An adaptive antenna array system being investigated by NASA, Georgia Tech, and the University of Colorado at Boulder researchers could dramatically decrease the cost of building and maintaining ground stations, and yet disseminate Earth-observing satellite data more widely and quickly. Amazingly, the proof-of-principle arrays are constructed from PVC pipe, aluminum foil, and copper wire. Sophisticated signal-processing techniques are the key. Satellites such as Earth Observing-1 (EO-1) are typically in contact with antenna systems five to eight times a day for 10 min. at each transmission. Their data are downlinked to various 11 m dishes, primarily in the Arctic Circle. The systems cost about $4 million each, and require resident crews to maintain their complex aiming mechanisms. Replacing the existing system with inexpensive arrays of 0.75 m dishes that have no moving parts and use software instead of mechanical aiming would be a major advance. Dan Mandl, mission director for NASA's EO-1 program at Goddard Space Flight Center, envisions a "continuous cell-like network around the world that would provide almost unlimited opportunities to downlink data." The arrays could potentially communicate with more than one satellite at a time, with software separating the signals.
In the new arrays, signals from four antennas are analyzed using adaptive processing that combines scattered and reflected versions of the signal and suppresses noise and interference. Front-end hardware and antenna aiming are eliminated, and ground station location becomes more flexible.
The small apertures will be connected with digital signal processing, Ingram says. "A smaller aperture has a wider beam, so the tracking requirement won't be as great. They may pick up interference, especially in tracking a satellite at a low elevation angle, but because we combine multiple apertures, we can null out the interference."
Although the individual arrays will not offer the data rate of NASA's larger structures, their numbers and wide deployment will compensate. For instance, two ground stations with seven 0.75 m dishes or eight electronically steered antennas could match the data rate of one 11 m dish. After developing a filter to block interference from terrestrial repeater stations used by satellite radio broadcasts, the researchers were able to downlink EO-1 data in the S-band. The next step will be to ramp up to X-band, with 300 Mbps rates and eventually to implement Ka for even higher rates.
The team is also looking at ways to extend satellite reception time with techniques such as array-based synchronization and optimization of the tilt angles of the antennas' planar apertures. Success could quadruple the download capacity for an 8-antenna ground station.