Inside the Inmarsat-4 Inmarsat has used the seven years since its last satellite launch to develop a new breed of super-satellite - the I-4 - which will usher in the next generation of broadband mobile satellite services. Working with an international team of space technologists, Inmarsat has created the biggest and most powerful commercial communications spacecraft ever. From the giant wingspan of its solar array wings, to the raw power and sophistication of its digital signal processor, the I-4 is a unique satellite.
	I-4 at a glance Constructed of flight-proven technology that promises at least 10 years of service life, the Inmarsat-4s will speed through space at 6,875mph. The satellites will sit in geostationary orbit, where they will be synchronized with the rotational speed of the Earth and appear to 'hover' over the Equator. They will bring about a 16-fold increase in the traffic-bearing capacity of the Inmarsat network - and will extend high-speed data into space, creating a truly global broadband network. Much of the I-4 traffic will be carried as Internet Protocol (IP) packet-switched data - the 'language' of the Internet. But the network will also be powerful enough to support circuit-switched applications, such as voice, ISDN and fax. All the benefits of broadband will be open to Inmarsat users, including simultaneous voice and data, web browsing, e-mail, file transfer and access to local area networks (LANs). Aside from their raw power, the uniqueness of the I-4s lies in their ability to generate hundreds of high-power spot beams. These can quickly be reconfigured, and focused anywhere on Earth to provide extra capacity where needed. The contrast with their predecessors, the Inmarsat-3s, is massive. Each I-4 will generate 19 wide beams and more than 200 narrow spot beams, compared with only seven wide beams on the I-3. It will also bathe the Earth in a single global beam, which provides an initial signaling link for all services. Key features Every aspect of the spacecraft has been tailored to meet the demands of 24/7 operations in the harsh vacuum of space, where temperatures can fluctuate within minutes from -150 degrees centigrade in solar eclipse to +150 degrees in the full heat of the Sun. Some of the spacecraft's most impressive features include: The I-4 body - approaching the size of a double-decker bus at 7m x 2.9m x 2.3m Solar arrays - approaching the width of a football pitch, with an immense wing span of 45m  Solar panels  - combining conventional silicon with advanced gallium arsenide (GaAs) cells for optimum efficiency Digital signal processor - controlling the antennas, beam forming and channel allocation Reflector - 9m wide and designed to unfurl in orbit like a giant flower Antennas - 120 helix elements combined in a single flexible array Thrusters - both chemical and plasma ion for orbital station keeping
	Building the Inmarsat-4 As befits an historic achievement that will benefit worldwide communication, a multi-national team of space technologists from the UK, France, Germany the USA and Canada has collaborated on the Inmarsat I-4 project. In May 2000, Inmarsat contracted European satellite specialist EADS Astrium as the lead contractor to build three spacecraft. The contractor's tried-and-trusted Eurostar series provided the blueprint for the Inmarsat I-4. The main body of the satellite was constructed in Britain. The bus, the onboard rocket engine that positions the spacecraft in orbit (also known as the service module), was built at the EADS Astrium facility in Stevenage. The payload - the satellite's communications powerhouse - was assembled at the company's facility in Portsmouth. The other main elements of the spacecraft - the antenna, the solar arrays and the 9-metre reflector - were manufactured in Canada, Germany, and the USA and transported to the EADS Astrium facility in Toulouse, France, for integration with the bus and payload.
