GAPS - User Guide
9 Operating GAPS
Every operation starts by the initialization and the survey configuration of the system (see
section 5.1). During the survey, you can control the incoming data (see section 9.1).
Do NOT sail faster than 12 knots with the GAPS head in the water. It is the mechanical limit
of the GAPS unit.
9.1 Deploying the Transponders
The transponder to be positioned is either mobile on an underwater vehicle or attached to a
mooring. The deployment of an underwater vehicle is specific to that particular vehicle. Please
refer to the specific User Guide of the transponder for more operational details.
9.2 Launching the Survey
9.2.1 INITIALIZING THE INS
● During the first 5 minutes after powering-on, the system performs a coarse alignment:
> Inertial sensor data (accelerometers and gyrometers) are computed to estimate
heading, roll and pitch angles.
> No estimation of position or speed, nor errors, are done by the INS during the rough
alignment: the data provided by external sensors (lever arm compensated) are used
directly.
● At sea, it is recommended that the system is kept as steady as possible during coarse
alignment: oscillations around a mean position or smooth drift are permitted but
accelerations should be avoided. Leaving the vessel adrift for 5 minutes would lead to
satisfactory conditions for coarse alignment.
● After the coarse alignment phase, the GAPS is ready for navigation. Kalman filter is
activated to compute and estimate position and speed with an optimal accuracy. The INS
switches to the “fine alignment” phase to improve the accuracy of roll, pitch and heading
estimations.
> During the fine alignment phase, movements of the vessel are required. 90 degrees
rotations are even recommended so that the Kalman filter assesses the sensors bias
on different axes.
> Error estimations from the INS Kalman filter are improved when optimal trajectories
are performed. The optimal trajectory to achieve fast fine alignment is “staircase”
shaped, as illustrated in figure below, with a typical duration of 3 to 5 minutes for each
step. Such a trajectory allows the Kalman filter to assess all sources of errors of the
system, to correct them and to achieve optimal performances at the end of the fine
alignment process.
> When a “staircase” shaped trajectory is performed, the fine alignment phase would
typically last for less than 20 minutes.
> Fine alignment requires the GPS to provide valid data to the system. the INS uses
both inertial sensors and external sensors to compute optimal estimates of position,
speed, attitude and heading.
MU-GAPS-AN-005-H - November 2019 141
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