Raymarine c97 - Radar Range and Image Quality

12.4Radarrangeandimagequality
Maximumradarrange
Theusablerangeoftheradarislimitedbyfactorssuchasthe
heightofthescanner,andheightofthetarget.
Maximumradarrangeisessentiallyline-of-sight,soislimitedby
theheightofthescannerandtheheightofthetargetasillustrated
below:
a
1
a
2
h
H
D16 43-4
R
ma x
R
ma x
= 2.23 ( h + H )
1
3
2
ItemDescription
1Radarequippedvessel.
2
Curvatureoftheearth.
3
Target(Cliff).
a
1
Radarhorizonofantenna.
a
2
Radarhorizonoftarget.
R
max
Maximumradarrangeinnautical
miles.
R
max
=a
1
+a
2
hRadarantennaheightinmetres.
HTargetheightinmetres.
Thetablebelowshowstypicalmaximumradarrangesforvarious
radarantennaheightsandtargetheights.Rememberthatalthough
theradarhorizonisgreaterthantheopticalhorizon,theradar
canonlydetecttargetsifalargeenoughtargetisabovetheradar
horizon.
Antennaheight
(meters)
Targetheight
(meters)
Maximumrange
(Nauticalmiles)
33
7.7
31010.9
5
38.8
5
1012
Radarimagequality
Anumberoffactorscanaffectthequalityofaradarimage,including
echoes,seaclutter,andotherinterference.
Notallradarechoesareproducedbyvalidtargets.Spuriousor
missingechoesmaybecausedby:
•Sidelobes.
•Indirectechoes.
•Multipleechoes.
•Blindsectors.
•Sea,rain,orsnowclutter.
•Interference.
Throughobservation,practice,andexperience,youcangenerally
detecttheseconditionsveryquicklyandusetheradarcontrolsto
minimizethem.
SideLobes
Sidelobepatternsareproducedbysmallamountsofenergyfrom
thetransmittedpulsesthatareradiatedoutsidethenarrowmain
beam.Theeffectsofsidelobesaremostnoticeablewithtargetsat
shortranges(normallybelow3nm),andinparticularwithlarger
objects.Sidelobeechoesformeitherarcsontheradarscreen
similartorangerings,oraseriesofechoesformingabrokenarc.
D16 38-5
1
2
2
3
4
5
6
6
ItemDescription
1Mainlobe
2
Sidelobes
3Antenna
4Arc
5
Trueecho
6
Sideechoes
IndirectEchoes
Thereareseveraltypesofindirectechoesorghostimages.These
sometimeshavetheappearanceoftrueechoes,butingeneralthey
areintermittentandpoorlydened.
D16 41-5
1
3
2
4
5
6
ItemDescription
1Falseecho
2Trueecho
3Passingship
4
Mastorfunnel
5
Trueecho
6Falseecho
MultipleEchoes
Multipleechoesarenotverycommonbutcanoccurifthereisalarge
targetwithawideverticalsurfaceatacomparativelyshortrange.
Thetransmittedsignalwillbereectedbackandforthbetweenthe
targetandyourownship,resultinginmultipleechoes,displayed
beyondtherangeofthetruetargetecho,butonthesamebearing.
D16 4 2-4
1
2
ItemDescription
1Trueecho
2Multipleechoes
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