Furuno 841, 821 manual Interpreting the Display, Radar Wave and Radar Horizon, Super-refraction

3.INTERPRETING THE DISPLAY

As an aid to navigation, radar can be a very valuable tool. No other electronic naviga- tion aid can give you the ability to spot ves- sels coming at you in the fog, or tell you the location of the inlet to the harbor in the pitch black of night.

To help you understand what your radar can (and cannot) do for you this chapter covers

•the characteristics of the radar wave

•target properties and radar wave reflec- tion

•range and bearing resolution, and

•false echoes.

3.1The Radar Wave and Radar Horizon

How the radar wave travels

The radar wave tends to travel in straight lines at the speed of light. However, it is subject to bending or refraction in the atmo- sphere, the amount depending on region and density.

Super-refraction

Super-refraction is a condition in which there is an upper layer of warm dry air over a surface layer of cold, moist air. Radar waves bend downward and thus increase the ranges at which targets may be detected.

Sub-refraction

Sub-refraction is the reverse condition of super-refraction; a layer of cold air is above a layer of warm air. Radar waves bend up- ward and thus decrease the ranges at which targets may be detected.

Radar horizon

Radar is essentially a “line-of-sight” phe- nomenon. That means you have just about the same range to horizon with a radar as you do with your own eyes. However under normal atmospheric conditions, the radar horizon is 6% greater than the optical hori- zon. Therefore, if the target does not rise above the horizon the radar beam cannot be reflected from the target.

The distance to the horizon from the scan- ner, under normal conditions, is calculated by the following formula.

Rmax = 2.2 x h1 + h2

Where Rmax: Radar horizon (mile), h1: An-

tenna height (meters), h2: Target height (meters)

D	Horizon
h2	Wave
h1	path

Figure 3-1 Radar horizon

3.2Target Properties and Radar Wave Reflection

Generally, larger targets can be seen on the radar display at greater ranges, provided line-of-sight exists between the scanner and target. However, a large target with poor reflecting properties may not be detected as easily as a smaller target with better reflect- ing properties. For example, you might ex- pect a lighthouse to be a good radar target because of its size. In actuality the return echo is weak since the conical shape diffuses most of the radiated energy.

A ship whose hull is made of conducting materials, such as steel, will return a rela- tively strong echo.

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