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Different information has different importance in respective sectors. Information can be about security, about monitoring, about observations or about detection. Similarly depending on the information sensitivity different methods of Information retrieval and algorithms are used with the help of suitable retrieval systems. This paper first discuss about Information retrieval systems based on retrieval methods and their use in different areas and then starting from the birth of radar, the evolution and advancement in radar systems as information retrieval system is studied to retrieve the required and suitable information for the purpose of security, monitoring, observation and tracking. Finally based on the technologies, electromagnetic consumption and the FCC standards, radar based time a line of the information retrieval systems is studied. [ABSTRACT FROM AUTHOR]

While deploying a sensor network is necessary for proof-of-concept experimentation, it is a time-consuming and tedious task that dramatically slows innovation. Treating sensor networks as shared testbeds and integrating them into a federated testbed infrastructure, such as FIRE, GENI, AKARI, or CNGI, enables a broad user community to benefit from time-consuming deployment exercises. In this paper, we outline the challenges with integrating sensor networks into federated testbeds in the context of ViSE, a sensor network testbed we have integrated with GENI, and describe our initial deployment experiences. ViSE differs from typical embedded sensor networks in its focus on high-bandwidth steerable sensors. [ABSTRACT FROM AUTHOR]

In the last few years, there is a great interest in developing an identification system capable to make a reliable classification of aircrafts into different groups (friendly, hostile or neutral). Depending on the context in which these systems are deployed, incorrect identification may lead to serious problems, such as fratricide or engagement of civilian aircrafts. Different techniques have been researched to face this problem, but non-cooperative ones have awakened more interest because they do not require aircraft collaboration. Non Cooperative Target Identification (NCTI) using radar is a complex task, mainly due to the fact that a database of possible targets is needed. To populate this database, Radar Cross Section (RCS) predictions produced by computer simulation seem to be the most feasible way to perform this task, since measurements alone cannot cover the vast range of targets, configurations and required aspect angles. These predictions are typically performed in the frequency domain and a specific processing must be done to obtain both High Resolution Range Profiles (HRRPs) and 2D Inverse Synthetic Aperture Radar (2D-ISAR) images. This paper shows a methodology to face the NCTI task, which use both synthetic HRRPs and 2D-ISAR to achieve an accurate identification. [ABSTRACT FROM AUTHOR]

This chapter introduces to the microphysical properties of clouds and precipitation, such as particle number concentration, size, and shape, and their relationship with atmospheric aerosols. The role of clouds and aerosols into the climate system and models is briefly discussed, with an emphasis on the associated uncertainty and the current level of understanding. Potential feedback effects of an increasing aerosols load are discussed. The chapter also explains the need for multi-sensor strategies, using radiometer, radar, and lidar, for the accurate and reliable measurement of cloud properties and finally it places some questions on the implications for common model assumptions. [ABSTRACT FROM AUTHOR]

An airborne ground looking radar sensor΄s performance may be enhanced by selecting algorithms adaptively as the environment changes. A short description of an airborne intelligent radar system (AIRS) is presented with a description of the knowledge based filter and detection portions. A second level of artificial intelligence (AI) processing is presented that monitors, tests, and learns how to improve and control the first level. This approach is based upon metacognition, a way forward for developing knowledge based systems. [ABSTRACT FROM AUTHOR]

Several approaches to the detection of Unexploded Ordnance (UXO) in the ground are discussed. Methods exploiting the coupling of sound into the earth are shown to have promise. These approaches can use both linear and non-linear phenomena as clues. Also discussed is the potential of a ground penetrating radar method that is based on a nonlinear phenomenon. [ABSTRACT FROM AUTHOR]

The detection and mitigation of unexploded ordnance (UXO) is recognised to be a serious global issue. Many millions of landmines have been deployed in recent conflicts, with few records of what has been laid and where. As well as landmines, other types of UXO include unexploded shells, mortar bombs and missiles, scatterable mines fired from mortars or artillery or dropped from aircraft or helicopters, and cluster munitions. Not only do such weapons cause injury and death to innocent civilians, but also they deny the use of substantial areas of land for agricultural and other economic purposes, which may be critical in countries where the threshold of poverty is already low. Ground-penetrating radar (GPR) is one of a family of sensors that may be used to detect UXO. In addition, GPR may also be used to detect other classes of target such as Improvised Explosive Devices (IEDs), weapons caches, and tunnels; further applications of GPR include archaeology, forensics, and the detection of buried pipes and cables. The purpose of this chapter is to present an account of the principles of ground-penetrating radar and their use in detecting buried UXO. [ABSTRACT FROM AUTHOR]

