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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
3. STAR IMAGE SELECTION 
As related above, this on-calibration method is based on star 
images. But not all star images are identified successfully by 
the reasons of the asymmetry distribution of stars in the 
celestial sphere, or the insufficiency of the number of the 
detected stars, or the limitation of the star identification 
algorithm etc. (Liebe, 1995). Only the identified star images can 
reach the basic requirement for calibration, in other words, the 
number of star image points of each calibrated image is more 
than three at least. 
In this foundation, the distribution of star image points of each 
calibrated images should be considered, because the calibration 
accuracy has correlation with the distribution of control points 
(Feng, 2002). Ten sequential images in one calibration period 
are shown in the figure 3. It is seen from it that different image 
has different distribution, and the calibration accuracy based on 
different image may be different. 
In this paper, the convex area method is used to evaluate 
whether this image is eligible for calibration. Take the eighth 
star image as an example, which is shown in figure 4. The steps 
of this method are as follow: 
(1) The convex is constructed by the star image points on the 
edge of the image by the convex algorithm (Jin et al., 1999; 
Zhou et al., 2003; Rourke, 1998). 
(2) As the coordinates of connection point is known, so then the 
area of the convex is computed. 
(3) The area percent of the convex in the whole image is got. If 
the percent is more than fifty percent, which indicates the 
distribution is relatively even, the distribution of the image is 
considered good and the image is selected. 
From figure 3, the result evaluated by the convex area method 
is the distribution of figures (e), (g), (h), (j) are good and figures 
(b), (c), (d) are bad. 
(e) The fifth image (f) The sixth image 
(i) The ninth image (j)The tenth image 
Figure 3. The distribution of part of images 
Figure 4. Convex constructed by star image points 
4. EXPERIMENT 
The star catalogue Tycho-2 (J2000)(Li, 2006)was downloaded 
for experiment, whose data types include star index, magnitude, 
the right ascension (hour, minute, second) and the declination 
(degree, arcminute, arcsecond) etc.. Average positions of the 
stars in standard epoch are transformed to intraday visual 
position in real attitude determination. The FOV is 8°x8°, and 
the size of array CCD is 512*512 pixels, and each pixel size is 
13 urn. The upper and lower limit of magnitude is 0 and 6.0 
(mv), and then there are 5001 stars selected in star catalogue for 
these experiments. Supposed one stellar image is got by the 
stellar camera each 0.1 second, and the calibration period is 2 
second in this experiment in the experiments.