**7.4. External background interference/noise**

Man-made radio interference may lead to radar signatures similar to those produced by a tsunami, leading to false alarms. In some cases, this interference can be mitigated; in others, it may not be possible. This problem can often be solved on a case-by-case basis, having collected a database of received signals. This is the biggest factor apart from the intensity of a tsunami that limits its detection and can give rise to false alarms if the threshold is too low. Even sporadic interference can cause the *q*-factor pattern-recognition algorithm to give rise to false alarms. Initial radio surveys for new radar sites are required to check for interference. Selection of a clean frequency band can improve the situation for all sites.

Methods are needed to detect interference, along with filtering where possible to remove it. If it cannot be removed, then passing along to a tsunami warning center a disclaimer with any *q*-factor alerts during high interference periods, or raising the threshold, is critical.
