Navigation
REMUS operates within an array of acoustic transponders that are deployed in the area of study for the duration of a mission. The vehicle calculates its position by computing its range to each of the acoustic transponders. REMUS uses three principal modes of navigation: Ultra Short BaseLine (USBL), Long BaseLine (LBL), and Dead Reckoning (DR). If the type of navigation is not specified in a REMUS mission, the vehicle will determine the most appropriate mode of navigation based on the type of mission objective, the environmental conditions, and the vehicle's proximity to the transponders.
The USBL transducer, located in the front end of the nose cone, employs a four-channel hydrophone to interrogate the transponders. The hydrophone then measures the range and bearing to the transponders from the received signal. Acoustic range and bearing are combined with vehicle pitch, roll and heading information to get a more accurate absolute fix on the vehicle's position. USBL navigation (+/- 1 m) is more accurate at short range than LBL navigation (+/- 10 m), and is used for homing in on a transponder or docking station.
The LBL transducer, located in the aft region of the nose cone on the underside of the vehicle navigates using the principles of triangulation. Therefore, two or more transponders must be deployed in order for REMUS to operate in LBL mode. The latitude and longitude of each of the transponders is pre-programmed into the REMUS mission file. The vehicle interrogates all the transponders, and those within range respond with unique acoustic signals. The position of REMUS can then be calculated based on the time it took for the return signals to reach the vehicle. LBL navigation is employed for most large area missions.
When acoustic fixes are not available, REMUS navigates in DR mode. When operating in this mode, the vehicle relies on its compass heading and last known location to navigate to its next programmed location. To estimate its location and navigate accordingly, REMUS incorporates data from the ADCP on current speed and direction, vehicle velocity based on propeller RPM and bottom-tracking Doppler signals, and heading information from the vehicle's yaw sensors. REMUS continually calculates the distance it has moved and adds that to its last known position. This is the least accurate mode of navigation and is only employed when the vehicle cannot get a good acoustic fix from the transponders. DR navigational accuracy will depend to a certain degree on the oceanic conditions but is estimated to be +/- 40 m per 1000 m traveled by the vehicle (2.3 degrees error in heading per km).
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