Koden Electronics RB716A Marine Radar RA42C User Manual 05

Koden Electronics Co., Ltd Marine Radar RA42C 05

05

2CHAPTER 2. USING RADAR FOR THE FIRST TIMEThis chapter describes basic information on radars and explains technical terms usedin radar operation for those who is using a radar for the first time.2.1 What is a radar ? A marine radar is one of the navigation equipment installed on a ship. It emits a radiowave in very high frequency called a microwave from its antenna and receives the reflectedradio wave from objects on the sea (e.g., other ships, buoys, and lands). The received radiowave is converted into an electric signal which is displayed on a display screen to indicatethe presence of such objects. Although it is very difficult to find other ships or the destina-tion coast with human eyes at night or in thick fog, a radar helps you detect objects on thesea helping you avoid danger when sailing. The antenna turns 360 degrees as it radiateswaves, allowing you to grasp ambient conditions around your ship at a glance.The radio wave radiated from the antenna is called a pulse wave and the radar performstransmission and reception alternately. Several hundred to several thousand pulse wavesgenerally are transmitted while the antenna rotates one turn.AntennaThere are many types of antennas generally used for aradar. For example, these include a parabolic antenna anda slotted-array antenna. The performance of the antennadetermines that of the radar. The dominant factors are theantenna's beam width and side lobe level. The narrowerthe beam width, the higher the resolution of the angledirection. The lower the side lobe level, the fewer the effectof a false echo.Side lobeA beam in one direction in which the strongestradio wave is radiated from the antenna is called themain lobe and beams in other directions are called"side lobes". The side lobe level refers to the differ-ence in level between the largest side lobe and themain lobe. Buoy Other ship Radar wave Your ship Antenna (Rotating) Radar display Fig.2-1 What is a radar? Beam width Side lobe level Main beam Side lobe Antenna Fig.2-2 Antenna pattern
3Beam widthA beam width is defined as the width of the main lobe at an angle where the radi-ated power is halved as measured from the position from which the strongest radiowave is radiated.2.2 Characteristics of Radar Wave Radio waves from the radar propagate while bending slightly along the terrestrial sur-face. This characteristic varies dependent on the density of the atmospheric air. The sightdistance D of a radar generally is said to be approximately 6% longer than the optical sightdistance and is calculated using the equation below : D (NM) = 2.22 ( h1 + h2 ) where, h1= antenna height in metersh2= target height in metersFig.2-3 Radar waveTargets difficult to display on screenThe intensity of the reflected wave from a target depends on the distance, height,and size of the target, as well as its material and shape. Targets constructed withFRP, wood, or other low-reflectance materials or those that have a small incident angleare difficult to display on a screen. Therefore, FRP and wooden ships, sandy beaches,and sandy or muddy shallows all are difficult to catch and require attention whenmonitoring on the screen. Especially, coast lines on the radar image appear to be pre-sent more apart from the ship than they are actually located. Therefore, it is importantnot to misinterpret the available data.Shadow zones of radarRadar waves are characteristic in that they propagate straight ahead. Therefore,if the ship's smokestack or mast is located near the antenna or there is a tall ship ormountain at the side of the ship, such an object generates a shadow behind it. In this Apparent coastline Actual(invisible) coastline Invisible Visible 3 1 HU Fig.2-4 Targets difficult to display on screenh1 h2Line of sightRadar Radio WaveEarth
4case, some objects produce a complete shadow and some produce a partial shadow. Inan extreme case, the shadow of an object may extend to a position far away and can-not be displayed on the screen at all. Since these shadows can be discovered when in-stalling an antenna, the problem can be avoided by changing the place of antenna in-stallation to minimize the shadow. Targets in shadow zones are difficult to display onthe screen.False echoesA false echo of an actually nonexistent object may sometimes appear on thescreen when sailing. The following explains the cause of each of such phenomena.A. Ghost echoesIt sometimes happens that one large object near the ship appears at two differentbearings. One is the actual echo and other is a ghost echo generated as the wave is re-reflected from the ship's own smokestack or mast. The former appears at the correctdistance and bearing on the screen and the latter appears behind the smokestack ormast. This type of false echo is also generated by re-reflection of waves from bridgesand quay walls other than the ship itself.B. Multiple echoesIf there is a large vertical reflecting plane near the ship as in the case when yourship passes alongside a large ship, the wave is repeatedly reflected back and forthbetween your ship and the other object. For this reason, two to four images appear onthe screen at equal intervals in the same bearing. A false echo that is generated bysuch multiple reflections is called multiple echoes. In this case, an image appearing atthe nearest position is the real echo. Multiple echoes disappear as the ship movesaway from the reflecting object or its bearing changes. Therefore, it is not difficult todetermine the correct image. Target Direct reflection path Secondary reflection path Mast etc. Real echo Ghost echo Direction of ghost echo 3 1 HU Fig.2-5 False echoes of radar (Ghost echoes)
