Communications requirements derived from the human and robotic exploration vision dictate a sophisticated, layered architecture that is dramatically different than what was required for the low-rate, point-to-point communications of the Apollo era. Emerging, but relatively near-term, exploration initiative needs include teleoperation and autonomous robotic missions, lunar reconnaissance and orbiting relay satellites, cooperative spacecraft, lunar surface wireless local area networks, extremely wide bandwidth links to support hyperspectral imaging, synthetic aperture radar, and other novel applications, such as high-definition television and telemedicine. For example, data rates on the order of 1 gigabit per second may be required from Mars areostationary relay satellites. Such enormous data rates and extreme link distances will require 10-meter-class microwave antennas, likely operating at Ka-band frequencies. The NASA Glenn Research Center has been involved in several efforts to meet these requirements. This year, Glenn and SRS Technologies developed a 4- by 6-m offset-parabolic inflatable membrane reflector. Glenn, SRS, and the Georgia Institute of Technology developed test apparatus and software to demonstrate that a novel ground station composed of an array of relatively small apertures can economically replace a single, expensive tracking ground station. In addition, Johns Hopkins University delivered a prototype 2-m version of an antenna based on a novel shape-memory composite structure.
Critical performance specifications for the very large antennas needed include low aerial density (~1 kg/m2), high packaging efficiency (i.e., a ratio of deployed-to-stowed volume greater than 10:1), accurate surface geometry to ensure high reflector efficiency (root mean square surface errors of less than 1/20th of the operating wavelength), and of course reliable deployment. The NASA Glenn Research Center is developing two parallel approaches: inflatable membranes with a rigidized torus and a hybrid shape-memory composite approach that incorporates a fixed central reflector that serves as a backup antenna in case of deployment problems with the main aperture. Glenn is partnering with SRS Technologies (Huntsville, AL) on the former and with Johns Hopkins University (Laurel, MD) and ILC Dover (Frederica, DE) on the latter. The goal is to develop large gossamer antennas for deep-space, Moon, and Mars applications that have very low mass, lower cost, and improved deployment reliability while maintaining very accurate surface tolerances over long deployed durations.
SRS 4- by 6-m inflatable reflector in Glennís near-field antenna range.
The 4- by 6-m offset-parabolic inflatable membrane reflector developed by Glenn and SRS Technologies (see the preceding photograph) produced a gain of about 48 dB at 8.4 GHz at Glennís near-field range. This corresponds to about 67-percent efficiency when compared with an ideal antenna using the same feed. Phase plots of the aperture more or less agree with the measured 3.5-mm average accuracy of the surface shape. The extreme antenna edges show some significant phase changes that are implicit with an inflatable lenticular geometry (i.e., flattened edges). Surface-shape accuracy can be improved through inflation pressure adjustment and/or perimeter attachment (catenary) tension adjustment.
During the late summer of 2005, Glenn, SRS, and the Georgia Institute of Technology developed a test apparatus and software to validate an adaptive beam-forming algorithm that synthesizes correlated aperture channels into a single signal. The goal is to demonstrate that an array of relatively small apertures can economically replace a single, expensive tracking groundstation. The experiment involved an array of four inflatable 1-m-diameter membrane reflectors mounted onto a very low cost tracking pedestal (see the final photograph). The novel ground station successfully collected data from the SAC-C low-Earth-orbiting satellite. Data analysis is underway.
Four-element inflatable membrane reflector array on a tracking pedestal at Georgia Tech as it prepares to track the SAC-C low-Earth-orbiting satellite.
The shape-memory antenna system combines a fixed parabolic dish with an inflatable reflector annulus. This antenna, based on a novel shape-memory composite structure, provides a scaleable high-gain antenna architecture and is competitive with mesh and membrane reflectors in terms of cost, aerial density, and packing volume. The prototype 2-m antenna was delivered by Johns Hopkins University to Glenn in April 2005. The dish surface profile was validated with a coherent laser radar capable of mapping the surface shape to ±25-μm accuracy. This proved to be a low-cost and highly accurate method to quickly validate the reflector and tune the perimeter attachments (catenaries) before proceeding with other tests. Initially, some wrinkling was observed near the inner ring and rim, but most wrinkles were eliminated upon retensioning and inflating the reflector to a pressure of about 5 in. of water. Testing indicated a root mean square surface error of 0.106 in. Removing problem areas near the outer rim resulted in a root mean square error of about 0.035 in.
Pearson, J.; and Romanofsky, R.: Thin Film Antenna Development and Optimization. AIAA-2006-2229, 2006 (to be published).
Ingram, M., et al.: Optimizing Satellite Communications With Adaptive and Phased Arrays. Presented at ESTC 2004, Palo Alto, CA, 2004.Glenn contacts: Dr. Robert R. Romanofsky, 216-433-3507, Robert.R.Romanofsky@nasa.gov; and Dr. Félix A. Miranda, 216-433-6589, Felix.A.Miranda@nasa.gov
Last updated: October 12, 2006
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