SFF find out the direction of the horizon by measuring thermal emission of the surface in the 10mm window region to determine the attitude of the rocket. The attitude is important for analyzing NTV and AGL data. In case of NTV, the disturbed region produced by the super-sonic speed of the rocket may affect to the measurement, and it is needed to know its position and shape by taking the attitude into account. In case of AGL, it is needed to know the zenith angle of the line of sight because slant line of sight usually results in larger apparent emission rate.
SFF is a small radiometer consisting of a Ge lens, a 7mm long-pass filter, a pyroelectric IR detector and electronics to process the data. Although the filter is a long-pass, the pass-band of SFF is determined mostly by the atmospheric window at 8-12mm. The field of view is 1° square. The pyroelectric detector is sensitive in the thermal IR region, and works under the room temperature without any cooling system. The line of sight is set in the direction 105° away from the spin-axis, so that SFF is usually expected to see the ground horizon twice in a spin period. The zenith angle of the spin-axis may be calculated from the time separation between such horizon-crossings. By combining information of the magnetic angle of the spin-axis from the GA sensor, the absolute attitude of the rocket may be determined.
Figure 1. SFF optics.
N. Iwagami (U Tokyo), S. Komada (U Tokyo), AD Co., Ltd.