The impact of new display technology on thedetection of small targets at sea by radar

P.D.L. Williams

Indexing term: Radar displays

Abstract: For nearly forty years radar displays in small and medium systems have been used by dark-adapted,dedicated radar operators or observers. Recently there has been a growing need for displays to be used in a'glance' mode, often under high-ambient-light conditions. The paper discusses the implications and outcomeof such usage with particular reference to small-target detection in sea-clutter conditions.

1 Introduction

Radar has been used to detect ships and obstacles since theliner Normandie fitted a centimetric set 1935.1 The use ofrotating aerials and plan position indicators (p.p.i.) usingcathode ray tubes (c.r.t.) began in the 1940s and was welldocumentated in 1947 by Ridenour2 and in 1951 by Soller,Starr and Valley3 in the MIT series of radar text books.

Modern reviews of radar displays include that by Hartleyand Williams4 and, in the specialist marine-navigationaldisplays field, Wylies and Harrison6 provide information onboth natural and synthetic radar displays.

The purpose of this paper is to draw attention to thedifferences in the actual probability of small target detectionwhen carried out by discontinuous observation of a conven-tional p.p.i. under high ambient light conditions by a ship'sofficer, compared to the detection obtained by a dedicatedradar operator, fully dark adapted, working with a conven-tional fluoride c.r.t. display, both using the traditionalmechanically scanned search radars. The use of a 1-bitdigital frame memory is shown to restore to the modernships' officers carrying out many duties the detectionperformance formerly achieved only by using a full-timeradar operator.

2 Conventional display systems

By definition, search radars can only spend a fraction of thetime for a complete search period examining or dwellingon a particular area. In practice, the aerial in a traditionalradar scans smoothly in azimuth and the beam-dwell time isa function of the aerial rotation speed and its horizontalangular resolution or beamwidth. Thus the p.p.i. has abright trace rotating in synchronism with the aerial and ap.p.i. picture is presented using the afterglow feature of asuitable phosphor. However, all phosphors have a lightoutput falling exponentially with time directly after theflash period. This results in the operator either followingthe trace round to examine that part of the p.p.i. directlybehind the rotating trace, or, if he wishes to concentrateon one particular area, he has to work in very low ambient

Croney, J.: private communication

Paper T438E, received 10th June 1979Mr. Williams is with Decca Radar Ltd., Davis Road, Chessington,Surrey, England

light level using a viewing hood and preferably become darkadapted for best detection results of small targets. In thesecircumstances the flash of the rotating trace can be anuisance, and various methods of reducting this have beenused. The oldest one is to use a c.r.t. with a double-layerphosphor giving a blue flash but yellow afterglow. Thisenabled the operator to view the c.r.t. through a yellowfilter which reduced the annoying flash. Later, a methodinvolving a mechanically rotating shutter to blank off theflash was proposed and tried experimentally by Croney*to allow inspection of the afterglow picture more comfort-ably but still at low level, although this could have beenimproved using a closed-circuit t.v. system or analogue-scan convertor.3 Radar operator or discontinuous user

The problems of the radar detection of small targets aregreatly increased when the observation of a conventionalc.r.t. radar display is only one of the duties of a ship'sofficer. As already discussed, dark adaption is necessaryfor good performance, particularly for small targets inthermal noise, as exhaustive trials' have shown that whereasa long-afterglow phosphor helps to integrate the separatepulse returns in a beam dwell time, weak signals do notpaint sufficiently strongly to create an afterglow which lastslong enough to integrate from scan to scan. In addition, inhigh ambient light, the afterglow is not seen if the p.p.i. isglanced at for a fraction of a scan. To illustrate this Fig. 1 is

tUnpublished work in 1971 and 1975

Fig. 1 Photograph of a p.p.i. display exposed for one full aerialrevolution

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0308-6984/79/060241 + 06 $0150/0

Fig. 2 A conventional p.p.i. seen at a glance

a photograph of a marine display taken with an exposuretime corresponding to 1 aerial scan, i.e. 2 s for a 30r/m inaerial rotation rate, i.e. all parts of the film are exposed tothe bright c.r.t. flash. Fig. 2, however, is the same p.p.i.taken with the same camera stop using a 1/30 exposure andshows only that arc occupied by the rotating trace. Thisillustrates the glance problem, as well as static photographspermit, and emphasises how the eye sensitivity is suppressed(or stopped down) at high light levels as found on a ship'sbridge during daylight.

