  Positive linear systems on arbitrary time scales are studied. The theory
developed in the paper unifies and extends concepts and results known for
continuous-time and discrete-time systems. A necessary and sufficient condition
for a linear system on a time scale to be positive is presented. Properties of
positive reachable sets are investigated and characterizations of various
controllability properties are presented. A modified Gram matrix of the system
is used to state necessary and sufficient condition of positive reachability of
a positive system on an arbitrary time scale.
