Spot beams make Ka-band different

Ka-band satellites will provide much larger capacity and flexibility than preceding designs. This characteristic makes them the first true broadband systems. What is fundamentally different with Ka-band is the satellite’s ability to reuse bandwidth many times over Figures la-1c show the antenna patterns of C-, Ku- and Ka-band satellites on the Earth. For a fixed size antenna on the spacecraft, each doubling of frequency results in roughly a four-fold increase in the number of independent beams, each with a separate data signal, that can be formed on the earth and a comparable increase in capacity.

Bandwidth is one of two limited resources on a satellite — the other is transmit power, supplied by the solar cells that charge on-board batteries. Satellite system designers often use transmit power as a surrogate for satellite cost as most of the cost drivers (solar array size, power amplifiers, spacecraft thermal dissipation, etc.) scale with increased transmit power. Fortunately for the satellite manufacturer, the much greater capacity of Ka-band does not require an increase in satellite power. Since each signal is now focused on a much smaller area, it takes less power per beam to illuminate that area at a given signal strength. Overall, the system transmit power is comparable to that of Ku-band satellites.

Terminal size and cost are also important factors in meeting user needs. The factor that drives the terminal size is the need to point the antenna at a desired satellite while avoiding interference from an adjacent spacecraft. The higher Ka-band frequencies make it possible to reduce user antenna size to one-half the minimum possible at Ku-band — less than 1 meter, which is suitable for residential and small office use. Table 1 shows how the resulting change in these parameters between C-, Ku- and Ka-bands dramatically affects the size and market size of the resulting product.

Mature technology enables broadband satellites

On-board processing payloads have been designed into government satellites such as ACTS and Milstar since the early 1980s. Recently, the technology required has reached the maturity and scale needed to be practical for commercial use. Two breakthrough steps have been the development of high volume monolithic microwave integrated circuits (MMIC) and radiation-hardened digital components that approach the speed and density of computer parts used in personal computers. Originally developed for military applications, the rapidly growing use of MMIC devices for cost and performance enhancements in cellular phones and embedded wireless devices has provided the manufacturing volume needed to produce a large variety and volume of satellite transmit and receive components at low cost. Space-qualified digital devices, long hampered by speed constraints that came with radiation hardening, have also reached parity with commercial performance and are enjoying the same gains in volume production. Densities of millions of gates allow development of the highly programmable processors needed to provide the flexibility desired for a long-lived payload design. Upgrades to router parameters and operation are typically made via uploads of new software from the ground.

The need to be at the leading edge has other benefits, with satellite electronics specialists like TRW spinning off their technologies into terrestrial telecommunications markets. Products like high performance microwave chips that extend the talk time of cell phones, high bandwidth radio links that can interconnect buildings without fiber, ultra-high-speed optical communications devices and more are, in reality, the offshoot of extensive government and commercial satellite research and development. Even as these products are bringing ever higher data rates, lower cost and improved functionality to terrestrial markets, the original technologies are well on their way to building the next generation of satellites targeting the same markets but with much broader geographic scope.

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