User Manual

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. The sight distance D of a radar

generally is said to be approximately 6% longer than the optical sight distance and is calculated using the

equation below :

D (NM) = 2.22 ( h1 + h2 ) where, h1= antenna height in meters

h2= target height in meters

Fig.2-3 Radar wave

Targets difficult to display on screen

The 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 with FRP, wood, or other

low-reflectance materials or those that have a small incident angle are 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 when monitoring on the screen. Especially, coast lines on

the radar image appear to be present further from the ship than they are actually located. There-

fore, it is important not to misinterpret the available data.

Fig.2-4 Targets difficult to display on screen

Shadow zones of radar

Radar 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 or mountain at the side of the

ship, such an object generates a shadow behind it. In this case, some objects produce a complete

shadow and some produce a partial shadow. In an extreme case, the shadow of an object may ex-

tend 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 an-

tenna installation to minimize the shadow. Targets in shadow zones are difficult to display on the

screen.

False echoes

A false echo of an actually nonexistent object may sometimes appear on the screen when

sailing. The following explains the cause of each such phenomena.

Line of sight

Radar Radio

Wave

Earth

Apparent coastline

Actual(invisible)

coastline

Invisible

Visible