Detection of landmines using electromagnetic induction (EMI) techniques is well established and a range of metal detectors is commercially available. Recent developments using dual sensor technology combining EMI and ground penetrating radar (GPR) have enabled improved discrimination against small metal fragments to be demonstrated in live minefields. Reductions of up to 7:1 compared with the standard metal detector have been achieved in the field by hand held systems such as the UK-German MINEHOUND/VMR2 system and the US AN/PSS-14 (formerly HSTAMIDS: Handheld Standoff Mine Detection System). Stand off vehicle based radar systems are now being trialled in realistic conditions. Airborne systems have also been trialled, but as yet have some way to go to deliver useful performance. These three distinct modes of operation pose fundamentally different challenges in terms of the physics of propagation and the radar system design and will be discussed. End user expectations in terms of performance are challenging and at present only the hand held detectors are approaching these needs. This chapter reviews the high-level performance requirements from an OA perspective in order to set the performance envelopes of the radar designs. We also address the fundamental challenges in terms of propagation, proximity to the ground surface; ground topography and signal to noise and signal to clutter bandwidth of operation with reference to both close in and stand off landmine and IED detection. A review of the performance of GPR systems at the higher TRL levels is provided. A key issue in comparing the published results of controlled trials relates to statistics of the depth of cover, the soil propagation characteristics, and the type of landmine, the sample size, the physical placement of the landmine as well as the characteristics of the clutter. This chapter will also highlight the future engineering challenges to achieve not only detection but recognition and identification using GPR. [ABSTRACT FROM AUTHOR]

In the evening of 20 August 2006 a severe thunderstorm hit Budapest causing 4 people΄s death and leaving seven hundred people injured who were on the streets on the occasion of the traditional Constitution Day΄s fireworks which attract thousands of people every year. This entailed deep and detailed analysis of the weather conditions and the development of this thunderstorm, as well as of the operation of observational now casting systems of the Hungarian Meteorological Service (HMS). The investigation of radar and flash characteristics was made by interactive and automatic cell segmentation and tracking methods developed at HMS for comparative analysis. Both methods provide the following attributes for every radar cells with different dBZ thresholds: centre and maximum dBZ values and their coordinates, area, radar rainfall intensity, water production and total water. The centre, the maximum values of flash density and their coordinates, area, flash activity and total flash count for every flash cell with different flash density thresholds are also calculated. In the course of the comparative analysis the radar and flash characteristics of the thunderstorms of the 3 August 2006 were also computed. On this day the highest flash activity was observed in Hungary since the SAFIR type lightning location network is in operation. The authors are underlying the importance of using such methods in the operational practice of HMS. [ABSTRACT FROM AUTHOR]

Introduction The German invasion of Poland on 1 September 1939 led to British and French declarations of war two days later, but Britain's war aim was to end the Nazi menace, not to preserve the status quo in Eastern Europe. This aim endured through a succession of military disasters and the extension of the war to the Mediterranean and the Far East through the intervention of Italy on 10 June 1940 and of Japan on 7 December 1941. From the French armistice with Germany on 22 June 1940 to Hitler's invasion of the Soviet Union exactly a year later the British Commonwealth and Empire fought alone. Hopes of victory depended upon supplies of food, raw materials and munitions from the United States, and therefore on victory over the U-boats as well as on American aid through the Lend-Lease Act of March 1941. Although the Japanese attack on Pearl Harbor brought the United States fully into the war, it was not until the American naval victory at Midway in June 1942 that the Japanese were decisively checked. The summer of 1942 was the nadir of British fortunes: Singapore had surrendered on 15 February; India was threatened. The Commonwealth armies in North Africa were heavily defeated by the Germans and Italians, and it seemed that Egypt too might be lost. The tide turned in late 1942, with the battle of El Alamein (23 October – 5 November), the Anglo-American landings in North Africa (8 November) and the Soviet relief of Stalingrad (19–25 November). [ABSTRACT FROM AUTHOR]

