What is the smallest signal that can be detected and processed as a target by the receiver?

The smallest signal that can be detected and processed as a target by the receiver is best described as a usable echo. This term refers to the minimum detectable signal that the radar system can identify and analyze, distinguishing it from background noise. The usable echo must be strong enough to stand out against the inherent noise present in the environment and the radar system itself.

In radar operations, echoes are reflections of the transmitted signal from targets, and a usable echo indicates that the signal has sufficient strength and clarity to be reliably processed. This makes it an essential concept in understanding radar performance and target detection capabilities.

The other terms relate to different aspects of signal processing and radar functionality. The noise threshold refers to the level of noise in the system where useful signals are distinguishable but doesn't directly define the smallest detectable signal. Signal eigenvalue is a term more associated with signal processing techniques and doesn't specifically denote a usable radar target. Target return typically refers to the reflected signal from a target after transmission but does not specify the minimum signal level that is actionable for the radar system. Thus, the term usable echo is the most accurate choice for defining the smallest signal that can effectively be detected and processed as a target.