  This work demonstrates that a middle size ring laser gyroscope (RLG) can be a
very sensitive and robust instrument for rotational seismology, even if it
operates in a quite noisy environment. The RLG has a square cavity, $1.60\times
1.60$ m$^2$, and it lies in a plane orthogonal to the Earth rotational axis.
The Fabry-Perot optical cavities along the diagonals of the square were
accessed and their lengths were locked to a reference laser. Through a quite
simple locking circuit, we were able to keep the sensor fully operative for 14
days. The obtained long term stability is of the order of 3~nanorad/s and the
short term sensitivity close is to 2~nanorad/s$\cdot$Hz$^{-1/2}$. These results
are limited only by the noisy environment, our laboratory is located in a
building downtown.
