A. Van Zyl

Calibration artefacts in radio interferometry - I. Ghost sources in Westerbork Synthesis Radio Telescope data

This work investigates a particular class of artefacts, or ghost sources, in radio interferometric images. Earlier observations with (and simulations of) the Westerbork Synthesis Radio Telescope (WSRT) suggested that these were due to calibration with incomplete sky models. A theoretical framework is derived that validates this suggestion, and provides predictions of ghost formation in a two-source scenario. The predictions are found to accurately match the result of simulations, and qualitatively reproduce the ghosts previously seen in observational data. The theory also provides explanations for many previously puzzling features of these artefacts (regular geometry, PSF-like sidelobes, seeming independence on model flux), and shows that the observed phenomenon of flux suppression affecting unmodelled sources is due to the same mechanism. We demonstrate that this ghost formation mechanism is a fundamental feature of calibration, and exhibits a particularly strong and localized signature due to array redundancy. To some extent this mechanism will affect all observations (including those with non-redundant arrays), though in most cases the ghosts remain hidden below the noise or masked by other instrumental artefacts. The implications of such errors on future deep observations are discussed.

Using Page's Cumulative Sum Test on MODIS Time Series to Detect Land-Cover Changes

Human settlement expansion is one of the most pervasive forms of land-cover change in South Africa. The use of Pages cumulative sum (CUSUM) test is proposed as a method to detect new settlement developments in areas that were previously covered by natural vegetation using 500-m Moderate Resolution Imaging Spectroradiometer time-series satellite data. The method is a sequential per-pixel change alarm algorithm that can take into account positive detection delay, probability of detection, and false-alarm probability to construct a threshold. Simulated change data were generated to determine a threshold during a preliminary offline optimization phase. After optimization, the method was evaluated on examples of known land-cover change in the Gauteng and Limpopo provinces of South Africa. The experimental results indicated that CUSUM performs better than band differencing in the before-mentioned study areas.

Land-Cover Separability Analysis of MODIS Time-Series Data Using a Combined Simple Harmonic Oscillator and a Mean Reverting Stochastic Process

It is proposed that the time series extracted from moderate resolution imaging spectroradiometer satellite data be modeled as a simple harmonic oscillator with additive colored noise. The colored noise is modeled with an Ornstein-Uhlenbeck process. The Fourier transform and maximum-likelihood parameter estimation are used to estimate the harmonic and noise parameters of the colored simple harmonic oscillator. Two case studies in South Africa show that reliable class differentiation can be obtained between natural vegetation and settlement land cover types, when using the parameters of the colored simple harmonic oscillator as input features to a classifier. The two case studies were conducted in the Gauteng and Limpopo provinces of South Africa. In the case of the Gauteng case study, we obtained an average for single-band classification, while standard harmonic features only achieved an average . In conclusion, the results obtained from the colored simple harmonic oscillator approach outperformed standard harmonic features and the minimum distance classifier.

Systematic Luby Transform codes as incremental redundancy scheme

Systematic Luby Transform (fountain) codes are investigated as a possible incremental redundancy scheme for EDGE. The convolutional incremental redundancy scheme currently used by EDGE is replaced by the fountain approach. The results of the simulations performed for each incremental redundancy scheme show that the fountain approach outperforms the convolutional approach on the second retransmission when implemented on the EDGE platform. The results also indicate that if the packet sizes used by a specific platform is large enough the fountain approach will always outperform the convolutional approach.

The influence of fibre in the diet on growth rates and the digestibility of nutrients in the greater cane rat (Thryonomys swinderianus)

The greater cane rat Thryonomys swinderianus is a coprophagous rodent in which fermentation occurs in the large caecum. The extent to which a 45% increase in the fibre component of the diet influenced growth rates of cane rats and the digestibility of nutrients and energy was investigated in two feeding trials. Higher fibre levels in the diet reduced the digestibility of dry matter, protein and fat, while animals digested fibre components (neutral-detergent fibre, acid detergent fibre, hemicellulose and cellulose) with a comparable efficiency to those maintained on a low fibre diet. In one of the trials animals fed the high fibre diet exhibited significantly lower growth rates than animals fed the low fibre diet. Digestibility coefficients of the cane rats for neutral-detergent fibre and protein seem to be intermediate to high when compared to reported values for the porcupine, guinea-pig, degu and rabbit. It is suggested that the ability of cane rats to utilise large quantities of fibre enable them to survive periods when only dry grass is available.

Graded tensor products and the problem of tensor grade computation and reduction

We consider a non-negative integer valued grading function on tensor products which aims to measure the extent of entanglement. This grading, unlike most of the other measures of entanglement, is defined exclusively in terms of the tensor product. It gives a possibility to approach the notion of entanglement in a more refined manner, as the non-entangled elements are those of grade zero or one, while the rest of elements with grade at least two are entangled, and the higher its grade, the more entangled an element of the tensor product is. The problem of computing and reducing the grade is studied in products of arbitrary vector spaces over arbitrary fields.

