submarines. Itisestimated thatthereareatpresent over550nuclear-power reactors installed inshipsorsubmarines, whichismorethanareinstalled onland although ofcoursethelatter haveasignificantly greater aggregate poweroutput. 85.Nuclear-powered sUbmarines arecompletely independent ofthesurface, and theirindependence islimited onlybythephysical andpsychological endurance of theircrews. Formerly oftentheweaponofcountries notabletocontrol theSeas bysurface forces, thesubmarine isnowamajorcomponent ofthemostpowerful navies. SUbmarine performance hasimproved tremendously. Thefirstnuclear submarine, theUSSNautilus, commissioned in1954,achieved speedsofover20knots andcoulddivetoadepthof200m.Thedevelopment ofthetear-shaped hUll, hydrodynamically moreefficient, andtheinstallation ofmorepowerful reactors madeitpossible toreach30knotsinlaterseries. Now,titanium hulls,better hydrodynamic characteristics andevenmorepowerful reactors insomepresent nuclear submarines makepossible speedsinexcessof40knotsanddivesto1,000m anddeeper. 86.Nuclear submarines havethusbecome amostformidable weaponofnavalwarfare inthattheycanoftenoutrun anysurface shipandfoilpursuit byusingthedepth oftheseatoevadedetection. However, thenuclear sUbmarine isatpresent beyond thetechnical andfinancial reachofnearlyallcountries. Thetraditional diesel-powered sUbmarine stillpresents certain advantages. Beingbothsmaller and oftenquieter underwater thanitsnuclear counterpart andeasiertouseinshallow waters,itismoredifficult todetectincoastal areas.Itisalsolessexpensive tobuildandtomaintain. 87.Nuclear powerhasnotbeendeveloped soextensively insurface vessels. Because ofspecial reqUirements, suchasthenuclear technology andspecialized welding techniques, nuclear-powered vessels areabOuttwiceasexpensive as traditional onesandreqUire higherskillstoconstruct andtooperate. Theunited States hasfouraircraft-carriers andninecruisers whicharenuclear-powered. According toJane'sFighting Ships1984-85, theSoviet Unionhasbuilt twonuclear-powered heavycruisers, andanuclear-powered aircraft-carrier isunder construction; inaddition, ithasthreenuclear-powered ice-breakers inservice and afourth underconstruction. France planstousenuclear powerinitsnew aircraft-carrier duetobeoperational attheendotthenextdecade. Otherthan theice-breakers mentioned aboveandthreeexperimental commercial vesselS, nuclear propulsion hasremained theexclusive preserve ofwarships. /...A/40/535 English Page30 C.Theelectronic revolution 88.Before 1940,shipsdiffered fromcountry tocountry onlyinnumbers, the quality oftheirconstruction andcertain mainlyquantitative aspects: engine power,sizeofguns,thickness ofarmour andsoforth. Comparisons between navies weretherefore relatively easy,andtheTreatyofWashington in1922wasableto imposelimitations onthedisplacement andweapons equipment ofvessels, features thatwerespecific andeasytoverify. Nowsuchcomparisons aremuchmore difficult. Thedecisive criterion forevaluating theeffectiveness ofavessel and theweaponsitcarries isnolongerquantitative butqualitative; inthisrespect a majorelement istheelectronic systems thatgovernalloperations, fromnavigation tocommunications andfromdetection toguidance. 89.Electronic equipment hasbecomeanessential partofnavigational aids. Inertial navigational systems, firstusedinmissiles, arenowwidelyinoperation insubmarines whichnolonger needtosurface inordertofixtheirposition. They stillneedtoupdatetheirinertial guidance system fromtimetotimebymeansof information provided bysatellites. Surface vessels alsoincreasingly use navigational satellites. 90.satellites arealsousedforcommunications, wheretheyareextremely important. Fleets nolongeroperate independently oftheirbasesbutarein constant contact withshore,sending andreceiving adensity oftraffic oververy greatdistances thatonlysatellite transmissions makepossible. 91.Communication withsubmarines haslongbeenaproblem asnormalradiowavesdo noteffectively penetrate water. Uptonowfortransmission, submarines haveused aerials thatpiercethewater's surface andforreception, wireantennae (wire floating onthesurface); suchdevices arefarfromidealasanyfloating object, nomatter howsmall,canbedetected. Theuseofextremely low-frequency transmissions, whichareabletopenetrate watermuchmoreefficiently, isbeing
**Analysis**
The text primarily discusses the development and capabilities of nuclear-powered submarines and their electronic systems, with a focus on their performance, advantages, and limitations. It also touches on the technical and financial aspects of building and maintaining these vessels. Given this content, it seems likely that the text falls under the category of Science and Technology.

**Category**
Transport, Science and Technology