SOME LEO SATELLITE SYSTEM

SOME LEO SATELLITE SYSTEM



1. THE IRIDIUM SYSTEM


The Iridium System is not proposed to be a replacement for existing terrestrial cellular systems, but rather as an extension of existing wireless systems to provide mobile services to remote and sparsely populated areas that are not covered by terrestrial cellular services. It provides more capacity (large no of channels) and better quality of service (shorter transmission delays) to areas that currently receive mobile services from geostationary satellites. It can also provide emergency service in the event that terrestrial cellular services are disabled in disaster situations(earthquakes,fires,floods,etc.).

The concept of using a constellation of low earth orbit satellites to provide global telecommunication services to mobile users. Because the initial proposal called for 77 satellites in the constellation, the system was called IRIDIUM after the element, which has 77 electrons in its orbit.later studies indicated that only 66 satellite would be adequate to provide the targeted services and performance. The 66 satellites are are grouped in six orbital planes; there are 11 active satellites in each plane with uniform nominal spacing of 32.7”. the satellites have circular orbits at an altitude of 783 km, and for each plane an in-orbit satellite is provided.

Satellites in one plane are placed to travel out of phase with those in the adjacent planes. Except for the first and last planes, which are counterrotating where they are adjacent, all remaining planes are corotating, The distance between corotating planes is 31.6, and the distance between the counterrotating planes is 22. The reduced seperation between counterrotating planes is needed to compensate for the reduced coverage provided by satellites on counterrotating planes. In the Iridium system, each satellite is equipped with four two-way communication links(intersatellite links, or ISLs), one each with its neighbors in the same plane and with those in the adjacent planes.

Each Iridium satellite uses a 48-beam antenna pattern, and each beam, which has a minimum diameter of 600 km, can be individually switched. For example, only about two-thirds of the beams will be active at any given time because some the beams will be switched off when the satellites are in the vicinity of the poles, where beam patterns tend to overlap, or when the satellites are over countries or regions in which, Iridium does not have regulatory arrangements to operate. The switched of beams is referred a cell management. In a LEO-based system like Iridium, the beams are equivalent to cells associated with terrestrial mobile systems. However, in case of the Iridium system, it is the beams that move rapidly relative to the subscriber, who is considered to be stationary with respect to the satellite. Thus, switching of beams or cell management to provide continuity of an existing call is equivalent to handoff in terrestrial cellular mobile systems. This requirement for cell management is, of course, and additional complexity associated with LEO- based systems compared with MEO or GEO systems.

The Iridium system supports links of three types:up- and downlinks from the space vehicle (SV) to the gateway (GW) [or to the telemetry, tracking, and control (TT & C) center], using the ka band; up- and downlinks between the SV and the Iridium subscriber unit (ISU), using the L band; and two-way inter-satellite links between the SVs using the Ka band.














2. THE GLOBALSTAR SYSTEM

Globalstar is a global mobile satellite system based on a constellation of 48 LEO satellites. Unlike the Iridium system, Globalstar system does not use intersatellite links but rather depends on a large number of interconnected earth stations or gateways for efficient call routing and delivery over the terrestrial network. It is designed to complement the terrestrial cellular mobile networks to provide telephony and messaging services to subscribers in locations that are not covered, or inadequately covered, by conventional wireline or wireless networks.

Globalstar’s constellation of 48 LEO satellites is designed t orbit at an altitude of 1414 km above earth’s surface in eight orbital plans inclined at 52. With each plane to be occupied by six satellited with a provision for one in-orbit spare satellite in each plane. The nominal weight of each satellite is 450 kg, with a deployed span of 7 meters and working life of 7.5 years. Since Globalstar satellites do not employ intersatellite communication, they essentially provide only transponder functions, making their design and operation less complex and perhaps more reliable. Each satellite supports a 16-beam antenna pattern with an average beam diameter of 2250 km. To mitigate blocking and shadowing, Globalstar will deploy path diversity, whereby multiple satellites may be used to complete a call.

In the absence of intersatellite links, the Globalstar system makes maximum use of the international terrestrial networks (wireline and wireless). Calls from a subscriber are routed via a satellite to the nearest earth station/gateway, and from there they will be routed over the existing terrestrial network. To provide the interface between the ground segment (terrestrial networks) and space segment (Globalstar satellites) Globalstar design deploys 100 or more gateway stations distributed around the world with each station equipped with three or five antennas that can track the trajectories of the satellites. A Globalstar gateway is designed to serve an area 3000 km in diameter and will be designed to take into account the technical and administrative requirements of the coverage area. These requirements may include such factors as coverage, quality of service, and satellite visibility, as well as regulatory and contractual factors associated with national boundaries.

Globalstar uses two types of communication links: service links in the L/S band for communication between the terminals and the space vehicle, and the gateway links in the C band for communication between the earth stations and the space vehicle.





















]3. THE TELEDESIC SYSTEM

Currently the high bandwidth, high quality fiber connectivity needed to support Internet access, computer internetworking, video conferencing, and so on is restricted to major commercial and population centers. Outside these application areas, such facilities are either too expensive or simply not available. The aim of the Teledesic network is to extend the existing terrestrial, fiber-based infrastructure to provide advanced information and communication services anywhere on earth. Whereas the target application for Iridium, Globalstar, and ICO is voice, with support of low bit rate data for facsimile and messaging for mobile subscribers, the primary target application for the Teledesic system is the provide worldwide, seamless, fiber like connectivity to support multimedia, video, and high bit rate data services. In a strict sense, Teledesic does not fall in the category of global mobile satellite systems or GMPCS because its focus is not on worldwide terminal mobility, but rather on providing the so-called Internet in the sky function. The planned target for Teledesic service availability is end of year 2002. Rather than individual end users, primary customers for the Teledesic system will be service providers in countries around the world wishing to extend their network capabilities in terms of geographic scope and the range of services, and also multinational corporations needing to extend the capabilities of their enterprise networks.

The design of the Teledesic system has not been finalized. According to the original plans, the Teledesic satellite segment was to use 840 LEO satellites in 21 planes at altitudes of 700 km. The Teledesic system now intends to deploy only 288 active LEO satellites placed in 12 planes (24 satellites per plane) at altitudes around 1350 km. Each satellite in the Teledesic constellation will have connections to eight of its neighboring satellites through intersatellite links operating in the connectionless packet mode, with each satellite in this interconnected mesh network providing necessary switching functions. The Teledesic network is designed for dual-satellite visibility with at lest one insight satellite at a minimum elevation of 40. This high elevation angle ensures an unobstructed and omnidirectional view of the sky from most building tops where Teledesic terminals may be located. Besides eliminating shadowing effects from neighboring buildings and terrain, the high elevation angel greatly reduces the fading effects of rain at high frequencies.