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Fig. 9-26. Sample Characteristics for Sample FIreSat Sensor Described In Table 9-15 In Sec. 9.5.5. A shows the point spread function of the sensor with respect to the pixel size (shown as a horizontal bar). B shows the modulation transfer functions of the sample Instrument C Illustrates optical transmission of the atmosphere over the operating wavelength of the sensor. D shows the total upwelling radiance through the atmosphere across the operating bandwidth of the sensor.

A second challenge with the design is the relatively poor signal-to-noise ratio of 88. If we consider a pixel at die limit of the field of view, then radiometric information becomes indistinguishable from noise. From this point of view our current design is at the limit of feasibility and may require changes to meet the SNR requirements. Finally the F-number of the optics of 2.7 (driven by the focal length of 0.7 m and an aperture diameter of 26 cm) is quite a demanding optical design.

We now select a "similar instrument" from Table 9-13 for our FireSat example. We appear to have two options: the Thematic Mapper or the Multispectral Mid-IR instrument We tentatively select the Multispectral Mid-IR as our similar instrument and will scale from its fundamental parameters of 1.5 m x 1 m diameter, 800-kg weight and 900 W power, for its 1-m aperture. We first compute the aperture Tatio,

With this fundamental ratio, we now estimate the FireSat payload parameters as Size = 0.4 m x 0.3 m diameter Weight = 2 x 800 x 0.263 = 28 kg Power = 2 x 900 x 0.263 = 32 W

As described in Sec. 9.5.3, we have incorporated a factor of 2 to provide margin for having substantially scaled down the payload size. The estimate of the linear dimensions needs to be adjusted as well to allow for the size of the scanner which will need to be mounted in front of the sensor optics and electronics. A rough estimate of the scanner dimensions is the same size as the payload estimate. Thus, as summarized in Table 9-17, the budgeted dimensions for the optics plus scanner is 0.8 m long x 0.3 m diameter. Thus, our first guess is that the FireS at payload is a moderately sized instrument and could fit well on a small to medium-sized spacecraft

TABLE 9-17. Summary of FireSat Initial Parameter Estimâtes.

Parameter

Nominal Value

Comments

Altitude, h

700 km

Range = 600 to 800 km

Inclination, /

55 cteg

Coverage to 65 deg latitude

Swath width, 2Amax

24.2 deg

20.1 deg required

Nadir angle range, T)

±57.9 deg

0 0

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