Previous Table of Contents Next


Low-Earth Orbiting Satellites (LEOs)

Low-Earth Orbiting and Middle-Earth Orbiting satellites (LEOs and MEOs) are satellites which operate at low altitudes of several hundred miles in a variety of orbits, as compared to Geosynchronous Earth Orbiting satellites (GEO), which were discussed in Chapter 3. LEOs and MEOs are systems of satellites that are configured as constellations of small, low-power satellites. In combination, they provide full coverage of major land masses. Designed to provide wireless voice and data communications to remote areas and to mobile users, the various proposals include from 12 to 840 satellites. These systems are also known as Mobile Satellite Systems (MSSs), as opposed to the Fixed Satellite Systems (FSSs) in geostatic orbit.

Whizzing around the earth like electrons whizzing around the nucleus of an atom (Figure 12.10) , LEO and MEO networks are designed so that a satellite is always within reach of a terrestrial terminal. Once a connection is established between terminal and satellite, the satellites communicate directly to pass the call to the satellite closest to the target terminal, which completes the connection. The concept was conceived by Motorola, which named its proposed 77-satellite constellation Iridium, after the element iridium, which boasts 77 electrons whizzing around its nucleus. Subsequently, the proposal was pared down to 66 satellites, although the name Iridium stuck. (After all, Iridium seems to roll off the tongue better than Dyprosium, the rare earth element with an atomic weight of 66.)


Figure 12.10  Low-Earth Orbiting (LEO) satellite constellation.

Estimates are that the total satellite market could reach $50 billion by 2005. If LEOs and MEOs could capture just 5% to 10% of that market, the rewards could be incredible. The costs, however, are not low; terminals are expected to cost approximately $3,500, and airtime will be in the range of $3.00 per minute. The costs of the proposed networks range from $2.6 billion(Inmarsat) to $9 billion(Teledesic).

LEO networks are defined in two categories. Little LEOs operate in the 2 GHz (S-band) range and are intended to support voice and other PCS services. Big LEOs operate in the 1.5 Ghz to 1.6 GHz (L-band) and 2.4 GHz (S-band) range and are intended specifically for voice and data communications. Table 12.2 provides a sense of some of the more significant LEO and MEO proposals.

Table 12.2 Example proposed LEO and MEO constellations

PRIMARY COMPANY/CONSORTIUM PROJECT CONSTELLATION SIZE

American Mobile Satellite Corp. AMSC 2
Calling Communications Corp. The Calling Network 840
Celsat America Celstar 2
Constellation Corp. Aries 48
Ellipsat Ellipso 24
Inmarsat Inmarsat P TBD
Loral Qualcomm Globalstar 48
Motorola Iridium 66
TRW Odyssey 12

Selected projects, backers and estimated costs are as follows:

  Iridium (Motorola, BCE, Veba, et al), $3.4 billion
  Teledesic (McCaw Cellular and Microsoft), $9 billion
  Globalstar (Loral Qualcomm, AirTouch, Dacom, Vodafone, et al)
  Orbocomm (Orbital Science Corp., Teleglobe, TRI, et al)
  Inmarsat (BT, France Telecom, KDD, Norwegian Telecom, et al), $2.6 billion
  AMSC (American Mobile Satellite Corp.), $870 million
  Celsat (Celsat, Inc., Cellular Communications Inc.); $220 million per satellite, plus launch [12-39], [12-40], [12-41], [12-42], [12-43].

Personal Digital Assistants (PDAs)

Personal Digital Assistants (PDAs) are multifunction wireless devices that were first conceived by Apple Computer and appeared on the market in the mid-1990s [12-44]. Although the concept is an intriguing one, PDAs have met with very little market acceptance at the time of this writing. Assuming that they live to tell the tale, PDAs may achieve some reasonable level of success in the early twenty-first century, likely through PCS networking.

PDA Equipment

PDAs are lightweight (13.6 oz. to 1.8 lbs.) wireless devices that communicate via infrared transmission or various radio frequencies. Current costs are in the range of $500 to $1,500, depending on manufacturer and configuration options. As a result of disappointing sales, costs have dropped over the last few years. For example, Sony reduced the price of its Envoy PDA from $995 to $499 in August 1995. Proprietary operating systems are employed and Graphic User Interfaces (GUIs) are standard. Data entry is via a combination of handwriting and soft keys, with some devices supporting external keyboards. The devices will accommodate 1 or 2 PCMCIA cards for wireless networking, with most also supporting infrared communications. The PCMCIA slot(s), of which there are 1 or 2, support wireless modems, flash memory, applications and attachable keyboards. Manufacturers include Apple (Newton), BellSouth (Simon), Motorola (Marco), Sharp (Zaurua), and Sony (Magic Link and Envoy).

PDA Networking

PDAs are networked through the use of modems, which may be either external or in the form of a PCMCIA card. Wireless messaging/paging services are provided through PDA-specific network vendors including Apple Notification Service, SkyTel, MobileComm, and Motorola NewsCard. Transmission rates vary from 2.4 Kbps to 14.4 Kbps, although throughput generally is much less.

Networking, on some PDAs, is also supported via access to data-specific networks such as those offered by ARDIS, Metricom, RAM, and Mobile Data [12-44]. Some also provide for connectivity to cellular networks, through an external adapter. Additionally, some may be connected to the PSTN through an RJ-11 connection. Connectivity to desktop computers, LANs and printers is provided through internal or external wireless radio adapters, or via infrared technology.


Previous Table of Contents Next