  We present the Generalised Differential Image Motion Monitor. It is a compact
instrument dedicated to measure 4 parameters of the optical turbulence: seeing,
isoplanatic angle, coherence time and wavefront coherence outer scale. GDIMM is
based on a small telescope (28cm diameter) equipped with a 3-holes mask at its
entrance pupil. The instrument is fully automatic, and performs continuous
monitoring of turbulence parameters at the Calern Observatory (France). This
paper gives a description of the instrument, data processing and error budget.
We present also statistics of 3.5 years of monitoring of turbulence parameters
above the Calern Observatory.