Introduction For the purposes of military technology, the post-war period may be divided into an atomic phase and a thermonuclear phase, the latter beginning with the Americans' first hydrogen-bomb tests in 1954. This chapter deals with the first phase. As international relations came gradually to be dominated by the Cold War, British foreign policy aimed to co-operate with what Churchill in 1948 called three overlapping circles of ‘free nations’: the United States, the Commonwealth and Western Europe. Anglo-American relations were the most important for three reasons. First, Britain had merged her atomic research with America's in 1942 and had hopes of collaboration in the post-war period. Second, Britain had to borrow dollars to finance imports from North America while converting her industries from war to peace, and she hoped to co-operate with the United States in creating a stable and expanding international economy. Third, only the United States could counter-balance the power of the Soviet Union. In analysing defence policy after 1945 one has to remember that the course of international relations was not predictable. An important Foreign Office brief for the Potsdam conference in July 1945 recommended that Britain should stand firm diplomatically against Soviet designs, but did not assume that American support would be forthcoming, and concluded that Britain must base its foreign policy on the principle of co-operation between the United States, the Soviet Union and the British Commonwealth. [ABSTRACT FROM AUTHOR]

Introduction The period covered by this chapter begins with the Churchill government's decision in the summer of 1954 to produce a British hydrogen bomb, and ends with dependence on the American Polaris system to deliver it. The Americans exploded their first thermonuclear device on 1 November 1952, less than a month after the first British atomic bomb test, and between 1 March and 13 May 1954 they carried out a series of tests showing that they had mastered the techniques of making hydrogen bombs. A hydrogen bomb falling on a city could kill a million people, compared with the 50,000 fatalities to be expected from an atomic bomb. The Russians detonated a thermonuclear device in August 1953 and their first true hydrogen bomb in November 1955. The major consideration put forward by Churchill in Cabinet for developing a British hydrogen bomb was the effect it would have on Britain's influence in world affairs, and therefore on her ability to prevent precipitate action by the United States. The belief that Britain should have such influence was shared by the Leader of the Opposition. When the government's decision was belatedly announced in the annual Defence White Paper in 1955, in terms that left no doubt that Britain would use the deterrent rather than submit to Communism, Attlee remarked that, in his experience, possession of nuclear weapons did have an effect on the rulers of other countries. [ABSTRACT FROM AUTHOR]

Introduction British defence policy in the inter-war years may be divided into two phases: 1919 to 1932, when economic problems and the absence of pressing dangers to national security led to reductions in the armed forces; and 1932 to 1939, when the darkening international situation gave defence preparedness increasing political priority. However, many of the strategic problems encountered during the later 1930s were rooted in the earlier phase, and this chapter analyses the period 1919–39 as a whole. There is a danger in this approach, since policies in the 1920s may be judged unfairly in the light of later events, but that is true even of the 1930s, when British defence policy was designed to deter aggression over an indefinite period even if the Chiefs of Staff were planning from 1934 on the basis of being ready for war by 1939. In August 1919 the Cabinet decided that the defence departments should revise their estimates of expenditure for the coming year on the assumption that ‘the British Empire will not be engaged in any great war during the next ten years, and that no Expeditionary Force is required for this purpose’. The purpose of this ‘Ten Year Rule’, as it came to be called, was to assist the chancellor of the exchequer in securing the cuts in expenditure required to balance his budget and, in one form or another, the rule remained the guiding principle of defence policy until 1932. [ABSTRACT FROM AUTHOR]

This book has taken a broader approach to the study of war than conventional military histories, both in studying the interaction of technology, economics and strategy, and in emphasising that war is a single process. Arms cannot be used effectively unless backed by adequate economic resources. Air, land, sea and economic warfare are mutually reinforcing. Seen in this broader perspective, British grand strategy was extraordinarily ambitious and adaptable, involving defence of world-wide interests. When British economic resources were inadequate to sustain its armed forces, the resources of the United States were harnessed for the purpose. Arms Edgerton has suggested that the British elite believed that relative strength in science and technology could compensate for lack of economic resources. From his perspective, the chain of causality ran from strategy to technology, with Britain being the first naval power, the first aeronautical power and one of the first nuclear powers. It is possible to argue that the march of science left British policymakers with little choice but to be technically up to date. For example, even naval officers who did not share Fisher's enthusiasm for submarines agreed that the Royal Navy could not neglect them once they had been developed abroad. The Wright brothers hawked their aircraft round the great powers as soon as it had been invented. Nuclear physics was not confined to one nation. From this perspective, expenditure on increasingly expensive research and development, and weapons systems, was inevitable. [ABSTRACT FROM AUTHOR]