5C. False echoes caused by side lobeThe radiant beam emitted from an antenna contains side lobes in directions otherthan that of the main beam. Since the side lobe level is low, it in no way affects distanttargets. However, if there is a strong reflecting target near the ship, it sometimes ap-pear as a circular-arc false echo on the screen.When located near large targets such as land, theship's mast, etc. sometimes appears as a falseecho of circular-arc shape. 3 1 HU Real echo Multiple echoes Fig.2-6 False echoes of radar (Multiple echoes) 3 1 HU Antenna Main beam Side lobes Real echo False sidelobe echoes Fig.2-7 False echoes of radar (Caused by side lobe)CAUTION!
6D. Distant false echoes caused by duct phenomenonDepending on meteorological conditions, duct phenomenon sometimes occurs intemperature inverting layers of air. In such a case, the wave propagates erraticallyreaching a location surprisingly far away from the ship. In this case, a target presentat a distant location more than the radar's maximum distance range appears on thescreen presenting a false echo that can be misunderstood to be present nearer thanthe actual position. This phenomenon is attributed to the fact that since echo from thedistant target arrives late, it gets out of the pulse repetition frequency and is displayedon the screen as an echo in the next frequency. If the target distance changes as youswitch over the distance range, you can determine that it is a false echo.Radar interferenceIf a radar operating in the same frequency exists nearyour ship, interference noise may appear on the screenthat is caused by transmitted waves from that radar. Thisinterference appears in various ways. In most cases, how-ever, it appears as spiral or radial patterns.The RA40C/41C/42C radar has a function to elimi-nate interference. Use of this function helps you minimizeinterference.2.3 Terms Specific to Radar HM (Heading Marker)This is a line-shaped marker used to indi-cate the advancing direction of your ship.North MarkThis marker indicates the north direction.It is a short line approximately 1/6 of thescreen size. 3 1 HU Radar inrterference Fig.2-8 Radar interfer-ence 0.75 0.25 HU HM(Heading Marker) North Mark Fig.2-9 Heading Marker and North Mark
7Display modesThis refers to a radar's display modes. There are four display modes depending on thedirection in which the top of the screen faces with respect to the ship.Head Up (HU)In this mode, the ship's heading always indicates the upward direction of thescreen. This mode lets you know the relative positions of your ship and other ships orland.North Up (NU)In this mode, the north direction always indicates the upward direction of thescreen, allowing you to compare your ship position with a marine chart as you navi-gate.Course Up (CU)The ship's heading in a course-up mode always indicates the upward direction ofthe screen as the bearing toward the destination. In this mode, the ship can be ma-neuvered to sail the shortest distance to the destination by steering it in such a waythat its heading marker always directs to the upward direction of the screen. If theship drifts due to tidal current, care must be taken because the fixed targets move toother positions.True Motion (TM)In this mode, the ship is displayed as if it is moving on a marine chart while thefixed targets such as islands and seashores are fixed in position. When the shipreaches a certain position on the screen (approx. 2/3 of screen size), the ship is placedback to the opposite side on the screen. (The top of the screen faces north.)Note: Navigation equipment such as a gyrocompass or magnet compass must be con-nected to your radar system before it can be operated in NU, CU, and TM modes.(Refer to Section 3.9 for details on how to connect your radar to navigation equip-ment.) Ship's Heading North Scheduled course HM EBL North mark HU NU CU TM Ship's locus (not displayed on screen) North North Scheduled course 0.75 0.75 0.75 0. 25 TM 0.25 CU 0.25 NU 0. 25 HU 0.75 Fig.2-10 Display modes
8VRM (Variable Range Marker)This is a circular-shaped marker whose size can be changed as desired. You can usethis marker when you want to examine the distance of an echo from your ship.When measuring the distance of an echo from your ship, be sure to measure at apoint close to the center of the echo image on the screen.EBL (Electronic Bearing Line)This is a marker shaped like a straight line segment that can be changed to any direc-tion centering around the ship position. Use this marker to examine the advancing direc-tion of your ship and its relative angle with an echo. When measuring the angle of an echo,position the marker at the center of the echo. VRM Echo 0.75 0.25 HU VRM Fig.2-11 VRM 0.75 0.25 HU EBL Echo EBL Equal intervals Fig.2-12 EBL
9STC (Sensitivity Time Control)Since echo signals received by the radar are strong when they are coming from a shortdistance, it is difficult to compare signal strength between each reflected signal. To over-come this difficulty, signal strength is adjusted in such a way that the received signal levelscoming from a short distance are lowered and those from a long distance are raised. Thisfunction should prove useful when there are large reflected waves from sea surfaces duringrough weather.FTC (Fast Time Constant)When it rains or snows, fine noise may appear over the entire screen, making it diffi-cult to identify echoes. In such a case, echo images on the screen can be made easily distin-guishable by adjusting FTC. 0.75 0.25 HU STC OFF STC ON 0.75 0.25 HU Echo is suppressed around center Fig.2-13 STC0.750.25HUFTC OFF FTC ON0.750.25HUSmall noisesare reduced.Fig.2-14 FTC

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