	Better by design The Inmarsat-4 is the product of years of intensive design work by the company and its international collaborators, led by EADS Astrium. Key elements of the spacecraft - such as the payload, bus, reflector, digital signal processor, antennas and solar array - are skillfully combined to make the Inmarsat I-4 the largest and most sophisticated commercial communications satellite ever. The payload This is the communications engine of the satellite. It will handle massive volumes of traffic, and deliver voice and broadband services via the Inmarsat network. The payload incorporates advanced software and systems, enabling the I-4 to exploit fully those areas of the radio-frequency spectrum allocated to Inmarsat services. This optimizes network capacity and ensures the most efficient and economic use of resources. The satellite controllers' ability to reconfigure the I-4's powerful digital signal processor (DSP) in real time, to meet changing traffic needs, will also contribute to overall efficiency and performance. The bus This is based on EADS Astrium's Eurostar 3000 generic service module. It draws on the long and successful track record of the Eurostar series. A core element is the plasma propulsion subsystem (PPS), which provides thrust to keep the satellite on the correct orbital station. The PPS contains four Russian-designed stationary plasma thrusters. These flight-proven units are supplied by xenon gas and work by electro statically accelerating plasma ions through a discharge chamber. The PPS is effectively two power-processing units in one. Each contains one prime thruster and one back-up. A separate chemical propulsion system will be used to establish the I-4 in its final orbit, for some in-orbit maneuvers, and to back up the PPS. Reflector Astro Aerospace (an affiliate of Northrop Grumman Space Technology of the USA) built the reflector, which is based on a similar device successfully launched in 2000. But, the I-4 reflector has been redesigned to be smaller and lighter, and to be stowed into a space about the size of a large refrigerator. A deployment boom - unique to the I-4 - will unleash the reflector and cause it to bloom like a giant flower once the satellite achieves final orbit. Antenna array The Canadian division of EMS Technologies supplied 120 helix antenna feed elements for the 2.5-metre antenna array. This structure enables the I-4 to generate complex spot beam patterns using the spacecraft's 9-metre reflector. In tandem with the spacecraft's DSP, it will optimize bandwidth and channels to provide varied and high-quality broadband services. Digital signal processor At the heart of each spacecraft is a digital signal processor (DSP) that will govern the antennas, beam forming, gain control, switching and channel allocation. The product of intensive design work, this will be the most advanced commercial DSP ever to fly. The use of digital beam forming gives Inmarsat a flexible system, and allows each individual spacecraft to be used at any orbital location. The beams can be changed at Inmarsat's convenience and be continuously adapted to the needs of system traffic as they evolve. The DSP has the ability to combine 100 kHz channels to provide wider bandwidths, if required. For instance, two 100 kHz channels can be combined to give a useable bandwidth of 190 kHz. Solar array EADS Astrium designed a solar array for the I-4 that uses solar cells provided by RWE of Germany. The company created two types of solar panel - one containing gallium arsenide (GaAs) cells and the other high-grade silicon (Si) cells. The former are more efficient and are highly resistant to heat and radiation damage, and so ideally suited to use in space. The latter are lighter, more widely available, and proven to be reliable and cost-effective. The I-4 harnesses both technologies by including two GaAs and three Si panels in each wing of its solar array.
	Launch vehicles At six metric tons, the Inmarsat-4 is the largest commercial communications satellite ever launched, so a powerful vehicle is needed to lift it. The satellite has been designed for compatibility with several tried-and-tested heavy lift vehicles, including Atlas V, Delta 4, Proton Breeze M and Zenit 3SL. After a thorough assessment process, International Launch Services (ILS) and Sea Launch were selected by Inmarsat to provide launch services for the Astrium-managed I-4 launch programme. International Launch Services ILS was founded in 1995 to provide launch services using US Atlas and Russian Proton vehicles. Inmarsat has contracted with ILS for an Atlas V vehicle to launch the first I-4 satellite. ILS is a joint venture between Lockheed Martin of the USA and the Russian companies Khrunichev State Research and Production Center and RSC Energia. It uses launch sites at Cape Canaveral in Florida, and Baikonur in Kazakhstan. Cape Canaveral is perhaps the most famous launch site in the world. It and the Kennedy Space Center were immortalized by the Apollo missions to the Moon in the late 60s and early 70s, when millions of viewers worldwide tuned into television pictures of the NASA launches. When manned flights to the Moon ended in 1972, Cape Canaveral continued in use as the base for the Space Shuttle programme, and for numerous commercial launches. The Atlas series has become the 'workhorse' of the US space programme, with 586 launches since the first Atlas mission in 1957. Sea Launch The Sea Launch partnership was formed in 1995. It provides a unique service by launching spacecraft from a floating platform in the middle of the Pacific Ocean. The partnership includes Boeing Commercial Space Company of the USA; RSC Energia of Russia; SDO Yuzhnoye/PO Yuzhmash of Ukraine; and Kvaerner ASA of Norway. Operating from its home port of Long Beach California, it uses the Zenit 3SL launch vehicle. Before launch, the floating platform Odyssey is accompanied to a launch zone near the equator by assembly and command ship Sea Launch Commander. The launch location provides the most direct route to orbit, offering maximum lift capacity from the 'sling-shot' effect of launching near the Equator.
