The ROV is prepped on the boat.
Aided Inertial Navigation
The CDL MiniPos is an INS that is built around a Kearfott Ring Laser Gyroscope (RLG). A monolithic RLG provides highly accurate heading, pitch, and roll information in real time at a rate of nearly 20 Hz. This information, when paired with the data from linear accelerometers in three dimensions (x, y, and z) can produce an accurate dead-reckoning position for the INS in three dimensions. Unfortunately, a stand-alone INS has a tendency to drift because it does not sense constant velocity drift. Without aiding from an external source, an INS can only generate its own values for the constant velocity. The INS struggles to accommodate 3-D long-term drift, so CDL aides the MiniPos by providing it with an RD Instruments Doppler Velocity Log (DVL) and Druck pressure sensor. The DVL provides real-time constant velocity measurements that aid the INS to account for horizontal drift. The Druck pressure sensor, accurate to nearly 0.04% of actual depth, provides a vertical correction to aid the INS. These inputs, combined with the dead-reckoning solution from the RLG and linear accelerometers, are fed through onboard hardware and firmware that run a series of Kalman filtering algorithms that generate the aided inerwww.seadiscovery.com
tial navigation solution. The pairing (the INS and the DVL) mounted to an ROV instrument skid are run through a "training" or calibration routine prior to the actual survey so that the Kalman filters can be taught how to combine the information from the INS and the DVL to generate a highly accurate positioning solution. After calibration, the result is an instrument skid that is ready to be mounted to the underside of the ROV (in our case, the SeaEye Falcon), powered from the ROV, and capable of outputting a single data stream that contains the x,y,z positioning, the heading, pitch, and roll and the velocity of the ROV in real-time at an update rate of between 10 Hz and 20 Hz. The SeaEye Falcon DR, fitted with an instrument skid that has the CDL MiniPos INS and RD Instruments Workhorse DVL mounted. The calibration routine is actually performed with the skid separate from the ROV, towed from a survey vessel, so that a variety of velocity changes, angular accelerations, and turns can be performed quickly in order to train the Kalman filtering algorithms in the INS. So long as we know the real-world location (in coordinates) of our starting point, we can use the aided INS to generate a displacement from the start-point that takes
Marine Technology Reporter 27
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