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Wireless mobile phones are widely used for emergency phone calls. However, most cellular networks provide only the most basic 911 services. In response to this, cellular systems should be able to locate a cellular phone in connection of an emergency, with certain schedules and accuracy requirements. This paper addresses the problem of determining the position of GSM cellular mobiles in a cost-efficient way, i.e. without any change in the infrastructure or the handset. It uses the signal strength technique as a major tool to solve the mobile location problem. By using the Kalman filter algorithm, this technique is able to satisfy the E-911 requirement that asks for at least an accuracy of 125m 67% of the time. Furthermore, an enhancement method is proposed to increase the efficiency of the solution. Results obtained by simulation will be introduced to show the accuracy and shortcomings of the technique for a stationary and a non-stationary mobile station.