Big LEO overview

[ Introduction | Big LEO Overview | Big LEO Tables | GSM Overview | Acronyms | References | Background ]

[ Preface | Altitudes | Odyssey | ICO | Globalstar | Iridium | Teledesic ]


A number of non-geostationary mobile satellite communications systems are currently proposed by different organisations, and a number of general overviews are available [ANA94, ANA95, COM93, GAF94, GAF95, JOH95, LOD91, GIU94, MOB94, WOO96, WU94]. Additional references have been indicated where appropriate in the text.

This section summarises the Odyssey, ICO, Globalstar, Iridium, and Teledesic systems. With the exception of Teledesic, these systems are commonly known as Big or Voice LEOs; systems targeted at providing real-time voice services through hand-held mobile terminals. The descriptions are not intended to be exhaustive, and are limited to systems that fit the criteria of being global or near-global, non-geostationary, providing real-time services.

Further information on the development of these systems can be found at Lloyd's satellite constellations, which has sections discussing Iridium, Teledesic, ICO, Globalstar and Odyssey.

Orbital altitudes

The orbits figure below illustrates the relative orbital altitudes of the systems. Teledesic's redesign from 840 to 288 active satellites, not discussed here, raised its altitude from similar to Iridium to similar to Globalstar.

Although this figure shows the geostationary ring, it is not a specific intersection through any one great circle. Rings indicating altitudes are generalised and orbital rings are not shown at their actual inclinations; this is particularly important for the elliptical orbits shown.

Due to the effect of the solar wind on the Earth's magnetosphere, Van Allen belts are not perfectly symmetrical and are included in the diagram as a rough guide to their position only.

This diagram is also available in postscript for printing, in encapsulated postscript (ideal for Insert Picture... once show all files is selected), and in Adobe's proprietary Acrobat portable document format, for which you require a copy of Acrobat Reader. Reuse of this diagram in a publication requires a credit to these pages; please let me know about it.

Relative altitudes of constellations

The systems


Odyssey has now been cancelled in favour of ICO.

The Odyssey medium-earth-orbit (MEO) system [BAI96, SPI93, GAF94] employs mobile link antennas which are off-pointed from nadir to best cover populated regions by dynamically steering the satellite body. This satellite constellation provides full global coverage, with at least one satellite in view above 20 degrees elevation angle on all points of the globe. Directed coverage (off-pointing from nadir) is used to enhance coverage of land areas as needed to respond to traffic demand (busy hour service peaks in a region, for example). Directed coverage is not needed for global coverage, and does not preclude it. The capability to handover will be catered for. Due to the pointing antennas the need to reassign spread spectrum codes or frequencies is quite rare. CDMA was chosen for the multiple access method as it permits spectrum sharing with multiple service operators. The minimum elevation angle is 20°, and usually at least two satellites will be visible from the Mobile Terminal (MT). This enables an optimum link to be selected at call set-up. Path diversity is not employed; the MT communicates through only one satellite at any one time. Throughout a connection's lifetime the MT monitors the other satellites in view, and using a second receiver, the MT uses the second satellite to ensure a continuous connection. Earth Stations (ES) provide connections to and from the public network, and terrestrial links are utilised to complete calls to and from an Odyssey MT. The capacity in terms of voice circuits per satellite ranges from 3000 to 9500, depending on the mix of mobile and fixed terminals (9500 does not assume 100% fixed terminals).

Odyssey employs frequency-division multiplexing on the Ka-band feeder link, with distinct sub-bands being allocated for each beam. On the uplink, the satellite's task is therefore one of frequency-division multiplexing the signals prior to frequency translation from L-band to Ka-band. For the downlink, the satellite translates from Ka-band to S-band, prior to demultiplexing the feeder link and routing the appropriate portion of the feeder link to the correct beam. All communication processing is performed on the ground, keeping the payload design of the satellite simple and future-proof.

Odyssey has now been cancelled. For further information, see the Odyssey entry in Lloyd's satellite constellations.


The ICO system [ICO96, HAR95, INM95] is a TDMA MEO system, with ten satellites and two spares in two inclined circular orbits. The orbit is designed for satellite diversity, in that two or more satellites are in view of the Mobile Terminal (MT) at most times. The satellites relay calls between the MT and an earth station (ES). Twelve ESs and the subscriber databases are interconnected using a terrestrial facilities to form a network. The ESs are linked to the public switched network through gateways which are owned and operated by third parties.

ICO is a member of the GSM Memorandum of Understanding group, and ICO plans to reuse as much as possible the GSM technology in a narrowband TDMA satellite environment. MTs are planned both as single mode and as dual mode, where the MT will work with both the ICO standard and a regional terrestrial cellular standard (GSM in Europe, JDC in Japan, DAMPS in North America).

