Satellite Business Systems Serves Corporate Customers
||2.16 m (7 ft 1 in.)
||2.82 m (9 ft 3 in.)
||6.6 m (21 ft 8 in.)
|Initial on-station weight
||540 kg (1300 lb)
In March 1981, Satellite Business Systems began delivering integrated voice, data, electronic mail, and video communications transmissions over the first all digital domestic commercial communications satellite system operating in the 14/12 GHz K-band.
When purchased, the SBS satellites were jointly owned by subsidiaries of IBM, Comsat General Corporation, and Aetna Life and Casualty. The SBS spacecraft are among the Hughes 376 series of satellites built by Hughes Aircraft Company's Space and Communications Group, known today as Boeing Satellite Systems, Inc., and designed for SBS to provide private business communication services to large American companies. SBS ordered three satellites in December 1977. Two of the satellites, launched November 15, 1980, and September 24, 1981, respectively, are in synchronous orbit 22,300 miles above the equator. The third SBS satellite became the first to be placed in space by NASA's Space Shuttle on November 11, 1982, during its premier commercial flight. A fourth satellite, ordered in late 1981, was launched on the Space Shuttle on August 30, 1984. A fifth SBS was launched on an Ariane rocket on September 8, 1988.
Stowed (left); In Orbit (right)
Headquartered in McLean, Virginia, SBS was the first of a growing list of customers to purchase the Hughes 376, a spin-stabilized satellite with two concentric telescoping solar panels and a folding antenna that conserve space on the launch vehicle. With more than 20 years of Hughes technology behind it, the SBS satellite has 10 channels with a capacity for 1250 two-way telephone conversations per channel, 10 simultaneous color television transmissions, or a combination of the two. The data rate is 480 megabits per second. The satellite's signal beam covers the continental United States, with higher gains focused on the densely populated eastern and western portions of the country.
The system also includes small, radio frequency (RF) ground terminals, of which Space and Communications Group's Satellite Ground Equipment Division in El Segundo built 100. The terminals are equipped with either a 5.5 meter or a 7.7 meter antenna and are designed to be mounted on the rooftops of SBS and its customers' facilities, or on the ground adjacent to them.
Through its solar array of K7 solar cells, which generate 19.7 milliwatts per square centimeter, the Hughes 376 offers SBS 1118 watts of dc power at beginning of life during the autumnal equinox. The power level on the fourth satellite is reduced to 1078 watts through the use of K4 3/4 solar cells in conjunction with the K7 cells. Two nickel-cadmium batteries furnish power during solar eclipse. The design mission of each satellite is 7 years.
When stowed for launch in the Space Shuttle, the satellite is 7 feet 1 inch in diameter and 9 feet 3 inches high. During operation at its orbital slot, the satellite is 21 feet 8 inches in height. The aft solar panel deploys to more than double the power output, and the antenna erects. With a 250 pound load of propellant used to keep the satellite on station and properly aligned with the earth, the spacecraft weighs 1300 pounds at beginning of life. On-orbit stationkeeping is provided by four thrusters.
The satellite's transmit and receive beams are created by a 6 foot shared aperture grid antenna with two polarization selective surfaces. The front surface is RF-transparent to vertically polarized beams bounced off the rear reflector, which is RF-transparent to horizontally polarized beams. Separate microwave feed networks are used for the two polarizations. Signal strength exceeds 43.7 dBW in the primary eastern coverage zone. Each channel is 43 MHz wide, and the transponders use 20 watt multicollector traveling wave tube amplifiers.
The satellites used a McDonnell Douglas payload assist module (PAM), which is attached to the spacecraft and performs the conventional third stage rocket function of insertion into an elliptic transfer orbit. The apogee motor, used to place the satellite into a near synchronous orbit, is a Thiokol Corporation Star 30 solid propellant rocket. The satellites drift into final orbit and are placed in operating position through the use of hydrazine thrusters.