Chapter 27: Low Earth Orbit Satellites (LEOs)

     Satellite systems are employed for telephone and data communications. there are geostationary satellites flying in high orbit where they can maintain the same position above the earth's surface ate all times. the only problem, with such high-flying satellites is that there is a noticeable delay in real time communications, and the power requirements to communicate with the satellites is too high for portable devices.

     LEOs are more practical for mobile communication devices like mobile phones, PDAs,and automobile communication system. An LEO satellite orbits in a relatively low earth orbit of a few hundred miles. In this orbit, the round-trip time for transmission is minimal, as are the power requirements for earth-bound communication devices. the Downside of LEO satellites is that a fleet of them is required. Because of their low orbit, they move faster relative to a point on the surface, so a fleet of LEO satellites is required to maintain communications over a single point. as one LEO moves out of position, the other moves on. Each satellites covers an area that could be compared to a cell in a cellular system, except that the cell moves as the satellite orbits. 

Advantages and Disadvantages of LEO and GEO

TYPE	GEO	LEO
Description	Geostationary Earth Orbit	Low Earth Orbit
Height	36,000 km	200-3000 km
Time in LOS	24 hrs	15 min.
Advantages	1. Covers 42.2% of the earth's surface 2. Constant view 3. No problems due to Doppler 24 hrs 36,000 km	1. Lower launch costs 2. Very short round trip delays 3. Small path loss
Disadvantages	1. Larger round trip delays – 250 ms latency uıp/down link 2. Poor look angle elevations at higher latitudes 3. Must be pointed to acquire a satellite	1.Short life 2.Short LOS 3.Short call interruptions depending to the position

Benefits of LEO

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.

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.

Public benefits due to flexible design

The digital technology deployed allows the total connection for all voice and data services on a seven day by twenty−four hour basis. This allows the flexibility of service provisioning. The LEO overcomes some of the limitations of the higher transport systems, such as the delay in the round trip transmission. Because the satellites are low, the user set needs a lower power output device. This orbit has been selected to be the most flexible.

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.

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

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.