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TABLE 17.4
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Link-Budget Parameters Gateway-satellite Satellite-subscriber Uplink 148.95 7.5 11.3 26 2.4 Downlink 137.5 12.5 11.3 28.6 4.8
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Range 2730 km Frequency, MHz [EIRP], dBW Misc. losses, dB 1 [G/T], dBK Data rate, kbps
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Uplink 149.4 40 7.3 33.3 57.6
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Downlink 137.2 6.5 7.3 12.8 57.6
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Orbcomm, 1993.
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The messaging and data channels are located in the VHF band, the satellites receiving in the 148 to 149.9-MHz band and transmitting in the 137 to 138-MHz band. Circular polarization is used. In planning the frequency assignments, great care has been taken to avoid interference to and from other services in the VHF bands; the reader is referred to Orbcomm (1993) for details. In particular, the subscriber-to-satellite uplink channels utilize what is termed a dynamic channel activity
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Orbcomm/Microstar satellite. (Courtesy of Orbital Communications
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Corporation.)
Seventeen
assignment system (DCAAS), in which a scanning receiver aboard the satellite measures the interference received in small bandwidths, scanning the entire band every 5 s or less. The satellite receiver can then prepare a list of available channels (out of a total of 760) and prioritize these according to interference levels expected. The Orbcomm system is capable of providing subscribers with a basic position determination service through the use of Doppler positioning, which fixes position to within a few hundred meters. The beacon signal at 400.1 MHz is used to correct for errors in timing measurements introduced by the ionosphere (these errors are also present in the GPS system described in Sec. 17.5, and two frequencies are used in that situation for correction purposes). When used in conjunction with the VHF downlink signal, the beacon signal enables the effects of the ionosphere to be removed. It will be observed from Fig. 17.5 that the satellites carry GPS antennas, which enable onboard determinations of the positions of the satellites. This information can then be downloaded on the VHF subscriber channel and used for accurate positioning. One significant advantage achieved with LEOs is that the range is small compared with geostationary satellites (the altitude of the Orbcomm satellites is typically 800 km compared with 35,876 km for geostationary satellites). Thus, the free-space loss (FSL) is very much less. Propagation delay is correspondingly reduced, but this is not a significant factor where messaging and data communications, as compared to real-time voice communications, are involved. The Orbcomm system provides a capacity of more than 60,000 messages per hour. By using digital packet switching technology, and confining the system to nonvoice, low-speed alphanumeric transmissions, Orbcomm calculates that the service, combined with other LEO systems, will be able to provide for 10,000 to 20,000 subscribers per kilohertz of bandwidth, which is probably unmatched by any other two-way communications service. Although it is a U.S.-based system, because of the global nature of satellite communications, Orbcomm has signed preliminary agreements with companies in Canada, Russia, South Africa, and Nigeria to expand the Orbcomm service (Orbcomm, 1994). The Orbcomm Web site is http:www.orbcomm.com 17.7 Iridium The Iridium concept was originated by engineers at Motorola s Satellite Communications Division in 1987. Originally envisioned as consisting of 77 satellites in LEO, the name Iridium was adopted by analogy with the element Iridium which has 77 orbital electrons. Further studies led to a revised constellation plan requiring 66 satellites (Leopold, 1992). The original company, Iridium LLC went bankrupt in 2000. In December
Satellite Mobile and Specialized Services
of that year a group of private investors organized Iridium Satellite LLC, which acquired the operating assets of the bankrupt Iridium LLC including the satellite constellation, the terrestrial network, Iridium real property, and intellectual capital. The 66 satellites are grouped in 6 orbital planes each containing 11 active satellites. The orbits are circular, at a height of 780 km (485 miles). Prograde orbits are used, the inclination being 86.4 . The eleven satellites in any given plane are uniformly spaced, the nominal spacing being 32.7 . An in-orbit spare is available for each plane at 130 km lower in the orbital plane. Some of the other orbital characteristics are listed in Table 17.5. The satellites travel in corotating planes, that is, they travel up one side of the earth, cross over near the north pole, and travel down the other side. Keeping in mind that there are eleven equispaced satellites in each plane, it will be seen that both sides of the earth are covered continuously. The satellites in adjacent planes travel out of phase, meaning that adjacent planes are rotated by half the satellite spacing relative to one-another. This is sketched in Fig.17.6, which shows the view from above the north pole. Collision avoidance is built into the orbital planning, and the closest approach between satellites is 223 km. Satellites in planes 1, 3, and 5 cross the equator in synchronization, while satellites in planes 2, 4, and 6 also cross in synchronization, but out of phase with those in planes 1, 3, and 5. Although the planes are corotating, the first and last planes must be counter-rotating where they are adjacent. This is illustrated Fig. 17.6. The separation between corotating planes is 31.6 which allows 22 separation between the first and last planes. The closer separation is needed
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