Instruction Manual

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2 RA51/52/53/54/55 INSTRUCTION MANUAL – 05

CHAPTER 2. USING RADAR FOR

THE FIRST TIME

This chapter covers basic information and technical terms about radar for those who are using one

for the first time.

2.1 What is radar?

RAdio Detecting And Ranging

The word "radar" is an acronym for "RAdio Detecting And Ranging." In very simple terms, this is

how it works. A radio transmitter sends a quick microwave pulse, and then a receiver listens for

that signal's echo when it is bounced back from something in its path. The returning signal is

processed by a computer to determine its relative distance, position and bearing. This information

is graphically displayed on a screen for you to see. Other boats or ships, navigational markers,

landmasses and such are referred to as targets.

By knowing how long it takes for a signal to return, the distance to a target can be determined. As

the radar antenna scans through a 360-degree rotation, it can show where the target is relative to

your position. By repeated scans, you can see which direction another vessel is moving.

Fig.2-1 What is radar?

Antenna

How radar will perform is largely determined by its antenna or scanner. Increasing the size of the

antenna improves long-range performance and target discrimination, or the ability to distinguish

two separate targets at a distance. The critical factors are the antenna's beam width and side lobe

level. Typically, a radar antenna will radiate a tightly focused beam from the front of the array.

The longer the antenna array is, the narrower the beam width will be. Additionally, it will also

emit smaller amounts of energy to each side. The lower the side lobe level, the less the effect of a

false echo. The RA51 radars are equipped with a closed dome scanner, the RA52/53/54/55 has a

larger, open array.

Side lobe

The beam in which the strongest radio signal is radiated from

the antenna is called the “main lobe”. Those beams that are

radiated in other directions are referred to as the “side lobes”.

The side lobe level refers to the difference in level (signal

strength) between the largest side lobe and the main lobe.

Fig.2-2 Antenna pattern

Buoy

Other ship

Radar wave

Your ship

Antenna

Radar display

Beam width

Side lobe

level

Main beam

Side lobe

Antenna

Summary of content (7 pages)

PAGE 1
CHAPTER 2. USING RADAR FOR THE FIRST TIME This chapter covers basic information and technical terms about radar for those who are using one for the first time. 2.1 What is radar? RAdio Detecting And Ranging The word "radar" is an acronym for "RAdio Detecting And Ranging." In very simple terms, this is how it works. A radio transmitter sends a quick microwave pulse, and then a receiver listens for that signal's echo when it is bounced back from something in its path.
PAGE 2
.2 Characteristics of Radar Wave Radio waves travel out from the antenna while bending slightly along the earth's surface. The amount they bend depends on atmospheric conditions. The sight distance of a radar generally is about 6% longer than the optical sight distance and is calculated using this equation: Radar sight distance (NM) = 2.22 ( antenna height (m) + target height (m)) Line of sight Radar Radio Wave h1 h2 Earth Fig.
PAGE 3
antenna on the second bounce. The actual echo appears at the correct distance and bearing on the screen. The ghost echo appears somewhere behind your boat. This type of false echo is also generated by re-reflection of waves from bridges, quay walls or building along shore. Target 3 Direct reflection path Real echo 1 HU Secondary reflection path Ghost echo Mast etc. Direction of ghost echo Fig.2-5 False echoes of radar (Ghost echoes) B.
PAGE 4
Main beam 3 False sidelobe echoes 1 HU Real echo Side lobes Antenna Fig.2-7 False echoes of radar (Caused by side lobe) D. Distant false echoes caused by duct phenomenon The duct phenomenon sometimes occurs when meteorological conditions create a temperature inversion between layers of air. When this happens, radar waves propagate erratically and can reach a location considerably farther away from your boat than the radar's maximum distance range.
PAGE 5
2.3 Terms Specific to Radar HM (Heading Marker) This is a line-shaped marker used to indicate the forward direction of your boat. HM(Heading Marker) North Mark The marker to indicate the direction North is a short line approximately 1/6 of the screen size. It only appears when the radar is connected to a suitable heading source provided through NMEA, 10/12 bit serial or sin/cos. 0.75 0.25 HU North Mark Fig.
PAGE 6
True Motion (TM) In this mode, the boat icon is displayed as if it is moving on a marine chart while targets such as islands and seashores are fixed in position. When the icon reaches a certain position on the screen (approx. 2/3 of screen size), it is placed back on the opposite side on the screen. North is at the top of the screen. External heading and speed data are needed for this mode.
PAGE 7
STC (Sensitivity Time Control) Since echo signals received by the radar are stronger when they are coming from a short distance, it's difficult to compare signal strength between each reflected signal. To overcome this, signal strength is adjusted in such a way that the received signal levels coming from a short distance are lowered. This is helpful when there are large reflected waves from sea surfaces during rough weather. STC OFF STC ON 0.75 0.75 0.25 HU 0.