focuses on a radar technique that offers the opportunity to overcome these limitations and allow subsurface radar imaging of large areas using radar data gathered by remote sensing systems. This novel technique is known as Virtual Bandwidth SAR (VB-SAR). VB-SAR utilises changes in soil moisture over a series of SAR images to differentiate buried objects from objects on the surface. In addition to this differentiation, VB-SAR also allows extremely high (centimetre scale) subsurface range resolutions to be obtained from SAR images with range resolutions measured in metres. This research has experimentally demonstrated the basic feasibility of performing remote subsurface radar imaging with the VB-SAR scheme. Within the laboratory environment a buried target has been successfully imaged using VB-SAR and the fundamentals of VB-SAR have been verified. Dramatic increases in subsurface range resolutions have been demonstrated, as has the ability of the VB-SAR scheme to work correctly over a range of radar frequencies, observation angles and polarisations. This laboratory work has been enabled by use of the Tomographic Profiling (TP) imaging scheme. TP is a synthetic aperture based imaging algorithm, but unlike conventional SAR TP produces images with a constant look angle over the entire imaging scene. This enabled the performance of the VB-SAR imaging scheme to be easily evaluated over a range of look angles using a single radar dataset and simplified the experimental setup. In addition to the experimental work, simulation exercises have been conducted and image processors have been implemented. Simulation, using a simulator created as part of this work, has allowed testing of the VB-SAR scheme in a range of scenarios (sidelooking SAR, different soils, multiple buried targets). The image processor work has implemented a high performance TP processor and a practical VB-SAR imager.
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    Three-dimensional X-band SAR imaging of a small conifer tree
    (2006-05-04T15:25:08Z) Morrison, Keith; Bennett, J. C.; Cookmartin, Geoff; McDonald, A. J.; Race, A.; Quegan, Shaun
    High spatial resolution 3-D SAR imagery was recorded by the UK’s Natural Environment Research Council GB-SAR Microwave Measurement Facility at the University of Sheffield. X-band V V polarisation measurements were made using a near-field monostatic imaging system inside an anechoic chamber. The measurement process employs vector network analyser techniques to sample backscatteredsignals over a 2-D aperture, allowing a 3-D reconstruction of a target. This technique is used to provide a detailed 3-D map of the spatial scattering behaviour of a small Colorado Blue Spruce tree (Picea pungens glauca). The images produced are at a su“ ciently high spatial resolution ( ~ 5 cm) that individual plant components can be discerned. An ability to select any volume pixel from within the target allows features in the microwave reconstruction to be readily associated with structures in the tree. The scattering behaviour associated with the uppermost set of branches shows it to be dominated by scattering from the branch tips.
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    A very high resolution X- and Ku-band field study of a barley crop in support of the SWINTOL Project
    (2016-08-10) Bermejo, J. P.; Morrison, Keith
    SAR Wave INteraction for Natural Targets Over Land (SWINTOL) is a project funded by the European Space Agency. The study’s goal is to better understand the interaction of high frequency radar (> X-band) with vegetation and soils, in order to drive the development of a high-frequency electromagnetic model to simulate SAR imagery at high resolution (< 1 m). Existing models work well at C and X band frequencies, but do not work properly at higher frequencies. Cranfield University’s role in this project was to provide the field data necessary for model validation and development. Radar imagery was taken of a barley crop over an entire growing season. The portable outdoor GB-SAR system used the tomographic profiling (TP) technique to capture polarimetric imagery of the crop. TP is a scheme that provides detailed maps of the vertical backscatter pattern through a crop canopy, along a narrow transect directly beneath the radar platform. Fully-polarimetric imagery was obtained across overlapping 6.5 GHz bandwidths over the X- and Ku-band frequency range 8-20 GHz. This gave the opportunity to see the detailed scattering behaviour within the crop at the plant component level, from emergence of the crop through to harvesting. In combination with the imagery, full bio-geophysical characterisation of the crop and soil was made on each measurement date. Surface roughness characterisation of the soil was captured using a 3D optical stereoscopic system. This work details the measurements made, and provides a comparative assessment of the results in terms of understanding the backscatter in relation to biophysical and radar parameters.
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    Virtual bandwith SAR (VB-SAR) for centimeter-scale vertical profiling through a soil at C-band from space
    (IEEE, 2019-11-03) Morrison, Keith; Edwards-Smith, Alexander; Zwieback, Simon; Hajnsek, Irena
    The first experimental demonstration of the Virtual Bandwidth SAR (VB-SAR) scheme is provided. VB-SAR is a new technique that promises subsurface imaging of soils at ultra-high, centimeter-scale resolution at large stand-off distances applicable to aircraft and spacecraft. This paper reports on how a stack of C-band images were used to retrieve high resolution vertical profiles of the backscattering through a soil in the laboratory. The VB-SAR scheme captures the phase behavior of a soil across a stack of DInSAR images as the soil dries. The real frequency of the interrogating radar behaves as a higher, virtual frequency within the soil by virtue of its higher-than-air dielectric. As the dielectric changes with time, the DInSAR stack captures a virtual bandwidth. Using this scheme, it was possible to produce a vertical slice of the backscatter through a soil at 10cm resolution, much improved on the formal 1m resolution offered by the real 150MHz bandwidth.
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    Widespread occurrence of anomalous C-band backscatter signals in arid environments caused by subsurface scattering
    (Elsevier, 2022-04-22) Wagner, Wolfgang; Lindorfer, Roland; Melzer, Thomas; Hahn, Sebastian; Bauer-Marschallinger, Bernhard; Morrison, Keith; Calvet, Jean-Christophe; Hobbs, Stephen; Quast, Raphael; Greimeister-Pfeil, Isabella; Vreugdenhil, Mariette
    Backscatter measured by scatterometers and Synthetic Aperture Radars is sensitive to the dielectric properties of the soil and normally increases with increasing soil moisture content. However, when the soil is dry, the radar waves penetrate deeper into the soil, potentially sensing subsurface scatterers such as near-surface rocks and stones. In this paper we propose an exponential model to describe the impact of such subsurface scatterers on C-Band backscatter measurements acquired by the Advanced Scatterometer (ASCAT) on board of the METOP satellites. The model predicts an increase of the subsurface scattering contributions with decreasing soil wetness that may counteract the signal from the soil surface. This may cause anomalous backscatter signals that deteriorate soil moisture retrievals from ASCAT. We test whether this new model is able to explain ASCAT observations better than a bare soil backscatter model without a subsurface scattering term, using k-fold cross validation and the Bayesian Information Criterion for model selection. We find that arid landscapes with Leptosols and Arenosols represent ideal environmental conditions for the occurrence of subsurface scattering. Nonetheless, subsurface scattering may also become important in more humid environments during dry spells. We conclude that subsurface scattering is a widespread phenomenon that (i) needs to be accounted for in active microwave soil moisture retrievals and (ii) has a potential for soil mapping, particularly in arid and semi-arid environments.