AIRBORNE ANTENNAS FOR BROADBAND SATELLITE COMMUNICATIONS M. Lisi, P. Angeletti, C. Catallo, A. Cucci, A. Florio, G. Lucchi Alenia Spazio SpA Via Saccomuro, 24-00131 Roma (Italy) Ph.: +39641512586; Fax: +39641512389 E-mail: m.lisi@roma.alespazio.it INTRODUCTION Several systems are currently being studied for the provision of commercial broadband services to airplanes via satellite. Broadband is telecommunication that provides multiple channels of data over a single communications medium. It is important to airlines and customers because it provides high-speed connectivity and live entertainment services concurrently to multiple users. The services to be provided include: Real-time, two-way open access to the Internet Real-time, two-way access to company Intranets Live e-mail Premium television and radio Airline and destination information Shopping Live television Crew Information Services Airborne RF communication systems applications pose challenging problems to current antenna technology: influence of antenna installation on aircraft configuration; EMC/EMI with other RF and/or electronic systems on board the aircraft; interactions with the aircraft aerodynamics, mainly resulting in changes to drag, lift and controllability; very wide-angle scanning performance, so as to operate at the extreme latitudes required by polar routes; compatibility with other satellite systems and with terrestrial communications services (involving beamwidth, beam pointing accuracy and sidelobes performance). The ideal antenna should actually be high gain, broadband, low profile, highly reliable and low cost. Three different antenna architectures can be investigated: 1. electronically steered antennas (planar and conformal phased arrays); 2. hybrid steering antennas (mechanically augmented phased arrays); 3. mechanically steered antennas (passive arrays, array of reflectors, single reflector). Phased-array antenna systems are currently close to meet most of the above mentioned requirements. They are often too expensive to be affordable, but the cost issue could be overcome with mass production and automated manufacturing systems. Mechanically steered configurations are presently offering the best trade-off in terms of performance, cost and development risk. AIRLINE COMMERCIAL BROADBAND TELECOMMUNICATION SERVICES VIA SATELLITE Several systems are currently being studied for the provision of commercial broadband services to airplanes via satellite. These systems will effectively change the way people communicate and entertain themselves during travel by providing high-speed, two-way Internet and television services to aircraft in flight. Through the service, twoway, broadband (or high data-rate) connectivity is delivered directly to airline seats, providing passengers with personalized and secure access to the Internet, company intranets and live television and audio content. These new services allow customers in the air to communicate as quickly and easily as if they were on the ground, providing them a virtual Office in the Sky. In addition, they will also provide airline personnel with information that will enhance operational security and efficiency on the ground and in the air. The frequency bands presently envisioned for aeronautical broadband satellite communications are Ku, Ka and, with some capacity limitations, L-band. The delivery architectures vary for each system but will include geostationary (GEO), highly inclined orbit (HEO), low earth orbit (LEO) and mobile satellite systems (MSS).
Fig. 1: Aeronautical Broadband Satellite Communications A typical communications system at Ku-band, via satellite, for civil airlines would be mainly addressed to entertainment and business for passengers, including the primary service of multimedia data, in particular for Internet (bi-directional, as Tx/Rx) and the optional DBS service of TV broadcasting (airborne Rx) as upgrading. Considering the type of Internet service, the related communication link would be asymmetric: information data rate higher in Rx than in Tx, with a ratio of about five; an information data rate of about 2 Mbps is typically foreseen in Rx for the single aircraft. The space segment, as baseline, would be using geostationary satellites (GEO) for the coverage of large regions, such as Northern America, North Atlantic Ocean Region and Europe. Geostationary satellites, though, are not ideal for polar routes, since at very high latitudes would correspond very low antenna elevation angles. For this reason, geosynchronous satellites in highly inclined elliptical orbit (HEO) could be adopted, with the advantages of a reduced angular scan in elevation and a service coverage extended to the polar regions. Fig. 2: Constellation of HEO Satellites in Tundra Orbit
AIRBORNE SATELLITE ANTENNAS: KEY REQUIREMENTS The design of airborne antenna systems must address several problems: influence of antenna installation on aircraft configuration; EMC/EMI with other RF and/or electronic systems on board the aircraft; interactions with the aircraft aerodynamics, mainly resulting in changes to drag, lift and controllability; very wide-angle scanning performance, so as to operate at the extreme latitudes required by polar routes; compatibility with other satellite systems and with terrestrial communications services (involving beamwidth, beam pointing accuracy and sidelobes performance). Let us consider some of them in more detail. Influence of Antenna Installation on Aircraft Configuration Aircraft has limited "real estate", or structure and surface area, for mounting antenna systems hardware. Part of the structure and surface area is dedicated for use by propulsion systems, cockpit, landing gear, etc., and is not compatible with antenna installation. Modern aircraft require many antennas that may have conflicting mounting or viewing angle requirements. The antenna must have excellent wide-angle scanning performance and good gain properties over the full range of motion. Antennas may have to be located on a particular