From AMS Glossary
The electromagnetic radiation returned to a radar by a target. The target signal is characterized by its amplitude, phase, frequency, and polarization.
These are the properties that, with suitable equipment, are available for measurement and may be used to infer such target properties as reflectivity or Doppler velocity. A Cartesian component of the electric field vector of the target signal at a given distance from the radar varies with time t and may be written
where A is the signal amplitude, ϕ the phase in radians, and ω0 = 2πf0 with f0 the carrier frequency in hertz. The amplitude varies with time because of changes in the radar cross section of the target or, for distributed targets, because of movements of the separate scattering elements relative to each other. The phase will vary with time because of motion of the target toward or away from the radar. The Doppler frequency in hertz is 1/2π(dϕ/dt). The intensity I of the target signal is defined as the average value of the square of E, evaluated over one cycle of the carrier. Thus, I(t = A2)t/2.