Some of the sensors carried by an AUV can observe the same part of the seafloor in consecutive sensor measurements, such as a Synthetic Aperture Sonar (SAS), a 3D sonar, or a camera. By correlating these consecutive measurements of the same seafloor patch over a small time interval, a measurement of the change of the AUVs position between these measurement times may be derived. A Correlation Velocity Log (CVL) is another sensor that provides a similar measurement, although using a different measurement principle. These sensors provide a different measurement than velocity to integrate with the INS. This aiding technique is herein called micro delta-position aiding.
When the time interval is very short, the measurement more and more resembles a pure velocity measurement. This technique therefore provides redundancy to the DVL, but may also decrease the position error drift of the DVL-aided INS substantially if the measurement is sufficiently accurate. In this aspect, SAS is a particularly attractive sensor. SAS uses consecutive pings to synthesise a larger array. For this to work properly, the array position displacement between pings must be found with extremely high accuracy. This is called micronavigation or DPCA (displaced phase centre antenna) (Belletini & Pinto, 2002; Hansen et. al., 2003). By using this position displacement as a velocity measurement, it can potentially be an order of magnitude better than even the most accurate DVL. The technique is fully autonomous, but requires a suitable sensor onboard the AUV.
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