LOES of a high order state space model

How to get Lower Order (4th order) Equivalant System (LOES) of a higher order (>100 order) state space model using tfest/ssest ? Can it be done wihtouht excting the model and collecting I/O data?

Answer 1

[Rajiv Singh]

You could attempt this in frequency domain.

G = frd(OriginalModel,w) % w is a suitably chosen frequency grid

ReducedModel = ssest(G, 4) % reduction (in L2 error since) to 4th order