(GLAS) PRECISION ATTITUDE DETERMINATION - Center for ...

....* 30 *400base lineAAAAAAAAAAU* 3*4 01** 24* * *symmetry lineFigure 4.5: Example of a star distribution in a CCD star tracker frame.triangle is drawn by base stars (1 and 2) and a star at 3 0 (or 4 0 ).An isosceles4.1.3 MisidenticationUsing the PMA, the measured stars can be absolutely identied or unidentied. Forthe latter case, the angular velocity detected by the gyros will enable prediction of thespacecraft attitude until identied star data are available. When only three stars areidentied, there is a possibility of misidentication if they satisfyD 23m = D 13m : (4.10)This is the condition that three identied stars form an isosceles triangle with the thirdstar on or near the vertex in the symmetric line, as illustrated in Figure 4.5. When twobase stars are located at the positions 1 and 2 and the third star is located at position3, there is little chance of a misidentication. However, the misidentication likely occurswhen the third star is located on or near the symmetry line between the base stars. Thenthe three stars (1,2 and 3) form an isosceles triangle. If the distinctive magnitudes ofthe base stars are known by the magnitude test, the third star located at or near the40