ACTS Technology

Communication satellites provide a unique perspective with which to view the earth's surface. At the geostationary altitude of 22,240 miles above the equator, they appear motionless in the sky. Serving as giant relay towers, they interconnect users in vast areas of the world who are within their continental field of view.

Due to interference considerations, communication satellites must maintain a certain separation. Therefore, only a limited number of satellites can be placed in geostationary orbit to provide communications for a region such as the United States. In addition, only certain radio frequency bands, assigned by international agreement, are available for commercial communication satellite use. The extraordinary success of satellite communication in the late '70s and early '80s threatened to exhaust both the available frequencies and the geostationary satellite positions for many regions in the world. New technology was needed to provide for this projected increase in demand. In addition to finding ways to use the existing frequency bands more efficiently, operations in the next higher frequency band (the Ka-band) were deemed necessary.

NASA’s ACTS program provided new technology for increased efficiency using all radio frequencies including Ka-band. Increasing the spectrum efficiency was achieved by developing high-gain, multiple spot beam antennas and onboard switching and processing that allowed for a great increase in the number of times the same frequency could be reused by a single satellite [24]. In addition, the high-gain spot beams provided the very desirable benefit of allowing for smaller aperture user terminals at higher data rates.

NASA and the U.S. commercial satellite industry jointly defined the ACTS program. ACTS was not intended to be an operational system. It was designed to be a test bed for verifying those advanced technologies that were beyond the ability of any one satellite company to finance. In the early 1980s, the U.S. satellite carriers had great concerns about the reliability of ACTS’ advanced, high-risk technology. Companies felt that a flight test was necessary to prove the technology was feasible before they would incorporate it into their commercial systems.

The ACTS program was designed to allow U.S. industry the opportunity to meet the communication needs of the twenty-first century while remaining competitive in the international satellite communication marketplace. The motivation for the program and its merits are discussed in the Gedney et al book on ACTS, Chapters 1, 7, 8, and 9. This chapter describes the technological advances made.

ACTS System Overview

ACTS is an in-orbit, advanced communication satellite test bed, bringing together industry, government, and academia in a wide range of technology, propagation, and user application investigations. NASA’s Lewis Research Center (LeRC) awarded the ACTS contract in August of 1984 to an industry team consisting of:

	Lockheed Martin (then RCA), East Windsor, New Jersey - for system integration and the spacecraft bus
	TRW, Redondo Beach, California - for the spacecraft communication pay-load
	COMSAT Laboratories, Clarksburg, Maryland - for the network control and master ground facility
	Motorola, Chandler, Arizona - for the baseband processor
	EMS Technologies (formerly called Electromagnetic Sciences), Norcross, Georgia - for the spot beam forming networks

The contract was actually awarded to RCA Astroelectronics of East Windsor, New Jersey (which was subsequently acquired by General Electric (GE), then by Martin Marietta, and is currently part of Lockheed Martin). In 1988, as a result of a congressionally mandated program funding cap, Lockheed Martin (General Electric Astro Space at that time) assumed responsibility for completing the development of the communication payload. Subsequently, Lockheed Martin subcontracted with Composite Optics, Inc., in San Diego, California, for the manufacture of the antenna reflectors and part of the bus structure.

ACTS was launched into orbit by the space shuttle Discovery (STS-51) on September 12, 1993, and achieved geostationary orbit at 100 degrees west longitude on September 28, 1993. As of the printing of this book, ACTS is still operating, but the in-orbit stationkeeping fuel has been depleted. Operations continue with an inclined orbit, using an autonomous, onboard program that provides a bias in the roll axis to offset the inclination and maintain the spot beams properly located on the ground.

The ACTS system is made up of a spacecraft and ground segment [25-28]. The spacecraft consists of a multi-beam communication payload and the spacecraft bus. The key technological components of the communication pay-load are the multi-beam antenna (MBA) assembly, the base band processor (BBP), the microwave switch matrix (MSM), and Ka-band components. The spacecraft bus houses the communication payload and provides attitude control, electric power, thermal control, command reception, telemetry transmissions, and propulsion for stationkeeping.

A NASA ground station (NGS) and master control station (MCS), collocated at the Lewis Research Center (LeRC) in Cleveland, Ohio, transmit commands to the satellite, receive all spacecraft telemetry, perform ranging operations, and provide network control for all user communication. The NGS/MCS process and set up all traffic requests and assign traffic channels on a demand basis. A satellite operations center was located at Lockheed Martin Astro Space in East Windsor, New Jersey, and connected to the NGS/MCS via landlines.

ACTS deployment from space shuttle Discovery

In June of 1998, the Satellite Operations Center was transferred to the Lockheed Martin Communications and Power Center facility in Newtown, Pennsylvania. The Satellite Operations Center 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. The Lockheed Martin C-band command, ranging, and telemetry station at Carpentersville, New Jersey, provided transfer orbit support during launch and originally served as an operations backup to the Satellite Operations Center. In 1998, however, the backup function was transferred to the GE American Communications station in Woodbine, Maryland.