Multi-path interference

The radar beam can reflect back directly from the target or from a sea surface reflection of the target. When this multi-path interference occurs, the two signals will either reinforce one other or cancel each other out, creating a null.

Multi-path interferenceMulti-path interference usually occurs on simple targets (comprising a single reflector) such as buoys, in calm sea states where the water is acting like a mirror. Multi-path interference may produce a large number of signal nulls at short range that become less frequent as range increases.

In higher sea states, when the sea is rough and the water is less likely to reflect, or when the target is complex (comprising a number of reflectors), the effect of multi-path interference is less pronounced, so the nulls are less deep. As the height of the radar antenna (or target) increases, the frequency of the nulls also increases. The frequency of the nulls also increments with increasing radar frequency.

1. Actual target.

2. Reflection.