ICO considers the constellation choice as one providing high elevation angles, accommodating satellite spatial diversity, and with acceptable propagation delay. The MEO altitude of 10355 km (changed to 10390km, late 1998) also provides for slow-moving satellites as seen from the earth, leading to fewer and simpler handover arrangements than in a LEO system. ICO also claims that the system's technical risk is acceptable, as the ICO system will be based on more mature and tested technologies. The chosen MEO constellation also allows system growth by adding planes as capacity requirements increase. TDMA was chosen for multiple access as ICO argues that TDMA permits power-efficient modulation schemes, promising the ability to support peak traffic capabilities by increasing and switching the capacity within a beam to cover real life traffic distributions.

ICO will be targeted primarily at users from the existing terrestrial cellular market, which travel to places where terrestrial cellular coverage is incomplete, patchy or non-existent. Road transport, maritime and aeronautical communities are also anticipated customers, in addition to the demand for semi-fixed applications in rural areas or in developing countries.

For further information on ICO, see the ICO entry in Lloyd's satellite constellations.


The Globalstar system [GLO96, GAF94, MAZ93, ROU93] has 48 low earth orbit satellites in eight planes. The constellation is designed for 100% single satellite coverage between ±70° latitude, and 100% dual or higher satellite coverage between 25° to 50° latitudes. Globalstar will employ path diversity combining to mitigate blocking and shadowing; up to three satellites may at any one time be used to complete the call.

Globalstar chose Qualcomm's terrestrial CDMA technology for the mobile link, and for the feeder link FDM uplink and FDMA downlink. As for any satellite CDMA system, the chosen feeder link approach is bandwidth-consuming; each beam will require a full 16.5 MHz portion of the feeder link for full re-use between beams, due to the CDMA technique. CDMA was chosen to increase capacity on the mobile link through frequency re-use and voice activity detection, for the ability for spectrum sharing and for improved multipath performance. Globalstar offers data rates at 1,200, 2,400, 4,800 and 9,600 bps, and the vocoder rate is allowed to drop down to 1,200 bps when no voice activity is detected. This reduces interference and increases capacity, while maintaining synchronisation and conveying background comfort noise. Globalstar's antennas are shaped for elliptical beams aligned with the satellites velocity vector to increase the time a user stays within each beam.

The Globalstar system provides interconnection to the PSTN/PLMN (Public Switched Telephone Network/Public Land Telephone Network) through 100 to 210 ESs which will each interface an MSC for extension of terrestrial cellular call processing. Globalstar will sell access to the Globalstar system to local service providers, which will have an exclusive regional right to offer the Globalstar service, as well as an obligation to obtain necessary regulatory approval. Calls will only be established through satellite(s) when connections cannot be made over the terrestrial network. All calls that are connected through the Globalstar system will be connected through the regional ESs, giving the local service provider additional revenue and enabling local regulatory authorities to maintain control. Two satellite operations control centres (SOCCs) will track and control the satellites through TT&C units located in various ESs. Additionally, two ground operations control centres (GOCCs) are designed for dynamically allocating capacity among nearby regions, coordinating information received from the SOCCs, and collecting information for billings to service providers.

For further information on Globalstar, see the Globalstar entry in Lloyd's satellite constellations.


The Iridium system [IRI96 MAI95 HUT95, IRI92, GAF94] incorporates Inter-Satellite Links (ISL), a GSM based telephony architecture and a geographically-controlled system access process. The 66-satellite LEO coverage was seen as one offering low path delays and global coverage.

Of the Iridium system's 3,168 beams, only approximately 2,150 will be active at any one time, as some beams will be switched off around the earth poles where beam overlap occurs. Connections between the Iridium network and the public switched telephone network (PSTN) are provided via ES (gateway) installations. Each satellite is connected to its four neighbouring satellites through inter-satellite links, and these ISLs provide flexibility in where the ESs can be located. A MT originated call can be routed within the satellite network and connected to any MT located anywhere, or it can be connected to the public network through any ES.

The Iridium system patterns its call processing architecture after GSM, and the ESs incorporate a GSM MSC with the associated databases (EIR, HLR, VLR). Additional functions required for the Iridium system and not accommodated by the GSM MSC are also taken care of in the ES. Examples of such functions are control of the feeder link, ES management subsystems and messaging controllers. When a MT originates a call, the Iridium system will calculate the user's location. Each ES has associated with it a location area which the ES controls, and the MT location is used to assign the home (or visited, if the MT has roamed) ES which controls all aspects the call. If required, a PSTN/PLMN connecting ES is chosen based upon the MT's location and the location of the PSTN/PLMN party at the time of call set-up. Using the MT position, the ES will also be queried to ensure compliancy with national laws enforcing call restrictions on MTs. The ISLs remove the requirement for the ES to be continuously available within the satellite footprint, and the terrestrial charges can be kept at a minimum by routing a call to the ES closest to the origination or destination of the particular call. Iridium proposes time division duplex; both uplink and downlink employ a TDMA and FDMA mixture.

For further information on Iridium, see the Iridium entry in Lloyd's satellite constellations.


Since the papers that this description is based on were written, Teledesic has changed its design from 40 active satellites in 21 planes at 695-705 km altitude, to one consisting of 12 planes of 24 active satellites at 1350km altitude, and has moved to using laser intersatellite links. This change, announced when Boeing became involved, is not reflected in the following description, and system details discussed are likely to have changed. Further changes in the design may result from Motorola's involvement.

