Medium Earth Orbit Satellite
A Medium Earth Orbit (MEO) satellite is one with an orbit within the range from a 1,000 and 22,300 miles above the earth's surface between the two Van Allen belts. Satellites of this type orbit higher than low earth orbit (LEO) satellites, but lower than Geostationary satellites. A MEO satellite system can be used in telecommunications. They are mainly used in geographical positioning systems and are not stationary in relation to the rotation of the earth.
The orbital periods of MEO satellites range from about 2 to 12 hours. They drift slowly in longitude and must be tracked as they move through the sky. Some MEO satellites orbit in near perfect circles, and therefore have constant altitude and constant speed. Other MEO satellites revolve in elongated orbits. The perigee (lowest altitude) of an elliptical-orbit satellite is much less than its apogee (greatest altitude). The orbital speed is much larger near perigee than near apogee. As seen from a point on the surface, a satellite in an elongated orbit crosses the sky in just a few minutes when it is near perigee, as compared to several hours when it is near apogee. Elliptical-orbit satellites are easiest to access near apogee, because the earth-based antenna orientation does not have to be changed often, and the satellite is above the horizon for a fairly long time.
A fleet of several MEO satellites, with orbits properly coordinated can provide global wireless communication coverage. Because MEO satellites are closer to the earth than Geostationary satellites, earth-based transmitters with relatively low power and modest-sized antennas can access the system. Because MEO satellites orbit at higher altitudes than LEO satellites, the useful footprint (coverage area on the earth's surface) is greater for each satellite. Thus a global-coverage fleet of LEO satellites needs to have more members than a global-coverage fleet of MEO satellites.
Currently they are not used or Telecommunications. The 24 GPS (Global Positioning System) Satellite orbiting the earth at about 18,000 km are examples pf MEO Satellites. MEO Satellites have a smaller footprint then Geostationary satellites but require fewer transmitters to reach them
Low Earth Orbit Satellite
LEO satellites are lower in altitude under the Van Allen Belts and orbit the earth between 400 and 1,000 miles above the earth's surface. LEOs are mostly used for data communication such as e-mail, paging and videoconferencing. In relation to the rotation of the earth, LEOs are not fixed in space, hence, they move at very high speeds and as a result, data being transmitted via LEOs must be handed off from one satellite to the next as the satellites move in and out of range of the earth-bound transmitting stations that are sending the signals into space. Because of the low orbit, the transmitting stations do not have to be as powerful as those that transmit to satellites orbiting at greater distances from the earth's surface and the round trip delay is only a few milliseconds. LEO telecommunication systems are a promising technology because they provide the ability for underdeveloped territories to acquire satellite telephone service in areas where it is either too costly or not geographically possible to lay land lines.
In 1990 Motorola filed an application with the FCC asking for permission to launch 77 satellites (Iridium). The plan was later revised to 66. The Iridium satellite telephone system consisted of 66 satellites in low Earth orbit, and was advertised as providing worldwide coverage with a single handset. The system cost was about $5Billion. The satellite altitude was 750 km, in circular polar orbits. They are arranged in North-South necklaces, with a satellite every 32° of latitude. Each satellite has a maximum of 48 cells and a capacity of 3840channels, giving a total of 1628 cells and 253,440 in all. After seven years of acquiring financing, the partners launched the Iridium satellites in 1997 and service began in November, 1998.
An advanced feature of the Iridium system was the use of inter-satellite radio links, reducing the number of ground stations, but greatly increasing satellite cost. The handsets were large and bulky, with a typical price tag of about $1000. Service charges ranged from about $1.40 to $3.00 per minute.
While the Iridium system was technologically sophisticated, it was doomed to failure for several reasons. First, the rapid growth of land-based cellular systems provided service to large percentage of the population at rates that typically were a tenth that of Iridium (Typically about $0.35 per minute during peak times, often with free talk time during off-peak hours. Cellular handsets are usually free, or with a small nominal charge). Iridium officials claimed that their system was the only one to offer seamless coverage to people in lesser-developed countries, remote desert or mountainous regions, or even on the oceans. This was true, but they seemed to miss an important point – there are not many paying customers in those regions.