part of the aircraft to realize the desired pattern coverage without shadowing from aircraft structure or to achieve electromagnetic compatibility with other aircraft systems. The multiplicity of antennas makes even more serious the antenna location and mounting problem. The largest cost driver is the installation cost related to cutting into an aircraft's fuselage. EMC/EMI with other RF Systems On-Board the Aircraft The electromagnetic characteristics of the antenna system interact with other RF and/or electronic systems and equipment on board the aircraft. International safety regulations are very strict and impose to watch very carefully this problem. Interactions with the Aircraft Aerodynamics Antenna system weight and moments of inertia interact with the aircraft structure and its mass properties, and the location of other airborne equipment. Moreover, antenna system size and form interact with aircraft aerodynamics, resulting in changes to drag, lift and controllability. These interactions impact the aircraft configuration and must be considered during the design of both the aircraft and its RF systems. The problem is even more serious when RF systems are added to an aircraft that is already operational, since the degrees of freedom available to the designer are more severely limited. Satellite receiving antenna installations on airborne platforms must have a sufficiently low profile to prevent drag. Antenna Tracking From a functional point of view, the antenna beam, both in Rx and Tx, must be steerable, in order to assure the directional RF link in Ku band between the aircraft and satellite, during the aircraft flight and with the aircraft onground, trying to minimize the angular pointing (tracking) error. The dynamic environment, defined at the interface of the antenna aperture with the aircraft (mounting point), is originated by the aircraft linear (route) and angular (attitude) speeds, being related to aircraft maneuvers, air turbulence and aircraft flexible body deflections. Open loop and closed loop tracking approaches can be envisioned for both mechanically and electronically steered antennas. Maintainability and Reliability The maintainability and reliability of airborne antenna systems are the key factors to be considered for the choice of the antenna configuration. The ease of maintenance and high reliability criteria must be met to lower operational cost.
AIRBORNE SATELLITE ANTENNAS: CANDIDATE ARCHITECTURES Several architectures can be considered for the realization of cost affordable antennas for aircraft, commercial and leisure cruise ships, trains and ground transportation vehicles. They will all ask for the adoption of mass production and highly automated manufacturing approaches. For broadband communications at Ku or Ka band, where high gain are required, three different antenna architectures can be investigated (Fig. 3): 1. electronically steered antennas (planar and conformal phased arrays); 2. hybrid steering antennas (mechanically augmented phased arrays); 3. mechanically steered antennas (passive arrays, array of reflectors, single reflector). Fully Electronically Steered Phased Array Planar PA Dome Augmented PA Cone PA Mechanically Augmented Phased Array Rotor Integrated TX/RX Solutions Flat Plate Multiple Reflectors Single Reflector Fig. 3: Candidate Antenna Architectures Table 1: Antenna Architectures Trade-off Antenna Technologies Planar Phased Arrays Saddlebag Phased Arrays Offset Fed Reflectors Benefits High reliability Future upgrade to multiple beam aperture Schedule High reliability Schedule Integrated Tx and Rx aperture Drawbacks Scan limitations Scan limitations Weights, Costs 4 Radomes Height and drag Conformal Phased Arrays (CONE) Mechanically Augmented Phased Array (MAPA) Mechanically Augmented Phased Array (Rotor) High Reliability Future upgrade to multiple beam aperture Low sidelobes High Reliability Schedule High Reliability Schedule Low sidelobes Thermal performance Weights, Costs Thermal performance Weights, Costs Thermal performance Weights, Costs Luneberg Lenses Narrow band Passive Arrays Mech. Steered Integrated Tx and Rx aperture Low profile Reduced schedule and performance risk Poor sidelobes Relayability Beampointing error
ALENIA SPAZIO ACTIVITIES ON AIRBORNE SATELLITE ANTENNAS Alenia Spazio is currently developing an advanced airborne antenna subsystem, able to support high-speed satellite communications at all latitudes around the globe. Several antenna configurations have been considered. Some of them will be hereafter presented in more detail. Phased Array Antennas Figure 5 shows a simplified general block diagram of a future phased-array antenna. Note that a phased-array generally consists of two major sections: radiating elements and beam forming network. To meet the challenging requirements of airborne satellite antennas, the element types indicated in the figure need to be broadband, low profile, and lost cost. Similarly, different types of beam forming network are required as shown in the figure to meet the maintainability, high reliability, and low cost goals. Phased array antennas require a wide range of state-of-the-art technologies and a large number of electronic equipment, such as printed-circuit radiators, solid-state amplifiers, beam-forming networks, power supply units and power distribution networks. A system engineering approach to the design and development of phased arrays is hence mandatory in order to achieve a synergic integration of all functions and to implement large-scale production and testing techniques. Phased-array antenna systems are currently close to meet most of the requirements needed for aeronautical satellite telecommunications. One major problem derives from the wide elevation scan angle (10 to 90 degrees) required to operate with a geostationary satellite at all practical latitudes. Moreover, phased-array antennas are still too expensive to be affordable, but the cost issue can be overcome with mass production and automated manufacturing