A fixed lag smoothing IPDA tracking in clutter

We present fixed lag smoothing for integrated probabilistic data association (IPDA). The IPDA track state is hybrid and consists of target existence state and the target trajectory state. The Fixed Lag smoothing IPDA (FLs-IPDA) considers all feasible target existence sequences in last N scans to get the improvement in target hybrid state at fixed lag. This is illustrated by the fixed lag of 2 as a special case.