The Gamma-Ray imaging Device (GRID) is a NASA experiment to be flown on a long-duration

Chapter 0, Problem CP12.8

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The Gamma-Ray imaging Device (GRID) is a NASA experiment to be flown on a long-duration, high-altitude balloon during the coming solar maximum. The GRID on a balloon is an instrument that will qualitatively improve hard X-ray imaging and carry out the first gamma-ray imaging for the study of solar high-energy phenomena in the next phase of peak solar activity. From its long-duration balloon platform, GRID will observe numerous hard X-ray bursts, coronal hard X-ray sources, "superhot" thermal events, and microflares [2]. Figure CP12.8(a) depicts the GRID payload attached to the balloon. The major components of the GRID experiment consist of a 5.2- meter canister and mounting gondola, a high-altitude balloon, and a cable connecting the gondola and balloon. The instrument-sun pointing requirements of the experiment are 0.1 degree pointing accuracy and 0.2 arcsecond per 4 ms pointing stability. An optical sun sensor provides a measure of the sun-instrument angle and is modeled as a first-order system with a DC gain and a pole at s = -500. A torque motor actuates the canister/gondola assembly. The azimuth angle control system is shown in Figure CP12.8(b). Tlie PID controller is selected by the design team so that Gc(s) KD(s2 + as + b)