Least squares identification of time varying parameters: Issues and trends

In this paper, we review the literature on time varying system identification with particular emphasis placed on the recursive least squares method with forgetting factor. Even though the subject of identification of varying systems has been intensively studied over the last two decades, many problems still remain open. Herein, we revisit the most important results appeared in the literature and point out some issues which have not yet found a satisfectory solution and are therefore worthy to be considered in future investigations.