Overview

This application of SRI's evidential reasoning technology highlights its ability to simultaneously reason with hard constrains, such as island barriers and maximum speed of vehicles, in combination with diverse sources of soft constraints of varying credibility, such as reports from stationary sensors, mobile sensors, direct sightings, and indirect sightings (e.g., bubbles), to establish the likely paths of underwater vehicles.

Domain

Minisubmarines that use tank-like treads to crawl through shallow waters were reportedly used by the Soviets off the coast of Sweden (When Soviet Subs Lurk Near Sweden's Coast, Jack Anderson and Dale Van Atta, United Feature Syndicate). These waters are characterized by thousands of small islands that inhibit the use of conventional submarines. Sightings of bubbles by fishermen and periscopes by sunbathers, along with reported sensor contacts by Swedish military vessels and sensor nets, support this conjecture. However, such reports are scarce and difficult to interpret. The problem is to reconstruct what might have happened after the fact. How many subs must there have been to explain the data? Where were they?

Application

The first step in building this application was to develop a frame of discernment for the possible locations of these subs. We overlaid a grid on a map of the area of interest, determined which grid cells contained water at a sufficient depth to support these subs, and enumerated these cells as elements of the frame (upper left). Next, the cells were linked in a compatibility relation to capture how these subs could move from one cell to another with the passage of an hour (upper right). Since islands prevent direct transit between neighboring cells, they are reflected in the compatibility relation by the absence of connecting arcs. This frame and compatibility relation constitute Gister's gallery that delimits the range of possibilities for how these subs might move.

Each sub contact is represented in a Gister analysis as a report. Each indicates an area within which a sub was located, the time of the contact, and a degree of confidence that captures the credibility of the source. Utilizing Gister's projection capabilities, the reported probabilistic locations of subs can be moved forward or back in time via the compatibility relation. When multiple reports are synchronized, they are fused to form a probabilistic consensus. A byproduct of fusion is a numerical degree of conflict. Here it is used to cluster compatible reports into a single track, while incompatible reports are assumed to be referring to different subs. Given a cluster of compatible reports, an analysis (lower left) is constructed that calculates the tightest possible bounds on that sub's location over time. Each report probabilistically constrains the sub's location during each hour of the analysis.

Performance

This application of evidential reasoning highlights a number of capabilities: natural constraints, such as islands and maximum speed of travel, are captured as logical (certain) constraints within the gallery; uncertain constraints, corresponding to negative sensor net reports, are easily incorporated in the analyses; other diverse sources of probabilistic information are incorporated as reports with varying degrees of credibility. We demonstrate a semiautomated approach to determining the number of minisubs that best explain the data and a fully automated approach to subsequently determining their probable locations over time. The demonstration scenario includes two minisubs that give rise to five reported contacts from disparate sources spaced over a ten-hour period.

John D. Lowrance, <lowrance@ai.sri.com>
Artificial Intelligence Center
SRI International

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