Noise and the strong signal limit in radio astronomical measurement
  The random error of radioastronomical measurements is usually computed in the weak-signal limit, which assumes that the system temperature is sensibly the same on and off source, or with and without a spectral line. This assumption is often very poor. We give examples of common situations in which it is important to distinguish the system noise in signal-bearing and signal-free regions. 
H. Liszt (NRAO)