Introduction
1.3 The past and future
Figure 1.3 High gain reflector scanner for sea surveillance service at 9GHz.
Inverse cosec2 type, aperture 5.5 m, gain 4OdBi, switchable circular and horizontal polarisation. Steel tower also carries transmitter- receivers and communications equipment, plus infrared and CCTV cameras. Reproduced by permission of Easat Antennas Ltd, Stoke on Trent, UK
increasingly for drug, intruder and piracy interdiction by coastguard or gendarmerie services. The majority use frequencies in or near the marine 9 GHz band. Equip- ments vary from powerful (^250 kW) sets with massive reflector scanners derived from military practice and displaying in a VTS-like range control centre, down to the smaller deep-sea ship sets or even yacht radars, deployed in small rough-terrain vehicles. Performance can be predicted from the following chapters, but we shall not cover the specialist precision tracking pulse or frequency-modulated (FM) radars which follow the flight of the projectile and of its sub-munitions.
Figure 1. 4 Small 9GHz marine radars on surveillance duty. Sea Safety Group volunteers operate an expanding chain of coastal stations around the UK coasts. All Illustrations reproduced by permission of Captain A. R Starling Lark and Sea Safety Group, (a) Watchroom ofSSGRedcar Station, North-east England. A Furuno radar (left) augments the visual lookout. Combination with afluxgate compass speeds reporting of exact latitude and longitude of casualties to the Search and Rescue services, to which the fully trained volunteers are officially accredited. Roof- mounted scanner at 40 m above sea level has 15 nmi horizon range and can detect target as close as the adjacent beach, (b) View from the watch- room towards the busy Tees Bay with its commercial shipping, (c) Twin displays. SSG Great Yarmouth Station. Quite small radars may suffice for inshore safety surveillance. Furuno 1832 radar, 4JcW transmit- ter power, 60 cm scanner. Feeds two 10 inch rectangular monochrome raster-scan displays. One observer can quickly guide Royal National Lifeboat Institution lifeboat to a casualty while a colleague maintains general surveillance, essential when sudden bad weather catches many leisure craft unawares
practical with the available technology. An experimental centimetric apparatus on the French liner Normandie was able to detect presence of ships at several miles range shortly before that great ship was destroyed by fire, and Radar Type XAF was at sea from 1938 in USS New York, but lack of transmitter power stymied progress.
Figure 1.4 Continued
Meanwhile the storm gathered over Europe. Britain, fearing airborne bombing and invasion, hastily and secretly set up a shore-based air surveillance system, constrained to conventional radio wavelength (~ 12 m) by the power problem. These Chain Home radars, described with a good engineering outline by Latham and Stobbs [8], resembled broadcast radio transmitter stations, with their huge static wire antenna arrays. They drew on nascent television technology. Immediate success in the Second World War triggered concerted research into the crucial need for powerful microwave generators. Necessity is the mother of invention and a superb solution was found with remarkable speed. Building on low-power cylindrical split-anode magnetrons developed in Japan, the cavity magnetron of 1940 opened the way to practical centimetric radar, first on fighter aircraft and then warships. Submarine periscopes became detectable as early as 1943. Perhaps exceeded in technolog- ical effort only by atomic energy and code-breaking, wartime Anglo-American radar research yielded an excellent understanding of basic principles. Declassi- fied before the Cold War, the work was published around 1950, notably in the 28-volume MIT book series,1 much of whose content is still valid today. Late in the War thoughts turned to peacetime uses and the British were able to write a confident design aim for 'Post War Radar', including the first commercial marine radars. As early as May 1945, a 9GHz prototype navigational radar Type 268 was demonstrated [9] to civil marine interests aboard HMS Pollux, using a submer- ging submarine, HMS Umbra, as a variable-sized target - the wheel had turned full circle.
The first commercial (1946) marine radars typically came complete with own cabin and roof-mounted scanner. Initially they were not very reliable and it took time for deck officers to accustom themselves to the strengths and limitations of this radically new aid to navigation. Nevertheless, the basic concept was sound,
1 Massachusetts Institute of Technology Radiation Laboratory, McGraw-Hill. Some volumes have been re-published by IEE in UK and all are available in CD-ROM form from McGraw-Hill.
early problems were gradually beaten and radar became mandatory for all ships.
Wavelengths and frequencies are discussed in Chapter 2, but we mention here that frequencies around 3 GHz, wavelength 10 cm (S band, 3000MHz), was found best for detection in severe rain, in other conditions 9 GHz (3 cm, X band, 9400 MHz) being preferable. Some ships got the best of both worlds and carried both. Early suppliers were mainly large electrical firms who had been active in wartime radar.
We shall not often mention individual firms but must make an exception for Decca (now Northrop Grumman Sperry Marine), who later deservedly won a pre-eminent position. Their war work included the Decca Navigator, but they did not enter the radar market until 1950.
