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Altimeter radar backscatter intensity, in terms of the normalized radar cross section (NRCS), is known to be modulated by surface wind forcing and the state of wind-sea development. Based on a data...

Synthetic aperture radar (SAR) imagery from the sea can contain ships and their ambiguities. The ambiguities are visually identifiable due to their high intensities in the low radar backscatter background of sea environments and can be mistaken as ships, resulting in false alarms in ship detection. Analysing polarimetric characteristics of ships and ambiguities, we found that (a) backscattering from a ship consisted of a mixture of single-bounced, double-bounced and depolarized or diffused scattering types due to its complex physical structure; (b) that only a strong single- or double-bounce scatterer produced ambiguities in azimuth that look like relatively strong double- or single-bounce scatterers, respectively; and (c) that eigenvalues corresponding to the single- or double-bounce scattering mechanisms of the ambiguities were high but the eigenvalue corresponding to the depolarized scattering mechanisms of the ambiguities was low. With these findings, we proposed a ship detection method that applies t...

Detection of springtime near-surface soil thaw events in mid-latitude areas is important for understanding the near-surface earth surface system. Radar backscatter time series, such as QuikSCAT Ku-band, are valuable for surface thaw detection in permafrost areas, but their usefulness in mid-latitudes remains unverified. In order to validate the application of radar backscatter time series for surface thaw detection in mid-latitude areas, we propose a new multi-step method based on the significant signature of radar backscatter time series when springtime soil thaw events happen. The method mainly focuses on the estimated geographical boundary of thaw events and the detection of the primary thaw date; the duration of the freeze–thaw transition period is a lesser focus. The method is applied to soil freeze–thaw conditions in a mid-latitude area – northern China and Mongolia – including humid land and dryland, and achieves a reliability of R2 = 0.678 ± 0.021 P < 0.01 by comparison with ground truth measurements. The results indicate that elevation/temperature and soil moisture conditions are the key drivers of the timing of springtime soil thaw events. The success of our study demonstrates the potential for our method to be applied in mid-latitude areas, and widens the application potential of radar backscatter time series beyond high north and permafrost studies.

The temporal characteristics of radar response from rice fields were analysed throughout the 1999 summer flooding season in the agricultural area of the central Korean Peninsula. Multiple scenes of Radarsat SAR images were acquired before, during, and after the flood inundation. From the interpretation of colour composite imagery of the multi-temporal SAR data, as well as from the temporal profiles of radar backscatter, three types of rice fields were separable according to flooding and post-flooding recovery conditions. The actual flooding damage to the standing rice crop depends on the duration of the flood and on subsequent recovery status. We found that image data acquired during and after flooding is necessary to assess accurately flood damage to rice fields.

Radar imaging is a valuable tool for the monitoring and management of tropical ecosystems. One of the obstacles to its successful usage is the variation of the radar backscatter with incidence angle caused by the side-looking mode of the sensor. Available research has modelled the effect of changes in incidence angle for soil, grass and scattering mechanisms in tree-covered areas. The effect varies depending on the scattering mechanism. To model the radar backscatter behaviour of a natural landscape, therefore, implies precise knowledge of the land-cover composition. A method for estimating and correcting the effect of changes in look angle on backscatter data is consequently proposed; it requires little field knowledge and encompasses the effects on most land-cover types in the study area. The method is based on the assumption that each line in azimuth direction contains a similar composition in regard to land-cover types. The backscatter frequency distribution of each azimuth line can then be employed to model and correct for the effect of variation in incidence angle. The results are evaluated using an existing land-cover classification to extract mean backscatter values for individual land-cover classes before and after the correction procedure. The correction procedure is shown to successfully adjust backscatter intensities to a nominal incidence angle for the vegetation communities in a coastal tropical savanna landscape in Australia's Northern Territory.

The use of spaceborne synthetic aperture radar (SAR) systems to estimate stem volume and biomass in boreal forests has shown some promising results, but with saturation of the radar backscatter at ...

C-band radar backscatter generally seems to be inversely related to accumulation rate across Greenland's dry snow zone. A coupled snow metamorphosis-backscatter model was constructed in order to investigate the observation. Accumulation rate and mean annual temperature were the coupled-model inputs. In a local area where accumulation rate is known very accurately the model predicts the backscatter level along several transects within the calibration uncertainties of the ERS-1 SAR data. The analysis was applied to the remainder of the dry snow zone. The model showed that both accumulation rate and temperature can modulate surface backscatter across Greenland's dry snow zone with accumulation rate being the primary influence.

The aim of this paper is to assess the use of RADARSAT data for rice field mapping and monitoring. The specific characteristics of RADARSAT data, i.e. the radar polarization, the spatial resolution, the range of incidence angles and the repeat cycle, have been studied. Experimental data have been collected over one of the most important irrigated flood plains of Indonesia and analysed along with RADARSAT data. The radar backscatter coefficient sigma of rice fields appears to have a significant temporal variation. However, the dynamic range of RADARSAT data is found lower than that of the ERS data due to a higher backscatter at HH than at VV polarization at early stages. At a higher incidence angle (42 compared with 23), the sigma of the fine resolution F3 mode reaches a saturation level earlier compared with the standard resolution mode, which appears consequently to be most appropriate for rice field mapping. The mapping result, based on the temporal change of the radar backscatter, was found to have an ...

