The Blueprint Subsea Seatrac USBL, or Ultra Short Baseline System, is another acoustic position tracking device. The operating principle uses a topside transducer beacon at a known location and a vehicle transducer beacon that responds to “pings” from the topside. The system interprets the phase shift of the returning “ping” within the sensor head and calculates the position of the ROV. This system can be used in conjunction with the DVL to fill the gap between the surface and the 75 meters (or 250 feet) limit of the DVL when working in deeper water.