Mutual Radar Interference is caused by other radars in the locality operating on a similar frequency to your ship’s radar. The interference shows up as bright spots scattered over the screen, or as a distinctive pattern of dotted lines curving outwards from the centre of the screen. It is more common on longer range scales as on shorter range scales only a few of the interfering pulses will be displayed.
If only one other radar is involved this is not too serious, but in busy traffic areas the clutter can be dense enough to cause confusion. An interference rejection circuit can minimise this problem. It works by rejecting any echo which does not return from any two successive pulses. While large targets will not be effected by IR, some small echoes may be lost. There is no IMO symbol for IR.