science missions. Much of the foregoing discussion regarding telescopes, pointing and scanning mechanisms, and electronics technology is equally applicable to both active and passive Earth observation sensors, but the design trade-offs and sizing constraints for active sensors are also heavily influenced by the technology of the active sources themselves—most specifically, lasers. In this regard, the overall "DC to photons" energy conversion efficiency is probably the most important factor in accommodating active Earth observing sensors on small satellites. This technology encompasses everything from high-efficiency solar arrays and DC to DC power converters to the efficiency of the laser devices themselves. These efficiencies vary widely depending on the wavelength of interest, but are typically in the range of a few percent for laser sources that are of interest to space science. The efficiency of the energy conversion process is the fundamental issue that will determine the future feasibility of small satellites to serve as platforms for sensors such as the Laser Atmospheric Wind Sounder.

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) is increased, transmitted power can be lowered, but the physical size of the antenna must grow proportionately to achieve the higher gain. There is thus a size/mass/power trade-off for active microwave systems. Pointing and scanning design issues comparable to those related to optical instruments are also involved.



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