  The rotational Doppler effect associated with light's orbital angular
momentum (OAM) has been found as a powerful tool to detect rotating bodies.
However, this method was only demonstrated experimentally on the laboratory
scale under well controlled conditions so far. And its real potential lies at
the practical applications in the field of remote sensing. We have established
a 120-meter long free-space link between the rooftops of two buildings and show
that both the rotation speed and the rotational symmetry of objects can be
identified from the detected rotational Doppler frequency shift signal at
photon count level. Effects of possible slight misalignments and atmospheric
turbulences are quantitatively analyzed in terms of mode power spreading to the
adjacent modes as well as the transfer of rotational frequency shifts.
Moreover, our results demonstrate that with the preknowledge of the object's
rotational symmetry one may always deduce the rotation speed no matter how
strong the coupling to neighboring modes is. Without any information of the
rotating object, the deduction of the object's symmetry and rotational speed
may still be obtained as long as the mode spreading efficiency does not exceed
50 %. Our work supports the feasibility of a practical sensor to remotely
detect both the speed and symmetry of rotating bodies.
