| Abstract: |
Passive microwave remote sensing is currently utilized by NASA, NOAA, USGS, and others to conduct earth science missions, including weather forecasting, early warning systems, and climate studies. Due to budgetary constraints and lack of reliable access to medium-class lift vehicles, the current trend in the space industry is towards smaller, cheaper, and more frequent missions. Nano-satellites, such as CubeSats, are gaining in popularity due to their low cost and ease of deployment. These miniaturized platforms impose severe constraints on the size, weight, and power (SWaP) of the payload. Conversely, relatively large apertures are required to achieve desired spatial resolution at microwave frequencies. Phase Sensitive Innovations’ (PSI) distributed aperture array sensor technology has the potential to dramatically reduce the SWaP of such microwave radiometers, thus enabling deployment of large RF apertures on spaceborne platforms. Our innovative approach employs distributed aperture imaging (DAI) with optical upconversion of the incoming microwave radiation and subsequent coherent optical reconstruction of the microwave scene. The electro-optic modulation technique converts received millimeter-wave radiation into sidebands on an optical carrier, a process which preserves amplitude and phase information from each sensor node as required for subsequent image reconstruction. Additionally, the optical upconversion approach is extremely broadband; our lithium niobate electro-optic phase modulators demonstrate high conversion efficiency over a range from <1 GHz to over 300 GHz, enabling radiometer operation at many of the current and future sounding frequencies. Furthermore, optical upconversion allows for the use of lightweight, flexible fiber optics for the routing of optical energy both before and after millimeter-wave encoding, thereby eliminating the need for bulky and lossy RF distribution cables. Optical processing techniques are utilized to provide real-time correlation engines using simple lenses and cameras, thus avoiding the need for significant post-processing with requisite power-hungry computational resources. The imaging radiometer provides the benefits of large apertures (i.e. high resolution) in an essentially two-dimensional form factor that does not scale volumetrically with aperture size, as do conventional contiguous aperture or focal plane array imagers. Since the phase of each receiver node can be freely adjusted at high update rates, the array can be phase-locked to implement the interferometric imaging technique. Arbitrary phases can be applied as required for beam forming, beam steering, focusing, and point spread function engineering. Furthermore, these phases can be adjusted as needed to compensate for mechanical deflections of the supporting structures. These properties make the aperture synthesis approach superior to the real aperture approach – no ultra-precise surface accuracy, alignment, or mechanical scanning is required. Besides easing implementation on small satellites, PSI’s imaging radiometer provides capabilities beyond those currently available on conventional microwave sensors, most notably the ability to generate real-time, two-dimensional radiometric imagery with no mechanical scanning. Our microwave photonic technology not only greatly reduces the SWaP of these radiometer instruments commensurate with deployment on emerging platforms, but also reduces the cost and complexity while increasing reliability and performance.
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