The system and the transmitter
2.1 The operator and the system
2.1.1 Scope of chapter
This chapter outlines radar operation in general terms, and then describes the transmission systems of the relatively large radars used in deep-sea ships, vessel traffic service (VTS) systems and firing-range surveillance. Receiving systems are described in Chapter 3. Chapter 4 onwards detail the various facets of the detection problem, including quantitative analysis.
Figure 2.1 shows the whole radar/target/environment system. A person is study- ing the traffic situation at the display console. No mere passive observer, this officer adjusts the radar controls to optimise the display of targets of most current impor- tance. Stressing this interaction, we refer to the person as the operator. The display itself, sometimes still called the indicator or scope, with associated controls forms the human-machine interface (HMI) between the radar and operator and one task of this book is to consider how the machine can best help the human perceive the targets - apprehend them within the mind to gain situational awareness. Figure 2.2(a) shows a traditional deck-mounted console, while Figures 2.2(b) and (c) depict alternative formats suited to building into operator workstations.
2.1.2 Operators afloat
Radars on merchant ships, including vessels subject to IMO's high speed craft (HSC) code, are primarily operated by the officer of the watch (OOW), who is the ship's master or a qualified deck officer. The radar(s) are one of the principal tools which aid navigation of the intended route, avoid collision with other craft and confirm positions determined by satellite and other means. The OOW may be the sole person
Figure 2.1 The radar system. The operator controls the radar to best observe the target of interest within its environment. The system elements interact;
all affect detectability. Radars and targets may be afloat or groundfast on the bridge during daylight, but at night or in thick weather must be assisted by a seaman lookout, perhaps posted at the bow in telephone contact, but usually on the bridge, keeping visual and aural watch for ships and other hazards but never using the radar. HSC always have two navigators on duty. Figure 2.3 shows a typical bridge layout with displays and controls available to either officer's chair.
A pilot with special local knowledge may be hired to advise the master and often conns the ship, using the radar as would the OOW. The Master or 0OW then monitors the proceedings, partly by observation of the radar secondary viewing display, but remains in charge. A seaman helmsman or quartermaster may actually steer the ship under orders, but never uses the radar.
Pilots in some VTS systems are provided with portable laptop computers incor- porating modems and radio links giving copies of the current VTS display for the local area, independent of the ship's radar, perhaps revealing targets masked from the ship by bends in the waterway, and annotated with VTS alpha-numeric data. It is less usual for shore radars to transmit data direct to the portables, dispensing with a VTS centre but providing all ships with a common overall high-quality view of the traffic situation. AIS radio-based systems such as the Tideland Signal AIMS Base are now available, offering radarless VTS, claiming all the precision, accuracy and reliability without the costs and maintenance. It will be interesting to see how well they catch on.
Pilots and OOWs hold Certificates of Competency or 'tickets', awarded by a national authority. Radar operation and display interpretation are taught and examined prior to award, standards according with IMO's Convention on Standards of Training,
Atmosphere Radar
Scanner height H May roll, yaw or pitch
Feeder, if fitted
Transmitter/
receiver
Range, R Echoes, clutter and noise
Processing and display Operator
Sets controls
Observes display, makes decisions
Environment
Precipitation
Target height h may move
Sea surface
Waves reflect unwanted clutter ^ Depend on Forward reflection at grazing point / wave height
Figure 2.2 Deep-sea marine radar. BridgeMaster E Series. All reproduced by per- mission of Northrop Grumman Sperry Marine Ltd, New Maiden UK.
(a) Traditional deck-mounted console. Controls immediately below dis- play screen, transmitter and receiver in base cabinet. For standing operator, substantial bracing handles for heavy weather. Menu-drive controls below display, (b) Desk-top display for use seated or stand- ing, (c) Flat panel display on RCCL cruise ship Brilliance of the Seas.
Bow 3 and 9GHz scanners for berthing, main scanners above bridge, (d) Main navigation workstation. Radar and chart displays, with engine and steering controls to hand by navigator s chair, Brilliance of the Seas, (e) (overleaf) Bridge wing workstation, again with radar and chart displays, Brilliance of the Seas
Certification and Watchkeeping (STCW). Deck officers frequently transfer from ship to ship and may be presented with unfamiliar models of radar, so the IMO Marine Radar Performance Specification includes detailed requirements for uniformity of display depiction and of controls and their labelling.