4 Bright display — a new generation of users

Radar displays now need to be viewed not only in daylightconditions but by nondedicated operators. These peopleneed to assimilate information and achieve satisfactorydetection probabilities on small targets with an acceptablylow false-alarm rate. Ideally this needs to be achieved onrelatively inexpensive radar equipment not having thebenefit of sophisticated target extractors to feed whollysynthetic displays as used in some of the large collision-avoidance radars reviewed by Wylie.s or in air-traffic-control systems described by Jackson et al.7

This type of synthetic radar display, together withcomputer visual display units (v.d.u.) appears to have solvedthe problem of radar detection. This is only partly true andbreaks down for poor target to noise or clutter ratios whena good operator has hitherto been required and his selectiveobservation needs to be replaced by a satisfactory processorto work under both high-sea and rain-clutter conditions aswell as in thermal noise.

5 Comparison of three forms of p.p.i. presentation

Discussion has so far revealed that the satisfactory storageof the results from a complete field or area of search has tobe achieved if a true bright display is to be obtained. It willbe evident that one of the first methods was to photographa p.p.i. with one film frame per aerial revolution. TheKelvin Hughes photoplot was an early realisation of thattechnique;8 other analogue methods included the directview storage tube and various direct-coupled-scan convertortubes both using a storage mesh. An example of a modern,optically coupled, scan convertor recently installed on aradar on the English north-east coast is shown as Fig. 3. Inthis photograph the light-output graduation is less than a

Fig. 3 Example of t.v. display driven by an optical scan convertorwith a 20r.p.m. aerial

snapshot photograph of a conventional p.p.i. shown in Fig.2, but not as good as the photograph in Fig. 1 where thefilm emulsion provides an area memory of equal sensi-tivity on all bearings. By contrast the radar-display photoshown in Fig. 4 is taken from part of a t.v. monitor drivenby a digital-frame store minutes after the primary radar hadbeen switched off to show the infinite storage facilityoffered by such a digital memory read out in a nondestruc-tive mode, a freedom not available with the analogue d.v.s.t.bright displays where the flood beam destroys the chargestored on the picture mesh. The exposure, as in Figs. 2and 3, is once again 1/30 second to simulate a quick glanceby a naval officer or other observer. Fig. 5 is another viewof the t.v. output of the same digital 1-bit memory showingfalse radar returns from the sea as viewed in a daylit roomallowing other work to proceed. The actual brilliance leveldepends, of course, on any particular t.v. monitor and itscontrol settings, but the important feature of this picturememory is that it is operated on only one amplitudequantising level. This has been made possible by the use ofa suitable and sufficiently robust distribution-free detector9

and trials to date have shown that not only is small-target

Fig. 4 Radar picture read out for 1 minute after primary radarsignals had been stopped

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Fig. 5 Sea clutter intentionally unsuppressed-as shown on a t.v.monitor having passed through a 1-bit digital memory

Note the false reporting of sea-clutter as strong targets when notusing a CFAR processor

Fig. 6 Large target clear of sea-clutter residue

detection in noise unimpaired but small boats in sea clutterare rendered more visible due to the ease in which the spatialdecorrelation of sea clutter is improved by using relativelyslowly turning aerials, whereas real targets are seen to overlaytheir positions on the last scan more clearly. Thus, the wholeprocess acts as a powerful pattern-change detector whenviewing any part of the t.v. radar displays as the aerial beamscans through it. Separately, a quick glance at the t.v.monitor shows all targets as last painted at full t.v. brilliancelevel; irrespective of amplitude, a function of the radarused, their range and each target size and aspect.

To illustrate this technique Fig. 6 shows one quadrant ofa p.p.i. with the radar position depicted in the bottom left-hand corner with several hundred false targets caused bythe spiky nature of sea clutter close to the radar, whereas asingle, fairly large target in the right-hand corner is suf-ficiently strong as to extend over several radar pulse lengthsand also aerial beamwidths. There is thus no problem inrecognising this particular target in this sea-clutter residue.Without the c.f.a.r. processing the whole area would havebeen swamped in a complete mass of sea-clutter return.

