Exploring Array Reception of the EO-1 Downlink

Mary Ann Ingram, Thomas Pratt, Paul Steffes, and John Langley

A new adaptive antenna array architecture for low-earth-orbit satellite ground stations has been investigated. These ground stations are intended to to have no or a minimum of moving parts and could potentially be operated in populated areas, where terrestrial interference is likely.

The architecture includes multiple, moderately directive phased arrays. The phased array technology considered in this project is the space-fed lens (SFL), which consists of a low-cost printed circuit forming the lens, and an arrangement of feed antennas on the focal surface. For this project, we considered electronic steering in elevation only, so the feed antennas were arranged on a focal arc. With each SFL steered in the approximate direction of the satellite, the SFL outputs are adaptively combined to enhance the signal-to-interference-plus-noise (SNIR) of the desired satellite. Parameters in the system design include the size of each SFL, the number of SFLs, and the number of feeds per SFL.

An additional benefit of the multi-feed architecture of the SFL, as well as lens-type antennas in general, is that more than one satellite can be tracked at a time. If two co-channel satellites are in the field of view of the SFL, then the illuminated feeds can jointly demodulated. If there is any crosstalk between the signals because of non-zero sidelobes of the SFL beam pattersn, then multiuser detection techniques can be employed to suppress the crosstalk. When the satellites are in the same band, this approach amounts to the application of space division multiple access (SDMA) to LEO satellites.

In Phase I of the project, fixed directional elements are adaptively combined in a prototype to demodulate the S-band downlink of the EO-1 satellite. This phase was reported in the paper entitled "Optimizing satellite communications with adaptive and phased array antennas.”

Two additional demonstrations were performed: one using an array of inflatable dish antennas and the other using a space-fed lens. Both demonstrations used the X-band transmissions of the SAC-C satellite. The inflatable dishes were mounted on a single planar frame, which was steered by a Mead positioner. The Mead positioner had a telescope in the center of the array, and its calibration for each pass was performed the evening before on selected stars. The purpose of the SFL demonstration was to show that our tracking system worked. The SFL pointing direction was calibrated using GPS coordinates of the SFL and of one of the prominent buildings in the Atlanta, Georgia skyline. Then the ephemeris provided by the Heavenscape software was used to program a microprocessor controller of a stepper motor (for azimuth steering) and a switch network (for elevation beam switching).

To lower costs and reduce latency, a network of adaptive array ground stations, distributed across the United States, is considered for the downlink of a polar-orbiting low earth orbiting (LEO) satellite. Assuming the X-band 105 Mbps transmitter of NASA's Earth Observing 1 (EO-1) satellite with a simple line-of-sight propagation model, the average daily download capacity in bits for a network of adaptive array ground stations is compared to that of a single 11 m dish in Poker Flats, Alaska. Each adaptive array ground station is assumed to have multiple steerable antennas, either mechanically steered dishes or phased arrays that are mechanically steered in azimuth and electronically steered in elevation. Phased array technologies that are being developed for this application are the space-fed lens (SFL) and the reflectarray. Optimization of the different boresight directions of the phased arrays within a ground station is shown to significantly increase capacity; for example, this optimization quadruples the capacity for a ground station with eight SFLs. Several networks comprising only two to three ground stations are shown to meet or exceed the capacity of the big dish. Cutting the data rate by half, which saves modem costs and increases the coverage area of each ground station, is shown to increase the average daily capacity of the network for some configurations. The paper reporting these results is "LEO download capacity analysis for a network of adaptive array ground stations.”

Last revised on May 26, 2006.