  Given n qubits prepared according to the same unknown density operator, we
propose a nondestructive measuring method which approximately yields the
eigenstates. It is shown that, for any plane which passes through the center
point of the Bloch sphere, there exists corresponding projective measurement.
By performing these measurements, we can scan the whole Bloch sphere like radar
to search for the orientation of and determine the eigenstates. We show the
convergency of the measurements. This result actually reveals a mathematical
structure of the n-fold Hilbert space .
