6.1 Soil Sensors
Analog Sensors, Sun Presence Detectors, Digital Sensors, Fine Sun Sensors
62 Horizon Sensors
Sensor Components, Horizon Sensor Systems 6J Magnetometers 6.4 Star Sensors
Overview of Star Sensor Hardware, BBRC CS-103 V-Slit Star Scanner Jbr OSO-8, BBRC CT-401 Fixed-Head Star Tracker 6J5 Gyroscopes
Rate Gyros, Rate-Integrating Gyros, Control Moment Gyros
6.6 Momentum and Reaction Wheels
6.7 Magnetic Coils
6.8 Gas Jets
In this chapter we describe representative examples of spacecraft hardware used for both attitude determination and attitude control. Extensive hardware experimentation has taken place over the 20-year history of spaceflight. Although this experimentation add development is still continuing, a variety of basic functional types of attitude hardware have emerged. This chapter describes the physical characteristics and operating principles of a variety of sensors. The mathematical models associated with these sensors are presented in Chapter 7. Additional summaries of attitude hardware are given by Fontana, et al., , Hatcher , and Schmidtbauer, et al., . A summary of attitude hardware for specific spacecraft is given in Appendix I.
6.1 Son Sensors
Sun sensors are the most widely used sensor type; one or more varieties have flown on nearly every satellite. The Sun sensor owes its versatility to several factors. Unlike the Earth, the angular radius of the Sun is nearly orbit independent and sufficiently small (0.267 deg at 1 AU) that for most applications a point-source approximation is valid. This simplifies both sensor design and attitude determination algorithms. The Sun is sufficiently bright to permit the use of simple, reliable equipment without discriminating among sources and with minimal power requirements. Many missions have solar experiments, most have Sun-related thermal constraints, and nearly all require the Sun for power.* Consequently, missions are concerned with the orientation and time evolution of the Sun vector in body coordinates. Attitude control systems are frequently based on the use of a Sun reference pulse for thruster firings, or, more generally, whenever phase-angle information is required. Sun sensors are also used to protect sensitive equipment such as star trackers, to provide a reference for onboard attitude control, and to position solar power arrays.
The wide range of Sun sensor applications has led to the development of numerous sensor types with fields of view (FOV) ranging from several square arc-minutes (10~7 sr) to 128 by 128 deg (approximately ir sr) and' resolutions of several degrees to less than an arc-second. The three basic classes of Sun sensors are analog sensors, which have an output signal that is a continuous function of the Sun angle and is usually monotonic; Sun presence sensors, which provide a constant output signal whenever the Sun is in the FOV; and digital sensors, which provide an encoded, discrete output which is a function of the Sun angle. A summary of sensor types manufactured by the Adcole Corporation is presented in Table 6-1.
Analog sensors are frequently called cosine detectors because a common type is based on the sinusoidal variation of the output current of a silicon solar cell with Sun angle as shown in Fig. 6-1. Specifically, the energy flux, E, through a surface of area dA with unit normal n is where P is the Poynting vector, which gives the direction and magnitude of energy flow for electromagnetic radiation. Thus, the energy deposited in a photocell and, consequently, the output current, I, is proportional to the cosine of the angle of incidence of the solar radiation.
Small transmission losses due to Fresnel reflection, the effective photocell area, and angle-dependent reflection at the air-cell interface are omitted from the simple model given by Eq. (6-2).
Fig. 6-1. Cosine Detector Sun Sensor n photocell
♦ Spacecraft (hat do not use solar power include the Pioneer missions, which use nuclear power because of the 1 /r2 decrease in solar flux with distance from the Sun.
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