The world's largest commercial communications satellite is in orbit

Tim Furniss/LONDON


Galaxy XI, the world's biggest commercial communications satellite, has been operating for PanAmSat in geostationary orbit (GEO) following its launch in December aboard an Ariane booster. Its purpose is to carry video and telcommunications services to North America and Brazil.

The vehicle, designated HS-702, is the first to be launched from a new generation of spacecraft which Hughes Space and Communications (HSC) began to develop in October 1995. Development will be continued by Boeing, which is to acquire HSC under a $3.25 billion deal announced last month. The HS-702 is an evolution of the three-axis, stabilised HS-601, offering satellite operators a giant of a spacecraft in terms of size, performance and cost efficiency, HSC says.

The acquisition of HSC will make Boeing the leading manufacturer of communications satellites and boost its space business to $10 billion a year. Its sale will accelerate "the transformation of Hughes into an entertainment, data information services and distribution company", says chairman and chief executive Michael Smith.

Satellite backlog

HSC, meanwhile, has a backlog of 36 satellites in production, worth $4 million. The company has built 182 spacecraft, of which 160 have been communications satellites. The flagship Galaxy XI is equipped with 64 transponders and operates at 10kW. There are 40 Ku-band transponders and 24 operating in the C-band.

Galaxy XI is the first of seven new satellites Hughes is manufacturing for PanAmSat. Another two Galaxys will be HS-702s and the rest will be HS-601s or spin-stabilised HS-376 models. PanAmSat's network of 19 satellites use various spacecraft buses. All the Galaxys will be launched by the middle of next year. Hughes is building nine other HS-702s as part of the 36-spacecraft backlog, most of which are HS-601s.

The giant Galaxy XI is 31m (100ft) long and 9m wide. It weighed 4,500kg (9,900lb) at launch, slimming to 2,775kg once it used its propellant to reach its operational position.

HSC began development of the HS-702 to deliver payloads exceeding 90 active transponders, in any communications frequencies that customers require, with power levels starting at 10kW, climbing to a maximum of 15kW. The craft is adaptable for use in medium Earth orbit and GEO.


A standard on the HS-702 is a xenon ion propulsion system (XIPS) pioneered by Hughes. This is used to maintain the spacecraft in the correct position to place its antennas for customers' receivers.

XIPS is 100 times more efficient than conventional liquid bi-propellant systems. The XIPS' four 25cm thrusters require only 5kg of propellant each year, a fraction of what chemical or arcjet systems consume. Its fuel efficiency allows up to 90% of the spacecraft's propellant mass to be eliminated. Customers can apply the weight savings to increase the revenue-generating payload, to prolong service life or to use a less expensive launcher when cost is based on satellite weight.

Another of the HS-702's features is the solar array. Angled reflector panels along both sides of the solar wings form a shallow trough and concentrate the sun's rays on the solar cells. These high efficiency, dual gallium-arsenide cells supply twice the power of traditional silicon solar cells.

Efficient solar cells

HSC says the dual-junction solar cells are among the most efficient available, able to convert nearly one quarter of the sun's rays into spacecraft power. Separating the bus and payload thermal environments and substantially enlarging the heat radiators achieves a cooler, more stable thermal environment for bus and payload, increasing unit reliability.

Deployable radiators use flexible heat pipes, which increase the packageable radiator area and make it possible to use more of the radiator's surface for heat dissipation.

Source: Flight International