  In many practical cases, the engineer has access to prior knowledge like
rough values of the DC-gain or the main time constant of the system. In order
to improve the accuracy of subspace-based identification techniques using the
model Markov parameters, we derive in this short paper the direct links between
these impulse response coefficients and this prior information. The next step
will consist in introducing this prior knowledge explicitly in Kung's algorithm
thank to dedicated equality and equality constraints.
