  A research paper in this journal vol. 61, no. 3, pp. 427-434, 2012, by M.
Darouach, provides a functional observer design for linear descriptor systems
under the partial impulse observability condition. The observer design is
correct, but there was a flaw in the algebraic criterion characterizing partial
impulse observability. In the present paper, we derive a novel characterization
of partial impulse observability in terms of a simple rank condition involving
the system coefficient matrices and an alternative characterization in terms of
the Wong sequences.