Due to its large coverage, satellite based SAR (Synthetic Aperture Radar) has proven very useful in large scale oil spill detection and monitoring. However, radar backscatter values from oil spills are not unique. Other phenomena may dampen out the capillary and short gravity waves which are sensed by the radar, and cause dark areas (generally called slicks) in the SAR imagery. At present, operational satellite SAR oil spill detection systems are based on an operator's experience in slick classification. Discrimination between oil and lookalikes includes analysis of shape, size,dB-values, gradients, texture, instantaneous wind, currents, platform-, ship lane- and natural seepage locations. However, data showing the temporal evolvement of the wind (i.e. the wind history) during the day(s) before SAR imaging, may also contribute significantly in the interpretation process. The geometrical shape of the slick is matched against wind history to estimate slick age. A procedure for such use of wind history in SA...

Multi-temporal radar backscatter characteristics of crops and their underlying soils were analysed for an agricultural area in south-western Ontario, Canada using nine dates of ERS-1 SAR imagery acquired during the 1993 growing season. From the calibrated data, SAR temporal backscatter profiles were generated for each crop type. The results indicate that small changes in incidence-angle can have strong impacts on radar backscatter. Thus, attention must be given to local incidence-angle effects when using ERS-1 SAR data,especially when comparing backscatter coefficients of the same area from different scenes or different areas within the same scene.

In this paper we demonstrate SAR system capability for detecting and characterizing marine surface slicks. During an aircraft measurement campaign over the Gulf of Genoa (Italy), a multi-frequency SAR system, operating in P-, L- and C-bands, explored a sea area heavily covered by slicks. At the same time in situ measurements were performed with an interferential microwave probe, installed on board a small boat, capable of measuring high resolution sea spectra up to frequencies of capillary waves. By plotting SAR pixel intensity versus sea wave Bragg frequency we obtained wide portions of the sea spectrum region affected by the surface film damping. Spectra derived from SAR imagery and from gauge data present comparable slopes and furthermore the ratio between clean to slicked water spectrum obtained with the two techniques were surprisingly similar. This demonstrates the multi-frequency SAR systems ability to detect and characterize sea surface films assuming the Bragg mechanism in the radar backscatter. ...

A full understanding of radar backscatter from urban areas is necessary in order to develop a robust methodology for monitoring and classifying urban characteristics using remotely sensed Synthetic Aperture Radar images.This paper examines the dominant backscattering mechanisms such as single bounce from roofs, double bounce from wall-ground structures and possibly triple bounce from wall-wall-ground structures, and their relative contributions to the backscatter. With the use of quad-polarized image data such as those acquired by the NASA/JPL AirSAR system, the backscatter can be decomposed into components caused by different backscattering mechanisms, offering a promise for urban monitoring and classification.

SIR-C SAR data were related to the above ground biomass of regenerating tropical forests in Amazonia, Brazil. C- and L- band SAR data in the conventional polarization configurations showed no signi...

This paper presents the results of field testing a radar model which relates leaf area index to radar backscatter for ERS-1 C-band VV polarization SAR data. Ground truth measurements of leaf area index and soil moisture content were made in selected sugar beet fields, with simultaneous acquisition of ERS-1 SAR image data. Radar backscatter coefficients were derived from the calibrated ERS-1 SAR data. The Leeuwen and Clevers expression of the water cloud model was fitted to determine the in situ relationship between radar back-scatter and leaf area index. The model can be inverted analytically to calculate leaf area index from radar backscatter. The results show considerable potential for the operational application of ERS-1 SAR data in crop monitoring.

Complex dielectric constants determined by inversion of the polarized returns of radar images acquired in wet conditions clearly delineate the distribution of saline soils in the Tragowel Plains Irrigation Area of Victoria, Australia. There is good agreement between the areas delineated as having anomalous dielectric constants by the radar backscatter inversion techniques with saline areas as defined by geophysics and as inferred from dielectric constants determined in the field. The radar-determined complex dielectric constants are significantly smaller than might be expected from the known moisture contents of the soils at the time of image acquisition. The magnitudes of P-band radar-delermined dielectric constants most closely approach those expected from field determinations although the distribution of L-band-determined dielectric constants give the best discrimination between saline and non-saline areas as seen at the surface. Modification of the inversion algorithm to allow for the effects...

An algorithm for geocoding and radiometric correction of ERS–1 Synthetic Aperture Radar images by using Digital Elevation Model data is described a.nd applied to images over a mountainous area in Norway. Several images covering the same area are calibrated and geocoded into a UTM map projection and the results are evaluated against map and i/1 situ measurements. The map to SAR geometric precision is given by a rms value of 54m in the casting direction and 45m in the northing direction. The SAR to SAR geometric precision is given by a rms value of 43m in the casting direction and 44m in the northing direction. Estimates of the radar backscatter coefficient are extracted from the geocoded SAR backscatter images. Obtained values for wet snow–13dB and for dry snow–7dB which agrees favourably with earlier radar measurements and theory.

