has become one of the significant problems of gravitational
wave detection experiments.
Generally, the vibration isolation methods are mainly
classified into active vibration isolation and passive
vibration isolation. The former is usually used for precise
weak-ground-tilt vibration isolation systems because the
low-frequency vibration is difficult to be isolated and the
damping reduces vibration isolation efficiency. Monitoring
low frequency weak-ground-tilt vibration with high
precision is a key technology. In 1997, the VIRGO group
measured ground tilt seismic spectrum of laboratory with
frequencies above 1 Hz using a rotational accelerometer,
and the experiments showed that the ground vibration
noise was 3 10–7 m/
ffiffiffiffiffiffi
Hz
p
at 1 Hz expressed by angular
displacement, but below 1 Hz its SNR (signal to noise ratio)
was very low [6]. AIGO laboratory designed the ground tilt
vibration isolation device based on WOS (walk-off angle
sensor ) to detect ground tilt seismic spectrum within
2–10 Hz in two perpendicular directions [7–9]. However