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Variations "from the quiet solar wind values occur frequently. Figure 5-10 shows observed solar wind velocity distributions as observed by the Vela 3 spacecraft from 1965 to 1967. Other parameters listed in Table 5-7 probably vary, but the correlation of their variation with velocity is poorly known. One explanation of the variations is the sporadic occurrence of "high velocity streams" in the solar wind. The velocity increases over the period of a day to typically 6.5 X 10s m/s, and then declines over several days. High densities occur for the first day, followed by several days of abnormally low densities. The temperatures vary proportionally to the velocity. The direction of the wind moves west of radial up to about 8 deg near maximum velocity after being east of radial the same amount at the leading edge of the stream [Hundhausen, 1972].

High-velocity streams are associated with energetic solar storms, but the exact relationship is unknown. These must be regarded as unpredictable at this time. The solar wind appears to be split in regions (sectors), which may be connected with the high-velocity stream phenomenon. These sectors, each 30 to 180 deg across, are


Fig. 3-10. Observed Solar Wind Velocity Distribution as Recorded by Vela 3 Spacecraft


Fig. 3-10. Observed Solar Wind Velocity Distribution as Recorded by Vela 3 Spacecraft best defined by the alternating direction of the interplanetary magnetic field within them, as shown in Fig. 5-11. The sector structure lasts for several months.

Data on the solar wind at distances other than 1 AU in the ecliptic plane is sparse. Pioneers 10 and 11, which took measurements of solar wind velocity from 1 to 5 AU, found that the mean velocity was essentially constant and that the velocity variation decreased with increasing distance [Collard and Wolfe, 1974], Nothing is known about the solar wind outside the ecliptic plane.

-► magnetic; ield oirection sector boundary

-► magnetic; ield oirection sector boundary

Harris and Lyle [1969])

5.4 Modeling the Position of the Spacecraft

John N. Rowe

To determine attitude reference vectors for nearby celestial objects such as the Sun, the Earth, or other planets, it is necessary to have an accurate model of the position of the spacecraft itself. In this section, we discuss both definitive orbits as they are generated at NASA's Goddard Space Flight Center and a simple orbit generator using the basic equations presented in Chapter 3. The latter method is satisfactory for most aspects of prelaunch attitude analysis and for generating simulated data. However, the analysis of real spacecraft data generally requires use of the ephemeris files generated by one of the much more sophisticated orbit programs.

The orbit of a spacecraft is determined from observations of its position or its distance and radial velocity at different points in its orbit; distance and radial velocity are the most commonly used and are often referred to as range and range rate, respectively. Because six elements are to be determined, at least six pieces of information are required. This means pairs of right ascension and declination or pairs of distance and radial velocity at a minimum of three points in the orbit. Usually such data is obtained at more than three points, and a differential correction procedure (see Chapter 13) is used to estimate the elements.


Spacecraft Ephemeris Files. When the position of the spacecraft is needed for attitude determination, it is normally obtained from files generated by numerical integration incorporating all significant forces. This is accomplished at Goddard Space Flight Center using the Goddard Trajectory Determination System (GTDS), a detailed discussion of which is beyond the scope of this book. (See Capellari, et al., [1976].)

GTDS generates two types of files. One type is the multilevel direct access or ORBIT file, which contains the spacecraft acceleration from which the position and velocity may be recovered. This file is read with the standard utility routine GETHDR (Section 20.3). Table 5-8 shows the contents of the two header records and Table 5-9 shows the contents of the data file. The header information is returned in arrays HDR and IHDR of GETHDR according to the following scheme: bytes I through 608 of header I and bytes I through 608 of header 2 are returned in that order in HDR; bytes 609 through 1092 of header I and bytes 609 through 1092 of header 2 are returned in that order in IHDR.

The second type of GTDS file is the sequential EPHEM file, which contains the spacecraft position and velocity at regular time intervals. The position and

Table 5-8. Goddard Space Flight Center ORBIT File Header Records. Contents of bytes marked "internal use" are given by Cappellari, et at., [1976] and Zavaleta, et at., [1975].




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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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