Radar receiver
3.8 Display principles
The whole purpose of the radar is to communicate information to an operator by means of the display. The display, screen, or scope is the human-machine interface and is arranged to suit the operator - it must be user friendly. It should display as much as possible of what the operator needs - targets and associated alpha-numeric data - with as little extraneous clutter and noise as possible. Echoes displayed on the screen are often called paints or blips.
3.8.1 Display format
On ships, IMO demands relatively large screens, 250 mm, to 340 mm active diameter on the bigger tonnages, to reduce eyestrain when alternately looking out of the window, viewing other bridge instruments and refocussing on the screen at moderate viewing distance, and to facilitate two persons viewing together, for example, the master and OOW when evaluating a manoeuvre. The very large 'conference displays' of some military systems are not used-perhaps merchant ships cannot muster enough deck officers! Screen area must have some influence on mental perception of echoes, but data is lacking.
Current echoes or declared target plots are displayed as spots of light at scale range and bearing from the scanner position. This map-like form of display is called a plan position indicator (PPI); Figure 3.13. Targets are shown in polar or R9 0 (range, bearing) coordinates. The simplest PPI presentation keeps own ship or station at the centre of the display while all target paints, including coastlines, move past in relative motion. Ships' displays may be aligned to suit the traffic situation: North-Up, Ship's Head Up or Course Up (differing from head up if the ship makes leeway). Own ship may be offset from the centre to extend the view ahead. In the important True Motion mode, the display is ground stabilised and all targets and own ship move as across a map, resetting when approaching the display rim. VTS displays are of course always referenced to a fixed datum. They are sometimes aligned to the view from the station window - for example, the West Coast port of Vancouver, whose station lies to the North of the harbour, has found South-Up displays convenient.
The operator measures target position relative to the scanner by intersecting a variable range marker (VRM) and electronic bearing line (EBL) over the plot of interest, digital readouts giving range and bearing relative to own scanner. Alterna- tively approximate position can be judged from a set of electronically generated range rings at say 1 nmi intervals and a bearing scale or pelorus (compass rose, nowadays
Figure 3.13 Display including radar data only: coastlines, targets (some with trails), offset with range rings, heading marker and copious alpha- numeric information. Original in full colour. Reproduced by permission of Kelvin Hughes Ltd, Ilford UK
Figure 3.14 Display with chart data superimposed. Original in full colour. Repro- duced by permission of Kelvin Hughes Ltd, Ilford UK
electronic rather than engraved, avoiding parallax) around the circumference. It is usually possible to lay a line between a pair of targets of interest and read off their relative range and bearing. A cursor can also be placed over any feature of interest, its coordinates being displayed alpha-numerically. Choice of presentation, although very important to the operator, rarely affects the detectability problem and we shall not consider it further.
Displays of relatively unprocessed radar broadband video signals are called raw radar, but it is now usual to show processed or 'synthetic' information on the screen.
Formerly, each radar had its own stand-alone display, the only non-radar data being a few short alpha-numeric statements of course, speed etc. Nowadays, partly because the display occupies prime real estate on the bridge, selected parts of the system electronic navigation chart (SENC) are routinely added to radar displays, Figure 3.14, and plots are exported as a radar layer within the ship's electronic chart and display system (ECDIS). There is scope for addition of AIS data and plots transmitted by VTS stations, although care must be taken not to present the operator with an inassimilable mass of material - information overload is dangerous, the mind tending to dump data in a rather arbitrary manner. The trend is for black box radars to export plots and tracks to multifunctional displays at operator workstations forming part of an integrated bridge system (IBS). Data fusion has been discussed by Lee [2] and the same fusion and association problems arise as when radar tracks are combined, Section 3.7.2.
The radar market is much too small to support development and manufacture of special screens, so high quality computer and air traffic control devices are used, available in a restricted size range up to about 580 mm diagonal.
3.8.2 Cathode ray tube
Many displays still use cathode ray tubes (CRT), a specialised triode valve or elec- tron tube. Within the rear neck of an evacuated glass/metal conical envelope are three electron guns for the three primary colours, a single gun sufficing for the older
monochrome displays. Each gun has a red-hot metal cathode operated at a high negative voltage of about — 1OkV. Cathodes are coated with a mixture of oxides facilitating electron emission, under control of nearby 'grid' electrodes. Positive- going grid bright-up voltage signals permit cathode rays of high-velocity electrons,
~ 100 |xA, to fly to the glass screen at the front of the tube, drawn by an earthed metal cylindrical anode (plate) within the neck. The rays are electrostatically focussed to sharp dots and can be steered by the time base system to any part of the screen by application of voltage to pairs of deflector plates (or by current to pairs of coils) surrounding the neck. The screen is made as flat as possible consistent with forming part of a pressure vessel and carries sets of electro-luminescent phosphor coating picture elements or pixels on its inner face which fluoresce in primary colours or the predetermined monochrome hue when struck by the beam, the display being intensity modulated by the grid signal. Colours, if used, are standardised by IMO (for naviga- tional data) and IHO (International Hydrographic Organisation for charting features), differing for day and night illumination.
Modern screens have very short afterglow and are scanned raster-fashion like TV screens, the bright-up signals coming from the frame stores, which here perform an R, 0 to Cartesian coordinate conversion, giving a flicker-free display, Section 3.9.
Older CRTs laid down raw radar R, 0 video pulses sweep to sweep and scan to scan.
Long-afterglow phosphors integrated the real time scan to scan to give a reasonably steady image, Section 3.10.
3.8.3 Other display devices
CRTs are bulky and are rapidly being replaced in television and PC monitors by various kinds of flat screen low voltage semiconductor arrays, including liquid crystal displays (LCD), which are produced by the million. Application of a low voltage switches certain organic molecules between transparent and opaque states. In LCDs, a very thin film of the active liquid is sandwiched between glass sheets bearing a near-transparent pattern of conductors to form pixels, controlled by transparent thin film transistor (TFT) arrays. Backlit and colour variants are available. LCDs consume very little power. Long widespread in laptop computers, they were first taken up by the radar industry in small craft radars. Problems of available brilliance range, adequate area and wide angle of view are being overcome and price is falling.
As a result, high resolution LCDs are replacing CRTs in new big-ship marine radars.
Figure 3.15 shows part of a modern screen and shows the high resolution available.
Advantages include low bulk, screen flatness, low power consumption and avoidance of high voltage. Sufficiently large screens or screen arrays for VTS stations are now becoming feasible, at a price. TFT screens drive each pixel continuously, not once per screen scan, improving brilliance and eliminating flicker.