Keith Morrison

High-resolution measurements of scattering in wheat canopies-implications for crop parameter retrieval

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.

Laboratory Measurement of the DInSAR Response to Spatiotemporal Variations in Soil Moisture

Differential interferometric synthetic aperture radar (DInSAR) has traditionally been used for the detection and accurate monitoring of surface movement in a scene and has found applications in fields such as mining subsidence and earthquake deformation. In these studies, the phase is understood to directly relate to the radial component of the physical deformation of the surface. In this paper, however, we use a novel combination of microwave and optical laboratory measurements to demonstrate the presence of persistent and coherent phase changes in a temporal sequence of DInSAR images, related solely to moisture change in a soil. This is confirmation of recent reports suggesting that, in some circumstances, the DInSAR signal may include a significant soil moisture signal. Laboratory measurements were used to obtain a set of high-resolution C-band DInSAR images of a sandy soil sample of an area of 2.0 m × 1.8 m and a depth of 0.2 m, with the fractional volumetric water content varying between 0.1 and 0.4. To independently monitor the soil surface for physical movement, a time-lapsed set of high-resolution digital optical images was continuously acquired. Although the soil underwent a large moisture change, the soil surface was static to within ±0.1 mm over the majority of the experiment. The DInSAR sequence displayed dynamic and complex variations of the phase, although a linear relationship with moisture change was evident when the mean phase change was considered. The work raises the possibility that DInSAR could be used for the monitoring of soil moisture change in a scene, a parameter of significant economic and environmental importance.

The SARALPS-2007 measurement campaign on Xand Ku-Band Backscatter of snow

The retrieval of snow parameters, and snow water equivalent in particular, are key parameters in hydrology and climate research. Theory, ground-based signature research and analysis of spaceborne scatterometry suggests that the high- frequency combination of Ku- and X-band active microwave sensors is an excellent tool for the retrieval of snow physical properties. In order to validate this, a snow measurement campaign was carried out with the University of Cranfields portable Ground-Based Synthetic Aperture Radar (GB-SAR) System during the winter of 2006/7 at two test-sites in the Austrian Alps close to Innsbruck. Fully polarimetric X-and Ku-band backscatter signatures were acquired over a range of incidence angles (~20deg-70deg), with the active sensor operating predominately in SAR mode, but occasionally also in InSAR mode. Microwave signatures and snow properties were measured on seven different dates. Detailed complementary meteorological and snow metamorphic conditions were also recorded.

Three-dimensional X-band SAR imaging of a small conifer tree

High spatial resolution 3-D SAR imagery was recorded by the UKs Natural Environment Research Council GB-SAR Microwave Measurement Facility at the University of Sheffield. X-band VV 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 backscattered signals 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 sufficiently high spatial resolution (∼5cm) 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.

Using DInSAR to Separate Surface and Subsurface Features

We report on an investigation into the use of differential interferometric synthetic aperture radar (SAR) (DInSAR) for the discrimination between surface and subsurface features in a soil, undertaken at the Ground-Based SAR Microwave Measurement Facility. A temporal sequence of C-band VV SAR images of a drying soil containing a buried target was collected. While the phase record of the signal identified with the soil return showed almost no variation, in stark contrast, the phase from the buried target showed a strongly linear change with time. A model is presented, which describes the observed phase changes in terms of retardation of the signal by the soil dielectric properties, which are dependent upon the moisture content. The model confirms a strongly linear relationship between phase and volumetric soil moisture. The linearity promises to greatly simplify any exploitation scheme, and such a DInSAR scheme would be applicable at large standoff distances from airborne and spaceborne platforms, in contrast to current subsurface techniques which rely on close-in measurement to spatially isolate returns vertically in backscatter.

Tomographic Profiling—A Technique for Multi-Incidence-Angle Retrieval of the Vertical SAR Backscattering Profiles of Biogeophysical Targets

Tomographic profiling (TP) is a new imaging technique designed to provide vertical backscatter profiles through biophysical and geophysical target volumes, such as snow, ice, vegetation, and forest canopies. Data is collected as for normal synthetic aperture radar (SAR) imaging, but with the antennas aligned along the scan or along-track direction. The real antenna provides a wide beam in the along-track direction, which is sharpened by the addition of elemental measurements across a subaperture using a SAR-like processing scheme. A novelty of the scheme is the ability to produce an image transect in which the incidence angle is constant at every point. This is accomplished by incrementally sliding the subaperture across the full aperture, and utilizing the appropriate subaperture to provide the necessary viewing geometry at each pixel. This is in contrast to the SAR case, in which the angle of incidence varies across a scene. By suitable phasing between the subaperture elements, the synthesized beam can be steered in angle within the wide angular extent of the real beam, allowing post-measurement retrieval of the backscattering properties of the scene over a continuous range of incidence angles from a single scan. In the across-track direction, a narrow real beam is required to maintain good vertical resolution and limit the size of the horizontal footprint. Example TP experimental fieldwork results are provided for a 42-cm-deep snowpack, collected with a ground-based SAR system. Although the scheme was developed for ground-based applications, its application to the airborne and satellite cases is also discussed.

Polarimetric calibration strategy for long-duration imaging with a ground-based SAR

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.

The UK NERC fully portable polarimetric ground-based synthetic aperture radar (GB-SAR)

A novel, fully portable, ground-based polarimetric synthetic aperture radar (GB-SAR) is described. The system is to be used to investigate radar backscatter from soils and vegetation canopies, so providing fundamental support for the use of radar in environmental science applications. A summary of the configuration of the instrument is provided, including detail of the hardware employed in its construction. The data processing procedures are presented, together with a summary of the system operating parameters. Finally, SAR imagery is presented to demonstrate the performance of the instrument in an outdoor environment.

Development of a ground-based, polarimetric synthetic aperture radar

A description of an indoor SAR measurement facility is given and a series of results is presented to demonstrate the imaging properties of the system. The aim of the work is to understand better the microwave backscatter characteristics of vegetation and soils. Based around a vector network analyser, the system uses synthetic pulse techniques to obtain high resolution images. The principles of the technique are presented, together with a description of the imaging algorithm used. Calibration procedures are implemented successfully and the images provide useful quantitative RCS information. The polarimetric capability of the system is demonstrated for both metallic and vegetation targets. The system is a precursor to an outdoor polarimetric ground-based synthetic aperture radar (GB-SAR), designed to be easily and rapidly deployable at widely separated measurement sites.

Recovery of badly motion-degraded SAR imagery by the use of frequency-randomized waveforms

The use of SAR imaging is an important tool in the laboratory RCS characterization of signature critical platforms. Despite measures to the contrary, air turbulence and mechanical vibration can produce complex perturbations of the target during the imaging process. Model code was written to provide simulations over a wide range of representative target motions and imaging schemes. The slow swept-frequency data collection schemes of many laboratory radars mean that the target can undergo significant motion during and between pulses, leading to substantial and time-varying defocusing of range profiles. Conventional motion-correction schemes cannot be used as they rely on the presence of clearly defined range profiles which can be tracked over the imaging process. It was found that replacement of a monotonically increasing frequency waveform with one in which the frequency sampling order was repeatedly randomized could produce a significant recovery of the imagery, especially in combination with data averaging. The pattern of the image degradation was found to have a complex dependence on the radar waveform scheme and target motion characteristics.