The starting point for this study of British defence policy between 1904 and 1969 is the tendency for the costs of new weapons systems to rise more rapidly than the national income. Three main insights are offered. First, British defence policy was based upon technological innovation. Second, reductions in the size of the armed forces to accommodate new weapons systems in defence budgets were not evidence of a decline in power. Third, British grand strategy, incorporating economic as well as military responses to external threats, was much more ambitious than is commonly believed. I first approached the relationship between economics and strategy in my book British Rearmament and the Treasury, 1932–1939, which showed that Treasury attempts to influence strategy reflected concern about Britain's ability to sustain a long war, and were related to trade and industry as well as money. Since then there have been a number of case studies of interaction between economics and strategy. For example, David French and Avner Offer have described how British strategic planning before 1914 assumed that naval blockade would cause the German economy to collapse, while Britain's access to raw materials and her industrial power would enable her to supply continental allies with munitions. David Edgerton has challenged assumptions about British military backwardness by putting forward a broad-arching thesis of Britain as a pioneer of technologically focused war, possessed of a powerful military-industrial-scientific complex that emerged in the first decades of the twentieth century and was cut back only in the late 1950s and the 1960s. [ABSTRACT FROM AUTHOR]

Results from a recent survey on MSCT practice in Germany indicate that 2-slice scanners (which are mainly former Elscint Twin scanners) are used at dose levels comparable with modern SSCT scanners, while dose values related with 4-slice scanner protocols are currently significantly, but not alarmingly, higher. The main reasons are: (1) reduced slice thickness, which tempts the users to increase the electrical current-time product in order to compensate for increased noise, (2) overbeaming due to penumbral effects, which are most pronounced at narrow slice collimation, and (3) reduced transparency of the implications of parameter settings on dose. The key factor to reduce dose at MSCT systems with N ≥ 4 to a level comparable to modern single-slice and dual-slice scanners is to appreciate the improved detail contrast achieved with thin slices due to reduced partial volume effect. This overcompensates for the drawback of increased noise. Thus, at least the same dose level as for modern SSCT scanners should be attainable. Furthermore, new technical means have the potential for dose reduction. It should be emphasized, however, that technical means are only a prerequisite, but no guarantee for dose reduction. Of greatest importance is appropriate training and guidance of the medical and technical staff operating the MSCT scanner with respect to the specific factors determining patient exposure and its reduction. [ABSTRACT FROM AUTHOR]

DEFINITIONS AND LAWS Naval blockades can be traced back at least to the end of the sixteenth century, when vessel design had improved sufficiently to permit Dutch war ships to conduct a “blockade-like” operation against Spanish ports in Flanders. In 1630, in a major proclamation, the Dutch extended “to the investment by sea of coast towns the principles applicable to a siege in land warfare,” thus claiming a right to confiscate all neutral commerce. The proclamation stated that all ships sailing to or from those enemy ports would be confiscated. Not surprisingly, the concept of a legal blockade – a deployment of vessels whose missions were supposedly governed by a set of legal rules that covered the maritime activities of the warring parties as well as those of any neutral countries that became involved – was not immediately accepted by all governments. However, the terminology contained in various treaties among the Dutch, English, French, and Swedes between 1674 and 1679, as well as in the Treaty of Whitehall of 1689 make it clear that, even then, a “blockade” was widely understood to refer to the lawful exclusion of all commerce from an invested port or coastline. Since that time, the debate over the legal rules governing blockades has been almost continuous; while, at the same time, serious questions about the possible effectiveness of such naval deployments have been raised. [ABSTRACT FROM AUTHOR]

INTRODUCTION By World War I, improvements in technology, particularly the innovation of long-range coastal defense guns, mines, and submarines, had all but ruled out “close blockades” of the type that characterized warfare from the English-French continental wars of the eighteenth century through at least the American Civil War. Earlier, most blockades were “close blockades,” that “must be confined to ports and coasts belonging to or occupied by the enemy.” By 1914, however, “close” blockades had been replaced by “distant” ones. The blockading vessels were now almost always positioned further out to sea, with a corresponding potential for more contact with nonbelligerent shipping; however, as long as the blockade can adequately restrict movements into and out of enemy ports, this form of interdiction is considered legally valid. Another important change is that although in the twentieth century most blockades have been enforced by government-owned-and-operated vessels, in the past the blockading vessels have included private ships operating under letters of marque as government approved privateers. Historically, the distinction between private and government blockades raised a number of issues including legal questions of controls over appropriate behavior and domestic questions involving the relative size of peacetime and wartime navies. LESSONS FROM THE PAST: WORLD WAR I Given its precursors and the size of the conflagration, it is not surprising that policies adopted during World War I almost entirely rewrote the rules of naval blockades; and the history of the war itself appeared to confirm that, in the future, the convoy was to become the ultimate weapon against a “distant” blockade. [ABSTRACT FROM AUTHOR]

