In Section 2.1
the viability of commercially available technologies that can provide or aid localisation are discussed. In Section 2.2
the findings are summarised and the localisation technologies used in the present work are selected.
2.1. Analysis of Relevant Localisation Technologies
Global navigation satellite systems (GNSS) are common place on marine vehicles but must be ruled out in the present case due to the fact that most facilities will be indoors. Even if GNSS was available, the nominal accuracy of a good quality GNSS is around 1 m, degrading at approximately 0.22 m for each 100 km from the broadcast site [12
Computer vision (including the use of monocular, stereo and RGBD cameras) can be used to provide odometry [13
] as well as simultaneous location and mapping (SLAM) [14
] both above and below the waterline. However, visual systems fail to function reliably in environments with too few or too many salient features, where lighting is poor or changeable and during rapid movements of the camera [15
]. As stated in Section 1.1
, for the present problem scenes are often feature-sparse, water may be turbid and good lighting cannot be guaranteed; therefore a vision system cannot be employed without a reliable fall back system in place. For example, the AR drone [16
] chooses between two visual SLAM algorithms dependent on conditions and has a fall back if neither algorithm can produce reliable localisation.
A LiDAR paired with an appropriate SLAM algorithm can provide highly accurate localisation above the water surface and has fast refresh rates [17
]. LiDARs use light emitted by the sensor and the results do not deviate greatly in variable light conditions. The largely 2D environment in which the ASV operates and the presence of high surrounding walls makes use of 2D LiDAR a favourable choice. LiDAR technology is mature and a range of high accuracy, waterproof units in suitably sized packages are readily available. To compliment the 2D LiDAR, a variety of well tested 2D SLAM algorithms are also available to give pose information from the laser scan. Underwater LiDARs have recently become commercially available but are large, expensive and have safety constraints [18
Inertial measurement units (IMUs) have very high refresh rates and can be used to improve the data obtained from an absolute measurement, such as LiDAR or GPS. By fusing the IMU with an absolute localisation method, the measurement accuracy of acceleration, velocity and displacement may be improved [17
]. The IMU may also be used to fill gaps between the measurements of an absolute sensor, which is useful for fast acting control systems [22
] and to rebuild a 2D or 3D scan [23
]. 9-axis IMUs are composed of a 3-axis accelerometer, 3-axis gyro and 3-axis compass. As discussed in Section 1.1
, magnetic disturbance is to be expected from the environment and the onboard electric motors, meaning that compass readings are likely to be noisy and unreliable; although, the compass is not necessary if another system, such as LiDAR localisation, is able to provide the ASV’s yaw angle. Accelerometers and gyroscopes are unaffected by the operational environment and with recent improvements in MEMS devices, reasonably priced, high accuracy, low power devices are now available.
Underwater acoustic systems (active sonars) paired with an appropriate SLAM algorithm can be used for absolute localisation [23
]. Most systems are designed for and used in marine environments and their performance in confined highly structured environments is unquantified. Acoustic systems can suffer from multi-path issues or multiple echoes [18
] and this will be amplified in confined environments. In comparison to a LiDAR, beam widths are wide and refresh rates are low, generally between 1–10 Hz [18
]. A key benefit of acoustic systems is that they are unaffected by water turbidity [18
]. Sonar localisation using beacons at known locations must be ruled out in the present case due to the requirements for minimal infrastructure and fast deployment. So called, sonar SLAM, using a mechanically scanned imaging sonar (MSIS) [4
] would be possible, and given the typically slow movement required in the current context sonar SLAM may be a viable option, although difficult to make the case for if above surface LiDAR SLAM is available.