Lighthouse a65 - Radar Range and Image Quality

20.4Radarrangeandimagequality
Maximumradarrange
Theusablerangeoftheradarislimitedbyfactors
suchastheheightofthescanner,andheightofthe
target.
Maximumradarrangeisessentiallyline-of-sight,so
islimitedbytheheightofthescannerandtheheight
ofthetargetasillustratedbelow:
a
1
a
2
h
H
D1643-4
R
max
R
max
= 2.23 ( h + H )
1
3
2
ItemDescription
1Radarequippedvessel.
2
Curvatureoftheearth.
3
Target(Cliff).
a
1
Radarhorizonofantenna.
a
2
Radarhorizonoftarget.
R
max
Maximumradarrangein
nauticalmiles.
R
max
=a
1
+a
2
hRadarantennaheightin
metres.
HTargetheightinmetres.
Thetablebelowshowstypicalmaximumradar
rangesforvariousradarantennaheightsandtarget
heights.Rememberthatalthoughtheradarhorizon
isgreaterthantheopticalhorizon,theradarcanonly
detecttargetsifalargeenoughtargetisabovethe
radarhorizon.
Antennaheight
(meters)
Targetheight
(meters)
Maximumrange
(Nauticalmiles)
33
7.7
31010.9
5
38.8
5
1012
Radarimagequality
Anumberoffactorscanaffectthequalityofaradar
image,includingechoes,seaclutter,andother
interference.
Notallradarechoesareproducedbyvalidtargets.
Spuriousormissingechoesmaybecausedby:
•Sidelobes.
•Indirectechoes.
•Multipleechoes.
•Blindsectors.
•Sea,rain,orsnowclutter.
•Interference.
Throughobservation,practice,andexperience,you
cangenerallydetecttheseconditionsveryquickly
andusetheradarcontrolstominimizethem.
SideLobes
Sidelobepatternsareproducedbysmallamountsof
energyfromthetransmittedpulsesthatareradiated
outsidethenarrowmainbeam.Theeffectsofside
lobesaremostnoticeablewithtargetsatshort
ranges(normallybelow3nm),andinparticularwith
largerobjects.Sidelobeechoesformeitherarcson
theradarscreensimilartorangerings,oraseriesof
echoesformingabrokenarc.
D1638-5
1
2
2
3
4
5
6
6
ItemDescription
1Mainlobe
2
Sidelobes
3Antenna
4Arc
5
Trueecho
6
Sideechoes
IndirectEchoes
Thereareseveraltypesofindirectechoesorghost
images.Thesesometimeshavetheappearanceof
trueechoes,butingeneraltheyareintermittentand
poorlydened.
D1641-5
1
3
2
4
5
6
ItemDescription
1Falseecho
2Trueecho
3Passingship
4
Mastorfunnel
5
Trueecho
6Falseecho
MultipleEchoes
Multipleechoesarenotverycommonbutcanoccur
ifthereisalargetargetwithawideverticalsurfaceat
acomparativelyshortrange.Thetransmittedsignal
296aSeries/cSeries/eSeries