Satellite Technology

The ACTS system is made up of a spacecraft and ground segment. The spacecraft consists of a multibeam communication payload and the spacecraft bus. The key technology components of the communications payload are the multibeam antenna (MBA) assembly, the baseband processor (BBP), the microwave switch matrix (MSM), and Ka-band components. The spacecraft bus houses the communications payload and provides attitude control, electric power, thermal control, command reception, telemetry transmissions, and propulsion for stationkeeping.

The ground segment is comprised of the spacecraft and communication network control stations and the user terminals. A Master Ground Station located at NASA Glenn Research Center (GRC) in Cleveland, Ohio transmits commands to the satellite, receives all spacecraft telemetry and provides network control for all user communications. As part of network control, it processes and sets up all traffic requests, assigning traffic channels on a demand basis.

A Satellite Operations Center was located at Lockheed Martin Astro Space in East Windsor, New Jersey (later transferred to the Lockheed Martin Communications and Power Center facility in Newton, Pennsylvania in 1998) connected to the Master Ground Station via landlines. It has the prime responsibility for generating spacecraft bus commands and for analyzing, processing, and displaying bus system telemetry data. Orbital maneuver planning and execution are also handled by the Satellite Operations Center.

ACTS is a three-axis stabilized spacecraft weighing 3250 pounds at the beginning of its on-orbit life. It measures 47.1 feet from tip to tip along the solar arrays and 29.9 feet across the main receiving and transmitting antenna reflectors. The ACTS multibeam antenna is comprised of separate Ka-band receive and transmit antennas, each with horizontal and vertical polarization subreflectors. The 7.2-foot, 30 GHz receive antenna collects uplinked signals, while the 10.8 foot, 20 GHz transmitting antenna radiates downlink signals. Antenna feed horns produce narrow spot beams with a nominal 120-mile coverage diameter on the surface of the earth.

Fast (less than 1-microsecond), beam-forming switch networks consisting of ferrite switches, power dividers and combiners, and conical multiflare feed horns provide sequential hopping from one spot beam location to another. These hopping spot beams interconnect multiple users on a dynamic traffic demand basis. A separate 3.3 foot, mechanically steered antenna, receiving uplink and radiating downlink signals, is used to extend the ACTS communication coverage to any location within the hemispherical field of view from ACTS' 100 degree West longitude position. Beacon signals at 20.2 GHz and 27.5 GHz are radiated from two small, separate antennas.