Low Earth Orbit Satellites (LEOs)

The low Earth orbit, LEO, satellite concept complements these terrestrial cellular standards and is based on the use of small, low-cost, satellites orbiting near to the Earth. To cover the whole of the Earth’s surface requires many satellites; each supporting a large number of overlapping cells as shown in Figure 1, because of the low orbit, users are able to communicate with the nearest satellite using only a small handset. Big LEOs, defined later, communicate directly with each other at millimetric wavelengths to provide backbone links for the network.

Mobile Satellite Services

Personal communication via satellite for maritime users or land-based users with no access to terrestrial systems is not new. For example, driven principally by the maritime mobile user community, Inmarsat consists of a consortium of 65 nations. Originally, serving maritime users, Inmarsat is now authorised to provide aeronautical services. In the future, Inmarsat is likely to be authorised to offer land-based services and has already put forward a proposal for a LEO network called Project-21.

However, Inmarsat is based on geo-stationary satellite technology which, because of satellite height, requires high-power terrestrial terminals and highly directional dish antennae which must be accurately pointed to the satellite at all times. In contrast, new satellite designs in low Earth orbit can supply services to low-power hand-held terminals using small, unobtrusive, omni-directional antennae.

The reduced path length (the distance between a terminal and a LEO) of a geo-stationary compared to a LEO satellite is the prime reason LEO systems can get away with smaller terrestrial equipment. A geo-stationary satellite is positioned at a fixed distance from the Earth of 36,000km, while LEOs orbit at an altitude of less than 900km. The path loss is reduced by between 22 and 28dB, so that a signal reaching a LEO satellite is over 100 times stronger than that received by a geo-stationary satellite, assuming comparable transmission power by the terrestrial equipment. It is therefore possible to reduce the power of the terrestrial transmitter and use an antenna of much lower gain while still achieving good received signal strength at the LEO satellite.

Terrestrial cellular operators can only expand their coverage areas on the basis that it is cost-competitive to do so. This forces operators to use cost competitive technologies (of course, this applies to LEO operators as well!) and to look closely at the revenue that can be generated from each new cell. As a typical cell might be only 20 miles in diameter it is unlikely that a seamless, ubiquitous, coverage of a country is achievable, let alone the whole globe.

Benefits of the LEO concept

Big & Little LEOs

Pushed by several US based consortia and the US delegation at the World Administrative Radio Conference, WARC-92, held in Torremolinos, Spain in February, proposals were made for two types of low Earth orbiting systems: little LEOs and big LEOs. Both have in common global coverage through the use of a number of satellites in a low orbit in the range of 700 to 12,000km as shown in Figure 2.

Little LEOs

Little LEOs are a small, low-cost, class of satellites weighing between 50 and 100 kilograms. They have been allocated the bands of 137-138MHz and 400.15-401MHz for space-to-Earth down-links, and 148-149.9MHz for Earth-to-space up-links.

These bands have been used for meteorological satellite, space research, and mobile and fixed services.

They will now also be shared by little LEO systems for use with slow data-communications, paging, store-and-forward, and messaging services. Small LEOs exclude voice services. These systems are an order of magnitude cheaper and do not seem to carry the technological and commercial risks of big LEOs.

Little LEO proposals include ORBCOMM, Starsys, and VITA amongst others. Volunteers in Technical Assistance, VITA, is a private non-profit making organisation aiming to improve the environment, health, education in developing countries. In January 1992, VITA was the first to be awarded status ‘pioneer preference’ for its LEO proposal by the Federal Communications Commission, FCC.

Big LEOs

Big LEO systems, with satellites in the 350 to 500 kilogram class, are aimed at data communications and real-time voice into hand-held units.

At WARC-92 the US proposed a new allocation of spectrum between 1610 and 1626.5MHz currently used by aeronautical radio navigation services and the Radio Determination Satellite Service, RDSS in Europe.

Big LEOs can carry voice and high-speed data services, unlike little LEOs. Also, big LEOs will use new, untried, technologies in the form of on-board processing and inter-satellite backbone linking at millimetric (30GHz) frequencies making them a much more risky technology.

Big LEO proposals have so far been put forward by six companies: Iridium (66 satellites), a consortium comprising Motorola, Lockheed, BAe, Deutsche Aerospace, and Matra-Marconi; Globalstar (48 satellites), comprising Loral Space Systems, Qualcomm, Alcatel, and Aerospaciale; Project-P from Inmarsat (P for portable); Odyssey (12 satellites) from TRW; Aries (48 satellites) from Constellation Communications; and Ellipsat (24 Satellites), from Ellipso.

Iridium

In 1989 Motorola announced a proposal to launch a series of 77 satellites in low Earth orbit starting in 1994. Iridium Inc. was formed to manage the programme, which aimed to offer world-wide mobile phone services. Although initially announced with little detail of operation, many of the gaps have since been filled in. Early in 1991, several technical and commercial points were made clear for the first time.

