Context awareness will become increasingly important in future domestic consumer electronics. In many domestic context aware applications, there is a need for location and orientation information about persons, devices or objects in the home. This information can be provided by a dedicated indoor location system. A consumer location system should be robust, safe, easy to set up, low cost, and should have a minimal infrastructure. In this paper, a step towards consumer location systems is taken by proposing an ultrasonic positioning method that needs just a single compact base station to measure 3D positions of mobile devices in a room. The method employs ultrasound time-of-flight tri-lateration to estimate device positions, using a base station containing an array of three ultrasound transducers. Five potential problems of the proposed method are identified; the main problem being line-of-sight path occlusion. The method has been prototyped, and initial results show an accuracy of 1.41 m or better for 95% of the position estimates, in case of a good line-of-sight path. It is concluded that the method is promising for providing 3D position information at around 1 m accuracy for context aware applications, but that the problem of line-of-sight occlusion requires further investigation.