As well as gaming hands-on experience at sea, navigators are taught on full mission or radar simulators ashore at nautical colleges. Simulators can replicate numerous scenarios, exercising the most effective operation and interpretation of
Figure 2.2 Continued
Figure 2.3 High speed craft command workstations. Typical layout with displays and controls available to either officer s chair. As always, a clear view forward is essential. Reproduced by permission of Kelvin Hughes Ltd,
Ilford, UK
the radar. Students sometimes emerge ashen-faced from close-quarter situations they hope never to encounter at sea. Complete ship's bridge simulators take the process further by inclusion of life-like and interactive views of the surroundings, with a full suite of navigational controls.
Naval officers are trained and examined in navigation much as their merchant navy cousins. Although naval bridge teams are larger, warships often take civilian pilots in unfamiliar harbours. Skippers and mates of fishing vessels (FV) are often
part-owners, or at least share voyage profits. Time is money to them, and they make full use of radar on passage. FVs are unmanoeuvrable while fishing and careful watch is kept for collision risks from approaching shipping. Owners of private leisure craft are not usually required to carry radar or be trained in its use, but will want to get the best out of an expensive gadget they have chosen to buy out of their own pockets.
2.1.3 Integrated bridge systems
Beside radar, operators gain situational awareness from the view from the window, radio traffic now including AIS, sound signals and maybe night vision equipment;
VTS may include radio direction finders and closed circuit television. The impor- tance of the radar display varies sharply between, say, night in thick weather and heavy traffic, and daytime in fair weather with little traffic, when the display may legitimately go almost unregarded.
Formerly, the navigation aids on a ship's bridge were almost autonomous, with minimal interconnection. Links to the radar were confined to heading and speed feeds from the compass and log for the True Motion and North-Up display modes. The radar(s), compass, log and other instruments each had their own displays, positioned in a rather uncoordinated manner. Nowadays the trend is to provide each member of the bridge team with definite seated work-stations, each having economically designed controls and displays appropriate to the member's function, see Figures 2.2(d), (e) and 2.4(a)-(c). The screens may be capable of displaying some electronic chart and other data as well as the radar picture. The radar, less display, then forms a sub-system of an integrated bridge system (IBS), being sometimes termed a black box radar.
The ship's voyage data recorder (VDR; bright orange, but sometimes called a black box nevertheless) is used for incident investigation and training purposes.
Among much else, it is required by IMO to record all the information currently presented to the operator on the master display of one radar, including range rings, radar status data (e.g. range scale), navigation alarms, etc., but not gain and other control settings.
2.1.4 Operators ashore
VTS may cover conflicting traffic flows in a navigationally difficult sea area. The traffic area of port VTS usually extends well to seaward of the harbour area. A small team of operators, sometimes called watchstanders, is led by a supervisor who may be the Harbourmaster. Methods of operation vary with port size, traffic patterns, local practice and the legal regime. There may be half a dozen sectors, each with its radar or radars, target data being handed from operator to operator as the ship transits the area. Beside radar, the operators use other sensors and information to build up situational awareness of the current and intended movements, anticipating conflictions and advising traffic to take appropriate actions; for example requesting or requiring a small vessel to keep clear of the deep channel while a supertanker passes.
Except perhaps in extremis, for legal and other reasons VTS operators are not generally responsible for fine detail of movements or collision-avoidance manoeuvres - they
Figure 2.4 Liner RMS Queen Mary2, Cunard Line. A Il courtesy Kelvin Hughes Ltd, Ilford UK (a) The largest passenger ship afloat, 150 000 gt. Entered service between Southampton and New York 2004. The bridge occupies prime space, the top floor forward. (Artists impression.) (b) Bridge console contents. This comprehensive outfit omits to mention the all- important Mark 1 Eyeball, (c) Main radar and pilotage consoles, shaded in(b)
Speed Compass
Depth Heading
Azipod Indicators
Internal comms Monitoring systems
Speed Compass
Depth Heading
Azipod Indicators
Compass mon Chart table Nav. equipment
Echo sounder
Speed Compass
Depth Heading
Azipod Indicators
Multi-function display DGPS Engine controls VHF and internal
communications CCTV
GMDSS Communications
VHF HF SAT-C
Auto pilot Int. comms Engine controls
Bow thruster VHF Steering wheel
Radar ECDIS DP-CSS Monitoring
Conning Multi-function
display DGPS Engine controls VHF and internal
com munications CCTV
do not seek to drive the ship. Training standards meet the need of the particular port.
Some states have national standards, others do not, since individual VTS systems vary so widely in complexity. Internationally recognised unified training standards are however being introduced through IALA and IMO. A surveillance system with many similarities to VTS was mentioned and illustrated in Chapter 1, Section 1.2.5.
There is a tendency to provide shore pilotage assistance from VTS centres, along the lines of Air Traffic Control. Only one aspect of this vexed question concerns us, registration of the targets displayed on the ship and shore radars, viewed by the 0OW and by the shore pilot, respectively.
