Radar receiver
3.9 Raster scan display
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.
Figure 3.16 Raster display. Shown on 12nmi range scale with lOOOpps. Target position is found by intersecting the variable range marker and
electronic bearing line on its paint
of phosphor pixels in primary colours. Raster is Latin for rake. Operation is digital in character. The beam is steered in a quick zigzag path, raking the screen with about 70 complete rasters per second, like TV or PC displays, giving sufficient brilliance for viewing in full daylight. Use of a raster coarsens the basic R, 0 footprint at close range. Kelvin Hughes have experimented with spiral scans to maintain footprint so
Path of cathode ray 70 full-screen rasters/second Screen with pixel array of tricolour short persistence phosphors Target plot Brief pixel bright-up each raster
Fed from frame store, refreshed each scan
Electronic range and bearing markers for target position measurement Target trails from previous frame stores Presence and length controllable Own radar
May be offset from screen centre to give longer view ahead Supports fast graphics and text in distinctive colours
Figure 3.15 Portion of display. Target trails shown as varying tones in background.
Original in full colour. Reproduced by permission of Kelvin Hughes Ltd, Ilford UK
detection cell area is proportional to range, but this has not so far fully caught on, perhaps because television display technology is unsuitable. Monochrome displays are sometimes used, see Chapter 2, Section 2.1.6, Figure 2.6. Oftener, full colour is used. The palette may be selected to suit daylight or night viewing conditions.
The electron beams take up one of usually 16 or 32 digitally determined intensities, giving the pixel 16 or 32 grey scale (brilliance) values. The digital frame store data is converted from R9 0 to Cartesian format by a digital scan converter. The scaling and raster pitch are sufficiently fine for the eye to perceive the display as a flicker-free continuous range of tones and hues. Monochrome displays usually use a green phos- phor, least tiring to the eye. The phosphors fluoresce during the electron pulse, but do not phosphoresce afterwards - they have short persistence, to cope with dynamic navigational situations and evolving alpha-numeric messages.
To support text and chart data, and to avoid degradation of the echoes, radar dis- plays need higher resolution than TV. The PPI itself occupies a circle or square field, up to 340 mm diameter, the whole field being 380 mm square as minimum on the bigger ships. The remaining space of the rectangular tube forms a convenient area for display of secondary alpha-numerics such as course and speed, control menus, etc.
The pelorus is generated electronically on the display face, avoiding parallax when reading target bearings. Trackforming (generation of vectors) is performed electron- ically before paints are laid down, so any nonlinearities of scale caused by changes of deflection sensitivity near the rim of the tube do not affect calculations of CPA etc.
The tube grids permit beam current to flow and a bright-up of appropriate colour to be generated only when the raster is at an address whose target register cell contains a target, or where secondary information is to be displayed. Each frame store cell is addressed each raster, although of course its input is updated only once per scan in an R, 0 manner. Scrutiny of a moving echo shows it to advance one step per scan.
Target trails (see Section 3.10.1) are generated from the frame stores and are under the operator's control. Trails are shown in Figure 3.15.
Necessary resolution is around a couple of million pixels. For example on the 12 nmi (22 200 m radius) scale, pulselength may be 0.1 |xs, equivalent to 15 m range cell. The diameter therefore contains 22 200 x ~ = 2960 cells and ideally the tube would have this number of lines with the same number of pixels per line, pixel size being 0.13 mm x 0.13 mm for 400 mm (16 inch) tube width. In practice, the eye cannot resolve such small pixels at normal viewing distance and pixels are made about 0.2 mm x 0.2 mm. About 1000 lines x 1500 pixels per line are used for ships' radar. Some large VTS displays have higher resolution, partly to support many small alpha-numeric target legends or 'tags'.
Being reconstituted from stored and manipulated data, the display is synthetic, although there is a confusing tendency to reserve this term for tracks, terming plots as 'raw', even when they are the output of a processor. The screen is scanned frequently, giving enough brilliance for daylight viewing. The digital format of the input facilitates hooking-in secondary displays using local area network (LAN) techniques. At one time differing colours were used to indicate echo strengths. This practice has been discontinued, since strength is not navigationally significant. The phosphor persistence is too short for smearing when own ship manoeuvres or the range scale is changed.
Beside brightness, important features of raster scans include their ability to display in different colours, and quickly to amend:
• the radar picture (plots and tracks);
• text, enabling targets to be tagged with identification symbols (also operator's menus for controls);
• radar graphics such as predicted tracks (from ARPA, etc.), range and bearing markers for position measurement, target alpha-numeric identification tags;
• non-radar graphics such as charts (from ECDIS, etc.).
By lessening the operator's mental task, raster systems give much more consis- tent detection performance and free the operator for the tasks that no machine can replicate - intelligent decision-making.