The paper characterizes all linear function observers for descriptor s
ystems. With the function observers formulated in the frequency domain
, the problem of finding all function observers is reduced to that of
solving a proper stable linear matrix equation. Parametrization of its
general solution results in the desired characterization of the funct
ion observers. These function observers are then realized in the state
space and it is shown that if an appropriate state space transformati
on is introduced, the class of the reduced order observers can also be
derived through this realization.