The ACTS Revolution
The contributions of the Advanced Communications Technology Satellite (ACTS) to the field of satellite communication can only be called revolutionary, when compared with the incremental progress previously made in commercial communication satellites.
A brief review of the history of satellite communications will serve to illustrate the significance of ACTS. Satellite communication began in 1960 with the passive Echo balloon, followed in 1962 by the Telstar satellites, developed and funded by AT&T. Telstars were equipped with active repeaters and demonstrated transmission of voice, fax, and television signals from an elliptical orbit.
To prove the feasibility of using the geo-synchronous orbit for worldwide communications as proposed by Sir Arthur C. Clarke, NASA and the US Air Force sponsored the Syncom series of geo-synchronous satellites, which provided national coverage in 1963 and international TV transmission from the Tokyo Olympics in 1964. With the ATS satellites in 1966, NASA pioneered the use of the C Band (4-6 GHz) frequencies and helped set into motion a commercial satellite industry.
In 1976, in conjunction with the Canadian Department of Communications, NASA placed in orbit the first CTS satellite, which opened the Ku frequency band (12-17 GHz) for commercial use. By this time, the Washington D.C.-based International Satellite Consortium (Intelsat) along with private operators had launched more than thirty satellites into geo-stationary orbit, using the technology base established by NASA.
As new satellites were built, incremental improvements were made in the number of communication channels (transponders) on board, power output was increased, spacecraft life in orbit was extended, and cross-polarization was used to double the number of active transponders in a satellite at a given orbital location. Over time, largely due to the success of the commercial players, NASA turned its attention to space applications other than communication.
The lack of new developments by American industry raised concerns about foreign competition and the worry that the US might lose its technological lead in space-based communication. These concerns, along with the expected saturation of the orbital arc, caused the U.S. President Jimmy Carter to direct NASA to give further attention to satellite communication research and development, including the exploration of new electromagnetic frequencies that could be used in communication via space.
In the late seventies, NASA undertook a series of studies, surveys and industry consultations to decide what projects might be undertaken. It conducted proof of concept tests with several advanced technologies and components that led to the establishment of the ACTS experimental platform. ACTS would make voice, data and video communication available via satellite directly to users, bypassing installations that could only be afforded by the telecommunications carriers, a concept that would revolutionize earth/space/earth communication.
The project faced numerous technical, financial and political hurdles in its long development period, but following a successful launch into geo-synchronous orbit at 100 degrees West in 1993, ACTS went on to successfully demonstrate an unprecedented number of breakthrough technologies. These included onboard processing and switching which helped to integrate digital services and applications, hopping spot beams that enabled frequency reuse, transponder bandwidths of 1 GHz which made satellite competitive with fiber optic transmission, and the opening of the Ka frequency band at 20-30 GHz for public/commercial use in both fixed and mobile communication. Results of experiments with ACTS technologies have been the inspiration for a new generation of wideband systems proposed to operate in Ka Band around the world.
The NASA-sponsored ACTS Experiment Program was terminated in 2000 and the ACTS satellite should have come to the end of its useful life. Instead, NASA offered it to any institution that would use it for education, as long as it could pay the operating costs. The spacecraft was parked in a geosynchronous orbit at the gravity well at 105.2 degrees West longitude, where it remains operational and out of the way of active satellites.
A consortium led by Ohio University took up NASA's challenge and has been operating ACTS from that location, from which the satellite is still providing service to its original spot beam locations by autonomously adjusting its pointing to counteract the effects of the orbit's inclination. Ohio University has integrated ACTS into its curriculum and assists the members of the Consortium in conducting their ongoing communications experiments.
The purpose of this special issue of the Online Journal of Space Communications is to provide a retrospective of the ACTS program, by making available on the web some of the best works that detail its development. Our hope is that this issue will be a resource for satellite professionals, for faculty, for students and for others around the world who want to learn about ACTS and the latest techniques in satellite communication.
Within these pages, the managers, scientists, engineers and policy makers who were involved in these significant events document the ACTS development history, its technologies, its key experiments and applications and note their significance. Since the Journal is online, we are eager to have your input. Please let us know how we can improve the usefulness of this issue to you and to our other readers.
NASA's Advanced Communications Technology Satellite (ACTS)
From the Guest Editor