Abstract: Apparatus and a method using ultrasonic signals to determine the distance between the displacement of two points in space. An ultrasonic distance measurement unit (1) generates frequency modulated ultrasonic waves using binary shift keying. The signals are transmitted by the transmitting transducer (2) towards a reflector assembly (30), preferably comprised of two reflectors (31, 32). One of the reflectors (31) is smaller than the other reflector (32), and the smaller reflector (31) is positioned in front of the larger reflector (32). The reflected signals are received by the receiving transducer (3). The unit (1) phase digitizes and processes the received signal by recording time stamps of zero-crossings of the received signal. The speed of sound may be determined by a temperature measurement system (5) and/or by measuring the transit time difference between the two reflectors (31, 32) of the assembly (30).