In this paper, we consider that the disturbance of a STAP system is composed of a low-rank Gaussian clutter and a white Gaussian noise. From this model, the theoretical SNR Loss of a Robust Low-Rank STAP filter based on the Normalized Sample Covariance Matrix is derived by means of a perturbation analysis. Theoretical results are shown to be in good agreement with simulations for a realistic STAP configuration. Our analysis extends previous result on the SNR loss of Low-Rank STAP filtering based on the Sample Covariance Matrix.