	Ready for take-off EADS Astrium checks the readiness of each Inmarsat-4 in the months before it leaves the integration facility in Toulouse, France, for transport to the launch location. Exhaustive launch-environment tests cover vibration, acoustic noise and shock, and are designed to replicate the harsh conditions the satellites will experience during launch. Thermal vacuum testing ensures that the spacecraft can withstand the rigors of geostationary orbit. It confirms that the overall workmanship is sound, and that electrical and other systems will perform as expected. They are exposed to heat and cold at 10oC above and below the temperatures anticipated during service life. The assembled I-4s are maneuvered into giant chambers at the Astrium factory in Toulouse, France, for the tests, which can take more than two months. The chambers are huge airless refrigerators, capable of replicating the extremes of space. On arrival at its launch site, each I-4 will undergo more tests to ensure that all systems are functioning normally following the journey from Toulouse. After fuelling and integration with the launch vehicle, further checks will be made to confirm the spacecraft is ready for the grueling flight into space. During launch, the spacecraft will be exposed to forces in excess of five times normal gravity. In orbit, temperatures can plummet to about -150 degrees Centigrade during solar eclipse, and then soar to more than +150 degrees as the satellites pass into the rays of the Sun.
	Flying the Inmarsat-4 Although they will be alone in space throughout their mission of at least 10 years, the I-4 satellites will have 'company' every minute of every day. Satellite controllers will keep a constant watch on the fleet through a network of monitoring stations around the world, collecting data about the condition and efficiency of the spacecraft and making adjustments when needed. Network operators will also keep watch, prompting the spacecraft to focus their spot beams onto parts of the Earth's surface where extra capacity is needed.
	Stations in the sky Each of the Inmarsat-4 satellites will have its own precise position in geostationary orbit above the Equator. From the ground, a spacecraft in geostationary orbit (also know as the Clarke Orbit) appears to be fixed in the sky, enabling Inmarsat users to locate the satellite easily and point their mobile devices in the right direction. To stay in geostationary orbit 35,786km (22,237 miles) above the Equator, the I-4 spacecraft will need to travel at a speed of 11,064 kmph (6,875mph) to synchronize with the rotational speed of the Earth. The first I-4 will be stationed at 64 East over the Indian Ocean, and the second is planned to be at 54 West above northern Brazil. A team of satellite controllers will work around the clock, 365 days a year, to monitor and control the satellites from the Inmarsat headquarters in London.