Minimum error land cover separability analysis and classification of MODIS time series data

Three minimum-error land cover classifiers are compared on coarse resolution MODerate-resolution Imaging Spectroradiometer (MODIS) surface reflectance data for discerning between vegetation and settlement classes. It is shown that good class separability can be achieved using only the seasonal component of Normalized Difference Vegetation Index (NDVI) data, or the mean component of several other MODIS land bands. It is also shown why particular classifiers fail in certain spectral bands. Finally it is shown that after NDVI, band 2 has the highest separability of all the MODIS land bands, and band 5 has the lowest separability.

Digestibility of Nutrients and Aspects of the Digestive Physiology of the Greater Cane Rat, Thryonomys swinderianus in Two Seasons

The greater cane rat, Thryonomys swinderianus, utilizes high fibrous plant material and is an important meat source in West Africa. An insight in its digestive physiology will enhance our understanding of its feeding habits. Digestibility coefficients of the food were determined during two seasons before the animals were euthanased. The distribution and concentrations of nutrients and energy in different parts of the gastrointestinal tract were determined at the time of day when animals practised coprophagy. Trial 1 diet in the wet season consisted of 36% neutral detergent fibre (NDF), 11% protein and 49% dry matter, while the Trial 2 diet in the dry season consisted of 53% NDF, 8–9% protein and 89% dry matter. The Trial 2 animals on the poor diet increased their daily nutrient intake, possibly increased the volume of digesta and practised frequently cophrophagy, so that faecal production was reduced and digestibility coefficients were relatively high. Coprophagy increased both protein and energy intake as soft pellets in the distal colon had higher protein and energy content than the hard faeces. Energy, protein and acid-detergent fibre were retained in the caecum of Trial 1 females and the caecum and proximal colon of the Trial 2 animals 16 hours after feeding, illustrating the importance of these two regions in the fermentation process. Water was absorbed in the distal colon as dry matter content of digesta increased 53%, 4% and 56% from the proximal colon to the distal colon. Animals produced hard faeces with only 16% and 5% lower moisture content in the dry season, compared to that produced in the wet season, as water was not a limiting factor during the trials. It was concluded that an increase in daily food intake, an increase in coprophagy and the presence of a colonic separation mechanism (that retains small particles) enable the greater cane rat to utilize high fibrous plant material. These digestive strategies seem to be comparable to those observed in other hystricomorph rodents.

Aspects of the anatomy and histology alimentary canal of the greater Thryonomys swinderianus, with reference feeding physiology

Aspects of the anatomy and histology of the alimentary canal of Thryonomys swinderianus were studied to gain a better understanding of this animal’s ability to digest large quantities of fibre. Morphometric measurements of the gut regions were taken with the aid of light, transmission and scanning electron microscopy. The stomach is fully glandular and unilocular with a low pH and is possibly the initial site of protein digestion. The duodenum, with its closely packed columnar cells with many mitochondria, may be the main site of enzymatic digestion. Owing to coprophagy, food and fermentation products are recycled for further digestion in the small intestine. The caecum is by far the largest region in mass and the apex ceci may be the main site of fermentation. A ridge-lined furrow is present inside the proximal colon. Thick muscle layers are present in the vicinity of the ridge-lined furrow. It is suggested that the furrow is part of the colonic separation mechanism that transports bacteria back to the caecum. The longitudinal folds and long villi in the proximal colon enlarge the surface area, where the columnar cells of the villi have many mitochondria, probably to absorb fermentation products and to transport water and electrolytes across the mucosa. In the distal colon many mucous cells open in tubular glands of Lieberkühn. Reabsorption of water probably occurs through the dense microvillous border.

Postnatal growth of the greater cane rat Thryonomys swinderianus (Thryonomyidae: Rodentia) in Gauteng, South Africa

Postnatal growth of captive Thryonomys smnderianus was measured for males and females on two experimental diets during 1994-1996 in Gauteng, South Africa. The experimental diets presented, differed 10-12 % in neutral detergent fibre content. Feeding trials were conducted to determine daily food intake and animal feeding efficiency. Six litters were born during the study with an average litter size of less than four. Logistic equations provided a closer fit to the growth data than Gompertz equations. The growth rates and body mass of males were significantly higher than that of females. At five months of age males attained a 23 % higher body mass than females. The postnatal growth of females and males flattened off at 300 and 370 days respectively and their feeding efficiency (gain/ intake) decreased, indicating that it is uneconomical to market them much later than this age. The two experimental diets did not affect postnatal growth. During two feeding trials, animals 247-289 and 419-454 days old, selected the food components of the two diets and their bedding differently, and their nutrient intake between the two diets did not differ significantly.