The Role of Small Satellites in NASA and NOAA Earth Observation Programs (2000) / Chapter Skim
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Appendix B: Effects of Technology on Sensor Size and Design
Appendix B: Effects of Technology on Sensor Size and Design
Pages 69-76
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From page 69... ...
SCANNING MECHANISMS Many sensors employ a scanning mechanism in order to cover a wide field of view in object space while utilizing narrow field-of-view telescope optics. The current generation of Geostationary Operational Environmental Satellites, Polar-orbiting Operational Environmental Satellites, Defense Meteorological Satellites, and Land Remote Sensing Satellite (Landsat)
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From page 70... ...
, because the extended focal planes of such designs~oupled with relatively short focal lengths lead to unworkable field angles and geometric distortion problems. Moving the entire spacecraft with the telescope assembly remaining fixed relative to the body of the spacecraft is yet another scanning alternative, and is particularly suitable for single-sensor small satellites.
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From page 71... ...
The effects are further magnified because the layout dimensions and alignment tolerances strongly influence the size and mass of the associated optical metering structure the structure that holds the optical elements in their required positions with the requisite precision. Most of the remote sensing instruments currently in development or production have exploited advances in optical design techniques in order to minimize size and mass.
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From page 72... ...
Specifically, sparse aperture or partial aperture systems can provide diffraction limited performance that is comparable to that of filled aperture systems-albeit at the expense of photon-collecting area and, hence, signal-tonoise ratio. This may be a reasonable trade-off for systems in which resolution is more important than radiometric sensitivity.
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From page 73... ...
As filter packaging techniques have improved, there have been some modest reductions in the volume of the aft optics and focal plane assemblies of these sensors, and some designers have explored direct deposition of spectral filters onto detector arrays. Although this is technically feasible, it is not in widespread use for radiometric instruments because it effectively cascades two complex processes, resulting in lower yields and consequently higher costs; the minor gains in packaging efficiency are generally not worth the added cost and risk.
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From page 74... ...
Technology now permits the design and production of integrated detector arrays that include first-level preamplification and multiplexing as part of the focal plane; this in turn permits simplification of the sensor's other electronic subsystems. For example, the focal plane arrays on the MODIS sensor utilize integral capacitive transimpedance amplifiers to integrate and amplify the detector signals; this eliminates the need for much larger off-focal-plane electronic modules that would otherwise be required to perform that function.
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From page 75... ...
ELECTRONICS TECHNOLOGY Advances in electronics technology probably offer the most broadly and readily applicable avenue for further reductions in the size/mass/cost of space sensors. A substantial fraction of nearly every sensor design is devoted to the electronics that provide preamplification, filtering, analog-to-digital conversion, on-board signal processing, and high-speed multiplexing functions, as well as command, control, telemetry, and conditioned power.
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From page 76... ...
Similar efficiency issues pertain to active microwave sensors, although there are additional degrees of design freedom wherein transmit power and antenna gain can be traded to achieve equivalent levels of effective isotropic radiated power. As antenna gain (directivity)
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