In this paper, we investigate the issue of radar emission scheduling for target track maintenance and quantify the benefits of multiple sensor tracking regarding emission constraints. In fact, the target track results from the fusion of passive detection obtained from ESM or IRST sensors and active measurements delivered by the radar when it is pointed towards the target. The tracking algorithm is based on Singer model developed in polar coordinates. We propose to control state covariance estimate provided by the Kalman Filter and compare it to a predefined requirement which is supposed to take into account both operational (tactical situation accuracy) and technical (association ambiguity and radar pointing efficiency) needs. When this threshold is reached, the radar is activated. Theoretical analysis is conducted and sensitivity to several parameters considered, including target dynamics, specified covariance threshold, and radar detection probability. Advantages of multiple (active and passive) sensors tracking is pointed out, especially when emission constraint is required. Finally, simulation results are provided in an air-to-air scenario in which ESM, IRST and Radar sensors are available onboard the own aircraft.