Wireless localization has been an active research area over the last decade due to its endless applications. In this paper, we investigate received signal strength (RSS) based estimation of a mobile station's location, through monitoring the RSS at multiple reference software defined radio (SDR) receivers. Uplink transmission power of the mobile stations and path loss exponent are assumed to be unknown parameters. Universal software radio peripheral (USRP) devices are used to capture the RSS from uplink transmission of a mobile user. Subsequently, a linear least square localization technique is used to obtain the location estimate of the mobile station. Experiments with real mobile phones are conducted to demonstrate achievable localization accuracies in indoor environments with GSM technology.