  We provide an observation method for gravitational waves using a pulsar
timing array to extend the observational frequency range up to the rotational
frequency of pulsars. For this purpose, we perform an analysis of a perturbed
electromagnetic wave in perturbed spacetime from the field perspective. We
apply the analysis to the received electromagnetic waves in a radio telescope,
which partially composes the periodic electromagnetic pulse emitted by a
pulsar. For simple observation, two frequency windows are considered. For each
window, we propose gauge-invariant quantities and discuss their observations.
