Introduction
1.1 Purpose and scope
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.
1.1.1 Purpose
The art, science and skill of the navigator is to get a ship from A to B without hitting anything. Ships come in many shapes and sizes, Figure 1.1, from pleasure yachts to half-million tonne tankers. A and B may be berths in harbours, but B may be a pilot cutter or a shoal of fish. Among the things to be avoided are coastal features, other vessels, heavy flotsam and ice. Radar is an essential tool used to locate these hazards, assisting the navigator to make timely manoeuvre decisions.
Radar, an acronym for radio detection and ranging, detects objects of interest by transmitting radio signals in known directions from a narrow-beam antenna or scanner which scans the horizon, then timing the instants of reception of returned echoes from these 'targets'. (This Second World War term dates from development of radar for gunnery control and other military tasks; like much of the early jargon, it has stuck.) Each detected target is displayed map wise at its correct range and bearing.
Additional to the anti-collision task, prudent seamen also still use radar to cross-check their position against known shore targets, despite the advent of satellite navigation systems as the prime onboard position finder. Ashore, vessel traffic service (VTS) radars depict the traffic situation in and around busy harbours and traffic lanes, and
Figure 1.1 Ships come in all sizes
range surveillance radars check that offshore gunnery ranges are clear to commence firing.
The radar's first task is to pick out targets against a background of unwanted electrical fluctuations within the radar receiver and unwanted reflections from objects of no interest to the observer, such as waves and rain. In the jargon, targets must be detected in presence of noise and clutter. Only when this has been done can the radar go on to display target range and bearing or generate tracks of the target's movement to provide the operator with useful information.
Much of this book treats the technical circumstances enabling a target to be dis- played but one must never forget that even the finest radar is merely a tool which can be ignored, misused or misunderstood. It is always up to the operator to decide whether and how to use the radar and its information. The human-machine inter- faces are the radar controls and the display screen. We shall step outside the realm of electronic engineering when considering how operators should utilise radar when performing their safety-related tasks, but steer clear of purely navigational matters, for which readers should turn to specialist books, such as those by Burger [1] for deep-sea ships, Bole and Dinelley [2], which concentrates on plotting aids and Wylie [3], which though old and out of print, remains surprisingly relevant in many respects, with many illustrations of radar displays of targets and clutter and much observational experience soundly based on theory. See Bartlett [4] for small vessels.
The present book stems from a paper published in the Journal of Navigation [5].
It is primarily offered to those, perhaps neither radar specialists nor mariners, who need an understanding of how well civil marine shipborne and shore-based radars are likely to detect their targets under practical conditions. Targets may be 'natural' obj ects such as ships and coastlines which happen to reflect radar echoes, or may be specially prepared devices intended to return good radar signals. These include passive radar reflectors, racons and radar target enhancers (RTE), the latter two being secondary radars, speaking only when spoken to, and hence 'active' targets. The book discusses all aspects of the detectability problem and helps the reader answer such questions as:
• Is higher transmitter power worth its cost to me?
• What is the best height for my scanner?
• Which radar band should I choose?
• Would carriage of an RTE significantly improve detection of my craft?
• Which supplier's VTS proposal is best for my harbour?
• Do trials results match theoretical expectations?
• How does weather affect performance?
Radar is useless unless it can detect the targets of interest to the user, yet manufac- turers in their data sheets rarely claim any specific detection ranges for their products.
This is because the environment and many targets fluctuate in ways which are difficult to quantify or measure. Reports from seafarers (e.g. as quoted by Bell and Starling Lark [6]) stress the variability of observed detection range and the difficulty of detect- ing small targets in bad clutter; we shall explain why this is so. Although precision is impossible, we provide methods of calculating the likely detection performance of all the radar/target/clutter combinations likely to be met in practice, with spread- sheets enabling readers to calculate the performance of their own particular systems on a personal computer.
1.1.2 Scope
We consider only radars used for civil shipborne navigation and collision avoidance and for VTS and related tasks such as sea surveillance of gunnery ranges, drug inter- diction and sea traffic research studies. These all employ centimetric, non-coherent, pulsed, low pulse repetition frequency surface to surface surveillance radars with directional rotating common transmit-receive scanners. 'Non-coherent' means that our radars use only the amplitude of echoes, ignoring signal phase. In general, they also operate unambiguously, with only one pulse in play between radar and target at any instant.
Other forms of radar exist, differing to lesser or greater extent, but are currently seldom used in marine contexts. Over the horizon radars and some others, although of course sharing the same underlying physical principles, differ so radically that only parts of this book apply directly. Much of the book is applicable to warship surface surveillance radar used for detection of surface targets, although we have completely excluded all the specifically military problems, such as jamming, which so dominate military radar design. Instead, we concentrate entirely on the civil field, where a modern treatment of detectability is lacking. Accuracy of positioning and tracking, which demands good detectability, is discussed towards the end of the book. The many other important radar qualities, such as size, cost and ease of use, are considered only where they influence detectability.
The linked components of the radar - transceiver, scanner, display - are frequently referred to as a system. We however prefer to treat the whole radar as part of a wider system which also contains the other main interacting elements: the marine environ- ment, the target and the operator. We point out some of the problems facing designers and indicate solutions. Likewise, we hope better understanding of the factors in play will help operators get the best from their equipment, and so contribute something to the safety of life at sea.