Radar receiver
3.10 Cursive display
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.
use green or orange. The phosphor fluoresces momentarily when struck by the beam, after which it continues to phosphoresce with an afterglow for several seconds, not necessarily in the same colour. Cascade screens combine long- and short-persistence phosphor layers with sealed-in optical colour filters.
In these older radars, each time the transmitter fires, the beam sweeps radially out- ward from own radar's position near the centre at a rate scaled to suit the transmission in a spoke-like or polar cursive manner, returns painting at scale range. Operation is wholly analog. There is no scan conversion and the screen is uniformly coated and not divided into discrete pixels. The sweeps rotate synchronously with the scanner, so the surveillance area is covered by a couple of thousand sweeps once per scan.
The pelorus is engraved on a collar surrounding the display and is subject to parallax error, especially near maximum range, where curvature of the tube face is the greatest.
Trackforming is generally performed manually using a reflection plotter, and any non-linearities of display scaling affect the trigonometry used in the calculation of CPA, etc.
The trace on later sets is made less dim on short-range scales by addition of a store from which the trace is repeatedly painted. The small (~7 dB) brightness range of long persistence phosphors is augmented by a soft limiter in the video amplifier to better indicate relative signal strength, helping show targets in precipitation clutter and noise.
Positive-going analog video bright-up pulses are applied direct to the CRT grid.
Each pulse makes the phosphor at the scale location of the scatterer glow dimly, fading away after 20 s or so. If the bright-up is a clutter or noise spike, it is unlikely that the same point will be further brightened on succeeding sweeps or scans. But if the scatterer is a target, the same or an adj acent phosphor point is likely to be brightened on these sweeps or scans. The more hits, the brighter the phosphorescence; the phosphor integrates the returns sweep to sweep and scan to scan. Noise and clutter are not integrated and appear as dim speckles. The eye's excellent pattern recognition skill
Figure 3.18 Cursive display. Brightness variation as strobe sweeps display can be mesmeric. Decca 9GHz radar, 1.5 mile range scale, Inverness.
Original monochrome orange. Author, 1983
then readily sorts echoes from noise, unless severe. Brightness is a measure of echo strength, within the limits set by the dynamic range of the phosphor, augmented by previous soft limiting applied to the video.
Thus, the tube cut-off grid voltage performs the threshold function and the phos- phor integration characteristic provides pulse to pulse and scan to scan integration.
The phosphor persistence maintains the display between scans and automatically pro- vides afterglow trails of target track history, whether this information is wanted or not. Spot brilliance to some degree indicates echo strength. Minimum spot size is
~0.5 mm, which may slightly degrade the display resolution when short pulses are used with a big scanner on a long-range scale.
Radar sensitivity is optimised by biassing the grid just to extinguish the beam in absence of signal, adjusting the receiver gain to give faint background noise speck- ling, following the principle of setting false alarm rate to the maximum tolerable by the decision processor, here the operator's brain. The operator's mental pattern recognition capability is then used to sort the frequent noise or clutter spikes from faint but more persistent echoes. Long-persistence cursive displays well suited the earlier radars before modern computer technology was developed.
3.10.2 Cursive display problems
The scan causes a rotating strobe of background brilliance variation, all too soporific when no targets seem in sight. Moving targets automatically leave trails of dimin- ishing brilliance, useful unless they mask other targets, but of course not under the control of the operator. The display smears badly when own ship yaws or manoeu- vres, unless set to North-up presentation, and also takes half a minute or so to clear after change of range scale or off-centring. Display of tracks is rather unsatisfactory because minor scan to scan track variations due to noise cause smearing. A phosphor point is illuminated once only per scan, yielding insufficient brilliance for daylight viewing, necessitating either blackout curtains or a viewing hood which precludes two or more persons viewing together and disaccommodates the eye. Attempts at more brilliance lead to an objectionably bright 'initial flash'. Radars of the cursive era take feeds from the ship's log and compass, but otherwise tend to stand in proud isolation from other bridge equipment. Indeed, the earliest bulky equipments like the BTH RMS-I came in their own special cabin, the operator telephoning voice-told plots to the bridge in the tradition of the crow's nest lookout. Later, a junior officer acted as radar observer at the back of the wheelhouse. The dim display and its daylight screening preclude one-man bridge operation.
Tracks are drawn by hand on an anti-parallax reflection plotter over the screen.
The plotter is a semi-silvered transparent screen of opposite curvature to the tube face.
Labour-intensive and permitting only a few targets to be followed, at least this chore focusses the operator's attention on nearby shipping movements. Display of alpha- numeric data is difficult. One manufacturer (AEI) went so far as to provide a separate 400 mm short-persistence green alpha-numeric display, superimposed visually on an equally large orange long persistence radar tube, the pair being combined by a half-silvered mirror, hardly a compact arrangement. It was soon withdrawn.
Continued observation of a cursive system in strong clutter demands close concentration and is tiring. The eye takes time to accommodate to the dim display.
Gain and brilliance controls must be kept in careful adjustment for good results. It is all too easy to take the lazy option - operate at high brilliance, low gain, fine for strong echoes, eliminating noise and clutter - unfortunately setting the grid bias threshold so high that small targets are eliminated as well.
3.10.3 Detection performance
Under ideal conditions, raw radar detection performance is little inferior to that of modern equipment and the same detectability calculations can be employed, with an operator correction to account for: (a) inappropriate control settings and (b) non- perception of targets displayed, perhaps weakly, on the screen. These human factors are difficult to quantify. They may be allowed for by assuming that the probability of detection threshold is high, or by addition of an operator loss term, Lo p, in the radar range equation. The following has been adapted from a suggestion of Skolnik [3] but, as with all psychological effects, precision is impossible:
L0P - I O I o8( ^ d B . (3.5)
For example, if desired probability of detection is 0.9 or 0.5, Lo p ~ 2.0 or 4.5 dB, respectively.