Until the late 1960s, purely analogue circuits were used, the raw echoes being presented on a cursive plan position indicator (PPI) using a cathode ray tube having a dim monochrome long-persistence phosphor. Operators had to draw tracks represent- ing the movement of each target by grease-pencil on the Perspex face of a reflection plotter. Radars of those days contained several electro-mechanical devices - motors, relays, etc. - and the only semiconductors were a few rectifiers and the microwave detector 'crystals'. The hard work was done by about 50 valves (thermionic tubes in the United States, where the anode is known as a plate). Each valve was supported by a couple of dozen passive components, mainly resistors, capacitors and inductors, hand-soldered to insulating tags, and all carried on metal trays, connected by cable bundles. Each valve anode consumed about 2.5Wat250V and its heater dissipated another 2 W. The total heat was considerable and the dust attracted by the high volt- age could cause arcing failures. Digital technology was in its infancy. Hardware meant screws. Software? What have woolly jumpers to do with radar? The term did not exist. Detail design by pencil, paper and sliderule was laborious and of variable quality, necessitating much 'cut and try' prototype testing.
In 1977, after a spate of accidents causing pollution, political pressure caused the United States Coast Guard to issue, after discussion, regulations requiring all ships entering US waters to carry and use a collision avoidance system, to include continuous evaluation of the echoes of all ships posing a collision risk. This demarche quickly resulted in the international adoption of ARPA, the computer-based automatic radar plotting aid so widely used today. The remainder of the radar soon went solid state, although retaining a few analog circuits. Displays now often carry chart material from an electronic chart system (ECS).
1.3.2 Secondary radars
Ship superstructures tended to be angular. To a radar engineer they were corner retro- reflectors riveted together, so were inherently good targets, helping to give the early sets useful range. From the first it was realised that lighthouses ought to be made radar- conspicuous, using transponders called racons (radar beacons), which early British experiments showed to be superior to ramarks radar markers. After initial setbacks, the first chain went into regular service around the British Isles in the late 1960s. Early sets weighed some 300 kg and consumed between 45 and 450 W, see Figure 1.5, but advent of solid-state electronics soon brought weight below 20 kg and consumption
Figure 1.5 Early 9GHz racons. (a) Bell Rock lighthouse. This historic structure, which first exhibited its light on 1 February 1811, carried one of the first racons to enter regular service, 1968. (b) Lighthouse engineer inspecting antenna. Transmit and receive full-height WG16 waveguides, four slots giving ±10° elevation beamwidth, flares giving omnidirectional azimuth response ±2dB, gain 7dB. Perspex X/2 radome. Reproduced by per- mission of the Northern Lighthouse Board, (c) Circuits rack. Duplicated transistorised receive-transmit units. Tuneable magnetron transmitters.
Automatic monitoring and changeover unit above. Power consump- tion 45 Wfrom diesel generator. Sub-units transportable by ship's boat.
Reproduced by permission of the Northern Lighthouse Board
Figure 1.6 Early low-power racon. GEC-AEISea-Watch 300; here in transponder duty on support ship s mast, 1973
to 1W, extending use to buoys; Figure 1.6. The market is well under 1000 a year, so racons remain rather costly. According to ITU,2 the world population is about 6000, of which rising 60 per cent are dual band, the remainder being 9 GHz types.
In the 1980s racon technology was used as a basis, initially in Japan, for search and rescue transponders (SART) for the specialist task of marking liferafts. Being a mandatory carriage requirement, some 50 000 are carried by shipping, although it is hard to find evidence that they have saved many lives.
The echoes of small Craft have for long been augmented by metallic reflectors, which are necessarily rather bulky. Active reflectors - containing electronic circuits - have now appeared, called radar target enhancers (RTE). Problems remain, partic- ularly provision of adequate radar cross section for heeling yachts. The potential user base is very large, exceeding that for small-craft radar, and prices are falling as products becomes better established and the regulatory authorities encourage carriage.
1.3.3 VTS
Progressive port authorities quickly adapted ships' radar to monitor shipping move- ments from shore, initially to regulate traffic during and after fog. Although marine radars are still sometimes used for this task, 'harbour radar' has evolved into a special- ist VTS discipline. Powerful radars with big scanners feed extensive data-handling adjuncts. Stations may have to combine echoes received by land-line or radio link from a dozen or more remote radar heads, and add data from other sources such as closed circuit television and shipping databases to provide the operator with full situa- tional awareness of the traffic. The market is small - perhaps 100 stations a year - and
2 International Telecommunications Union document ITU-R SE34(99)1 Annex 3 (SE(99)TEMP 171 revl). Compatibility studies between existing and proposed new radio services in the band 2700-3400MHz.
equipments are often custom-built to port requirements, so unit cost tends to be high.
