A multiple-model (MM) estimator with a variable structure, called likely-mo
del set (LMS) algorithm, is presented, which is generally applicable to mos
t hybrid estimation problems and is easily implementable. It uses a set of
models that are not unlikely to match the system mode in effect at any give
n time. Different versions of the algorithm are discussed. The model set is
made adaptive in the simplest version by deleting all unlikely models and
activating all models to which a principal model may Jump so as to anticipa
te the possible system made transitions. The generality, simplicity, and ea
se in the design and implementation of the LMS estimator are illustrated vi
a an example of tracking a maneuvering target and an example of fault detec
tion and identification. Comparison of its cost-effectiveness with other fi
xed- and variable-structure MM estimators is given.