	Satellite tracking Inmarsat directs its 'eyes and ears' towards space from ground stations around the world. These tracking, telemetry and control (TT&C) antennas are located at ground stations in Fucino (Italy), Beijing (China), Lake Cowichan and Pennant Point (Canada), Burum (The Netherlands), Auckland (New Zealand) and Eik (Norway). To prepare for the new generation of satellites, Inmarsat has upgraded both these facilities and its London-based Satellite Control Centre (SCC) and Operations Back-up Centre (OBC). The SCC is linked to the TT&C stations through a network of digital leased lines. State-of-the-art software Inmarsat has commissioned new state-of-the-art software to control the Inmarsat-4s and the rest of its in-orbit fleet. It will also be used to control and monitor TT&C ground stations. Known as I4S (Inmarsat Storm Satellite Support System), the software has been developed in partnership with L3-Storm Control Systems, a company that specializes in advanced satellite control systems. I4S is a powerful and user-friendly system, which Inmarsat can also offer on a commercial basis to external customers for control and monitor of their satellites. Put to the test For the launch of the I-4s, Inmarsat has been testing all flight operations procedures on a digital spacecraft simulator to ensure they will work as expected. This exercise has covered all the on-station procedures, as well as the mission-specific procedures to be used during the 'transfer orbit' phase (also known as launch and early operations - LEOP). A series of mission rehearsals have been conducted at the SCC to test LEOP procedures over a realistic period of time. Particular focus has been placed on deployment of the I-4 AstroMesh reflector, which is a major new LEOP sequence. This has been exhaustively modeled and tested by manufacturer Astro Aerospace, an affiliate of Northrop Grumman Space Technology.
	In orbit operations After lift-off, about 30 minutes elapse before the Inmarsat-4 satellite separates from the launch vehicle. At this point, the spacecraft enters the transfer orbit phase. Essential control and propulsion systems are automatically switched on and, a few minutes later, the I-4 becomes visible to Inmarsat ground stations. The first station to acquire the satellite is Hassan in India, shortly followed by Beijing in China. The satellite is now in a fuel-efficient super-synchronous orbit. Effectively a giant ellipse, its apogee (highest point) is 90,000km (55,926 miles) above the Earth and its perigee (lowest) 300km (186.5 miles). The satellite fixes its position in relation to the Sun using onboard solar sensors. Then the solar array panels partly deploy to provide power to the satellite's electrical systems, which have been running off batteries. At this point, about four days into the mission, satellite controllers begin a week-long series of firings of the on-board thrusters at apogee and perigee. These bring the I-4 into a uniform geostationary orbit of 35,786km (22,237 miles).                              The satellite's solar arrays are then fully deployed and its reflector panel unfurled. Ground control then performs a series of operations to establish full control over its position and attitude during orbit. They include calibration of gyros to control attitude, and Earth acquisition, which fixes its position in relation to the Earth. At this point in time the satellite is 'visible' from Fucino in Italy, and ready for in-orbit testing of the payload, scheduled to take about four weeks. Once tests are complete, the satellite is drifted on to its final orbital station, over the Indian Ocean. The four-week process is controlled by short firings of the thrusters, which fix its exact distance from the Earth's surface and its position on the north-south and east-west axes above the Equator. With testing and maneuvers successfully completed, about 70 days after launch, the I-4 will be ready to begin taking commercial traffic.   x
	Solar sailing Inmarsat's Satellite Control Centre in London will monitor and control the Inmarsat-4s throughout their service life, but relatively few operations should be needed to keep them on station. In 'solar sailing' mode, the spacecraft station-keeping strategy makes use of the pressure exerted by particles from the sun upon the solar arrays, while minor adjustments to their attitude will be made using the onboard gyros. If necessary, satellite controllers can reposition the spacecraft on either their north-south or east-west axes by firing their plasma or chemical propulsion thrusters. Around the spring and autumn equinoxes, the satellites will enter solar eclipse as the Sun appears to pass directly behind the Earth. The longest periods of eclipse occur at midnight on the equinoxes, when the satellites are deprived of the Sun's rays - their primary source of electrical power - for 72 minutes. Satellite controllers will plan for these periods, and ensure that the spacecraft remain on station and their batteries charged while they cannot 'see' the Sun.
	Shared expertise Inmarsat's long history of satellite operations has created a wealth of experience. This expertise is available to third parties through its TOPS (Transfer Orbit and Payload testing Support) program. In addition to the nine Inmarsat satellites launched between 1990 and 1998, the company's satellite control team has successfully played a key role in post-launch transfer-orbit operations for more than 10 satellites for external clients.