Attitude control and determination along with the fix and navigation of the spacecraft is the fundamental functional
elements in the space scientific research tasks. Nadir vector is usually applied in the attitude determination of spacecraft
relative to the Earth, and it is also commonly used as a reference vector in autonomous navigation especially in the
satellites' preliminary coarse orbit determination.
In the paper, we begin with the shape of the Earth as seen from the space, which is defined by the Earth horizon as
seen from the position of the observer. The geometric model of the nadir vector pointing from the spacecraft to the Earth
center is established basing on three-dimension earth spheroid model; the effect of Earth flattening on the attitude
determination and positioning is discussed, the flattening and irregularity of the Earth ultraviolet layer model are the
major error sources resulting in poor accuracy of attitude determination and fix. An error modification method is
presented to compensate for the Earth flattening error.
Several crucial techniques in determining earth center and nadir vector basing on CCD sensor are stressed in the paper.
Noise elimination and image processing means are introduced to restraint the influences of harmful signals in the image
acquisitions; edge detection is one of the most important aspects in the process of determining the Earth center,
Randomed Hough Transformation method and least square method are effective way to check the edge of earth horizon