A locator system for use in connection with at least one mobile person includes a mobile locator system adapted to be worn by the person, wherein the mobile locator system includes an inertial navigation system. The locator system further includes a reference locator system to provide information about a position of the mobile locator device relative to a coordinate system. In several embodiments, the reference locator system comprises an ultrasonic system. For example, the reference locator system can include at least one ultrasonic transmitter and at least one ultrasonic receiver. One of the at least one ultrasonic receiver and the at least one ultrasonic transmitter can be positioned at a known position on the coordinate system and the other of the at least one ultrasonic receiver and the at least one ultrasonic transmitter can be worn by the person. The reference locator system can also include a plurality of ultrasonic transmitters and a plurality of ultrasonic receivers. Each of the plurality of ultrasonic receivers or each of the plurality of ultrasonic transmitters can be positioned at known positions on the coordinate system. Each of the other of the plurality of ultrasonic receivers and the plurality of ultrasonic transmitters can be worn by one of a plurality of persons.