• Instead of using the ship's radar to detect local targets, the VTS radar must display all of a group of distant targets in correct register to the piloted ship, demanding particularly high performance. A degree or so bearing error, trivial on the ship's display, might translate into a quite unacceptable relative positional error.
• Displays used by the shore pilot and the OOW should both contain exactly the same set of targets. Given a pair of weak targets A and B ahead of the piloted ship, there is rich possibility of confusion should the ship detect Abut not B, while the VTS, with its different aspect, detects B but not A.
Surveillance radars on coastal gunnery and missile firing ranges primarily ensure the hazard zone is clear of non-participating vessels, secondarily control movements of military craft participating in exercises. The civilian or military operators are trained and drilled in radar operation, interpretation and safety procedures. As members of the Range Safety Officer's team they operate to standing instructions which stress safety to all. Modern ranges take safety seriously. A UK Ordnance Board officer once told the author that they classed as a 'frequent occurrence' a life-threatening hazard predicted to arise once per 10 000 years. On the other hand, one has heard of a tanker master finding a deep indentation in the deck plating after passing a certain Mediterranean rocket range.
Fixed or mobile surveillance radars, often adapted ships' radars, are increas- ingly employed by Coastguard or Police forces on anti-terrorist or drug interdiction missions, again after suitable training. Feeds may also be taken from VTS instal- lations, where the security dimension is becoming an important factor in system design.
2.1.5 Basic radar operation
Conventional marine and VTS radars generate a steady train of pulses - bursts of oscillation - of microwave power. An antenna transmits the energy in a continuously rotating beam as shown in Figures 2.5(a) and (b). Any object in its path scatters the radiation reaching it. A very little returns to the radar. Object bearing is that of the antenna, range being measured by the delay before reception.
Let us look at the process in a little more detail, giving some typical shipborne radar performance parameters - like many of those quoted later on, these are approximate and vary from radar to radar. The pulses have quite high power of 1OkW but very
Figure 2.5 Radiolocation and ranging
short duration, 1 |xs or less. A pulse is transmitted at the speed of light, 300m/|xs, sweeps out and strikes any scatterer on or above the sea surface lying in its path, indicated by the direct path of Figure 2.1. Some of the incident energy is absorbed within the scatterer. The remainder is scattered through a broad solid angle. The tiny part returning to the antenna forms an echo. Knowing that transmission and echo each propagate at the speed of light, the elapsed time to reception measures echo range, Figure 2.5(c), with uncertainty inversely proportional to the pulselength. The two-way scaling is 150 m/|xs or 6.67 |xs/km. In radar work, time and range are often interchangeable. Each transmitter pulse is in effect 'time stamped' for measurement of echo delay.
After waiting long enough to receive the echo from a possible scatterer at the longest range of interest, another pulse is transmitted, the time between successive transmissions being the sweep time or pulse repetition interval, typically 0.001 s or 1 ms. A steady train of such pulses is emitted, the pulse repetition frequency (prf) being 1/0.001 = 1000 pulses per second (pps); pps is preferred to Hz to stress the extremely non-sinusoidal waveform. Sometimes prf varies with control settings. A few ancillary displays may operate ambiguously, with two transmissions simultaneously in flight.
The directional antenna radiating the pulses is called a scanner. Its beam rotates continuously at 25 rpm and typically covers 25° in elevation to cater for roll of the platform (ship carrying the radar), but is only 1° wide. Any particular scatterer is therefore scanned every 60/25 = 2.4 s for a period of 2.4/360 = 0.0067 s, being illuminated by a packet of 0.0067/0.001 = 6.67 successive sweeps, say half a dozen, Figure 2.5(Z)). Any echoes received during this period are assumed to come from objects lying on the known azimuth bearing currently being illuminated, azimuth accuracy approximating the beam width, Figure 2.5(d).
Vertical
Reference bearing (North or ship's head) Scanner
location Target bearing
Target range
Rotating fan beam Reflecting target
Range, km Max instrumented rang©
Pulse 1 Transmission
Slope = velocity of light (300m/|ls)
Echo 300m/ns
Pulse 2
After max range of pulse 1 Time, jus
Elapsed time measures range (c) Ranging
(a) Perspective view
Reference bearing Scanner
location
Half a dozen sweeps per scan
(b) Plan (d) Track on ppi display
Own radar
Bearing as scanner Predicted position - at time of scan 10..
