Low-Earth Orbit satellites

Chapter 27: Low-Earth Orbits satellites

                                                                                     

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 . This would provide a continuous

line−of−sight coverage area from any point on the globe to virtually any other point on the globe,

using a spot beam from the radio communications services on−board each of the satellites. The use

of this spot beam concept, which had been discussed for years in the satellite industry, allowed for

high frequency reuse capacities that had not been achieved before. Iridium wanted to provide the

services outlined in Table 27−2. Motorola also suggested that an interconnection arrangement

would be set up with all providers around the world through an arrangement with the local Post

Telephone and Telegraph (PTT) organizations. The concept was sound, and the approach would

have provided for the coverage that was lacking in the past to remote areas. In the table, the two

columns are used as exclusive of each other. The services can be provided in any of the coverage

areas regardless of which service is selected.

A communications satellite in orbit 400 to 1600 miles above the earth. Being much closer than 22,282 mile-high geosynchronous satellites (GEOs), LEO signals make the round trip from earth much faster. Thus, low-powered "pizza dishes" and handheld devices can be used. LEOs are also better suited to interactive conferencing. Unlike GEOs, which travel at earth speed, LEOs revolve around the globe every couple of hours, and any single LEO is in view for only a few minutes. In order to maintain continuous communications, multiple LEOs must be used. From 48 to 66 LEOs are needed to cover the earth.

Advantages to LEO Satellite Telemetry

• Global applications in developing a remote environmental monitoring communication system.
• Two-way communications
• Easy to set up and low maintenance costs
• Low profile, non-directional whip helix antenna
• Easy access to data
• No FCC or other governmental agency requirements for data transmission
• Coverage in very remote areas
• Data is proprietary
• Stevens is able to quickly detect any problems with transmission of data
• Systems verifies that data has been transmitted which minimizes risk of missing data.
• Event notification on line, by pager, etc.
• Lower power transceivers compared to GEO transmitters.
• LEO Transceivers hardware system is lower in cost than a GEO transmitter system.

Disadvantages to LEO Satellite Telemetry

• Monthly service fee which could be expensive with frequent transmissions of data.
• Power outage at GCC would shut down the communication server, which could delay transmission of data to end user until power is restores. However, no data is lost.
• LEO satellites have a much shorter life span (five to eight years) than GEO satellites.