The PDF document makes reference to its navigation accuracy relative to the last GPS or USBL update. USBL is an underwater positioning system in which the topside (boat) notifies the vehicle of its (the vehicle's) position. First, the topside's acoustic transducer pings the vehicle and determines the direction and distance of the vehicle's response ping. Using some trig and its own GPS position, the topside calculates the vehicle's position, and sends it in an acoustic message down to the vehicle. Coupled with a high-accuracy internal navigation system on the vehicle, this allows the vehicle to stay at depth and hold its survey course for long periods.