Surface Forces: Bottom Feeders

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December 30, 2019: A Japanese firm, IHI, has developed a new autonomous underwater mine detection system. The IHI system does what several existing systems do but does it more quickly, efficiently and without much human involvement. The IHI system consists of a five meter (16 foot) long AUV (Autonomous Underwater Vehicle) similar to a torpedo, but with four steering fins near the front. Depending on equipment carried it will have several other items protruding from the chassis. The IHI AUV is designed to move slowly as it scans the ocean floor for mines. The top speed is about seven kilometers an hour. Endurance is 24 hours and it can operate up to 3,000 meters down, but requires some expensive accessories to do that. This AUV is basically built to operate in coastal waters where most mines are placed on the ocean floor. Thus most IHI AUVs would be configured to handle depths of 200-600 meters (630-1900 feet).

The AUV  normally carries a side-scan sonar for searching the seabed below for suspicious objects. In addition, there is a multi-beam sonar and a high-resolution digital vidcam to capture more detailed images of suspicious objects. This AUV is also notable for its navigation and communications capabilities. Basic navigation is handled by GPS and INS (Internal Navigation System). GPS can only operate when the AUV is at the surface and when it does that the GPS location can update the less accurate INS. What makes this so useful is that INS has become a lot more accurate and cheaper in the last decade. INS has existed for nearly a century but has gotten smaller, cheaper, and more reliable as have most other electronic components since the 1960s. Basically INS uses three gyroscopes and three accelerometers to constantly measure changes in direction and changes in velocity. With that, the INS will always know where it is in relation to its initial starting point, which can be obtained initially via unjammed GPS or older means. Miniature INS devices have long served as a backup for GPS guidance in ships, aircraft and weapons. But while GPS guidance can land a bomb or missile within 10 meters (32 feet) of a target, INS can only achieve 30 meter accuracy.  On the upside, INS cannot be jammed or deceived. When searching for bottom mines on the ocean floor, accuracy describing the location of a suspicious object is important.

After 2010 American researchers created new concepts and technology that greatly improved INS accuracy and lowered cost. By 2013 prototypes proved they could be nearly as accurate as GPS and almost as small. Cost was still a factor, with the new INS still costing more than 10 times what GPS does. But this is all a big improvement over what has been available before. The new INS can now be used to monitor GPS and alert the operator that their GPS has either developed a problem or is being jammed. The new INS is also useful for some fast missiles that often lose their GPS signal as they maneuver. Another urgent chore for INS is to alert users that their GPS is being spoofed (sent a false signal that is luring the user away). Thus, even with the ability of anti-jamming tech to keep up with jammer technology, there is still a demand for a new INS. For AUVs the latest INS tech means the AUV does not need to surface as often to compare GPS location with the calculated INS one. This is essential for a mine detection system because those locations have to be avoided by ships and manned submarines, and also be accurate enough so s smaller AUV, carrying explosives, can be sent down to destroy some of these bottom mines. Normally it’s good enough just to know where these mines are and avoid them. But if there is a line of them blocking a harbor entrance or narrow shipping channel, you have to eliminate some of these mines.

Another job of these AUV mine detectors it to regularly scan mined areas to see if storms, tides or underwater “weather” have moved some of these mines. The data collected by the AUV automatically updates the navigation charts. Another thing to look for are self-propelled mines. Some of the more elaborate bottom mines can move. These are rare because they are expensive and not always useful. But with something like the IHI AUV regularly scanning shipping channels, it becomes more difficult for an enemy to use naval mines effectively.

The IHI AUV has other navigation devices including DVL (Doppler Velocity Log) which more accurately measures the movement of the AUV relative to the sea bottom. There is a similar Ultra-short baseline system which makes it possible for a highly maneuverable AUV, like the IHI model, to reach a specific location underwater. The AUV also can transmit, rather slowly, data to a smaller AUV near the surface, using an underwater acoustic modem. The smaller AUV also has antennae extending above the surface to transmit the collected data to a nearby ship or shore station. These data packets are usually only sent when the larger AUV suspects it has found a mine. The search AUV has a lot of computing power onboard as well as a digital library of what various bottom mines look like to sonar and vidcams.

The IHI AUV is designed to easily accept new hardware and software as well as upgrades. Mine detecting tech has been rapidly improving since the 1990s and most mine detecting AUVs are now built to work with the new tech and not be made obsolete by it.

 


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