The significance of several key multi-frequency, polarimetric back-scatter parameters extracted from calibrated and noise-corrected NASA/JPL DC-8 SAR data are examined. The data were collected during the 1989 MAESTRO-1 campaign over the Flevoland agricultural test site. Calibration uncertainty estimates are used to specify minimum separations between features. Thirteen different backscatter types were identified from the test site data, including eleven different crops, one forest and one water area. Using the parameters with the highest separation for a given class, a hierarchical algorithm was developed to classify the entire image. All three frequencies and all polarizations were used to construct the rules for the classifier. Results indicate that multi-frequency, polarimetric radar backscatter signatures can be useful in classifying several different ground cover types in agricultural areas.

We applied the Santa Barbara canopy backscatter model to model radar backscatter from mangrove forest stands in the Ganges delta of southern Bangladesh, and assessed the feasibility of delineating flooding boundaries within the stands. Modelled L-band (0-235 m wavelength) HH backscatter showed that canopy volume scattering dominated for stands under nonflooded ground surface. Double bounce trunk-ground term were enhanced by the presence of water under trees. For flooded mangrove forest, the trunk-ground term was dominant at small radar incidence angles; the trunk-ground term dominancy reduced as the incidence angle increased. Shuttle Imaging Radar (SIR-B) data and model results showed that for the mangrove forest, radar data with small incidence angles should be used to delineate the flooding boundaries.

Initial observations on the effects of wildfires in black spruce forests on radar backscatter are presented. Airborne and spaceborne SAR imagery are utilized to illustrate two distinct fire signatures. A theory is presented to explain these differences.

The similarities and differences between imagery acquired at X-, C- and L-bands are examined by calculating the correlations between X-, C-, and L-band SAR data acquired over an agricultural area near Melfort, Saskatchewan in western Canada with the CCRS SAR-580. The SAR data were acquired on two separate occasions during the growing season (July and August, 1983) with vertical polarization in both transmit and receive. It was found that the correlations between the C- and L-band and between X- and L-band data were very low indicating that the radar backscatter at the different frequencies is caused by different mechanisms. The correlation between X- and C-band data was low for grains, which have a well defined vertical structure. For canola and field peas, which are both broadleaved plants with little definite structure, there was a higher correlation between the measured back-scatter values. For L-band data the primary cause of the radar backscatter is probably from the underlying soil and thus...

Measurements of radar backscatter from an artificial tree were made in the laboratory at 6 and 10 GHz at horizontal incidence. The system had a resolution cylinder 18 cm in diameter and 11 cm long, so about half of the tree (30.5 cm high) was within the beam. The mean returned power from the target with leaves was always higher than that from the target without leaves, as expected. The signal faded with an exponential distribution when the artificial tree was rotated about its trunk, with no angular trend apparent. When the leaves were present, tilted polarizations gave results favouring an angle corresponding with that of the branches. When leaves were absent, the trunk of the tree dominated the radar backscatter at all polarizations.

Helicopter-borne scatterometer measurements of various forest and surface types were conducted in spring 1988 in Espoo, near Helsinki. The scatterometer operates at 5-4 and 98 GHz and employs four linear polarizations (VV, HH, VH and HV). At each of the eight channels the radar can measure the backscattering properties of a target with a range resolution of 1 metre. The experimental results indicate that the location of backscattering sources within the forest canopy varies substantially with tree species. By applying principal-component analysis to the radar return versus range spectrum, identification of tree species is shown to be possible.

The current state of backscatter modelling of forested regions is reviewed, with a view to outlining possible future developments if suitable models are to be available to support research with for...

During the 1984 and 1985 winter seasons, radar backscatter measurements were performed on artificial sea ice at the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) at Hanover, New Hampshire. Radar data were collected at selected frequencies in the 4-17 GHz region for incidence angles from 0 to 60° with like and cross polarizations. These measurements were performed on smooth, rough, bare and snow-covered saline ice and open water. Backscattering from ice increased with its thickness until the ice was about 1 cm thick and then decreased gradually with further growth. Rough ice and snow-covered ice gave similar returns at 13-6 GHz, but the scattering coefficients of snow-covered ice were lower than that of rough ice at 9-6 GHz. Depolarized scattering from smooth, thin ice and water were much lower than from rough ice and snow-covered ice.

Radar backscatters from loam with a dry bulk density of 0·6g/cm3 have been measured at 9·9 GHz using both linear and circular polarizations. The sensitivity of radar return to soil moisture content has been obtained at five polarization combinations, HH, VV, HV, LR and LL (L and R denote the left-circular and the right-circular polarizations, respectively). Comparison of the moisture sensitivities shows that the sensitivity of HV is the highest among five polarizations and the sensitivity of LL is slightly higher than that of HH, VV and LR. Surface scatter theories are discussed in relation to the moisture sensitivities of five polarizations.