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Satellite ConstellationThe Globalstar constellation consists of 48 LEO (low-Earth-orbiting)
satellites, plus an additional four satellites in orbit as spares.
Each consists of an antenna, a trapezoidal body, two solar arrays
and a magnetometer, and operates at an altitude of 1414km (876 miles).
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| The satellites are placed in
eight orbital planes of six satellites each, inclined at 52 degrees
to provide service on Earth from 70 degrees North latitude to 70 degrees
South. Because of this configuration, the polar regions, including
most of Greenland, small parts of Alaska, Canada, Scandinavia, Siberia,
and regions in the Southern Hemisphere, including Antarctica and parts
of South America are not covered. |
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| "Bent-Pipe" Technology |
| Like "bent-pipes", or mirrors
in the sky, the Globalstar constellation of satellites can pick up
signals from over 80% of the Earth's surface. Several satellites pick
up a call, and this "path diversity" assures that the call does not
get dropped even if a phone moves out of sight of one of the satellites.
If buildings or terrain block your phone signal, a "soft-handover"
takes place, and the call's transmission is switched to an alternate
satellite with no interruption. This satellite now maintains transmission
of the original signal to one of several terrestrial Globalstar Gateways.
Because all the switches and complex hardware are located on the ground
in the Gateways, Globalstar satellites are relatively simplistic,
leading to dramatically increased system reliability. |
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| Additional advantages of using
low-Earth-orbiting satellites within the Globalstar Communications
System include lighter, smaller, all-in-one handsets and no perceptible
voice delay. LEO satellites also minimize call transmission time delays.
Long transmission delays cause noticeable gaps between the time a
caller speaks and when the listener hears what is being said. |
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| Satellite Footprint |
| The satellite coverage beams
for satellite phone communication links and the Gateway communication
links are quite large. The picture to the right gives you a sense
of scale for the coverage of a typical thirty-six hundred mile diameter
beam footprint. |
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| Each footprint moves rapidly
across the Earth's surface. A satellite that passes directly overhead
is visible to that spot for only about 15 minutes. |
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| When a user places a Globalstar
satellite call, the nearest satellite picks up the signal. Globalstar
satellite phones can operate with a single satellite in view, though
typically two to four satellites will be overhead. This simultaneous
coverage by multiple satellites is called "path diversity," which
enhances the quality of the Globalstar satellite call. |
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| Path Diversity |
| Path diversity is a method
of signal reception that combines multiple signals of varying power
strengths into a single coherent signal. Satellite phones communicate
with as many as three satellites simultaneously, combining those signals
into a single, static-free signal. Globalstar satellite phones also
alter power levels to compensate for shadowing and interference as
needed. |
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| As satellites move in and out
of view, they will be seamlessly added to and removed from the calls
in progress, reducing call interruption. This enables the Globalstar
system to provide superior service to a wide variety of locations
with less potential for signal blockage from buildings, terrain, or
other natural features. |
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