  A simply structured distributed observer is described for estimating the
state of a discrete-time, jointly observable, input-free, linear system whose
sensed outputs are distributed across a time-varying network. It is explained
how to construct the local estimators which comprise the observer so that their
state estimation errors all converge exponentially fast to zero at a fixed, but
arbitrarily chosen rate provided the network's graph is strongly connected for
all time. This is accomplished by exploiting several well-known properties of
invariant subspaces plus several kinds of suitably defined matrix norms.
