  The indirect approach to continuous-time system identification consists in
estimating continuous-time models by first determining an appropriate
discrete-time model. For a zero-order hold sampling mechanism, this approach
usually leads to a transfer function estimate with relative degree 1,
independent of the relative degree of the strictly proper real system. In this
paper, a refinement of these methods is developed. Inspired by indirect PEM, we
propose a method that enforces a fixed relative degree in the continuous-time
transfer function estimate, and show that the resulting estimator is consistent
and asymptotically efficient. Extensive numerical simulations are put forward
to show the performance of this estimator when contrasted with other indirect
and direct methods for continuous-time system identification.
