US Patent 8352173 Two-level grouping of principals for a collision warning system

One embodiment of the present invention provides a system that hierarchically groups principals to facilitate collision warning. During operation, the system obtains information on a number of principals and constructs one or more first-level groups of principals. Within a first-level group, the system constructs one or more second-level groups, thereby facilitating an assessment of probability of collision between a second-level group and a primary principal.