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NASA¡¯s Evolution to Ka-Band Space Communications for Near-Earth Spacecraft
Over the next several years, NASA plans to launch multiple earth©\science missions which will send data from low-Earth orbits to ground stations at 1©\3 Gbps, to achieve data throughputs of 5©\40 terabits per day. These transmission rates exceed the capabilities of S-band and X©\band frequency allocations used for science probe downlinks in the past. Accordingly, NASA is exploring enhancements to its space communication capabilities to provide the Agency¡¯s first Ka©\band architecture solution for next generation missions in the near©\earth regime. This paper describes the proposed Ka-band solution¡¯s drivers and concept, constraints and analyses which shaped that concept, and expansibility for future needs.
For over five decades, NASA has launched and operated spacecraft for studying Earth and space phenomena. As both scientific knowledge and instrument technology have advanced during this period, the observation capabilities of these spacecraft have grown as well. The transmission rates and daily volumes of data collected by science probes orbiting the earth have correspondingly increased by several orders of magnitude. As the science data downlink rates have increased, so, too, have the signal bandwidths required to support these transmissions.
NASA, like all users of the radio spectrum, must respect frequency allocations established by the International Telecommunications Union (ITU). In ITU parlance, science data downlinks are designated as space-to-earth communications for the Earth Exploration Satellite Service or for the Space Research Service. The ITU has
allocated about 5 MHz for such purposes at S-band (approximately 2-4 GHz), 10 MHz for Space Science links and 375 MHz for Earth Science links at X-band (about 7-12 GHz), and 1.5 GHz for direct downlinks and 2.25 GHz space-to-space links at Ka-band (about 19-36 GHz). With growing bandwidths needed to support increasing data rates, downlink designs have evolved over the decades from S-band to X-band, and near-term future needs compel advancing further to Ka-band.
In advance of anticipated needs for Ka-band downlinks in the near-Earth region, NASA has developed a design for its networks operating in that region to support corresponding capabilities. This approach leverages three missions, presently under development, as references for understanding relevant needs and constraints for a solution enabling Ka-band near-Earth missions of the future.
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