  We analyze the problem of sending, in a single transmission, the information
required to specify an orthogonal trihedron or reference frame through a
quantum channel made out of N elementary spins. We analytically obtain the
optimal strategy, i.e., the best encoding state and the best measurement. For
large N, we show that the average error goes to zero linearly in 1/N. Finally,
we discus the construction of finite optimal measurements.