6 Further improvement of target detection in seaclutter

To improve the sea clutter residue shown in Fig. 7 it isnecessary to fully integrate the result of several radarscans and the effect has been reproduced in Figs. 8—10where it will be seen that, whereas the target is comingup at full brilliance level, the clutter residue in the mainsea-clutter field has only achieved full correlation over

Fig. 7 First scan Fig. 8 Second scan plus first scan

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a few dozen points in Fig. 9. Fig. 10 shows a furtherimprovement as the process continues. This techniqueuses the property of an integrator counting the finalthreshold crossing of a one bit digitiser translating theM count into synthetic amplitude levels and, hence,whereas full brilliance of non-fading targets is achieved,there is an opportunity that targets fading according to theSwerling 1 or 3 target models10"12 still have a chance ofbeing detected, albeit at a lower brilliance level. Thus, onehas achieved digitally a very reasonable synthesis of the useof an operator using a conventional c.r.t. with a longafterglow phosphor but whose disadvantage is the necessityto use it under darkened conditions. For medium and largesized radar targets integration over only one aerial scanproduces a dramatic improvement in false alarm reductionfrom sea-clutter as the c.f.a.r. threshold circuits may beadjusted to back off the clutter associated with the non-Rayleigh sea-clutter amplitude distributions associated withhigh-resolution radars13'14. For really small targets such atechnique is not permitted and the c.f.a.r. threshold circuitshave to be set up to allow a high residue of sea-clutter spikesto show as false alarms on the display.17 Fig. 11 has beentaken from a medium resolution radar where on the three-quarter mile range quadrant shown there are many falsealarms from sea-clutter as well as an echo from a sphericalsmall plastic 'buff buoy. These are used to mark fishermansnets or mooring ropes, and they cannot have a radar cross-section (r.c.s.) greater than their optical cross-section. Asthis particular buoy had a diameter of only half a metre ther.c.s. will be approximately 0-2 m2 with its echo centre veryclose to the sea surface, thus representing the smallest

practical target requiring detection. It should be noted that0-2 m2 represents a peak value, the median10 value maywell be 10 dB down, i.e. 0-02 m2. Fig 12 already shows areduction in the false alarm from sea clutter but targetdetection is still not practical. Fig. 13 is the result ofintegration over one more aerial revolution and outside thefirst quarter-mile calibration ring about a dozen possibletargets are shown, and, not until the integration process hasbeen carried out over eight scans, is the small target clearlyand unambiguously shown as a bright dot between thesecond and third calibration rings.

Integration over eight scans has two penalties: first theneed for a 3-bit store, and secondly the need for an accuratetrue-motion system to allow these small static targets tointegrate spatially whilst the radar equipped vessel (oraircraft) proceeds.

7 The case for multiamplitude displays produced ona single-aerial-scan basis

For the entirely different case of radars being used tointerpret large tracts of ice, land or other extensive areatargets, at least 3-bit quantising is needed, and it is import-ant that the use of radar for collision avoidance involvingsmall discrete targets is never confused with its imagingqualities, for example the SEASAT photographs recentlypublished17 or photographs of an s.l.a.r. used for ice or oilslick imagery as illustrated by Ekengren.16

For an airborne radar it may well be desirable to have a1-bit mode of operation for small-target detection, whileallowing multiamplitude quantisation when the equipment

Fig. 9 Three scans integrated

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Fig. 10 Eight scans integrated

ELECTRONIC CIRCUITS AND SYSTEMS, NOVEMBER 1979, Vol. 3, No. 6

is used for ice interpretation, where the penalty for missinga small target is far less serious than aboard a ship using itsradar for collision-avoidance purposes. True motion aboardaircraft may well constitute a serious problem if it has to bedriven from an air-speed indicator and compass, but may bemore readily achieved by using a true ground-speed sensorsuch as a Doppler navigation aid.18

8 Conclusions

The 1-bit digital p.p.i. memory coupled to an appropriatenaval radar provides a powerful technique for enhancing thedetection of small targets, such as yachts10 and small motorboats thereby reducing the collision hazard. The removal ofgray tones in radar displays corresponds to the realisationof a threshold detector as used in automatic radar targetextractors, so that the onus of weak target detection isremoved from the human operator, but the same operatorcan use his judgment in accepting a fading target by virtueof a given number of positive detections presented to himas a percentage of aerial scans observed and so his choice offalse alarm level is more clearly presented to him.

