page_title: Short baseline acoustic positioning system
section_title: Example
hierarchical_section_title: Short baseline acoustic positioning system / Example
caption_attribution_description: The SCINI ROV, a special vehicle designed at Moss Landing Marine Lab, sits on the ice near Heald Island in Antarctica. The skinny, torpedo shaped body makes it possible to drop the vehicle through an 8" hole in the ice.
context_page_description: A short baseline acoustic positioning system is one of three broad classes of underwater acoustic positioning systems that are used to track underwater vehicles and divers. The other two classes are ultra short baseline systems and long baseline systems. Like USBL systems, SBL systems do not require any seafloor mounted transponders or equipment and are thus suitable for tracking underwater targets from boats or ships that are either anchored or under way. However, unlike USBL systems, which offer a fixed accuracy, SBL positioning accuracy improves with transducer spacing. Thus, where space permits, such as when operating from larger vessels or a dock, the SBL system can achieve a precision and position robustness that is similar to that of sea floor mounted LBL systems, making the system suitable for high-accuracy survey work. When operating from a smaller vessel where transducer spacing is limited, the SBL system will exhibit reduced precision.
context_section_description: An example of SBL technology is currently (since 2007) underway in Antarctica, where the Moss Landing Marine Laboratory is using a PILOT SBL system to guide the SCINI remotely operated vehicle.  SCINI (figure 2) is a small, torpedo-shaped tethered vehicle (ROV) designed for rapid and uncomplicated deployment and exploration of remote sites around Antarctica, including Heald Island, Cape Evans and Bay of Sails. SCINI system is designed to be compact and light-weight so as to facilitate rapid deployment by helicopter, tracked vehicle and even man-hauled sleds.  Once on site, its torpedo shaped body allows it to access the ocean through small (20 cm dia.) holes drilled into the sea ice. The mission's science goals however demand high accuracy in navigation, to support tasks including running 10-m video transects (straight lines), providing precise positions for still images to document the distribution and population density of benthic organisms and marking and re-visiting sites for further investigation.  
  The SBL navigation system (figure 3) consists of three small, 5 cm diameter sonar baseline transducers (A, B, C) that are linked by cable to a control box (D). A small (13.5 cm L x 4 cm D), cylinder shaped transponder is mounted on the SCINI vehicle. Accuracy is optimized by making use of the flat sea ice to place the baseline transducers well apart; approx. 35m for most SCINI deployments. 
  Figure 4 reviews SCINI operations guided by the SBL system.  Figure 4A is an improvised ROV control room, in this case in a cabin hauled on top of an ice hole at Cape Armitage.  From left, the displays are the ROV controls screen (A), the main camera view (B), the navigation screen (C) and the science display (D). The ROV pilot will generally watch the main camera view. He will glance at the navigation screen (C), which shows the current ROV position and track overlaid on a chart, for orientation and to guide the ROV to the location instructed by the scientist.  The scientist, shown here seated on the right is provided with the science display (D), which combines the ROV imagery with position, depth and time data in real time.  The scientist types written or speaks audible observations into the computer to provide a context for the data, note objects or evens of interest or designate the start or conclusion of a video transect (figure 4B).
  A typical investigation of a site will span several dives, as tasks such as initial investigation, still image acquisition and video transects are gradually completed. A critical element in these dive series is to show prior-dive search coverage, so that a successive dive can be targeted at a previously unvisited area.  This is done by producing a cumulative coverage plot of the dive site (figure 4C).  The plot, which is updated after every dive, is displayed as a background map on the navigation screen thus providing guidance for the ongoing dive.  It shows the prior ROV tracks with color used to indicate depth. Analysis of the track data displayed here yields the quality of positioning to provide a margin of error for measurements.  In this case, the typical precision has been established as 0.54m.
caption: Figure 2: The SCINI ROV next to its dive hole at Heald Island, Antarctica
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