Measuring indoor trajectory can be useful to monitor different cases. Currently, different methods exist to measure indoor trajectory. They can be classified according to the type of physical waves, the signal processing used and the use of not of a mobile device used as a beacon. The objective of this work is to explore the possibility of carrying out a low-cost trajectory measurement through ultrasound in air, derived from the biomedical ultrasonic echography method. A mobile sensor, mounted on a motor, of central frequency f(0) equal to 40 kHz, was used. The narrow frequency bandwidth is less than 2 kHz. Signals were sampled with a conventional audio soundcard at f(s) equal to 44.1 kHz using the alias (f(s) - f(0)) spectrum. This configuration was tested with a person walking slowly, at a speed of one step every two seconds, in a room measuring 7 meters deep by 4 meters wide. The main result was that it was possible to follow the person's displacement until 4 m during 90 s using low-cost ultrasonic air echography and without the use of a mobile device.