The Doppler Spectra of Medium Grazing Angle Sea Clutter; Part 1: Characterisation

This paper is concerned with the characterisation of Doppler spectra from high range resolution X-band radar sea clutter observed from an airborne platform over the range of grazing angles, 15° to 45°. It is observed that when looking up or down wind there is a strong correlation between mean Doppler shift and local spectrum intensity. When combined with random fluctuations of spectrum width, these characteristics give the spectra a temporal and spatial variability. This behaviour has previously been observed in low grazing angle data and these results confirm the wider applicability of the models developed using that data. The modelling method is also extended here to capture the bimodal behaviour observed with high intensity returns from breaking waves looking up or down-wind

The Tail of the HI Mass Function

The contribution of extragalactic objects with HI masses below $10^8 M_\odot$ to the HI mass function remains uncertain. Several aspects of the detection of low-mass sources in HI surveys are not always considered, and as a result different analysis techniques yield widely different estimates for their number density. It is suggested at one extreme that the number density of galaxies follows a shallow Schechter power-law slope, and at the other extreme that it follows a steep faint-end rise like that found for field optical sources. Here we examine a variety of selection effects, issues of completeness, and consequences of LSS. We derive results for the large Arecibo Dual Beam Survey which indicate that the field mass function does rise steeply, while within the Virgo Cluster environs, the slope appears to be much shallower. Dependence on the local density of galaxies may partially explain differences between surveys.Comment: 8 pages, presented at Mapping the Hidden Universe: The Universe in HI. eds Kraan-Korteweg, Henning, Andernac

A comparative assessment of solar thermal electric power plants in the 1-10 MWe range

The candidate power system technologies were ranked in terms of the cost of electric energy each system produces. In all cases, it was assumed that development programs would result in mature power plant systems that could be commercially manufactured. The results of the study, a brief description of the systems examined, and the methodologies used are presented

The appearance of writing and ist (disappearing?) authority

Throughout history, the design and layout of the writing support, as well as the appearance of written characters have almost always been limited by various factors. For example, the scarcity and, consequently, the huge costs of parchment (and later paper for the first couple of centuries after its invention) made it economically nonsensical to use exclusively very large writing or typography. Production methods of the writing tools and the technological abilities at the disposal of the producers further limited the appearance of writing. The tool for writing on clay tablets, the triangular stylus, made it difficult to draw curves or round drawings in clay which resulted in wedge-like impressions in cuneiform writing. These limitations eventually led to standardization, just as the idea of using movable type for printing with Gutenberg’s press did. The appearance of writing was soon charged with semantic meaning that could for instance demarcate Latin from the vernacular or the prestigious text from the mass-market product. Nowadays, however, many e-reading devices or internet browsers enable the reader to choose a font. In my paper, I will summarize some of the historical developments, especially in the transitional phases of the book, and argue that with the almost unlimited abilities that the digital age has to offer, the representation of typography on e-reading devices might lose its authority because of the omnipresence of options for the reader. It remains to be seen whether this will be a vital loss for readers, writers and publishers or whether the authority of typography has been overstated and is no longer needed.   In my paper, I will summarize some of the historical developments, especially in the transitional phases of the book, and argue that with the almost unlimited abilities that the digital age has to offer, the representation of typography on e-reading devices might lose its authority because of the omnipresence of options for the reader. It remains to be seen whether this will be a vital loss for readers, writers and publishers or whether the authority of typography has been overstated and is no longer needed.  

Image Ellipticity from Atmospheric Aberrations

We investigate the ellipticity of the point-spread function (PSF) produced by imaging an unresolved source with a telescope, subject to the effects of atmospheric turbulence. It is important to quantify these effects in order to understand the errors in shape measurements of astronomical objects, such as those used to study weak gravitational lensing of field galaxies. The PSF modeling involves either a Fourier transform of the phase information in the pupil plane or a ray-tracing approach, which has the advantage of requiring fewer computations than the Fourier transform. Using a standard method, involving the Gaussian weighted second moments of intensity, we then calculate the ellipticity of the PSF patterns. We find significant ellipticity for the instantaneous patterns (up to more than 10%). Longer exposures, which we approximate by combining multiple (N) images from uncorrelated atmospheric realizations, yield progressively lower ellipticity (as 1 / sqrt(N)). We also verify that the measured ellipticity does not depend on the sampling interval in the pupil plane using the Fourier method. However, we find that the results using the ray-tracing technique do depend on the pupil sampling interval, representing a gradual breakdown of the geometric approximation at high spatial frequencies. Therefore, ray tracing is generally not an accurate method of modeling PSF ellipticity induced by atmospheric turbulence unless some additional procedure is implemented to correctly account for the effects of high spatial frequency aberrations. The Fourier method, however, can be used directly to accurately model PSF ellipticity, which can give insights into errors in the statistics of field galaxy shapes used in studies of weak gravitational lensing.Comment: 9 pages, 5 color figures (some reduced in size). Accepted for publication in the Astrophysical Journa

Technological change and productivity growth in the air transport industry

The progress of the civil air transport industry in the United States was examined in the light of a proposal of Enos who, after examining the growth of the petroleum industry, divided that phenomenon into two phases, the alpha and the beta; that is, the invention, first development and production, and the improvement phase. The civil air transport industry developed along similar lines with the technological progress coming in waves; each wave encompassing several new technological advances while retaining the best of the old ones. At the same time the productivity of the transport aircraft as expressed by the product of the aircraft velocity and the passenger capacity increased sufficiently to allow the direct operating cost in cents per passenger mile to continually decrease with each successive aircraft development