This paper proposes a hybrid observer for state estimation over a
network. The network provides delayed measurements of the output of the
plant at time instants that are not necessarily periodic and are
accompanied by timestamps provided by a clock that synchronizes with the
clock of the observer in finite time. The proposed observer, along with
the plant and communication network, are modeled by a hybrid dynamical
system that has two timers, a logic variable, and two memory states to
capture the mechanisms involved in the events associated with sampling
and arrival of information, as well as the logic in the estimation
algorithm. The hybrid model also includes a generic clock
synchronization scheme to cope with a mismatch between the clocks at the
plant and the observer. Convergence properties of the estimation error
of the system are shown analytically and supported by numerical
examples.