Scale range proportional to echo delay Successive target positions form echo trail
Scan 4 (current scan) Scan 3 (memorised) Scan 2
Scanl
The positions of all detected objects in range and bearing (polar or R, 0 coor- dinates) are therefore determined on each scan. Their echoes are laid down to scale as plots on a display screen called a plan position indicator (ppi) which informs the operator of their positions relative to the radar. Plots are refreshed by the new mea- surements taken on each scan. By following the progress of a plot over several scans, the operator can determine the object's track or course made good relative to the radar. Historic plots may be shown as trails, roughly indicating target course and speed during the last few scans, Figure 2.5(d).
Targets are all objects, such as ships, of current interest to the operator. Although the Collision Regulations are written round aspect (relative bearing of target cen- treline) as indicated visually by navigation lights, often the radar discrimination is too coarse separately to display the individual scatterers comprising the target object and thus its aspect. Heights cannot be determined by radar. Radar is valued for its ability to position targets in range as well as bearing, and its general independence of cooperative equipment at the target. Although good signal processing facilities do the donkey work in presenting the clearest possible display, only the operator can decide that vital question - what to do?
2.1.6 Target detectability
Targets can only be displayed and tracked when the echo power or signal can be distinguished or detected with reasonable certainty from competing clutter, electrical noise, and such man-made interference as the transmissions of other radars. Figure 2.6 shows a ship's radar display with clearly visible coastal fea- tures. Areas of speckling over the sea surface are clutter caused by rain squalls and would mask any small target echoes within. We will now briefly examine these and other factors which affect detectability. They are discussed in detail later in this book.
When examining its passage through the atmosphere, the transmitted beam is often regarded as a bundle of linear elements called rays. The atmosphere subjects the rays to loss or attenuation and to variable curvature in the vertical plane on both transmit and receive legs. Figure 2.1 indicates that there are both direct and indirect ray paths between scanner and target, the indirect path being formed by intermediate reflection at the sea surface. Interaction between the two rays causes constructive or destructive multipath interference. At long range, the horizon intrudes on the scanner-target path.
Having reached the target, the proportion of unit incident transmitter power reflected back towards the radar governs the apparent reflecting strength of a target and is called its radar cross section (RCS, defined in Chapter 7, Section 7.1.1, and sometimes called cross section area, CSA). Most targets, such as ships and coast- lines, are inanimate or passive. Racons, RTEs and SARTs (radar beacons, radar target enhancers or active reflectors; search and rescue transponders, Chapter 8) are active devices which include a reception and retransmission process. Although much mod- ified by environmental effects, transmissions reaching any target basically follow an inverse square law and returning echoes or responses again follow this law, so echo
Figure2.6 Ship's basic radar display. Atlas 9GHz monochrome (green) raster cathode ray tude display, 12nmi range scale, North-Up. VRM set to 5.35 nmi, measuring range of a ship target bearing 228°. Own ship heading 177°. Rugged cliffs of Cape Wrath, NW Scotland, to South; the lighthouse is not conspicuous. Rain squalls to NW and SSE would mask small ship echoes. Two blind arcs astern (North) from masts. Alpha- numeric data around edge of screen. Lighthouse tender Pharos. Author, reproduced by permission of Northern Lighthouse Board, 1997 power, S9 at the radar tends to follow an inverse fourth power law of range, R;
(S ex \/R4, discussed in Chapter 4).
Clutter arises from scatterers such as a volume of precipitation or an area of sea-waves, not interesting to the operator. Their returns clutter the display and so hinder perception of targets. Although we will often use signal loosely, properly speaking signals convey information, wanted or unwanted. Echoes are signals but transmitter pulses are not, for they contain no information. Each echo is tiny (10~6 to 10~12 W) and may fluctuate in strength, say as own ship or the target ship rolls.
The signal to noise-plus-clutter ratio, often shortened to SNR, is of great importance.
Unavoidable imperfections within the radar receiving system also generate clutter- like background power, called noise. Because noise and clutter are random in nature, detection is never clear cut. There is always some probability of detection (PD) less than unity, associated with a finite probability of false alarm (PpA)- AS to be expected from information theory, high SNR raises PD for any given PFA-
For detection on a single sweep with acceptably high PD (>0.5) and low PFA (< 10~6), echo amplitude must exceed the adjacent noise and clutter by a large margin (SNR at least 10:1 power ratio or 10 dB) - there must be adequate contrast. Candidate events are winnowed by thresholding, only returns above a predetermined strength passing to the signal processor following the receiver, where they are assigned to the appropriate one of an array of detection cells or bins in range and bearing. Detection is improved by having several sweeps per beamwidth and averaging or integrating them. Echoes, being associated with a definite position, are statistically correlated and their counts build up more rapidly than those of clutter, noise and interference, which are more random in nature and decorrelated. Targets are declared valid when there is