A report discusses a proposal to use thermal (long-wavelength infrared) images of the Earth, as seen from spacecraft at interplanetary distances, for pointing antennas and telescopes toward the Earth for Ka-band and optical communications.
The purpose is to overcome two limitations of using visible images: (1) at large Earth phase angles, the light from the Earth is too faint; and (2) performance is degraded by large albedo variations associated with weather changes. In particular, it is proposed to use images in the wavelength band of 8 to 13 µm, wherein the appearance of the Earth is substantially independent of the Earth phase angle and emissivity variations are small. The report addresses tracking requirements for optical and Ka-band communications, selection of the wavelength band, available signal level versus phase angle, background noise, and signal-to-noise ratio. Tracking errors are estimated for several conceptual systems employing currently available infrared image sensors. It is found that at Mars range, it should be possible to locate the centroid of the Earth image within a noise equivalent angle (a random angular error) between 10 and 150 nanoradians at a bias error of no more than 80 nanoradians.
This work was done by Gerardo Ortiz and Shinhak Lee of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category.
NPO-40395
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Thermal Imaging of Earth for Accurate Pointing of Deep-Space Antennas
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Overview
The document titled "Technical Support Package for Thermal Imaging of Earth for Accurate Pointing of Deep-Space Antennas" (NPO-40395) presents an analysis of the use of thermal imaging technology to enhance the accuracy of pointing deep-space antennas. This initiative is part of NASA's Commercial Technology Program, aimed at making aerospace-related developments accessible for broader technological, scientific, and commercial applications.
The core focus of the document is on the application of long-wavelength infrared (LWIR) imaging to capture thermal emissions from the Earth. This thermal data can be utilized to improve the geometric centroid estimates necessary for precise antenna pointing. The document outlines the expected benefits of using full Earth images, which can significantly enhance the tracking capabilities of antennas used in deep-space communications.
The analysis includes a detailed examination of various infrared imaging technologies, such as VO2 and VOx microbolometers, Si microbolometers, and HgCdTe detectors, among others. These technologies are evaluated based on their performance metrics, including temperature sensitivity, wavelength range, noise equivalent power (NEP), and resolution. The document emphasizes the importance of selecting the appropriate technology to optimize the performance of thermal imaging systems in space-based environments, where background noise levels are considerably lower than those on Earth.
Additionally, the document discusses the challenges posed by background radiation from non-optical surfaces within telescopes, which can interfere with the detection of thermal signals. To mitigate this, standard practices such as placing low-temperature detectors in cold enclosures and using cold stops to limit radiation from out-of-field sources are recommended.
Future work is suggested to refine the analysis of signal and noise, particularly for deep space missions beyond Mars. This includes the implementation and validation of specific algorithms to enhance the integration of thermal cameras with communication systems.
Overall, the document highlights the potential of thermal imaging technology to revolutionize deep-space communication by providing accurate pointing capabilities, thereby facilitating more effective data transmission from distant space missions. The insights and recommendations presented aim to guide future research and development in this critical area of aerospace technology.
