TechTalk: Future Trends: Satellite Communication Antennas
by Eric Amyotte and Dr. Luís Martins Camelo, Macdonald, Dettwiler and Associates (MDA)
Predicting the future is a very uncertain science. Nevertheless, in the satellite business, the trend for ever-increasing capacity, flexibility and availability of service, as well as increasingly more affordable, more compact, lighter, and even more stylish and ergonomic ground and personal terminals, has become a clear reality of this communications market segment. There is no reason to believe that these trends towards perpetual improvement will change markedly in the future, and that competitive pressures to achieve these objectives earlier and for a lower price will ever disappear. Moreover, the initial conceptualization and design phases of future missions tend to start many years before the issuance of the first associated Requests for Proposal (RFP), and therefore they provide a rather reliable and solidly based window into the future of the business and of the technology.
Even if many such initial studies will never materialize, or will be repeatedly pushed further into the future, the underlying trends are nevertheless very apparent and a pattern of future evolution can be drawn. In addition, many missions are now in rather advanced stages of planning and conceptualization and their probability of being funded and becoming reality can be considered high.
Business ambitions tend to lead the available technical solutions, but the latter are constantly being honed and advanced by R&D efforts around the world, in a very competitive environment in which the financial stakes can be high and the challenges to be overcome are ever-present. Some of our thoughts on these future trends are expanded upon in the following sub-sections.
Increased Power Levels
The DC power capability of spacecraft platforms is always increasing as new solar array and battery technologies evolve and become more affordable. More power available on the satellite allows for ever improving link budgets, and payloads that fully use this capability become a necessity.
For satellite antennas this means a greater number of antennas on board (maintaining the thrust towards compact low-mass designs) and higher transmit (i.e., down-link) RF power levels with the consequent power handling issues. These include more challenging multipactor and Passive Inter Modulation (PIM) requirements as well as more efficient thermal management solutions.
Multipactor is an electron avalanche phenomenon that can be established in a vacuum between two surfaces when certain conditions are met, often meaning a relatively high voltage across a relatively small gap (measured in terms of the wavelength). This phenomenon can inflict serious permanent damage to the on-board equipment, and must, generally, be avoided. Meeting future multipactor requirements will not solely hinge on developing new designs with higher threshold voltages. It will also necessitate a better understanding of the phenomenon in multi-carrier environments as is usually the case in communications satellites, more accurate modeling, more extensive testing facilities and a more pragmatic approach to the requirements specifications.
Passive Inter Modulation occurs when multiple transmit RF carriers propagate in a non-linear medium, such as is obtained, for example, when dissimilar materials are put in physical contact with each other. The PIM products represent an undesired noise that interferes with the intended communications signals and might prevent the achievement of the required signal-to-noise levels. The challenge associated with PIM under higher power levels will be compounded by the extended operating frequency bands of future systems. Wider frequency bands enable the occurrence of lower order passive inter modulation products, generally implying a much stronger level for the PIM signals.
Wider Bands + V-Band
Higher capacity systems call for increased bandwidths, and since the level of difficulty when designing antennas increases with the bandwidth as a percentage of the center frequency, higher center frequencies are necessary. The Ka-band market is still growing, but higher frequencies such as V-band offer significant potential for increasing system capacity and are now starting to be exploited. The IEEE definition of V-band is 40 to 75 GHz, but for communications satellites this usually means transmit (down-link) signals in the range of 40-46 GHz and receive (up-link) signals in the range of 48-56 GHz.
Although the bandwidths as a percentage of the center frequency are generally lower at V-band, many other aspects are harder to achieve than at Ka-band (tighter manufacturing tolerances, higher RF losses, higher atmospheric propagation losses, much higher losses due to precipitation, and lower efficiency electronics, among others). Consequently, while the required V-band technological advancements are being pursued, cheaper Ka-band systems will continue to be preferred in the near future, as long as the frequency spectrum remains available. The first commercial use of V-Band may well be in gateway links for multibeam Ka-Band missions, replacing the currently used Ka-Band gateway links, and thus increasing the Ka-Band spectrum available for the user beams.
Combined Frequency Bands
As stated previously, satellite platforms are becoming increasingly more powerful, and their power/volume ratio is increasing. Consequently, missions are becoming limited by the real-estate available to mount antennas on a spacecraft. Combining antennas to save spacecraft real-estate and increase spacecraft revenues has become one of the trends, expected to last and intensify way into the future.
Combining Tx and Rx into the same antenna is already a prevalent feature of modern satellite designs, and this tendency will continue and strengthen in the future and will also lead to antennas combining more than one frequency band. The corresponding design challenges are already the object of many R&D projects around the world. The implementation of PIM-free multiband antennas will call for advanced low-loss multiplexer technologies such as the triplexers and quadruplexers that have been required on some recent programs.
Ever increasing gain requirements will call for ever increasing antenna aperture sizes, which can be most efficiently achieved with reflector antenna configurations. Unfurlable mesh reflectors are commercially available, have already been used in many satellite missions and, although their price is high, they are currently the most practical means to implement these large aperture diameters.
