Ula-klassens enkeltstråle ekkolodd - strålebredde og posisjonsbestemmelse

The Ula class submarines are presently being updated in the Norwegian Defense Logistics Organization’s project P64345. The plotting table and the inboard part of the single beam echo sounder are among the systems that are being replaced. FFI supports P6345 through its project 1305. Among other things, this project has investigated the effect of lever arms between the different sensors used for navigation. This work showed that for the echo sounder the beam width could be a larger problem than placement, especially since the echo sounder is used to determine position. The present single beam echo sounder has a very broad beam, up to 10 degrees alongship and 20 degrees across. The echo sounder uses the start of the echo to compute depth, in other words it measures the shortest distance to the bottom. The echo sounder is used both to ensure a safe distance to the bottom and for finding the submarine’s position by comparing measured depths with chart depths. The fact that the bottom depth measurement is from the closest point in the echo sounder footprint, and not from straight underneath the ship, may cause problems when using the measurements for positioning. The problems are that the position may be wrong with as much as 175 meters, and it may not be possible get a consistent estimate even if the chart contours have a recognizable pattern. Logged data from the echo sounder show examples of both these problems. In the new Ula class navigation system, the navigator can give these position estimates a standard deviation. This standard deviation should be set large enough to cover the footprint of the echo sounder. As the beam width (in meters) gets smaller with shorter distances, more weight should be placed on shallow measurements than deep ones. Due to the beam width, the single beam echo sounder is not well suited for use in an automatic terrain correlation system of the type that has been introduced on the HUGIN AUVs. The Ula class is also equipped with a bottom mapping sonar and a mine avoidance sonar, both use narrow beams. If the bottom depths and the tactical situation allow it, one of these could be used instead of the single beam echo sounder. The inboard part of the echo sounder will be replaced. If the signal into this part makes it possible, one could consider a function to get the average depth instead of, or in addition to, the smallest depth. If this is not possible, one might consider replacing the transducer to get narrower beams. Both alternatives would need a cost/benefit analysis, which must also look at the echo sounder’s primary task: To ensure that there is no unintended bottom contact.