Multipath Tracking Errors in Elevation-Scanning and Monopulse Radars

The tracking performance of elevation- scanning and monopulse radars in the presence of multipath propagation are compared. The key difference between these two generic types of radars is the way they respond to moving targets. There are no significant differences between their responses to pure specular multipath, nor to diffuse multipath for targets on radial courses. However, they are found to respond quite differently to the diffuse com ponent for low-altitude crossing targets. For these conditions the tracking errors for elevation-scanning radars may be several times those for monopulse radars.     

EXOSAT 2–10 KeV observations of M87 and the Virgo cluster

A series of observations of the Virgo cluster of galaxies with the Medium Energy experiment on board EXOSAT are presented. These take the form of two orthogonal scans meeting at M87. The observed count rates have been compared with those obtained with the IPC on board the EINSTEIN observatory and are found to be consistent with the the IPC surface brightness distribution. This fact together with spectral data obtained with Exosat indicates that the region around M87 is more or less isothermal (T 2–3 keV) out to > 80 arc min from the centre. Although a slight hardening is seen near the optical centroid of the cluster the results do not support a model which involves a hot ( 8 keV) intra-cluster medium. A mass of 5 × 10¹³ solar masses is derived for the inner 90 arc minutes of M87.     

Models for Radar Scatterer Density in Terrain Images

Statistical models for the density of strong scatterers detected in high resolution radar images of rural terrain are presented. The probability distribution of the density of these natural terrain detections was found to be negative binomial. The variance of the negative binomial depended strongly on the window size used to measure the density. This dependence indicates that these detections, like those of a Poisson process, are locally uncorrelated, but have a slowly varying mean density whose correlation distance is 1 km or more. Negative binomial parameters were computed using over 200 km2 of terrain image for densities measured using windows sized from 75 m × 75 m to 375 m × 375 m. Average terrain detection densities of 10-3 and 10-4 per resolution cell were evaluated on images with resolutions of 7 and 28 ft.     

Vague Reference and Approximating Judgments

"Mount Everest" is a vague name. That is (on the account here defended) there are many portions of reality all of which have equal claims to serve as its referent. We propose a new account of such vagueness in terms of a theory of what we shall call granular partitions. We distinguish different kinds of crisp and non-crisp granular partitions and we describe the relations between them, concentrating especially on spatial examples. In addition, we describe the practice whereby subjects use systems of reference grids as a means for tempering the vagueness of their judgments for example when they say that Libya straddles the Equator or that the meeting will take place between 2 and 3pm. We then demonstrate how the theory of reference partitions can yield a natural account of this practice, which is referred to in the literature as 'approximation'.