This paper presents an ultrasonic active sonar system in air constituting one emitter and four receivers. Target localization is achieved by means of intersection of four ellipsoids defined by the time of flight between emission and reception of the signal reflected by the target. This paper shows a proof of concept of the localization principle through some localization tasks conducted in a laboratory environment. The position of a spherical target is determined with an error between 1cm and 7cm depending on receiver configuration and target position. The purpose of the fully developed sonar system is to assist drones and robots in their autonomous navigation