HOS find out the direction of the horizon by measuring atmospheric thermal emission of CO2 in the 15mm 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 supersonic 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.
HOS is a small radiometer consisting of a Ge lens, a band-pass filter, a pyroelectric IR detector and electronics to process the data. The filter has a central wavelength at 15 mm with a FWHM of 2 mm. 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 103.8° away from the spin-axis, so that HOS is usually expected to see the CO2 horizon at around 44km 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. HOS optics.
N. Iwagami (U Tokyo), S. Komada (U Tokyo), AD Co., Ltd.