Decades ago, U.S. and European operators began planning mobile satellite communications (MSC) systems to provide voice and data connectivity around the globe, based on a vision to advance commercial and military communications in the new millennium. Terrestrial cellular was early in its development and ubiquitous adoption by society was not certain. Even if cellular became widespread, regions of the globe would be uncovered, regions important to industry, science and the military.
MSC DEVELOPMENT
In September 1991, Inmarsat became the first international geostationary Earth orbit (GEO) operator to announce a strategy to develop a MSC system, called Project-21. The highlight was a prototype handheld satellite phone based on the Inmarsat-P communication standard. Implementing this service would require a new satellite constellation in either medium Earth orbit (MEO) or low Earth orbit (LEO). This first MSC system, ICO Global Communications (formerly known as the Inmarsat-P Affiliate Company project) was established in January 1995 as a commercial spin-off of Inmarsat; however, it was not successful.
Other MSC proposals—Globalstar, Ellipso, Iridium, Odyssey and others—were based on big MEO or LEO constellations. Only Globalstar, Iridium and Odyssey were awarded licenses by the Federal Communications Commission (FCC) to operate in the U.S. on January 31, 1995. Odyssey, proposed by TRW, was a MEO solution with a constellation of 12 satellites orbiting 10,600 km above the Earth, equally divided into three orbital planes inclined at 55 degrees to the equator. At an estimated cost of $3.2 billion, Odyssey was to start service in 1999. The FCC license required TRW to begin building the first two spacecraft by November 1997; however, the company was unable to find another major investor to support the project, and Odyssey was abandoned in December 1997.
Iridium was conceived in late 1987 by three Motorola engineers.1 Initial system research calculated that global coverage would require a constellation of 77 satellites—hence the name Iridium—although further study after the name was announced reduced the size to 66. An Iridium LLC was formed in 1991 to pursue the venture. Motorola built the satellites, transforming the way satellites had historically been manufactured. After more than 20 launches, each carrying from two to seven satellites, and an investment of some $7 billion, Iridium auspiciously began operations on November 1, 1998, with a call between U.S. Vice President Al Gore and Gilbert Grosvenor, the great grandson of Alexander Graham Bell. However, the service was not widely adopted, and Iridium filed for bankruptcy after nine months, in August 1999. After much of the debt was erased through bankruptcy, private investors bought the assets and “relaunched” Iridium service in March 2001.2
In 2007, the resurrected company announced plans to replace the original satellites. The second-generation Iridium NEXT satellites were built by Thales Alenia Space and launched on SpaceX Falcon 9 rockets during the two-year period from 2017 to 2019. Service was switched to the new constellation in 2019.3
Iridium Communications Inc. has headquarters in Leesburg, Va., and is publicly traded on the Nasdaq stock exchange (IRDM).
IRIDIUM ARCHITECTURE
Figure 1 Overview of the Iridium MSC network.
Iridium provides truly global coverage, offering voice, facsimile, paging, data and tracking services and integrating GPS for satellite tracking. The various satellite “phones” developed by Iridium and its partners provide connectivity for companies and individuals who require mobility or access in regions without cellular coverage. Iridium can provide messaging for mobile tracking, including position, velocity and time (PVT) data. Iridium is a member of the GSM-MoU association, with agreements to provide complementary and value-added global roaming to augment terrestrial telecommunication networks.4
Compared to “little” LEO systems such as Orbcomm, Iridium has more power and bandwidth to provide services to subscribers. The larger size of the Iridium satellites enables more data processing in the transponders than simply storing and forwarding data. The enhanced capabilities of the Iridium NEXT satellites have expanded MSC services for maritime, aviation, land and government applications.
The Iridium system comprises three segments: space, ground and user (see Figure 1). A mobile or semi-fixed transceiver anywhere on Earth can communicate to a visible Iridium satellite via an L-Band radio link, using time-division duplexing to switch between receive and transmit. Like cellular systems such as GSM, the user is handed off, here between beams from the same satellite or, when required, from one satellite to the next. The satellites have intersatellite links (ISL) for satellite-to-satellite communication, which ensures continuous connection from users to ground Earth stations. These gateways connect to the public switched telephone network (PSTN) and other terrestrial networks.
SPACE SEGMENT
Figure 2 Iridium satellite orbits (a) and spot coverage (b). Source: Iridium.
The Iridium constellation comprises 66 operational satellites orbiting in six polar planes intersecting over the North and South poles, the 11 satellites in each plane serving as nodes in the communications network. The satellites, launched into a near polar orbit at an altitude of 780 km, travel more than 30,000 km/h and circle the Earth approximately every 100 minutes (see Figure 2a).
To cover the globe, each satellite has 48 overlapping spot beams, with each spot about 600 km in diameter (see Figure 2b). 66 satellites generate 3168 cells, of which only 2150 need to be active to cover the entire surface of the Earth. Each cell covers about 15 million km2, with each satellite simultaneously serving an average of 80 cells and a maximum of 240. Given the speed of the spacecraft, a user switches between adjacent beams about once per minute. The global throughput of the network varies between approximately 171,000 and 500,000 simultaneous calls.4
Figure 3 Iridium first- (a) and second-generation (b) satellites. Source: Iridium.
Figure 4 Iridium’s global coverage. Source: Lloyd Wood, L.Wood@society.surrey.ac.uk.
The first-generation of Iridium launched 14 additional satellites as spares to replace any failed satellites. 81 total Iridium NEXT satellites were built to support the life of the constellation; nine spares were launched and six are stored on the ground.
The two generations of Iridium satellites are shown in Figure 3. The Iridium NEXT satellites, with a mass of 800 kg, were designed to fly a 50 kg secondary sensor payload consuming 50 W average power. Iridium NEXT uses the Proteus Bus, developed by Thales Alenia Space, and has two deployable solar arrays. A 15-year battery lifetime was planned, with a minimum of ten years.
To connect to users, Iridium NEXT uses an L-Band phased array antenna to generate a 48-beam, 4,700 km diameter cellular pattern on the Earth’s surface (see Figure 4). K-Band (19.1 to 19.6 GHz) is used for the downlink from the satellites to the gateways, Ka-Band (29.1 to 29.3 GHz) for the uplink from the gateways. K-Band crosslinks (22.55 to 23.55 GHz) connect each satellite to the satellites immediately adjacent in the same orbital plane and two neighboring satellites in adjacent planes, i.e., front, back and two in adjacent orbits.5 Cross-linking the satellites forms a global mesh network, enabling communication from any location on Earth to any other location on Earth. Having multiple paths linking users to a GES improves system flexibility and reliability.
GROUND SEGMENT
Figure 5 Space, ground and user segments. The ground segment connects users to terrestrial networks.
The System Control Segment (SCS) and the GES or gateway make up Iridium’s ground segment (see Figure 5).6 The SCS manages the overall system, handling global operational support and control services for the satellite constellation, providing satellite tracking data to the gateways and performing the termination control function for messaging. The SCS system has three integrated components: the Operational Support Network, the Network Control Station and four telemetry, tracking and command sites.
The GES gateways, ground terminals with high gain parabolic antennas, connect the Iridium network to the PSTN and other terrestrial networks.7 The gateways generate and control the information for registered users, such as user identity, location and billing. They also track the satellites for operations and support management. Each GES connects directly to up to four satellites and, via cross links, to the other satellites in the network. Iridium has two commercial GES sites, located in Tempe, Ariz., and Fucino, Italy. The U.S. government owns and operates its own gateway, in Oahu, Hawaii.