ECONOMIC WARFARE During a war there are a number of alternative military and naval strategies that a belligerent power can pursue in that country's efforts to defeat its enemies. Obviously, one such strategy is conquest by force of arms in direct combat. Such a strategy involves the siege or the invasion of an enemy's territory, and it is aimed at the destruction, capture, or surrender of the enemy's armed forces and, perhaps, the permanent occupation of its territory. Economic warfare, by weakening the enemy's ability to pursue military action, can substitute for or complement a strategy of direct combat. Such an economic strategy is designed to sever the trading links between the enemy and his allies or with neutral powers, and, in so doing, to reduce the level of military and civilian goods that are available to support his military ventures. Historically, the blockade, usually sea-based but occasionally land-based, has been the most common form of economic warfare; however, in the more recent past, other forms of economic warfare have been utilized. They include the imposition of higher tariffs, nontariff exclusions, restrictions on capital movements, and policies aimed at encouraging the production of substitutes by the targeting and neutral nations – all tactics designed to reduce enemy exports as well as their imports. In addition, the scope of direct economic warfare has been expanded to include the aerial bombardment of economic objectives, sanctions designed to restrict trade to neutral countries, sabotage of economic targets, preemptive purchases of strategic material, and, more generally, psychological warfare. [ABSTRACT FROM AUTHOR]

The main objective of this chapter is to review and extend existing adaptive natural gradient algorithms for various multichannel blind deconvolution models. Blind separation/deconvolution of source signals has been a subject under consideration for more than two decades. There are significant potential applications of blind separation/deconvolution in various fields, for example, wireless telecommunication systems, sonar and radar systems, audio and acoustics, image enhancement and biomedical signal processing (EEG/MEG signals). In these applications, single or multiple unknown but independent temporal signals propagate through a mixing and filtering medium. The blind source separation/deconvolution problem is concerned with recovering independent sources from sensor outputs without assuming any a priori knowledge of the original signals, except certain statistical features. In this chapter, we present using various models and assumptions, relatively simple and efficient, adaptive and batch algorithms for blind deconvolution and equalization for single-input/multiple-output (SIMO) and multiple-input/multiple-output (MIMO) dynamical minimum phase and non-minimum phase systems. The basic relationships between standard ICA/BSS (Independent Component Analysis and Blind Source Separation) and multichannel blind deconvolution are discussed in detail. They enable us to extend algorithms derived in the previous chapters, in particular, the natural gradient approaches for instantaneous mixture to convolutive dynamical models. We also derive a family of equivariant algorithms and analyze their stability and convergence properties. Furthermore, a Lie group and Riemannian metric are introduced on the manifold of FIR filters and using the isometry of the Riemannian metric, the natural gradient on the FIR manifold is described. Based on the minimization of mutual information, we present a natural gradient algorithm for the causal minimum phase, the finite impulse response (FIR) multichannel filter. Using information back-propagation, we also discuss an efficient implementation of the learning algorithm for the non-causal FIR filters. Computer simulations are also presented to illustrate the validity and good learning performance of the described algorithms. [ABSTRACT FROM PUBLISHER]

After completing this chapter, the reader should be able to: Compare and contrast pulse-type and current-type ionization chambers; Explain the relationship between ionization current and electrode voltage; State four uses of ionization chambers in radiology and nuclear medicine; Describe the principle of operation of Proportional counters; Geiger–müller tubes; Solid scintillation detectors; Photomultiplier tubes; Liquid scintillation detectors; Semiconductor detectors. [ABSTRACT FROM PUBLISHER]

This chapter discusses several areas of interest that were not considered previously. The optimum detection problem is an obvious topic not discussed up to this point. Section 10.2 discusses five topics: target tracking, space-time processing for radar, space-time processing for wireless communications, matched field processing, and spatial spectrum estimation. These topics are closely related to the subjects in this book. [ABSTRACT FROM PUBLISHER]

