The document discusses low frequency modes in power systems, which refer to oscillations between 0.1-2 Hz caused by fluctuations in load. It is important to identify these modes to increase transmission capacity and ensure stability. Identification can be done offline using ambient data or online using phasor measurement data. Common online methods include FFT, Kalman filtering, and Prony analysis, but they have limitations. Noise space decomposition and modified Prony analysis are proposed for more accurate identification using phasor data from PMUs.