Fig. 1. (a) Sky background radiation simulation results at noon for urban summer with different detector directions; (b) Atmospheric transmittance simulation results at noon for urban summer with different detector directions when the visibility is 5 km

Fig. 2. Detectable star magnitude limit changes with focal length and diameter at sea level

Fig. 3. Cutoff wavelength optimization at sea level

Fig. 4. Detection capability estimation at sea level

Fig. 5. Average number of navigation stars in FOV (field of view) at sea level

Fig. 6. (a) Average number of navigation stars in FOV with fixed diameter of 200 mm and changing focal length; (b) Average number of navigation stars in FOV with optimal focal length at different altitudes

Fig. 7. Average number of navigation stars in FOV changes with focal length and diameter at 20 km

Fig. 8. Cutoff wavelength optimization at an altitude of 20 km

Fig. 9. Experimental system of star sensor with tracking mode

Fig. 10. (a) Infrared star image taken at daytime after non-uniformity calibration (magnitude –1.608, 9:45 a.m.); (b) Gray values of the boxed star area

Fig. 11. Signal to noise ratio of star images for different magnitudes