It is noted that, for both explicit and implicit linear systems, sever
al observer design problems, including system inversion, for systems w
ith or without unknown inputs can all be formulated as a single proble
m involving descriptor systems. The method used for solving this probl
em is based on recursively applying the reduction operation which cons
ists of identifying and discarding equations that do not provide usefu
l information for the estimation problem. The reduction algorithm prod
uces a reduced dimension system for which standard observers can be co
nstructed. The method presented can be implemented using numerically r
obust algorithms. One such algorithm is presented. (C) 1998 Elsevier S
cience Ltd. All rights reserved.