In this chapter, the authors study the angular correlation function, the memory effect, and the detection of object buried in random media. Two-dimensional simulations are presented in Section 2 and the angular correlation function and radar cross section are compared. In Section 3, three-dimensional simulations are presented. In Section 4, SAR imaging is studied. Angular correlation function and angular-frequency correlation function are used. [ABSTRACT FROM PUBLISHER]

The two major applications of RF and microwave technologies are in communications and radar/sensor systems. There are many other applications such as navigation and global positioning systems, automobile and highway applications, direct broadcast systems, remote sensing, RF identification, surveillance systems, industrial sensors, heating, environmental, and medical applications. Some of these systems are discussed briefly in this chapter. [ABSTRACT FROM AUTHOR]

Radar stands for radio detection and ranging. It operates by radiating electromagnetic waves and detecting the echo returned from the targets. The nature of an echo signal provides information about the target-range, direction, and velocity. Radar and sensor systems are big business. The two major applications of RF and microwave technology are communications and radar/sensor. In this chapter, an introduction and overview of radar systems are given. [ABSTRACT FROM AUTHOR]

This introductory chapter begins with a brief history of RF and microwave wireless systems. It provides discussions of frequency spectrums and wireless applications, including a simple example of a wireless radio systems. The chapter concludes with a brief outline of the organization of the book. [ABSTRACT FROM AUTHOR]

Introduction The invention of the transistor in 1947 ushered electronics into a new era. Although access to transistor technology was relatively open, France and Japan responded differently to the opportunities offered by the new technology. This chapter compares their national responses to semiconductor technology, particularly transistors, up to the mid-1960s. Semiconductor diffusion occurred faster in Japan than in France and this had important industrial competitiveness consequences. Partly as a result of historical patterns of state action (or lack thereof) and societal arrangements, by 1972 there was no French firm among the world's top ten semiconductor firms, and only one among the top twenty (Webbink 1977, p. 22). In contrast, there were two Japanese firms among the top ten and five among the top twenty. By 1960 Japanese transistor production was larger than French. Early technological choices shaped the future competitiveness of the national industry in semiconductors. Conventional views portray Japanese society and state as forward looking and fascinated with material progress. Since the Meiji restoration Japanese society has been eager to adopt modernity and build upon it. In contrast, France, her society and state, are portrayed as conservative with regard to new technologies at least until World War II. Formalistic explanations which analyse the cultural form of resistance hide the historically contingent and political nature of that resistance. To identify cultural, market or societal differences may be a sufficient explanation of resistance from a functionalist viewpoint but it fails to uncover the process characteristics of the phenomenon, which is an important resource for collective action. [ABSTRACT FROM AUTHOR]

The Renaissance, 1450–1530 Part of the charm of Renaissance writers is their firm conviction that they were living in a ‘golden age’. Their world was bigger and better than anything in the past and, they sometimes reflected, the heroes of antiquity would have been miserable failures as Renaissance men, even as Renaissance soldiers. ‘We must confesse’, wrote Sir Roger Williams, an English general of the later sixteenth century, ‘Alexander, Caesar, Scipio, and Haniball, to be the worthiest and famoust warriers that euer were; notwithstanding, assure your selfe, …they would neuer haue…conquered Countries so easilie, had they been fortified as Germanie, France, and the Low Countries, with others, haue been since their daies.’ We may smile at this characteristic Renaissance hyperbole, but in the field of warfare at least it was fully justified: the military realities of the sixteenth century were indeed far more complex and far more daunting than those of the Classical (or any previous) Age. European warfare was transformed between 1450 and 1530 by a number of basic changes. First came the improved fortifications of which Sir Roger Williams wrote, linked to the introduction of powerful new artillery. An entirely new type of defensive fortification appeared in Italy in the later fifteenth century: the trace italienne, a circuit of low, thick walls punctuated by quadrilateral bastions. The development of large siege-cannon – made of cast-iron from the 1380s and of bronze from the 1420s – rendered the high, thin walls of the Middle Ages quite indefensible. [ABSTRACT FROM AUTHOR]

This article focuses on demonstrating some basic principles used in weather radars. One should help young students understand how weather radar works and the difference between old radars that measure reflectivity, or intensity of precipitation, and newer Doppler radars that also measure winds. One has to choose several students to represent rain shows or snow showers. Other students should be allowed to be the radar pulse of energy that leaves the radar and reflects off of the rain or snow particles. One should use a stopwatch if possible and record the amount of time for the radar pulse to return.