The reflector diameter range covered by this technology is currently between 6m and 22m, although even larger reflectors will likely be available in the future. For Ku-band and higher frequency bands, the need for smaller diameters and for tight reflector surface tolerances has so far been fulfilled by solid reflector technology, often using Carbon Fiber Reinforced Plastic (CFRP) construction. Their size is currently limited, by the volume available inside the launch vehicle fairing, to about 3m in diameter.
High accuracy reflectors in the range of diameters between 3 and 6m have not yet been developed, and are likely to be required by future wideband multiple spot beam applications. Some additional folding and deployment may be required for the larger reflectors in this diameter range so as to fit within the allowable stowed envelope, once the spacecraft volume limit or the launch vehicle fairing dimensional limits are reached. In this case, the reflector may be built as several deployable solid parts rather than one single solid reflector structure. These larger solid reflectors may also incorporate semi-rigid parts into their construction.
The need for in-orbit reconfigurability has been gaining momentum in FSS/DBS communications satellites over the last few years. Operators would like to have the ability to reconfigure their spacecraft in orbit in order to cope with changing traffic requirements, or to be able to re-assign the spacecraft to cover a different service area or the same region from a different orbital location. These needs are accentuated by the long mission life of modern satellites, commonly reaching 15 years or more.
The market demands evolve substantially during that time and the original satellite configuration may no longer be optimal to meet them during the later stages of the mission. The design challenge is accentuated by the fact that, although operators are always keen on getting more flexibility, they do not necessarily want to pay substantially more, nor increase the risk profile of their program, in order to obtain this flexibility. Reconfiguring an existing satellite, if such capability has been built into the design from the start, is usually the most economical and less risky approach to meet evolving market demands.
Antennas can be reconfigured by mechanical means (whereby the original antenna configuration is typically modified by rotating, translating or mechanically changing the shape of a reflector or sub-reflector), by fully electronic means, or by using hybrid solutions that combine the two types of reconfigurability. Concepts using controllable reflectarrays have also been extensively studied and the technology may eventually become sufficiently mature for use in a commercial communications satellite.
Active Direct Radiating Array Technologies
Active Direct Radiating Array (DRA) antennas offer the potential for unequaled coverage flexibility from space, with significant commercial returns. However, they currently have high complexity, risk, and cost, and consequently are often bypassed in favour of more established lower-cost technologies, such as reflector-based architectures. This is especially true for geo-stationary (GEO) satellites, where reflector antenna solutions offer unparalleled technological maturity and are therefore hard to displace for many of the existing and planned missions.
For Medium-altitude Earth Orbit (MEO) satellites, and especially for Low-altitude Earth Orbit (LEO) missions, active arrays have already become the solution of choice in cases where wide angles of scan and moderate gain levels are required, consistent with a limited number of radiating elements (typically in the order of one hundred elements). For GEO applications a much higher gain requirement would mean a much greater number of radiating elements, however the small scan angles involved from GEO allow for a greater inter-element separation and for the use of sparse array concepts so as to limit the number of elements and the number of active controls across the array aperture.
In order to decrease risk and cost, a modular approach to building the array, comprising highly integrated tiles (incorporating RF radiating elements, feed networks and amplifiers, as well as power and control signal distribution and also structural and thermal management functions), is a promising strategy that greatly advances the feasibility of GEO based active DRAs. Advances in enabling technologies, which may include semiconductor technologies leading to higher RF power levels and higher DC-to-RF power conversion efficiencies, alternative beamforming technologies such as optical beamforming, cheaper and highly integrated electronics, low-loss phase shifting technologies such as those using Micro Electro Mechanical Systems (MEMS) or others, and so on, will make active DRA solutions increasingly more attractive in the future.
Much R&D work is proceeding around the world to address this type of antennas and, as is usually the case in the space industry, many research papers and prototypes are produced before these concepts are actually included in a commercial satellite payload. It is, however, a certainty that many of these designs will eventually be used in space. b
About the authors
Eric Amyotte is a well-known figure in the satellite communications antenna industry, with nearly 25 years of working experience at Macdonald, Dettwiler and Associates (MDA), EMS Technologies and Spar Aerospace. In addition to his industrial background, he has been featured as a speaker and a chairman in numerous workshops, conferences and seminars around the world. His experience covers a broad range of antenna types for numerous different space applications. Mr. Amyotte has been granted several patents on satellite communications antennas. He holds a B. Eng. degree from École Polytechnique de Montréal. Mr. Amyotte is currently Director of Antennas and Electronic Products at MDA.
Dr. Luís Martins Camelo has extensive experience in the design, implementation and test of satellite antennas, with close to 30 years of related work at Macdonald, Dettwiler and Associates (MDA), EMS Technologies and Spar Aerospace. Dr. Martins-Camelos experience includes reflector and array antennas for space applications, for geo-stationary orbit as well as for low-Earth orbit, and for communication satellites as well as for space-borne remote sensing radar missions. He has published many papers in the area of antennas, and for the past 25 years he has been teaching a comprehensive course on antenna theory and design at the École Polytechnique of the University of Montreal. Dr. Martins Camelo holds a PhD in Electromagnetic Theory and Antennas from the University of Michigan (1982). Dr. Martins Camelo is currently a Staff Scientist at MDAs Antenna Engineering Department, near Montréal, Canada.
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