systems. A good example of mass production approaches applied to phased-array antennas is the Small Satellites Center, the production plant developed by Alenia Spazio in the frame of the Globalstar program. Located in Rome near other Alenia Spazio production sites, this plant of new conception, expressly conceived for the large scale production of spacecraft s and phased-array antenna subsystems, covers an area of 6,000 square meter and includes a 4,000 square meter clean room (class 100,000), maintained at controlled environmental conditions. The Small Satellites Center is equipped with a complete set of electrical and environmental test equipment able to check spacecraft and antennas performance in their real operative conditions. More specifically, for the radiated testing of the active phased-array antennas, completely automated Near-field and Far-field test ranges were realized. Most of the test equipment is computer-controlled and networked to a Data Management System (DMS), which collects and analyzes test results. Radome Multilayer PCB Horns Horns supporting frame TTDL BFNs and structural support Fig. 5: Schematic Architecture of an Airborne Phased-Array Antenna
Array of Multiple Reflectors The antenna is composed of three identical squared paraboloids, displaced along a horizontal line (Fig.6). Fig.6: Multiple Reflectors Array Configuration Each paraboloid is onset-fed by a wideband and axial symmetric feed with an integrated OMT and transmits/receives two orthogonal linear polarizations: namely H (horizontal) and V (vertical). An RF network is housed just behind the paraboloids. Main functions of the RF network are: in Transmit mode: it feeds the three paraboloids to generate a linearly polarized antenna beam. The linear polarization slant angle is remotely controlled; in Receive mode: it collects the energy received from the three paraboloids in the two linear polarizations, amplifies these RF signals and generates two orthogonal circularly polarized outputs. The antenna is mechanically steered in azimuth and elevation. Main drawbacks of this configurations are the presence of grating lobes in the azimuthal plane and the need for a relatively complex feeding network. Elliptical Cassegrain Reflector Antenna Alenia Spazio, Mitsubishi Electric Corp. (MELCO) and Boeing (the so-called Tri-party Team) have participated in the definition of a mechanically scanned, Ku-band, TX/RX, Elliptical Cassegrain Reflector System for the ConneXion by Boeing SM program, meant to support high-speed communications at all latitudes around the globe (Fig. 7). Alenia Spazio is presently developing a configuration consisting of a mechanically scanned TX/RX reflector system, that, w.r.t. the Tri-party developed brassboard, offers substantially improved performance in terms of mass, dimensions, RF characteristics and reliability. The antenna aperture is a shaped Cassegrain dual reflector. The key components of the antenna system can be recognized in fig. 8: the quasi-elliptical reflector and sub-reflector and the circular corrugated feed integrated with the Ortho-Mode Transducer (OMT). The elliptical Cassegrain reflector performance is influenced by sub-reflector caused blockage losses and elevated near-in sidelobes. The major performance advantage of our elliptical Cassegrain reflector, with focal length to diameter ratio of 0.25, is in the extremely low far-out sidelobes.
Fig. 7: Tri-party Transmit/ReceiveElliptical Cassegrain Reflector Brassboard From the functional point of view, the Alenia Spazio Airborne Antenna System (AAS) can be decomposed in eight major assemblies hereafter listed and characterized by their location with respect to the aircraft skin (Fig. 9): Outdoor Equipment 1. Aperture Assembly: including reflector optics, feedhorn and all the RF components common to the receive and transmit paths down to the two channels rotary joint; 2. Electrical/Mechanical Assembly: including all the elevation over azimuth mechanics, the motors and the driving logic; 3. Radome; Indoor Equipment 4. Diplexer : including the two diplexers and the 90 hybrid for the TX path; 5. TX Chain: including the up-conversion stage, the electronics for both amplitude and polarization control and the RF power amplifiers; 6. RX Chain: comprising the LNAs and the down conversion and polarization control electronics; 7. Power Supply : providing the required DC power to all the equipment; 8. Antenna Controller : interfacing the units external to the AAS (DTR and Navigation) and providing all the controls required inside the AAS. Fig. 8: Alenia Spazio Elliptical Cassegrain Reflector Antenna
OUTSIDE EQUIPMENT Outside Aperture Assembly Inside Diplexer DIP Assy INSIDE EQUIPMENT TX Chain DIP HPAs DIP From DTS RADOME Feed Reflector OMT El Az RJ Az DIP TX Ctrl Up-Converter RFCA To DTS Az LNAs RX Chain Optional RFCA To DBS S-R Driver Mot Enc Mot Enc Antenna Controller Power Supply Electrical/Mechanical Assembly Power Supply 115 VAC Ethernet ARINC 429 Fig. 9: Airborne Antenna System Block Diagram CONCLUSIONS The paper overviewed the main features of the systems currently being studied for the provision of commercial broadband services to airplanes via satellite. It focussed on the key requirements of airborne satellite antennas and described the development activities being carried-on at Alenia Spazio. Three different antenna architectures have been examined: 1. electronically steered antennas (planar and conformal phased arrays); 2. hybrid steering antennas (mechanically augmented phased arrays); 3. mechanically steered antennas (passive arrays, array of reflectors, single reflector). Phased-array antenna systems are currently close to meet most of the service requirements and offer the best potential for future growth. They are often too expensive to be affordable, but the cost issue could be overcome with mass production and automated manufacturing systems. Mechanically steered configurations are presently offering the best trade-off in terms of performance, cost and development risk.