Developments are concentrated on data fusion from multi-head systems, with some convergence with air traffic control practice. Range surveillance systems are broadly similar to VTS. Argument raged for a long time whether VTS should control or advise.
The author soon found that use of VTMS (M for management) was likely to stir such a hornets' nest that the term is banished from this book. IMO is harmonising3 the training and certification of VTS operators and supervisors.
As already noted, there is a trend to colocate maritime rescue coordination centres (MRCC) with VTS stations to enhance the resources available to handle marine incidents.
1.3.4 The current generation of radars
All large fishing vessels, and all ships within SOLAS - broadly, all merchant ships - have to carry a 9 GHz radar. The bigger merchant ships have to have a second set, preferably at 3 GHz. It is remarkable that after half a century's development in a competitive international market the WW2 design concept is still followed, demon- strating fitness for purpose rather than complacency. Although there have of course been numerous improvements in detail, centimetric-wavelength short pulses are still generated by a magnetron and radiated by a rotating scanner as a narrow beam. The tiny echoes bounced back from targets are amplified in a form of radio receiver and displayed as a map representation of the area surrounding the radar. Range continues to be calculated from elapsed time and bearing from scanner pointing angle.
Instead of functioning as a stand-alone device, shipborne radars now tend to be treated as units within an integrated navigation-aid system, linked together to form an integrated bridge system or IBS. Radar designers seized on computer technology as it developed, particularly for processing the signal delivered from the receiver. This has enabled use of colour raster-scan television-style daylight-viewing screens, capable of displaying auxiliary data such as track lines and alpha-numerics, culminating in a complete superimposed electronic chart. Ever more computing capacity is now being harnessed, primarily to improve detection and tracking of multiple targets in clutter. Radars are now seen as sensors within an integrated data system, whether on the bridge in an integrated navigation system (INS), at the VTS centre or at the range control building. Most big-ship radars are no longer supplied as stand-alone items, but as part of a complete electronics package. In all cases the overall aims are improvement of the operator's situational awareness and enhanced detection of weak targets.
Marine and VTS equipments are simpler than some military radars, and very much cheaper. But they are not a poor relation. On the contrary; long continuous development in a fiercely competitive market gives the user choice of the optimal engineering solution for almost every requirement of performance, reliability, ease of use, size and price. The world market for deep-sea radar is only moderate - less than 25 000 a year. Technological development work is costly and radar improvements
3 IMO MSC Circular 1065, IALA standards for training and certification of vessel traffic services (VTS) personnel.
perforce ride on the back of mass produced items for communications, computers and television.
At the other end of the scale, miniaturisation and price reductions have enabled smaller craft to benefit from radar, and a large specialist yachting market has devel- oped, particularly in the United States and the Far East. There may be4 about 30 000 of 3 GHz radars at sea, predominantly on large ships and certain large fishing vessels, and 800 000 at 9 GHz, on vessels of all sizes.
Marine radar engineers, of course, keep fully abreast of developments in related fields. Current work programmes concentrate on integration of displays with elec- tronic charting and with integrated bridge systems generally, which all make wide use of digital technology, and this task is by no means yet complete. Emphasis is being placed on improving the echo strength of small vessels, some travelling fast, using passive or active reflectors. Radar is beginning to be employed to determine wave information for routeing assistance systems, to find the optimal route to minimise voyage time while avoiding seas bad enough to cause cargo damage, particularly when containers are carried on deck, or even to hazard the vessel itself.
Increasing pressure on the electromagnetic spectrum from telecommunications interests has recently forced the radar industry to pay more attention to unwanted out-band transmitted interference and further tightening of permissible emissions can be expected.
1.3.5 Future possibilities
Technical advances, evolving operational spectral and other constraints, plus desire for new facilities may induce substantial design changes in the fairly near future.
Professor Baker, who has been studying the possibilities, has very kindly, and bravely, contributed a concluding chapter reviewing a wide range of options for change. Some are evolutionary, continuing existing trends. More revolutionary re-design would discard much of the present 'pulse magnetron' configuration in favour of low power, long pulse modulated transmissions, active scanners, microwave data processing and other unfamiliar concepts. What will happen only time can tell. Such developments would demand extensive revision of IMO and other regulations, not only those relating to radar itself, but extending to SARTs, racons, RTEs and reflectors. If only for that reason, new concepts will not supersede conventional technology for at least a decade and equipment to the current concept will surely be around in a quarter of a century's time. The underlying laws of physics are eternal and well understood, so most of the content of this book will always remain valid, come what may.
Irrespective of the form radar may eventually take, it will be more necessary to combine marine or VTS radar data with that gathered from other sources, particularly the emerging AIS system. Sollosi [10] gives a useful summary of the VTS function and the problem of fusion of radar and AIS data.
4 From ITU-R, IALA standards for training and certification of vessel traffic services (VTS) personnel.