LEO vs. GEO Satellites when used with Mobile Satellite Services
A Low Earth Orbit or (LEO) is any Earth orbit up to 1,500 kilometers in altitude. LEO satellites rotate the earth and currently deliver significant voice quality over the Geosynchronous (GEO) satellite systems. Globalstar and Iridium constellations both use LEO satellites. GEO satellite systems orbit at an altitude of 35,000 kilometers (22,369 miles) above the earth’s surface. GEO satellites never change location they move with the earth. GEO satellites are basically stationary over a certain region of the earth. They are best used for Television transmission and high-speed data transmission.
The Globalstar Satellite constellation orbits 1,414 kilometers (878 miles) above the earth’s surface. A Globalstar satellite takes less than two hours to complete a full rotation. Their relative low orbit allows for the highest voice and quality available in the industry due to the short distance the signal must travel. Even using low powered hand held satellite phones a user can enjoy high quality voice clarity similar to a digital cellular phone.
The Iridium Satellite constellation orbits 485 miles above the earth’s surface. An Iridium satellite traveling at 17,000 miles an hour will orbit from pole to pole in 100 minutes. Unlike the Globalstar constellation the Iridium constellation communicate with each other using intersatellite links. This allows for the total planetary coverage only Iridium can offer. Each satellite had four intersatellite links two to communicate with satellites on either side and tow to communicate to other satellites for and aft in the same orbital plane. The disadvantage using intersatellite links is if the signal links through numerous satellites before transmission to an Iridium ground station the user may experience echo.
The Globalstar constellation consists of 48 LEO satellites that orbits have an inclination of 52 degrees compared with Iridium’s near-polar 86.4 degree orbits. Globalstar does not cover the poles due to the lower orbital inclination. The Globalstar satellites have no inter-satellite linking like the Iridium satellites have. The Globalstar satellites are simply bent pipe repeaters. The Iridium constellation consists of 66 active satellites in LEO orbit. Given the Globalstar and Iridium satellites speed and the vast number of satellites in orbit, any coverage gaps are rare and are corrected in minutes.
Both the Globalstar and Iridium satellites use Omni-directional antennas that do not require calibration like a satellite TV dish would require. Using an Omni-directional antenna only requires an 80% view of the sky to provide good service. Being able to use relatively small Omni-directional antenna allows the use of portable hand held devices. GEO systems use expensive, bulky directional antennas that must be pointed and calibrated to acquire a satellite.
Currently GEO satellites have limited capacity to service mobile users. The spectrum for Mobile Satellite Systems are scarce so a MSS system require satellite systems that can maximize the use of the limited spectrum while providing multiple satellites to share the system. Currently GEO Systems presently use Frequency division multiple access frequency modulation (FDMA-FM). This type of technology requires inefficient band segmentation to share the spectrum.
The Globalstar and Iridium LEO constellations will continue to perform even when a satellite in their constellation fails. This is due to the vast number satellites, constant rotation and speed at which the satellites travel. The GEO directional antennas must be pointed directly in the area of service for the satellite to work. If a GEO satellite fails an entire region could experience a systems outage. Globalstar and Iridium LEO satellites provide multiple paths of communications with a system constantly in motion.
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