Installation Instructions

ManualsBrandsKoden Electronics ManualsElectronicsMarine Radar RA41C

CHAPTER 2. USING RADAR FOR

THE FIRST TIME

This chapter describes basic information on radars and explains technical terms used

in 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 radio

wave in very high frequency called a microwave from its antenna and receives the reflected

radio wave from objects on the sea (e.g., other ships, buoys, and lands). The received radio

wave is converted into an electric signal which is displayed on a display screen to indicate

the 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 the

sea helping you avoid danger when sailing. The antenna turns 360 degrees as it radiates

waves, 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 performs

transmission and reception alternately. Several hundred to several thousand pulse waves

generally are transmitted while the antenna rotates one turn.

Antenna

There are many types of antennas generally used for a

radar. For example, these include a parabolic antenna and

a slotted-array antenna. The performance of the antenna

determines that of the radar. The dominant factors are the

antenna's beam width and side lobe level. The narrower

the beam width, the higher the resolution of the angle

direction. The lower the side lobe level, the fewer the effect

of a false echo.

Side lobe

A beam in one direction in which the strongest

radio wave is radiated from the antenna is called the

main 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 the

main 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

Summary of content (8 pages)

PAGE 1
CHAPTER 2. USING RADAR FOR THE FIRST TIME This chapter describes basic information on radars and explains technical terms used in 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 radio wave in very high frequency called a microwave from its antenna and receives the reflected radio wave from objects on the sea (e.g., other ships, buoys, and lands).
PAGE 2
Beam width A beam width is defined as the width of the main lobe at an angle where the radiated power is halved as measured from the position from which the strongest radio wave is radiated. 2.2 Characteristics of Radar Wave Radio waves from the radar propagate while bending slightly along the terrestrial surface. This characteristic varies dependent on the density of the atmospheric air.
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case, some objects produce a complete shadow and some produce a partial shadow. In an extreme case, the shadow of an object may extend to a position far away and cannot be displayed on the screen at all. Since these shadows can be discovered when installing an antenna, the problem can be avoided by changing the place of antenna installation to minimize the shadow. Targets in shadow zones are difficult to display on the screen.
PAGE 4
3 1 HU Real echo Multiple echoes Fig.2-6 False echoes of radar (Multiple echoes) C. False echoes caused by side lobe The radiant beam emitted from an antenna contains side lobes in directions other than that of the main beam. Since the side lobe level is low, it in no way affects distant targets. However, if there is a strong reflecting target near the ship, it sometimes appear as a circular-arc false echo on the screen. ! CAUTION When located near large targets such as land, the ship's mast, etc.
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D. Distant false echoes caused by duct phenomenon Depending on meteorological conditions, duct phenomenon sometimes occurs in temperature inverting layers of air. In such a case, the wave propagates erratically reaching a location surprisingly far away from the ship. In this case, a target present at a distant location more than the radar's maximum distance range appears on the screen presenting a false echo that can be misunderstood to be present nearer than the actual position.
PAGE 6
Display modes This refers to a radar's display modes. There are four display modes depending on the direction in which the top of the screen faces with respect to the ship. Ship's Heading HM Ship's locus (not displayed on screen) Scheduled course Scheduled course North North 0.75 0.75 0.75 0.75 0.25 HU 0.25 NU 0.25 CU 0.25 TM North mark North EBL HU NU CU TM Fig.2-10 Display modes Head Up (HU) In this mode, the ship's heading always indicates the upward direction of the screen.
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VRM (Variable Range Marker) This is a circular-shaped marker whose size can be changed as desired. You can use this 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 a point close to the center of the echo image on the screen. 0.75 0.25 HU Echo VRM VRM Fig.
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STC (Sensitivity Time Control) Since echo signals received by the radar are strong when they are coming from a short distance, it is difficult to compare signal strength between each reflected signal. To overcome this difficulty, signal strength is adjusted in such a way that the received signal levels coming from a short distance are lowered and those from a long distance are raised. This function should prove useful when there are large reflected waves from sea surfaces during rough weather.