The problem of observer design for descriptor
systems, or systems of differential algebraic equations (DAEs)
as they are also known, has been studied in the linear time
invariant case. However, those studies do not readily extend to
general linear time varying descriptor systems. Recently there
have been new theoretical results and algorithms for computing
completions of DAEs. In this paper we examine the application
of these ideas to the computation of reduced order observers
for linear time invariant and linear time varying DAEs.