This chapter reviews the status of technologies for the space segment of communications satellite systems. The discussions highlight changes since the 1992/1993 study and emphasize developments that are both new and important.
Although the tendency to emphasize satellites in any discussion of satellite communications was in evidence during site visits, a pleasant change was the recognition of the importance of satellite terminals both in the economics and user acceptance of systems.
Designers of systems have exercised the tradeoffs between satellite and earth terminals in the conscious attempt to achieve high system capacity while realizing a small and affordable terminal.
Some designs for mobile satellite systems have reduced the orbit altitude and employed a larger than familiar number of satellites to achieve continuous global coverage. The reduced altitude reduces the propagation (spreading) path loss, which can be traded for lower transmit radiated power. Although this design approach leads to smaller satellites, innovations have nevertheless been introduced, such as use of Global Positioning System (GPS) (satellite) receivers on satellites for autonomous station keeping; multiple crosslink (intersatellite link) antennas; phased array antennas for up and downlinks; and onboard baseband processing.
The assembly of satellites used to proceed on the basis "one of a kind - one at a time." Some manufacturers (notably for the Iridium and Globalstar satellites) have adopted techniques from the automobile industry by setting up assembly lines and generally reducing the extensive environmental testing conducted on satellites prior to launch. Manufacturers of geosynchronous earth orbit (GEO) satellites have also streamlined assembly by concentrating on standard buses.
While low and medium orbit constellations attracted much attention in the past few years, geostationary orbit communications satellites continue to thrive. The ability to keep coverage fixed and provide high capacity over long distances may offer the possibility of gradual market entry or market development. In any case, there is continued development of ever larger buses to support ever increasing antenna size and complexity, large numbers of transponders, and other dimensions in sophistication and complexity.
The trend in GEO satellites is increased power and increased number of transponders. Satellites with numerous C and Ku-band transponders are becoming commonplace. These increases have not resulted in a proportionate scaling of the weight of the satellites, since the use of shaped antennas eliminates the need for considerable microwave plumbing and the use of lighter structures has helped contain the weight of the satellites. Nevertheless, GEO satellites are becoming heavier and launch capability is increasing to accommodate the additional features of modern satellites. Increased power is driven by the desire to decrease the ground terminal size and cost, appealing features for end-consumer equipment. Figure 3.1 provides some comparison between GEO satellites of the 1970s, 1980s and early 1990s, and buses now in development in terms of mass and power. The figure shows on a related scale how the increases in power have allowed a steady reduction in terminal sizes. Given that a size on the order of a foot has been attained, further increases in power may be viewed as making larger capacities possible with these small terminals, or adding other features such as other frequency bands or crosslinks (i.e., as opposed to further reduction in terminal size).
Fig. 3.1. Satellite power/mass and terminal antenna size trends.
This chapter begins with a discussion of critical technologies of large GEO satellites where the primary power system is growing rapidly towards 20 kW and more. Satellite antennas are discussed in some detail since this is one of the most critical areas in measuring communications progress. This is followed by a discussion of onboard processing, progress in satellite traveling wave tubes and solid state power amplifiers, optical ISLs and some satellite bus issues (electric propulsion, thermal control and attitude control). Larger satellite antennas imply smaller beams and a need for tighter attitude control. In all cases emphasis is placed on what was learned in the site surveys.
Small and mini-satellites derive much of their technology from that of GEO satellites and therefore are not treated in detail. The major aspect of these satellites is the process adjustments made to transform the former one-at-a-time, hand made approach taking three to four years, to a more streamlined, production oriented approach for producing satellites.