A conical scanner having a detector is mounted on a spinning satellite with the scaner axis directed normal to the satellite spin axis for scanning the detector over a great circle passing through the poles of the spin axis of the satellite and producing horizon crossing information of the bodies in the scanning path of the scanner. The scan rate of the conical scanner is synchroized with the spin rate of the satellite and then by slightly increasing or decreasing the scan rate of the conical scanner, successive crossings are precessed through the field of view of the detector for determining the orientation of the satellite with respect to the earth or other celestial body such as the sun or the moon with great accuracy. When the satellite is operated at high spin rates, the conical scanner is operated at a slower rotational speed allowing gaps between successive scans which are filled by precessing the conical scanner to fill in the gaps after several satellite rotations.