Another serious problem with the Iridium system (and one that was never mentioned in their advertisements) is that Iridium handsets required a line-of-sight path to the satellite, meaning that it was rarely possible to use an Iridium phone in a building or vehicle. Land-based cellular telephone systems, working at lower frequencies with better link margins and propagation properties, work quite well in buildings and vehicles. Iridium declared bankruptcy in August 1999. A sad outcome to well-engineered system, but one that was not unexpected.
The satellites were purchased by an investor for 25 million and the service was restarted in March 2001. Iridium’s business was and is providing worldwide telecommunications services using hand-held devices that communicate directly with the satellites (voice, data, paging, fax and navigation)
The Globalstar satellite system consists of 48 LEO satellites, and is also designed to provide worldwide telephone coverage. System cost was about $4B. Globalstar handsets typically cost about $750, and service charges are about $1 per minute. Satellite altitude is 750 miles, with an expected space vehicle lifetime of about 7.5 years. The Globalstar system uses a different switching scheme than Iridium. Iridium’s switching scheme requires sophisticated switching equipment in the satellites. Globalstar uses a traditional bent-pipe design. The call form earth is sent back to earth to a ground station near where the call is routed via a terrestrial network to the ground and delivered by a bent-pipe connection. It puts much of the complexity on the ground, where it is easier to manage.
Service began in late 1999, and at the present time the Globalstar system is
struggling to meet its market projections. Globalstar requires about 500,000 customers to financially break even, but after six months of operation it had only 13,000. It, too, has trouble providing service to users in buildings or vehicles, and so suffers from the same type of problems as did Iridium. Globalstar's financial backers remain publicly optimistic, but we expect it to suffer the same fate as Iridium, perhaps at a slower pace.
The lesson here is that large constellations of LEO satellites still cannot compete with land-based systems that provide essentially the same service at a better price. Land-based facilities are much cheaper to build, install, and operate than satellites, and they can be much more easily modified, upgraded, and repaired. In addition, the quality of service (including factors such as coverage in buildings and vehicles, handset size, weight, and battery life) of land-based telephone systems is significantly better than that provided by satellite systems. This is ultimately due to the difference in link loss between satellite systems and land-based cellular systems – a fact of nature that no amount of marketing can change. Users will not pay substantially more for inferior service, even if the system can work worldwide. The same conclusion applies to data-oriented LEO systems, such as the proposed Teledesic system.
The Teledesic system is advertised as an “internet in the sky”, and is intended to offer broadband data services (64 Mbps downlink / 2 Mbps uplink). It was conceived in 1990 by mobile phone pioneer Craig Mc Caw and Bill Gates who were unhappy with the present ‘high bandwidth’ to computer users. The goal of Teledesic systems is to provide millions of internet users with an uplink of 100Mbps and a downlink of up to 720Mbps using a small, fixed V-SAT type antenna, totally bypassing the telephone system.
In 1994 Teledesic was proposing to use a constellation of 840 LEO satellites, and had targeted a unit satellite cost of $5.5M (other communications satellites typically range in cost from $50M to $100M). Besides the up and down link hardware, these satellites would also use inter-satellite relays. In a 1994 Forbes magazine article comparing proposed LEO satellite systems, Gilder claimed that the “big winner for the next decade is Teledesic”.
The original design was scaled down to 288 small foot print satellites, arranged in 12 planes just below the Van Allen Belts at an altitude of 1350km. this was later changed to 30 satellite with larger footprints. Transmission occurs in the relatively uncrowned high bandwidth Ka band. The system is packet-switched in space, with each satellite capable of routing packets to its neighbouring satellites with a speed of 50 msec.
The first launch was in 2000, and service to start in 2004. While there is a huge demand for broadband internet access, this LEO system suffers from the same fundamental problems as Iridium and Globalstar. Competing land-based broadband service can be provided by fiber cable, DSL lines, LMDS and DSS wireless, and others.
By the time Teledesic is operational, these services will have picked all the ‘low-hanging fruit’ of the large numbers of customers near urban and suburban areas. And these land based services will offer cheaper rates, with much lower overhead, than the Teledesic system. In the late 1990s, one of the Teledesic technical staff told me that they viewed Teledesic as “a machine that would effectively print money” for the shareholders. We’ll see!
Tannembaum, Computer Networks 4th Edition. Prentice Hall
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