Chapter 27

Low−Earth−Orbit Satellites (LEOs)

Overview

Quite a bit of discussion regarding wireless communications has already been covered in earlier

chapters. However, a combination of two separate services and technologies are merging as new services for broadband communications. These two services include long haul communications and the use of personal communications services (PCSs). The technologies include the use of satellite and the cellular concepts combined. Worldwide communications services can be achieved by these two combined services, therefore some diligent effort should be made to understand just what is happening in this arena. This truly brings home the concept of communications from anywhere to anywhere. The thought of being out in the middle of a lake and receiving a call, or rafting down a river and making a call, boggles the mind. This is especially true when thinking about some of the more rural areas in the world where no telephone service infrastructure exists today. Yet, in a matter of a few years these remote locations, on mountaintops, in forests, in valleys, or on the sea will all be reachable within a moment's notice. The infrastructure of a wired world will not easily lend itself to this need, due to timing and cost issues. Therefore, the use of a wireless transmission system is the obvious answer.

Low Earth Orbit (LEO) refers to a satellite which orbits the earth at altitudes between (very roughly) 200 miles and 930 miles.
Low Earth Orbit satellites must travel very quickly to resist the pull of gravity — approximately 17,000 miles per hour. Because of this, Lowe Earth Orbit satellies can orbit the planet in as little as 90 minutes.
Low Earth Orbit satellite systems require several dozen satellites to provide coverage of the entire planet.
Low Earth Orbit satellites typically operate in polar orbits.

Low Earth Orbit satellites are used for applications where a short Round Trip Time (RTT) is very important, such as Mobile Satellite Services (MSS).

Low Earth Orbit satellites have a typical service life expectancy of five to seven years.

Orbit	Number Of Competitors	Status
Low−Earth	8	Pending licenses granted Orbit (LEO) based on very specific areas of coverage
Mid−Earth	4	Experimental licenses Orbit (MEO) granted for specific areas of coverage
Geosynchronous	4	Licenses have already been Orbit (GEO) issued for some, are experimental with others

Low−Earth Orbit

In December 1990, Motorola filed an application with the FCC for the purposes of constructing,

launching, and operating a LEO global mobile satellite system known as Iridium. This was the hot button that sparked the world into a frenzy. Iridium was a concept of launching a series of 66

satellites [1] Originally the Iridium proposal was for 77 satellites, but Motorola amended this number after the World Administrative Radio Council meeting in the spring of 1992. around the world to provide global coverage for a mobile communications service operating in the 1.610 to 1.6265 GHz frequency bands. The concept was to use a portable or mobile transceiver with low profile antennas to reach a constellation of 66 satellites. Each of the satellites would be interconnected to one another through a radio communications system as they traversed the globe at 413 nautical miles above the earth in multiple polar orbits [2] The original concept was to use 7 polar orbits with 11 satellites in each. This would provide worldwide coverage, much similar to an orange slice concept .

In Figure 27−1, the concept of the LEO arrangement is shown. In this particular case, the satellites

are traversing the earth's surface at a height of 400+ nautical miles above the earth, in a polar orbit. In the polar orbit, the satellite moves around the earth's poles and passes over any specific point along its path very quickly. The satellites move at approximately 7,400 meters per second in different orbits. Therefore, as one target site moves out of view, a new one comes into view at approximately the same time. A handoff will take place between the individual satellites (using the Ka band).

The Benefits of These Service Offerings

Motorola established a list of benefits from the deployment of the Iridium network services, which at first glance may look biased toward their services. Upon further evaluation, these benefits can be derived from any network of this type. Therefore, generically these are addressed and kept in the context of any LEO network. The benefits lean toward the end user as shown. These are summarized as follows:

Ø  Ubiquitous services With continuous and global coverage, any−to−any connections can occur. As users travel either domestically or abroad, the service travels with them. It will eliminate the need for special access arrangements and special numbers that must be dialed. Users should never be out of range from their network. Remote areas with limited demand and finances now have the capability to connect anywhere in the world.

Ø  Spectral efficiency As already mentioned, the frequency reuse patterns for the bandwidth

allocation will be significant. No other satellite system has achieved these reuse ratios. Iridium was first to claim this capability of efficiency. RDSS portion of the Iridium network is

contained in the same spectral arrangement, freeing up 16.5 MHz of spectrum. This is a

quantum leap in the efficient use of the spectrum.

Ø  The potential to save lives It's common for the news media to publish stories of people stranded in remote areas with no life support systems who died because of their inability to communicate. The press today is filled with stories of cellular and PCS users notifying authorities of casualties. If only people in remote areas had a means of notifying authorities and/or rescue parties, their lives could be spared in the event they get into a life−threatening situation.

Ø  Capabilities of the vendor Motorola states that they are uniquely qualified to provide

these types of services due to their background in the development and sales of other

ancillary equipment that works in the wireless world. Specifically, they have been one of the

Ø  major developers in the production, research, and development of private mobile−radio

services.

Ø  LEO deployment promotes international communications The LEO networks deliver

modern digital−transmission services to remote areas of the world. The FCC and the U.S. government are attempting to use telecommunications as a strategic and economic tool to foster development in these areas. Their goals are to

·         Promote the free flow of information worldwide.

·         Promote the development of innovative, efficient, and cost−effective international

communications services that meet the needs of users in support of commerce and

trade development.

·         Continuous development and evolution of a communications services and networks

that can meet the needs of all nations, and specifically those of developing nations.

References:

http://www.tech-faq.com/low-earth-orbit.html

Broadband Telecommunications Handbook