Positive realizations on time scales

The problem of realization of a linear input-output map as a positive linear system on a time scale is studied. To state the criteria of existence of realization, modified Markov parameters corresponding to the input-output map are introduced. It is necessary for the existence of a positive realization that the modified Markov parameters be nonnegative. A necessary and sufficient condition for realizability is expressed in the language of positive cones in an infinite dimensional space. The sequence of modified Markov parameters generates one of the cones that appear in the criterion of realizability.