1. A state estimating apparatus comprising: a first processor element configured to: measure a value of a first state variable indicative of a state of an object; disperse at least some particles among a plurality of particles around previous positions in a state variable space defined by a second state variable; transit each of the plurality of particles to a current position according to one current model selected from a plurality of models, such that a plurality of particle groups which each include particles having a common current model are formed, wherein the current model common to the particles within each of said plurality of particle groups is a unique current model for that particle group which fluidly changes for each particle within said particle group; assess a likelihood of each particle relative to a current measured value of the first state variable; and estimate a current state of the object on the basis of the likelihood; and a second processor element configured to: allow particles from among the plurality of particles which have been assessed by the first processor element to have a likelihood greater than a predetermined high-likelihood threshold to remain or increase; extinguish particles from among the plurality of particles which have been assessed by the first processor element to have a likelihood less than a predetermined low-likelihood threshold, said low-likelihood threshold being equal to or less than the high-likelihood threshold; and then determine a next model from the plurality of models for each particle on the basis of the current model for each particle.