  Earth rotation sensing has many applications in different disciplines, such
as for the monitoring of ground motions, the establishment of UT1 and the test
of the relativistic Lense-Thirring effect on the ground. We report the
development of a 1 m*1 m heterolithic passive resonant gyroscope (PRG). By
locking a pair of laser beams to adjacent modes of the square ring cavity in
the clockwise and counter-clockwise directions, we achieve a rotation
resolution of about 2E-9 rad/s at an integration time of 1000 s. The
sensitivity of the PRG for rotations reaches a level of 2E-9 rad/s/rtHz in the
5-100~Hz region, currently limited by the detection noise, residual amplitude
modulation and the mechanical instability of the cavity. Our initial results
improve the reported rotation sensitivity of the PRGs and indicate that PRGs
have a great potential for high-resolution Earth rotation sensing.
