Aiming large satellite dishes can be difficult, even when locating a geostationary satellite for the first time. Now imagine aiming an 11-meter dish at a non-geostationary satellite as it passes overhead. That's the problem users of Earth observing satellites face when receiving high-resolution images from these orbiting satellites at high data rates. Currently data from satellites such as Earth Observing-1 (EO-1) is downlinked using motorized 11-meter dishes with complex aiming mechanisms, primarily located in the Arctic Circle. Mary Ann Ingram, a professor in Georgia Tech's School of Electrical and Computer Engineering and her team of researchers have come up with a way to receive data from these satellite using a phased-array of smaller, less expensive antennas and digital signal processing software to shape the antenna pattern, track the satellite and minimize interference.
The Georgia Tech research news article Data from Space: Adaptive Array Network Could Improve Access to NASA's Earth Observing Satellites has limited technical information on the system, but is worth checking out for the photos of the helical antennas constructed with PVC pipe and aluminum foil. The test system using these helical antennas works at S-band and is limited to lower data rates. "We have demonstrated the lower rates in S-band and during the upcoming year we will work on X-band for higher rates. Ultimately, we would like to target data rates in the range of 300 megabits per second. The technology could also be used in future research to implement Ka, thus enabling even higher data rates," said Dan Mandi, mission director for NASA's EO-1 program at the Goddard Space Flight Center.
In addition to the work with the phased antenna array, the Georgia Tech researchers are also planning to test inflatable dish antennas provided by NASA Glenn Research Center in collaboration with a contractor; demonstrate a 6 Mbps downlink from NASA's SAC-C satellite; and test electronically steered antennas being developed at the University of Colorado under a grant from NASA Glenn Research Center.
Commenting on the potential for reducing the need for those 11-meter dishes in the Arctic, Dan Mandi said, "If you are in the Arctic and the motor moving your dish breaks down, it may take a few weeks to fix it. If this could be done with no moving parts using techniques of digital signal processing and software radio, one of the most desirable features will be a high level of reliability. That's important for space applications and locations where you can just put equipment out there and not require an operator or maintenance crew."
A NASA Web site, Earth Observing-1 Sensor Web / Testbed Initiatives describes the research and includes a link to the paper Optimizing Satellite Communications With Adaptive and Phased Array Antennas that has more detailed technical information on the system, including system drawings and a spectrum analyzer plot of the received signal and local interference.
