Phasor measurement units (PMUs) have emerged as the sensors for smart grids because of their inherent capability to provide with synchronized current and voltage phasors at buses sprawling over large geographical area. Thus, PMUs or phasor measurement sensors (PMSs) constitute the edifice for wide area measurement system to provide with situational awareness (SA) for a complex interconnected electric grid by providing with real-time data. They assist in predicting future disturbances and also for decision-making of control actions. The PMS outage has adverse impact on grid operation. Therefore, their periodic maintenance must be carried out to ensure healthy and reliable operation to enhance SA. In this paper, optimized routine and self-supervision test intervals for ensuring maximum observability of the grid have been proposed. Since PMSs are taken out of service during maintenance, an intelligent maintenance scheduling algorithm has been proposed such that SA is maximum. Case studies pertaining to an example nine-bus, IEEE 14, 24, 30 and new England 39-bus system validate the efficacy of proposed approach. To prove the efficacy of results, general confidence intervals are reported for observability.