In one aspect, a method to assign observations includes receiving first observations of a first sensor system, receiving second observations of a second sensor system and assigning a set of pairs of the first and second observations predicted to correspond to the same physical position. The assigning includes using a likelihood function that specifies a likelihood for each assigned pair. The likelihood is dependent on the assignment of any other assigned pairs in the set of assigned pairs. The assigning also includes determining the set of assigned pairs for the first and second observations based on the likelihood function. The likelihood function uses a gate value determined from estimating a true volume using nearest neighbor distances determined from the first and second observations.