Saturday, April 08, 2006

Underwater, Unmanned, Untethered - A new paradigm in undersea surveillance

Introduction Before the advent of the Advanced Unmanned Search System (AUSS), researchers faced a tradeoff between using tethered and untethered undersea surveillance robots. The untethered robots obviously had more freedom and range of movement, without chances of wires getting caught or tangled, but they lacked the capability of transmitting real-time information to the user. Furthermore, an untethered vehicle can move at relatively high speeds and perform sharp, precise maneuvers, or it can hover stably without expending power fighting cable pull. The AUSS, developed by Richard Uhrich & James Walton at the Naval Command, Control and Ocean Surveillance Center of SPAWAR Systems Center in San Diego, is an underwater vehicle which is both unmanned and untethered. Communication with a surface ship is accomplished by means of underwater sound, by means of a sophisticated digital acoustic link. Its operation is similar to that of a space probe, with the robot proceeding on its own, and able to receive real-time instructions at any time. Advanced abilities of the AUSS include going to a newly commanded location, hovering at a specified altitude and location, executing a complete search pattern, or returning home on command.

Purpose The purpose of AUSS is to improve the Navy's capability to locate, identify and inspect objects on the bottom of the ocean up to depths to 20,000 feet. The vehicle utilizes sophisticated search sensors, computers, and software, and it is self-navigating. When commanded to do so, it can autonomously execute a predefined search pattern at high speed, while continuously transmitting compressed side-looking sonar images to the surface. The operators evaluate the images and supervise the operation. If they wish to further check out a certain object, they can order the vehicle to temporarily suspend sonar search and swim over for a closer look using its scanning sonar or still-frame electronic camera. Each camera image is also compressed and transmitted to the surface. If the operators see that the contact is not the object sought, a single command causes the vehicle to resume the search from where it left off. Once the object sought is recognized, a detailed optical inspection can be conducted immediately. The AUSS offers multiple options for this, including: - Previously transmitted images can be retransmitted at higher resolution. - New optical images can be requested from different altitudes and positions. - A documentary film camera can be turned on or off. - If the object of interest is very large or found to be highly fragmented, the vehicle can perform a small photomosaic search pattern, taking overlapping pictures guaranteeing total optical coverage of a defined area. How Acoustic Communication Works As opposed to physical cables like fibre-optic cables, the AUSS communicates via sound. The acoustic link transmits compressed search data to the surface at rates up to 4800 bits per second, and sends high level commands to the vehicle at 1200 bits per second. Given the robot vehicle’s intelligence, the operator does not have to supervise it at each step. Instead, higher level commands of what to do (rather than how to do it) are given. The AUSS autonomously performs each task until it is completed or until the operators interrupt with a new command. Navigation The vehicle's computers use a Doppler sonar and a gyrocompass to perform onboard navigation. Control Generally, all critical loops on the robot’s control system are closed. This means that the operator does not have to employ joystick-like control of every movement. The intelligence and navigation of the AUSS allows the user to instruct it to move to a specified location, say a few miles away, and have the confidence that it will successfully navigate itself to that location without further input. On the other hand, the operator has the freedom to give new instructions or interrupt some decision loop that is executing on the robot. This frees the AUSS from being limited to pre-programmed routines and allows the operator to apply his intelligence and experience to control the robot vehicle. Since images are fed back to the operator constantly, once there is something interesting the operator can instruct the AUSS to make closer investigation, in the variety of ways outlined above. Image sensors There are two ways of relaying search information back to the human operator – sonar and optical images. Sonar images can be generated faster and have a very large range, but the resolution is poor. Optical images, on the other hand, are taken at the range of a few feet and offer greater detail. Generally, if a human wants to take a closer look at any object, an optical image is necessary to confirm the status of the object. Other specifications The AUSS vehicle is designed to operate as deep as 20,000 feet. It is 17 feet long, 31 inches in diameter, and weighs 2800 pounds. The center section is a cylindrical graphite epoxy pressure hull with titanium hemispherical ends. The hull provides the central structure and all its buoyancy---no syntactic foam is used. The free-flooded forward and aft end fairings and structure are of Spectra, a nearly buoyant composite. At its maximum speed of five knots, the endurance of the AUSS silver-zinc batteries is ten hours. Recharging requires 20 hours. Typical missions have been ten to fifteen hours. Three sets of batteries would allow AUSS to operate indefinitely, with only 3-1/2 hours between 20,000 foot dives. Test results In the summer of 1992 the system performed a series of sea tests off San Diego culminating in a 12,000 foot operation. AUSS conducted side looking sonar search at five knots, and performed detailed optical inspections of several objects which it found. It proved capable of sustained search rates approaching a square nautical mile per hour, including the time spent investigating false targets. The image shows a World War II Dauntless Dive Bomber identified by the AUSS.

Issues At this point, I will raise some issues for consideration. Here, unmanned vehicle means that low-level instructions on navigating and doing pre-programmed search are unnecessary. However, the AUSS still requires human supervision to detect unique objects, and to decide which objects to further investigate. This may result in problems in missions where there has to be acoustic silence, or where manpower is insufficient. Can a better algorithm and control system be developed such that the AUSS be equipped with sufficient intelligence to discern between interesting and trivial objects? Also, given the kind of technology that the navy employs in such robots for undersea surveillance, could such vehicles also be used for scientific research and oceanography at the same time? The deep oceans are still relatively unexplored and the information returned by such robots could prove invaluable to the scientific community and mankind in turn. Reference links:

1 comment:

Assistive said...

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While it is definitely very desirable to have an unmanned vehicle that could perform searches with complete autonomy, it is very difficult to develop such an autonomous system. Firstly, as had been mentioned, it is difficult to transmit information at real-time to user with an untethered system. While the AUSS is able to periodically send compressed low resolution sonar image to the surface, operators are still needed to decipher if the image is that of the target object before a detailed optical inspection would be conducted. I guess the technology to transmit images through sonar means has yet to reach a level comparable to wired transmission, rendering the need to still have human supervision in such search operations. Nevertheless, the ability to search a much deeper and larger region at a high speed meant that search operations could be performed much more efficiently and effectively.

As to whether such robots could prove useful in scientific research and oceanography, I guess it is just a matter of time before they are employed in these fields. At this point in time, I believe cost is the biggest factor why they are not used in other areas yet. Another reason could be that the AUSS is not really designed to perform marine research which requires close scrutiny of marine lives etc. The AUSS, with a high speed maneuver capability is probably meant for search operations over large areas rather than slow examination of marine biology.