Firstly, Iridium announced that the design of the handsets had been simplified and now used a stripped-down version of time division multiple access, TDMA, as used in the GSM cellular standard. Simplification is possible because some of the more complex protocols are not needed. For example, there is no requirement for a highly-complex hand-off between cells. As each satellite has the capacity to control 37 cells, each of which is approximately 360 nautical miles in diameter, hand-offs will be infrequent.

Clearly, the use of GSM standards will be of great benefit to European market, as the same handsets could be used to access both terrestrial and satellite GSM cellular networks. When a user makes a call with such a dual-mode handset, it will first check for a local terrestrial signal. If it is unable to find one, then it switches to Iridium mode. This raises the question of whether users would need two telephone numbers, which would not be a good situation (understatement).

It was proposed that Iridium would utilise a 10.5MHz part of the spectrum between 1616 and 1626.5MHz. Motorola gained access to this part of the spectrum by purchasing the bankrupt Geostar Inc. in 1992. This company exclusively owned the licence for this part of the spectrum used for RDSS. This licence authorises positioning data and paging messages to be sent via satellites.

On a commercial front, Iridium plans to sign up as many operators as it can world-wide. Service would be marketed through existing cellular suppliers’ networks of distributors and dealers. Indeed, it even plans to lease capacity to one of its arch rivals, Inmarsat. This proposal might be intended to deflect criticism of a single US company being granted a world-wide monopoly of a part of the radio spectrum. Motorola also announced variations on the way the satellites are to be launched. At one time, a dedicated space-craft was to be used, but now the satellites will be launched a few at a time by commercial services such as the European Arian to minimise risks. Motorola also stated that a handset would cost $3,000 and a $3 per minute tariff would be levied. Their target user would be that breed of manager who spent many days of the month globe trotting.

In August 1992, Iridium announced enhancements to the system. There were two major changes: firstly, an Iridium LEO satellite will now illuminate 48 areas on the Earth’s surface, an increase of 11 over the original design. Secondly, because of this increase in the number of discrete beams, only 66 satellites are now. These will be deployed in six planes with eleven satellites in each plane. The total cost of the project remains the same at US$3,370 million.

In September 1992, the FCC turned down Motorola’s bid to build the Iridium network. The FCC’s view was that none of the plans for the proposed LEO systems was sufficiently advanced to justify awarding a priority "pioneer’s license", intended to lead to a full operating licence within two years. Motorola replied that it was still planning to launch five experimental satellites in the 1996 time frame.

Iridium Latest

The Iridium system 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, and 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 compliance 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.

Globalstar

The Globalstar proposal is from Loral Qualcomm Satellite Services Corporation, (LQSS).

Heavily backed by French interests, including Aerospaciale, Alcatel, and Alenia, LQSS will initially launch 24 satellites to initially provide an optimised service to continental USA. International operations will follow, after deployment of 24 additional satellites.

Each satellite should cover an area of 400 miles in diameter. Two kinds of user equipment have been envisaged. The first, a vehicle mounted terminal based on a car radio design, will be easily removable and easy to operate. The second is a hand held terminal that the user can take anywhere. Globalstar will be based on CDMA spread spectrum technology. This enables the average transmit power for a terminal to be less than 1 Watt and the use of unobtrusive antennas.

Like Iridium, LQSS plans to provide transmission services to organisations that will resell capacity to other re-sellers or end-users. Globalstar is intended to operate in conjunction with existing terrestrial networks through gateways. These consist of equipment, which interface signals from a Globalstar user to a local cellular network, in the UK this will be Vodafone. The cost of this equipment is projected to be in the order of one additional cellular telephone site.

Globalstar claim that they will be offering a single world-wide communications solution for voice and data. Digital data can be transmitted up to 2.4Kbit/s and position location services with better than 300 metre accuracy should be available nearly everywhere. Other services will include global paging, messaging, and international roaming.

Globalstar Latest.

The Globalstar system 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 satellite’s velocity vector to increase the time a user stays within each beam.

The Globalstar system provides interconnection to the Public Switched Telephone Network & Public Land Telephone Network (PSTN & PLMN) 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, co-ordinating information received from the SOCCs, and collecting information for billings to service providers.

Figure 4 - Globalstar Coverage

Some Issues

Big LEO projects encompass a great deal of vision and possibly represent the ultimate in personal communications. Although originally beset by crowds of doubters, the Iridium project in particular has begun to make progress and achieve some credibility.

Many questions still remain to be answered concerning the practicality of some of the technical proposals, commercial viability, and how LEO systems will co-exist with terrestrial systems.

Meetings took place between LEO operators and the FCC between November and March 1993 to set the rules that will govern the operation of LEOs. It would be very interesting to see the results of these meetings.

As usual the Yanks have it all sown up.

Due to the costs many of these companies change on a regular basis. Finding reliable information is tricky.

The companies are clearly overestimating demand for their products in Europe where terrestrial cell coverage is very good; I can see more demand in the USA though.