Landecker, "Operational Spacecraft Attitude Determination Using Data from a Spinning Sensor", The Journal of the Astronautical Sciences, Vol. 32, No. 2, April-June 1984, pp. 189-196, describes a hypothetical method of attitude determination of a spinning satellite orbiting the earth, using an on-board radiometer to sense stars each time the spinning satellite is not facing the earth. The present invention, on the other hand, uses satellite instruments on a three-axis stabilized satellite to perform image navigation. Differences of the invention compared with the reference include:
1. In the reference, the radiometer is spinning very fast, about 100 rpm, because the radiometer is on the platform which provides stabilization to the satellite. Thus, the radiometer is moving past the inertially fixed stars at a rate of 36,000.degree. per minute, compared with the 0.25.degree. per minute in the present invention. As a consequence, the present invention offers a vastly superior signal-to-noise ratio.
2. The present invention controls slew in two orthogonal dimensions to precisely home in on the specific desired star. The reference, on the other hand, steps the radiometer optics in only one dimension. As a consequence, for a small area scan (a scan of less than the entire earth), the reference is limited to looking for starts within that narrower field of view. The present invention is not so restricted.
3. The present invention is used as part of an image navigation system to determine long term instrument attitude and satellite orbit parameters which are then compensated for in an image registration system. The reference, on the other hand, is used just for attitude and orbit determination.
4. The reference system requires knowledge of the magnitude of the stars being observed in order to function. On the other hand, the present invention looks for preselected stars, but is not dependent on their magnitude (within the sensitivity limits of the instruments 1, 2).
5. The reference system looks for stars during backscan (away from the earth) while the present invention looks for stars just beyond the limb of the earth or other celestial body around which the satellite is orbiting.
The following four references allude in a general way to portions of the invention: (1) Graul, oral presentation accompanied by a posterboard display before the Environmental Research Institute of Michigan at its International Symposium on Remote Sensing of the Environment, Oct. 21, 1985; (2) Schwalb, "Envirosat-2000 Report; GOES-Next Overview", National Oceanic and Atmospheric Administration, September, 1985 (3) Koenig, "The GOES-Next Imager and Sounder", disseminated at American Meteorological Society Conference, Miami, Fla., Jan. 13-16, 1986; and (4) Juarez and koenig, "Infrared Imaging and Sounding from a Geostationary Body Stabilized Spacecraft", disseminated at AMS Second Conference on Meteorology/Remote Sensing and Application, Williamsburg, Va., May 12-16, 1986.
Secondary references are U.S. Pat. Nos. 3,952,151 and 4,300,159.