Deployment and Spacing of Satellites
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Motorola s concept of seven different orbits on a polar orbital path would lend itself to the potential of collisions in mid-air if the spacing were not correct Therefore, the spacing design had to take into account that the satellites were all traversing the same end-points where the paths would all cross One can imagine that the two poles of the earth are the midpoints in the orbit The Iridium network originally consisted of 77 satellites, however, a modification to the plans dropped the number of craft to 66 To assign or determine the best approach, Motorola took into account six critical criteria in determining the best approach These are the following 1 The need to provide a single global coverage over the entire earth s surface for availability at all times This means that each subscriber must have at least one satellite in view at all times to provide coverage 2 Some portion of each orbit must be available to allow for low power outputs, which accommodates the recharging of the communications power subsystems This minimizes the size requirements of the craft 3 The relative spacing of the satellites and the line-of-sight relationships had to allow the on-board systems to control cross system linkage 4 Costs for the entire constellation was a concern in the selection of the orbit and spacing requirements Minimizing costs was a portion of the decision-making process 5 The angle of incidence from the end user to the spacecraft as measured from the horizon to the line-of-sight communications process allowed link margins to accommodate the low-powered user device The slant angle was selected at 10 to meet these criteria 6 The final criteria was the operational latitude of the spacecraft Systems operating over 600 nautical miles were affected more by radiation, which drives the cost up for the systems Altitudes lower than 200 nautical miles required much more fuel and control over the positioning of the craft, thereby driving up the cost At 413 nautical miles, the fuel consumption and command and control over the craft position coupled with the better radiation performance, the cost and coverage ratios provided a better solution
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The satellites were spaced at 327 degrees apart, traveling in the same basic direction and moving at approximately 16,700 miles per hour from north to south and 900 miles per hour westward over the equator Given this path, each satellite was designed to circle the earth approximately every 100 minutes At the equator, a single device would provide coverage However, as the craft moved toward the poles, overlap occurred, increasing the levels of coverage above and below the equator The expected life cycle of the satellites was around five years The basic building blocks of the Iridium system provided the model for other networks ready to launch (such as Globalstar systems) In each case, the systems used proven technology for radio transmission in well-established frequency bands The basic system was composed of the following: 1 The space segment comprised of the constantly moving constellation of satellites in a low earth orbit 2 A gateway segment comprised of earth station facilities around the world 3 A centralized system control facility 4 The launch segment to place the craft in the appropriate orbit 5 A subscriber unit to provide the services to the end user
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The Space Segment
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The space segment consisted of small satellites, operating in the low earth orbit, which were all networked together as a switched digital communications system In Figure 25-4 , a sample outline of the beam coverage with Iridium is shown Each satellite used up to 37 separate spot beams to form the cells on the earth s surface Multiple relatively small beams allowed the use of higher satellite antenna gain and reduced RF power output The system was designed to operate with up to 250 independent gateways, although in its initial deployment only between 5 20 gateways Figure 25-5 is an overall view of the network communications through the gateways and the handsets
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