This introductory chapter explains the book's aims and methods, introduces the persons who operate radar, gives a brief historical background and outlines the regulatory framework under which marine radar is used.

As reciprocal devices, scanners receive incoming signals from the same volume of space illuminated on transmission. The receive and transmit gains, radiation patterns, sidelobes and polarisation are identical. Gain benefits both the transmit and receive legs, so increased gain is doubly beneficial to detection of weak targets. Most marine radars use linear polarisation in which the electric field exists in one plane, sometimes vertical but usually horizontal. Antennas can always efficiently receive the polarisation which they transmitted. Passage through the atmosphere does not significantly affect ray polarisation; most targets and sea clutter also more or less preserve the incident polarisation when they reflect. Any depolarisation to plane polarised signals is allowed for by a reduction of the nominal radar cross section (RCS) of the target or clutter. Targets and sea clutter are therefore received by plane polarised scanners without ostensible polarisation loss.

This chapter equips us to develop multipath factors M to describe the differences from free space propagation other than atmospheric loss for point targets and for extended targets. Insertion of atmospheric loss and multipath factor convert the radar range equation from free space to real conditions, allowing echo strength at all ranges to be calculated as accurately as our knowledge of the environmental conditions permits.

This chapter examines the mechanisms causing insulators and metallic shapes to reflect and goes on to discuss the reflection to be expected of point objects. It draws on material previously published by the author and by others who are acknowledged in the text.

Previous chapters considered point targets so physically small that only single pairs of direct and indirect rays exist, giving definite multipath structures within the interference region. Point targets also usually have well-defined and broad (often omnidirectional) spatial target pattern maps (TPM) or radiation patterns, so their RCS does not vary much as they tilt. Although sea-waves may modulate their multipath factor, received signal is reasonably steady. In this chapter we consider how multipath affects vertically extended targets, such as ships and coastlines, whose structure extends over a wide height bracket. Atmospheric and precipitation attenuations do not affect the arguments deployed in this chapter and will be ignored.

Preceding chapters enable the echo strength reaching the radar from passive or active targets to be calculated in presence of precipitation and in the various sea conditions. In this chapter we examine the noise and clutter which reach the detection system, competing with and perhaps masking the wanted target echoes. The competitors comprise thermal noise generated in the receiver and elsewhere, back-scatter from precipitation, sea-waves, ice or land, and possibly short-range internal feeder mismatch reflections. Their quantification prepares us to examine the signal to noise and clutter ratios necessary for calculation of target detectability in Chapter 12. Noise and clutter, by competing with wanted echoes at the radar, set a limit to the smallest detectable echo. Some aspects of precipitation and the sea were described in Chapter 5. The special characteristics of land and ice clutter were discussed in Chapter 10, Sections 10.12 and 10.13, respectively.

Catastrophic marine accidents are rare. Of those collisions and standings which do occur, many start as quite routine navigational situations; just another night, just another watch, just another target. Something then goes awry perhaps the radar fails to display an islet or a target's calculated closest point of approach (CPA) is erroneous jeopardising the crew, ship, cargo, passengers, other shipping or the environment, with losses running to a hundred million pounds. All significant targets have to be displayed to scale on the screen, often nowadays with chart features superimposed. We need to look in detail at the validity or accuracy of the plotted positions, for this accuracy, or lack of it, should always inform the navigators judgement of the situation and the prudent action to take. So far, we have concentrated on the process of detecting targets. Detection, although necessary, is not sufficient the navigator would give little thanks to be told that there was another target somewhere or other within busy Tokyo Bay (Chapter 5, Figure 5.14). We have looked at how plots of target current positions are laid down on the display, showing where targets and coastal features are relative to own ship at the present time. This basic information is a useful start and must contain little error, but the navigator needs more: are targets moving or manoeuvring? What have they been doing? Are they hazardously close? Will they approach to close quarters? Where? When? Most of these questions involve movement, which is the rate of change of position with time, necessitating data collection during two scans at the very least. Movement is represented as a track, which is the target velocity vector either relative to own ship (relative motion display) or to the ground or the slowly moving water mass.

This chapter presents hypothetical case studies of radar system detectability, using the spreadsheets of Chapter 14 to illustrate some of the factors meriting consideration when designing, specifying or analysing the detection aspects of typical systems. Likely importance of the major factors in play are indicated, but, rather than relying on the chapter's results as a rule of thumb, readers should take the plunge and run the spreadsheets for their specific systems. For brevity, this chapter excludes some of the re-iterations and alternatives necessary to real-life situations. None of the equipment represents any particular make or model.

This glossary gives the meanings usually borne in the text of the main technical terms, especially the less self-evident. Shades of meaning might differ in other radar contexts, for example, air traffic control.