The Teledesic satellite system [SHA95, GRI95, STU95, TUC94] is by far the most ambitious of the proposed systems. The cost is US $9 billion for 840 active satellites (924 satellites, including in-orbit spares) in 21 planes in a sun-synchronous, inclined circular low earth orbit.

Teledesic aims to provide high data rate (broadband) fixed and mobile services, continuous global coverage, fibre-like delay and bit error rates less than 10-10. Thus, rather than targeting voice and supporting low-bitrate data for fax and messaging as the Big LEOs do, Teledesic focuses on providing wireless broadband services with a fibre-like quality, focusing on data and supporting voice. The term Broadband LEO is therefore more suitable for describing Teledesic than Big LEO, as it's in a distinct category offering distinctly different services to Iridium et al.

A Ka-band mobile link was chosen to achieve sufficient bandwidth for the high bitrates Teledesic requires. Accommodating low propagation delays, a LEO orbit with a large earth terminal elevation mask angle of 40° was sought. This high minimum elevation angle is necessary to overcome fading due to rain in Ka-band. The elevation angle and low earth orbit together dictate the number of satellites for the system to provide continuous global coverage. The system is designed for dual-satellite visibility from a MT to enable load sharing between the satellites.

Each satellite has connectionless packet-oriented intersatellite links to its eight neighbouring satellites, and each satellite acts as a switch in the mesh network so constructed. All communication within the network is as streams of short, fixed length (512 bits) packets, described as using mechanisms similar to Asynchronous Transfer Mode (ATM). Each intersatellite link has a capacity of 155.52 Mbps. Gateways provide connection to the land fibre network and to the Teledesic support and database systems, to privately-owned networks and to high-rate terminals. Communication between the gateways and the satellites is treated as high-bitrate usage of the mobile link; there are no separate feeder links in the Teledesic design. The gateways have bitrates ranging from 155.52 Mbps and multiples of that rate, up to 1.24416 Gbps. Each satellite has the capacity to support a total of around 100,000 basic 16 kbps channels.

Teledesic uses steerable antennas and regional resource mapping to reduce the number of handovers required due to the satellites' motion and the earth's rotation. The earth's surface is mapped into a fixed grid of approximately 20000 supercells, each of which is again divided into 9 cells. The supercell is a square 160 km on each side. A satellite footprint encompasses a maximum of 64 supercells, or 576 cells, corresponding to one supercell per beam. The channel resources (frequencies and timeslots) are associated with each cell, and managed by the serving satellite. A mobile terminal will retain the same channel resources during a call, irrespective of which and how many satellites are serving the MT during the call's lifetime. Channel reassignments will thus become the exception rather than the rule. A database onboard each satellite defines the service types within each earth-fixed cell, and also ensures that interference to or from specific areas is avoided. Service areas may also be contoured to national borders or regional boundaries using this earth-fixed cell technique.

The multiple access method chosen by Teledesic is a combination of space-, time- and frequency-division multiple access. Each supercell has one receive and one transmit beam dedicated to it. Each beam is scanned cyclically over the nine cells in the beam's supercell, covering one cell at a time. The scan cycle is 23.11 ms long, and each scanning beam supports 1440 16-kbps channels. This results in TDMA between cells in a supercell, and SDMA (space division multiple access) is used between cells scanned simultaneously in adjacent supercells. Within each cell's time slot, terminals use FDMA uplink and ATDMA (asynchronous TDMA) downlink. Transmissions from the satellites are synchronised so that each supercell receives transmissions at the same time, and there is a guard band of 0.292 ms per cell to ensure that there is no overlap between signals from time-consecutive cells. Each terminal is allocated one or more frequency slots on the uplink for the call's duration. On the downlink, the 512-bit packet header is used to discriminate users rather than a fixed assignment. A standard 16 kbps terminal requires one packet per scan, and the satellite transmits only as long as it takes to transmit the packets queued for a cell. Due to the space separation of the Teledesic system, all supercells use all available frequencies simultaneously. However, only one out of the nine cells in a supercell is using all available frequencies at a time, and this corresponds to a re-use pattern of nine. The corresponding global re-use factor becomes 2222 (the 20000 supercells divided by the re-use pattern).

Teledesic proposes a wide variety of terminals, with bitrates ranging from a minimum 16 kbps, plus 2 kbps for signalling, up to 2.048 Mbps (128 basic channels of 16 kbps) for the mobile subscriber terminals. The mobile terminal antennas have a diameter ranging from 8 cm to 1.8 metres, and an average output power ranging from 0.01 W to 4.7 W. The antenna diameter is determined by maximum output power, maximum channel rate, climatic region and availability requirements.

A small number of fixed-site terminals operating between OC-3 (155Mbps) and OC-24 (1.2Gbps) can also be supported in the design - but those aren't mobile and can't be compared with anything in the other schemes here.

For further information on Teledesic, see the Teledesic entry in Lloyd's satellite constellations.

Lloyd Wood (
last updated 14 January 2000