  This paper investigates observability/controllability of a networked dynamic
system (NDS) in which system matrices of its subsystems are expressed through
linear fractional transformations (LFT). Some relations have been obtained
between this NDS and descriptor systems about their
observability/controllability. A necessary and sufficient condition is
established with the associated matrices depending affinely on subsystem
parameters/connections. An attractive property of this condition is that all
the required calculations are performed independently on each individual
subsystem. Except well-posedness, not any other conditions are asked for
subsystem parameters/connections. This is in sharp contrast to recent results
on structural observability/controllability which is proven to be NP hard. Some
characteristics are established for a subsystem which are helpful in
constructing an observable/controllable NDS. It has been made clear that
subsystems with an input matrix of full column rank are helpful in constructing
an observable NDS, while subsystems with an output matrix of full row rank are
helpful in constructing a controllable NDS. These results are extended to an
NDS with descriptor form subsystems. As a byproduct, the full normal rank
condition of previous works on network observability/controllability has been
completely removed. On the other hand, satisfaction of this condition is shown
to be appreciative in building an observable/controllability NDS.
