Satellite-communications (satcom) systems are not the most cost-effective means of connecting two points. They have limited bandwidth and capacity, to say nothing of the effort required to get a satellite into orbit around the earth. However, compared to terrestrial metal wire, optical fiber, and even wireless communications systems, satcom links do offer some advantages.
Satcom systems can connect with difficult-to-reach areas, operate with variable information rates, and be adapted to the needs of many different customers. Some customers—in particular, those needing communications links to new areas at short notice, such as in broadcast and satellite-news-gathering (SNG) applications—have found satcom services to be as invaluable as they are reliable. And military customers [e.g., those using satellites for surveillance functions or for remote control of unmanned aerial vehicles (UAVs) in hostile locations] are finding satellites difficult to replace.
1. Large, fixed satcom earth-station terminals such as this dish on Mount Rushmore provide reliable communications at high data rates. (Photo courtesy of Cobham plc.)
As a business, demand for satcom services is strong and growing. Because satellites can support reliable voice and video communications and high-speed data communications almost anywhere on the planet (including on ships, planes, and other moving vehicles), they have become the preferred means of communication for commercial and military customers alike. In fact, with many US military customers become more cost-conscious as their budgets are being tightened, satcom service providers are exploring novel possibilities—for instance, using one rocket to launch multiple satellites, or consolidating capabilities once handled by multiple satellites on board a single one.
Satcom systems consist of the space-based equipment [such as antennas, receivers, frequency converters, filters, low-noise amplifiers (LNAs), switches, transmitters, and high-power amplifiers (HPAs) for transmission on board a satellite], as well as the earth-based equipment based on the same lineup of components, stored in a fixed or movable earth terminal. Fixed-earth terminals (Fig. 1) can be integrated with earth-based communication infrastructure, including fiber-optic links and wireless networks. Mobile terminals (Fig. 2) offer the flexibility of portability and being able to make connection points where needed.
2. Portable satcom earth-station terminals such as this provide flexibility for mobile broadcast and SNG applications. (Photo courtesy of IABG.)
The components that support these satcom links operate at assigned frequencies, including C-, Ku-, and Ka-band frequency bands. These components must overcome path loss from the earth to the satellite (also known as the uplink) and from the satellite back to the earth (the downlink). Satcom systems employ higher frequencies for the uplink than for the downlink. In a C-band satcom system, for example, uplink frequencies extend from 5.925 to 6.425 GHz while downlink frequencies stretch from 3.70 to 4.20 GHz. In a Ku-band satcom system, the uplink frequencies span 14.0 to 14.5 GHz while the downlink frequencies are from 11.7 to 12.2 GHz. Satcom systems also work with S-band frequencies (from 2 to 4 GHz) and Ka-band frequencies (from 26.5 to 40.0 GHz).
For example, one of the world’s leading satcom service providers, Intelsat Global Services Corp., holds the rights to different sectors around the globe for satcom services at C-, Ku-, and Ka-band frequencies, with typically one frequency band per satellite. But the firm’s new Intelsat EpicNG series of satellites will feature an open architecture; they are intended to use wide beams and spot beams for high throughput data communications, in addition to operating with all three frequency bands per satellite. The Intelsat Epic satellites are being groomed for broadband connectivity across North America, including Internet access aboard commercial aircraft and ocean liners.
The first of the Epic satellites, to be built by leading satellite supplier Boeing, is scheduled for launch in 2015. Until then, Intelsat will be launching the first of its Boeing-built Ka-band Global Express satellites in 2013, with full global coverage achieved by 2014. The Global Xpress network is expected to provide downlink speed to 50 Mb/s and uplink speeds to 5 Mb/s, employing compact user terminals.
Satcom systems are constructed not only with many different frequencies, but with satellites at many different orbital heights. An excellent introduction to satcom technology is available free of charge from MITEQ, in the form of a PowerPoint presentation prepared by company President Howard Hausman. The presentation covers the different types of satellites used in satcom systems, including fixed-service satellites (FSS) and geostationary versions, as well as how satellite orbital heights affect coverage areas and transmission effects (such as transmission latencies and Doppler shifts that can impact the complexity of a satcom transceiver design). Geosynchronous satellites, for example, orbit 35,800 km (22,300 miles) above the Earth, at about 11,000 km/hour, to remain in the same location orbiting above the earth and providing radio coverage for about one-third of the planet.
Satellites that are not geostationary fly at greatly reduced distances above the earth. Low-earth-orbit satellites (LEOS) cruise about 100 to 300 miles above the earth, with reduced launch costs and much less path losses than geosynchronous satellites, but much less visibility of the earth. The IRIDIUM network is an example of a LEOS system. At somewhat higher distance from the earth, medium-earth-orbit satellites (MEOS) networks maintain their space vehicles about 6000 to 12,000 miles above the earth.
Orbital locations are regulated by the International Telecommunications Union (ITU). Satellites transmit over a 17.3-deg. beamwidth to achieve that one-third coverage of the planet. They are spaced at least 1.5 to 2.0 deg. apart so that antennas on satcom earth terminals will not illuminate more than one satellite at a time. Components that comprise the payloads on board these satellites must be extremely reliable, and guidance for what defines a “space-qualified” component can be found in documentation prepared for the US Department of Defense (DoD), such as guidelines detailed in the MIL-PRF-38534 Class K standards for commercial and government spaceflight equipment.
Requirements for satcom components differ, of course, for ground-based and space-based applications. While components used on satellite must be as small and lightweight as possible, similar components used in the terrestrial portions of satcom systems are not driven by the same needs. In some cases, satcom component suppliers such as MITEQ may even incorporate different technologies for the space-based and ground-based satcom terminals, such as solid-state power amplifiers (SSPAs) for in space and traveling-wave-tube amplifiers (TWTAs) for ground terminals.
Suppliers of complete satcom systems include some very large companies, such as Boeing, Northrop Grumman, and Lockheed Martin. Sources of RF/microwave components for those systems include MITEQ, its high-power-amplifier company MCL, and a large number of component and subsystem suppliers that design and produce hardware for both satellite payloads and earth-station terminals. These include Admiral Microwaves, Aeroflex Corp., Cobham plc, Communications & Power Industries, EM Solutions, Epsilon Lambda Electronics Corp., ESA, General Dynamics Satcom, Krytar, Linx Technologies, Inc., Marki Microwave, MCLI, Meca, Millitech, Narda Microwave, PMI RF, Q-par Angus Ltd., Spacek Labs, Teledyne Microwave, UKRF, and ViaSat. Ensuring that RF/microwave components are ready for use in space requires proper preparation but, as reported late last year, these firms are equipped with the testing and screening capabilities.
As noted, the design and construction of similar functions, such as antennas and amplifiers, can differ significantly whether that component is intended for earth or space use. Of course, in some cases, a single design may prove suitable for both applications, such as the model MT2100 TWTA from MCL that was engineered for airborne applications. The tube amplifier delivers as much as 125 W CW power from 6 to 18 GHz but weighs only 25 lbs. and can be adapted to military flight qualification requirements. The standard unit employs conduction cooling, but can be modified for air or liquid cooling.
Finally, in addition to offering a variety of high-power TWTA-based amplifiers for indoor and outdoor satcom use, the CPI Satcom Division of Communications & Power Industries also designs solid-state PAs for satcom applications at C-, X-, Ku-, and Ka-band frequencies. These include output-power levels to 100 W at Ku-band frequencies and 200 W at C-band frequencies.