In this paper we propose a low-cost radar based indoor security system with super-resolution capability. A high-resolution imaging algorithm, the TR (Time Reversal) method, is employed to realize a simplified radar system with only a single antenna. While the TR method provides some advantages, it requires prior accurate information about the multipath environment. Since this is usually difficult to achieve in practice, we propose a new method to estimate the multipath environment without any reference targets for calibration. The proposed method makes the TR method practical and effective in estimating target location without calibration for any particular indoor environment.