Technology
System Overview
 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.
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