  A world-wide network of interferometric gravitational wave detectors is
currently operational. The detectors in the network are still in their
commissioning phase and are expected to achieve their design sensitivity over
the next year or so. Each detector is a complex instrument involving many
subsystems and each subsystem is a source of noise at the output of the
detector. Therefore, in addition to recording the main gravitational wave data
channel at the output of the interferometer, the state of each detector
subsystem is monitored and recorded. This subsidiary data is both large in
volume as well as complex in nature. We require an online monitoring and
analysis tool which can process all the data channels for various noise
artefacts and summarize the results of the analysis in a manner that can be
accessed and interpreted conveniently.
  In this paper we describe the GEO600 Online Detector Characterization System
(GODCS), which is the tool that is being used to monitor the output of the
GEO600 gravitational wave detector situated near Hannover in Germany. We
describe the various algorithms that we use and how the results of several
algorithms can be combined to make meaningful statements about the state of the
detector. This paper will be useful to researchers in the area of gravitational
wave astronomy as a record of the various analyses and checks carried out to
ensure the quality and reliability of the data before searching the data for
the presence of gravitational waves.
