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We do not always know whether a given mission has passive or active subjects; in some cases, we can choose either type. For example, we could detect airplanes passively with an IR sensor or radar, or actively by listening for or interrogating a transponder on the airplane. Chapter 22 summarizes an alternative for sensing forest fires by using equipment on the ground and then relaying it to space—a technique possible for various mission types. Satellites that monitor the weather or environment could do complex observations or simply collect and relay data from sensors on the ground.

The next step is to determine whether we need multiple subjects (and, probably, multiple payloads) to meet our mission objectives. Using multiple subjects at the same time has several advantages. This approach can provide much more information than is available from a single subject and can eliminate ambiguities which occur when observing only one aspect On the other hand, multiple subjects typically require multiple payloads, which dramatically drive up the space mission's cost and complexity. Thus, a principal trade is between a low-cost mission with a single subject and single payload vs. a more expensive mission that achieves higher performance by using several payloads to sense several different subjects related to the same objective.

For FireSat we tentatively select the heat of the forest fire as the subject of the mission, keeping in mind that this may change as the design evolves. Of course, we should make these trades as rapidly as possible because they strongly affect how the mission is done.

Finally, as always, we should document the subject selection and review it frequently during the program's early stages, looking for other possible methods and subjects. Looking for alternative subjects is perhaps the single most important way to drive down the cost of space missions. We need to continually ask ourselves, "What are we trying to achieve, including the tacit rules of the program, and how can we achieve it?"

93 Background '

93.1 The Electromagnetic Spectrum

As Fig. 9-4 illustrates, the electromagnetic spectrum is a broad class of radiation. It includes gamma rays and X-rays, with extremely short wavelengths measured in angstroms (A=10"10 m), as well as visible and infrared (IR) wavelengths of 10~7 to 10~3 m and the microwave region from 0.1 to 30 cm. Finally, it ranges into the radio spectrum, with wavelengths as long as kilometers. As the figure shows, satellite systems operate over the entire spectral range. Normal wavelengths for comsats, radars, and microwave radiometers range from approximately 1 meter to 1 millimeter, whereas visual and IR systems operate from around 0.35 to 100 microns (1 micron = 10-6 m = 1 |im).

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