Raymarine Combined Pathfinder Radar, RL80CRC PLUS manual Identifying False Echo Returns, Side Lobes

Chapter 3: Standard Radar Operations				3-5
	.t
	Table 3-3:	Controls to Adjust the Radar Picture
	Control	Option	Function	Default Setting
	GAIN	GAIN	Controls the strength of the target returns	AUTO
		SEA	Reduces sea returns	AUTO-HARBOUR
		RAIN	Reduces close rain or snow returns	OFF
		FTC	Reduces distant rain or snow returns	OFF
	MULTI	TUNE	Fine tunes the receive	AUTO
	TARGETS	INT REJ	Turns interference rejection on/off	ON
	Soft Key	EXPANSION	Turns target expansion on/off	OFF
		WAKES	Turns wakes on/off and selects wakes length	OFF

Identifying False Echo Returns

Not all echoes are direct returns to the radar antenna. Occasionally signals appear at positions where there is no actual target. These are called false echoes, and may be caused by side lobes, ghost images, indirect echoes or multiple echoes.

This section briefly describes the echo patterns that can be produced by false echoes and the likely cause. It should be noted that the radar operator, through observation, practice, and experience, can generally detect these conditions very quickly. These effects can usually be minimized using the radar controls.

Side Lobes

A small part of the RF energy from each transmitted pulse is radiated outside the single narrow beam. This produces side lobe patterns, shown in Figure 3-3.

Side lobes have no effect on distant or small surface objects. However, the echo from a large object at short range may produce an arc on the radar screen similar to a range ring or a series of echoes forming a broken arc. Side-lobe echoes normally occur at ranges below 3nm, and can be reduced by adjusting the SEA control.

Main lobe

Side lobe Side lobe

Antenna

Identifying False Echo Returns

Arc True echo Side echoes

D1638-2

Figure 3-3: Side Lobes