Sea Height Research Articles

Radar Altimeter Optimization for Geodesy Over the Sea

The optimum processor and its accuracy limit for radar altimetry for geodetic use over the sea are studied with a model accounting for random surface reflectivity, sea height variation, additive noise, and pointing errors, and allowing for arbitrary antenna patterns, signal modulations, and other system parameters. The ?threshold? case solution (which can have any specified accuracy) dictates a signal modulation bandwidth just shy of resolving the sea height variation and/or illuminated sea area (as scaled into time delay and ?smeared? by pointing errors). For such a modulation a relatively complete solution is obtained. These results are used to determine practical radar altimeter designs, additionally accounting for antenna size, stability, and peak power restraints. Conditions allowing neglecting of limiting or complicating effects due to temporally varying reflectivity, sea height, and vehicle position are given and shown to be satisfied for a typical satellite.

Model for Prediction of Ocean Surface Reverberation

A model for reverberation is described which is based on a solution for scattering of a low-frequency acoustic plane wave by a time-varying trochoidal surface. It is shown that under certain approximations the Helmholtz integral for a time-varying pressure release boundary leads to the characterization of the surface as a continuum of uniform isotropic reradiators undergoing circular harmonic motion about fixed points on the mean-free surface. A farfield solution is obtained for the scattered field for arbitrary incidence and observation directions. The solution is seen to be dependent on the sea height, wave speed and direction, and the reradiating aperture size. The scattered field spectrum is, in general, asymmetric and consists of spectral lines positioned at multiples of the surface-wave orbital frequency. The model is employed to predict reverberation due to single-path propagation for a historic source-receiver configuration. These predictions are compared with spectra obtained from experimental data.

Prehistoric Roads and Tracks in Somerset, England: 2. Neolithic

The Somerset Levels are the largest area of low-lying ground in south-west England, covering an extensive region between the highlands of Exmoor, the Brendon Hills and the Quantock Hills to the west, and the Cotswold and Mendip Hills to the east (Pl. XXIII, inset). The Quantock Hills and the Mendip Hills directly border the Levels themselves, and reach heights of over 250 metres above sea level. The valley between extends to 27 metres below sea level, but is filled to approximately the height of the present sea by a blue-grey clay. The Levels are bisected by the limestone hills of the Poldens, and both parts have other smaller areas of limestone and sand projecting above the peat deposits that cap the blue-grey clay filling. In this paper we are concerned with the northern part of the Levels, an area at present drained by the River Brue.The flat, peat-covered floor of the Brue Valley is some six kilometres wide and is flanked on the north by the Wedmore Ridge, and on the south by the Polden Hills (Pl. XXIII). In the centre of the valley, surrounded by the peat, is a group of islands of higher ground, Meare, Westhay, and Burtle. These islands, which would always have provided relatively dry ground in the Levels, are linked together by Neolithic trackways of the third millennium B.C. Several of these trackways formed the basis of a paper in these Proceedings in 1968 (Coles and Hibbert, 1968), which continued the work of Godwin and others (Godwin, 1960; Dewar and Godwin, 1963).

Predicting Long Term Trends of Deck Wetness for Ships in Ocean Waves

A method of predicting the short term probability of deck wetness and the long term probability of “wet-deck navigation” is presented, along with results of its application to cargo ships operating on the North Atlantic.Relative bow motions have been evaluated theoretically for geometrically similar ships of various sizes at different headings to regular and irregular waves, based upon the linear strip theory and the linear superposition technique. According to those results, the relationship between the short term probability of deck wetness related to bow freeboard and the significant wave height of irregular sea has been determined in correlation to average wave period, heading angle and ship speed. Then, the long term probabilities of “wet-deck navigation”, where the short term probability of deck wetness will be larger than 1/10, are predicted for different seasons and for various wind forces by the aid of long term wave statistics on the North Atlantic.The following trends of deck wetness related to bow freeboard are concluded from the predicted results : (a) The probability of deck wetness is large in head and bow seas, and small in following, quartering and beam seas.(b) The probability of deck wetness decreases with decrease of ship speed, but the influence of ship speed is rather small in higher speeds beyond 10 knots.(c) A large sized ship has small probability of deck wetness.(d) A full ship has small probability of deck wetness.(e) The long term probability of “wet-deck navigation” is large in winter and small in summer on the North Atlantic.(f) The long term probability of “wet-deck navigation” increases with increase of wind force on the North Atlantic, but this trend is not so remarkable in extremely heavy weather.