|
2.3.2
MSS Development
Development of MSS, introduced in Chapter 1, started with Marisat in 1975 and
evolved through the various satellites used for maritime applications. Inmarsat,
originally an international organization like INTELSAT, was established in 1979
to provide worldwide mobile satellite communications for commercial, emergency,
2.3
Specialized Systems: DTH and Mobile
63
and safety applications at sea and in the air. Inmarsat services were extended to
land-based users with the Inmarsat M and Mini-M standards. The Inmarsat 4
satellites have sufficient power to support portable user terminals, which in the
case of B-GAN are the size of a laptop computer (Figure 1.7).
Domestic MSS satellite services began to appear in the mid-1990s, when Optus
Communications decided to include an L-band payload on its second-generation
Ku-band satellites. The Optus MobileSat team designed and implemented the
ground segment to make mobile telephone calls possible from conventional vehicles.
In the United States, AMSC was formed from eight applicants for domestic licenses.
The consortium floated its own series of stock on the public stock market to raise
sufficient capital to fund a satellite and the associated gateway Earth station. Even
with a dedicated high-power satellite, the AMSC network could serve only vehicular
and portable terminals (similar to the mini-M terminal of Inmarsat). It would take
satellites either much higher in power or, alternatively, closer to the Earth to close
the link to a hand-held type of MSS telephone instrument.
The real excitement in mobile satellite services was created by Motorola when
it announced the Iridium project in 1990. As discussed in Chapter 1, Iridium
employs a LEO constellation of 66 satellites (the original proposal was for 77
satellites, hence the selected name Iridium, which is the element in the periodic
table with 77 orbiting electrons). Loral followed suit with Globalstar, which was
based on CDMA technology from Qualcomm.
The Iridium space segment, which is still in operation at the time of this writing,
employs satellites similar in design to GEO satellites, with the exception of its
smaller size and configuration adapted to the orbit and operational requirements.
Built by the team of Motorola, Lockheed Martin, and Raytheon, the Iridium
satellites demonstrate the principle of nearly continuous coverage of the entire
globe in the delivery of a packet-switched voice and low-speed data service. The
intersatellite links, called cross links, add the benefit that a user with a handheld
Iridium subscriber unit can communicate directly with another such user who can
be located anywhere else on the Earth’s surface. Some of the same capabilities were
instilled in the Globalstar system, developed by Loral and Qualcomm. In this case,
the satellite is a bent-pipe design without the onboard processing and switching
found in Iridium. Using CDMA instead of TDMA, Globalstar has some added
features that make the system more flexible in terms of services within its coverage
footprint. Another LEO system called Orbcomm also went into operation this time
for the purpose of providing low-speed data relay.
The entire MSS business went through an upheaval during the late 1990s and
early 2000s, caused to a significant extent by the bankruptcy of Iridium, Inc., in
1999. Globalstar, which completed their system as well, was also the subject of
bankruptcy proceedings. Both systems have come out of bankruptcy with new
owners who offer services to a much smaller customer base than originally defined.
Meanwhile, MSS satellites to serve handheld devices were produced for regional
systems in the Middle East (Thuraya) and Asia (ACeS). Inmarsat developed the
more powerful and capable Inmarsat 4 series of satellites (Figure 2.13) that provide
both Internet access and ISDN services to portable and mobile users.
Another development on the MSS front was the creation of a hybrid concept
of satellite and cellular-type wireless communications in the same band, called
64
Evolution of Satellite Communication
Do'stlaringiz bilan baham: |