Actual signal/noise ratios needed to declare signals maynow be read from Marcum and Swerling's computedtables,11'12 which are now more conveniently recomputedand presented by Mayer and Meyer19 and for the first time,radar-performance forecasts in thermal noise may be madewith confidence for radar observers operating under highambient light conditions due to the automatic detection

circuitry now becoming available. Furthermore, asextended integration times become possible using digitalcircuitry and large capacity stores, troublesome sea clutterreturn with the long-tailed amplitude distributions may berendered more noiselike by the central limit theorem beinginvoked for long integration periods.

9 AcknowledgmentThe author in formally acknowledging permission to publishfrom the Director of the Development Laboratories ofDecca Radar, wishes to thank his many colleagues and inparticular the contributions of H.D. Cramp and M.R.Blythe.

10 References

1 Bulletin de la Societe Francaise Radio-Electrique,Aout-Septembre-Octobre 1935, pp. 110-118

2 RIDENOUR, L.N.: 'Radax system engineering', Vol. 1 of MITSeries (McGraw-Hill, 1947)

3 SOLLER, T., STARR, M.A., and VALLEY, G.E.: 'Cathode raytube displays', Vol. 22 of MIT Series (Mcgraw-Hill, 19XX)

4 HARTLEY, P.L., and WILLIAMS, P.D.L.: 'A review of displaydevices for radar applications'. Proceedings of the exhibition andconference 'Electronic Displays 75', September-October 1975

5 WYLIE, F.J.: 'Escape time: the crucial factor in collision avoid-ance situations and systems', /. Inst. Navig., 1978, 31, pp.438-451

6 HARRISON, A.: 'Marine radar today: a review', Radio &Electron Eng., 1977, 47, pp. 207-208

7 JACKSON, I.R., and KNOWLES, P.N.G.: 'Plot extraction forprimary and secondary radar' in 'Radar present and future'. 1EECon. Publ. 105,1973, pp. 333-338

Fig. 11 First scan Fig. 12 Integration of two scans

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Fig. 13 Integration over three scans Fig. 14 Integration over eight scans

8 PARSONS, S.R.: The marine radar photoplot system', /. Inst.Navig., 1962,15, pp. 289-293

9 WILLIAMS, P.D.L.: The detection of small low lying surfacetargets at sea by radar'. M. Phil. Thesis, University of London,1977

10 WILLIAMS, P.D.L., CRAMP, H.D., and CURTIS, K.: 'Anexperimental study of the radar cross-section of maritimetargets', IEE J. Electron. Circuits & Syst. 1978, 2, pp. 121-135

11 MARCUM, J.I.: 'A statistical theory of target detection bypulsed radar'. Rand research memo RM750, December 1947(also IRE Trans. 1960, IT-6, pp. 59-267

12 SWERLING, P.: 'Probability of detection for fluctuatingtargets'. Rand research memo, March 1954, (also IRE Trans.,1960, IT-6, pp. 269-308)

13 CLARKE J. and PETERS, R.S.: 'Constant false alarm ratedetector adaptive to clutter statistics'. Memorandum 3150

14 LEWIS, B.L. and OLIN, I.D.: 'Some recent observations of seaspikes' in 'Radar 77'. IEE Conf. Publ. 155, 1977, pp. 115-119

15 SCHLEHER, D.C.: 'Harbor surveillance radar detection perform-ance', IEEEJ. Oceanic Eng., 1977, OE-2

16 EKENGREN, B.: "Oil slick detection with real aperture side-looking radar' (Reidel Publishing Co., Dordrecht, Holland, 1978)

17 Aviation Week & Space Technoi, August 7th 1978, pp. 42-4318 BECK, G.E.: 'Navigation systems', (Van Nostrand Reinhold,

1971)19 MAYER, D.P., and MEYER, H.A.: 'Radar target detection'

(Academic Press, 1973)

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