Bidirectional reflectance effects in NOAA AVHRR data

The bidirectional reflectance effects in NOAA AVHRR data have been investigated for forest and pasture sites in New Zealand. The impact of surface anisotropy has been examined for channel 1, channel 2, and the derived Normalized Difference Vegetation Index (NDVI) over a 14-day period in the southern hemisphere summer of 1992/1993. Results show a bidirectional effect which persists through atmospheric correction processing and the generation of the NDVI. Comparison is made with previously published results and models, which show consistency for this limited data set.     

Contextual Pattern Recognition Applied to Cloud Detection and Identification

The use of contextuial information in classification of satellite imagery of clouds is considered. A general contextual classification rule is developed which is then simplified under some realistic assumptions made. for the label relationships of neighboring cloud image pixels. The resulting decision rule is defined in terms of transition probabilities between pairs of neighboring pixels. Experimental evaluation of the rule confirms its superiority over the conventional Bayes rule and provides justification for the assumption made.     

A System Design for a Multispectral Sensor Using Two-Dimensional Solid-State Imaging Arrays

The architecture of a system for the simultaneous capture of up to four images from solid-state two-dimensional image sensors is described. The system has been developed for remote sensing from light aircraft in the 400-1100-nm wavelength range. Software generation of sensor drive signals is incorporated in the system.     

Stand age retrieval in production forest stands in New Zealand using C- and L-band polarimetric Radar

Regressions of single-, dual-, quad-, and full-polarization L- and C-band synthetic aperture radar (SAR) against stand age from 403 radiata pine stands in Kaingaroa Forest, New Zealand have been carried out, using the National Aeronautics and Space Administration's Airborne SAR instrument. The regressions attempted to find products suitable for the separation of young (two years or less) from old stands (25 years or older), and for the estimation of stand age. Local incidence angle had no significant effect for C-band, but was always significant for L-band, giving a standard error reduction of 13% to 32% for log stand age. Stand density was highly significant for both bands, giving standard error reductions of 7% to 47%. Single- and dual-polarization products were severely biased, and it was impossible to separate young and old stands, except L-band horizontal-(HH)-plus-horizontal-vertical (HV). C-band quad-polarization gave less bias and lower error than for that L-band, when local incidence angle and stand density were excluded. C-band full-polarization using covariance magnitudes gave no improvement over C-band quad-polarization, but L-band did give a significant improvement. The C- and L-band full-polarization products with six polarimetric indices gave significant improvements in the standard error. The results show that regressions of SAR data with stand age are possible with full- and quad-polarization L- and C-band datasets, although the prediction limits increase rapidly with stand age. The smallest error in estimated stand age, with an RMS of 3.22 years, was for L-band full-polarization with six polarimetric indices, calculated from a validation dataset. Separation of young and old forest stands was only possible for full-, quad-polarization, and the L-band HH-plus-HV products.     

Correlation and relaxation labelling: An experimental investigation on fast algorithms

This paper compares experimental results between three popular matching functions: the cross-correlation coefficient (CCC); the sum of squared difference (SSD); and sum of the absolute value of difference (SAVD), within our newly developed correlation-relaxation (C-R) framework (Wu 1995). The C-R framework is a general method for determining optical flow and has been applied to determining cloud motion from satellite images. SSD and SAVD are simpler and faster functions to calculate, when compared with CCC, and their uses can lead to significant savings in computer time in the initial selection of displacement candidates. Given that the image distortion is Gaussian noise, and the motion is translational, the study shows that while computationally more expensive, the performance of the CCC function is better, or at least no worse, than using SSD and SAVD in the selection of initial displacement candidates. Similarly, the performance of SSD is better, or no worse, than using SAVD. Computationally, SSD is the fastest among the three functions. In the presence of high level distortion, however, the poor quality of initial candidates selected using SSD and SAVD usually means a large number of iterations of the subsequent relaxation labelling process. In contrast, the CCC function gives high quality initial candidates, and only a small number of iterations are needed. The CCC function also usually leads to better final quality in motion estimations than that produced using the SSD or the SAVD function in the C-R algorithm. In the presence of moderate and low level distortion, however, the performance of SSD can be adequate, and its use can lead to faster processing without much sacrifice to the overall motion estimation quality.     

A relaxation labeling technique for computing sea surfacevelocities from sea surface temperature

The paper applies a relaxation labeling technique to computing sea surface advective velocities from sequential satellite sea surface temperature (SST) images. The technique first identifies prominent feature points in a pair of time-lapsed SST images, and then estimates sea surface current velocities by establishing feature point correspondences between the two images. In the context of relaxation labelling operation the authors have constructed a compatibility coefficient expressing flow field spatial smoothness, or coherency, suitable for fluid motion. New results are reported and are compared to those obtained using two other methods: the geostrophic currents from in situ measurements and the maximum cross-correlation (MCC) method. The three kinds of velocities are shown to be consistent to one another     

Computing advective velocities from satellite images of sea surfacetemperature

A method of employing sequential satellite images of sea surface temperature (SST) patterns to estimate surface advective current velocities is described. SST images are obtained by processing data received twice daily from the Advanced Very High Resolution Radiometer. The current velocities are estimated by applying a maximum cross correlation (MCC) technique on two time-lapsed images. The MCC technique involves computing matrices of cross correlation coefficients and identifying correlation peaks. A two-stage multilevel statistical test over two-dimensional correlations is developed for determining the relative significance levels of velocity estimates. The test also identifies areas where the MCC technique cannot be effective. Aspects of implementation and limitations of the MCC technique for computing current velocity are also discussed. Advective velocity fields computed through MCC for the Chatham Rise area in New Zealand extending from (40.5°S, 173.5°E) to (49°S, 178°W) show good agreement with the known geostrophic flow patterns in this area     

Terrain influences on SAR backscatter around Mt. Taranaki, NewZealand

An L-band SAR image from the Japanese JERS-1 satellite has been analyzed for the effects of local surface orientation relative to the radar illumination direction. Orthorectification of the SAR imagery and determination of the local surface orientation is achieved with the aid of a high resolution digital terrain model. An improved method of determining scatterer density for a surface in three dimensions is introduced and used to correct radiometrically the image for terrain variation. Residual radiometric effects due to surface orientation are shown to be dependent on the ground cover class. Backscatter from the indigenous forest was more isotropic than that from the farmland. As accurate registration was required for this study, a method for identifying control points in the rectified imagery is described which alleviated the difficulty of identifying them in the raw image