Dave Larson of the US Coast Guard reviewed USCG search and rescue (SAR) metrics, focusing specifically on effective sweep width. Effective sweep width is an abstract planning metric that helps allocate SAR resources. Consider the situation of an airplane flying over an area in which there is a uniform, random distribution of SAR targets. A large SAR target very close to the airplane would be easy to detect and as the targets grow smaller or more distant, they are harder to detect. The effective sweep width describes this situation and helps planners determine, for example, how long it will take an asset to search an area to a specific probability of detection, the expected amount of time until a particular target is detected or how many assets to allocate to a search area. Effective sweep width can also be used to improve the probability of detection estimation based on observable factors as weather conditions or other searcher or target parameters.

A key determinant of the effective sweep width is the lateral range curve, which is a description of the probability of target detection as a function of range from an airborne searcher. A lateral range curve typically looks like a bell curve, with the highest probability closest to the searcher and long “tails” of low probability far from the searcher. Many conditions occur during a search that can change the value of the curve in flight. For example, when searching at night in starlight, night vision goggles amplify to the level of a cloudy, foggy day and if the moon comes up, the goggles amplify as if searching in bright sunlight. Also, the ability to detect is increased if the searcher looks out to the side, rather than straight ahead, giving a full range of vision. This means that the lateral range curve dynamically changes during a search, having implications for ongoing search planning. Applying sensors to human capabilities can fundamentally change the shape or characteristics of the curve. Forward Looking Infrared Radar (FLIR), for example, creates a very brief detection opportunity by limiting sweep width, but offers very high detection within that sweep – like looking through a soda straw.

The effective sweep width, however, is an enduring planning metric that is the key to improved search planning and evaluation. While it cannot improve the acuity of a searcher, it serves to improve the conceptualization of the search problem. Since better conceptualization means better planning, higher detection rates can therefore occur.