Satellite communications in Ku- and Ka-bands are beginning to see increasing interest in active and semi-active antennas, because of the flexibility they provide compared with the conventional passive approach. For mobile communications in L and S-bands, on the other hand, active antennas have been mandatory for a number of years, because the accommodation of the additional reflectors which would be necessary for a passive antenna approach is impossible.

EADS Astrium has been associated with Inmarsat programmes for many years, and in particular has provided the antenna systems for both Inmarsat 3 and Inmarsat 4. While these are architecturally similar, they are very different in scale and in the technology used in their implementation. The Inmarsat 4 mobile user link antenna comprises a 9m deployable mesh reflector fed by an array of 120 helical elements, each operating in both transmit and receive. Over 220 simultaneous RF beams of different sizes are created by applying vector weights to the feed elements, forming the beams from overlapping clusters of about 20 elements. This is achieved via a digital beamformer. Simultaneous transmission and reception requires the achievement of very low levels of passive intermodulation (PIM) products; this was a key focus of the antenna development. The mission requires continuous coverage over fixed ground cells for orbital inclinations of up to 3°. The fixed ground cells and the small beam size together necessitate very accurate control over pointing errors. Maintaining the fixed cells under inclined orbit conditions is achieved by continually adapting the antenna patterns by uploading revised element weights on a daily basis. A system of RF sensing beams is deployed in order to reduce the pointing error achieved and improve the overall communication performance.

The proposed paper surveys the key system design issues and technologies involved in the Inmarsat 4 user link antenna system. More recent satellite mobile antenna system and technology trends are then briefly discussed.

The paper presents the development and latest results of the qualification activities of the 12m unfurlable Reflector for the Large Deployable Antenna (LDA), jointly developed by the Prime Contractor Alcatel Alenia Space Italia and several Partners, under an ESA contract. The qualification campaign will be finalized within July 2006 opening near term opportunities for space applications relying on a European technology.

The Russian subcontractor, NPO EGS and RSC ENERGIA have designed, manufactured and performed the qualification campaign of the Reflector and its hold-down system.

The west European countries subcontractors have designed important components: a large mechanical hinge, a trimming mechanism and the antenna hold-down.

Four hinges have been developed by HTS - Switzerland. They are required to deploy the limbs and the reflector with the proper distance from the satellite and from the RF focal array.

A reflector trimming mechanism (SENER - Spain) is installed between the tip of the deployment arm and the Reflector permitting to correct thermo-elastic distortion if needed.

Two antenna hold-down (STEYR MAGNA-Austria) have been designed for stowage of reflector and arm during launch.

The LDA development phase will be presented, starting from the requirements definition, covering the antenna configuration description and the design analyses. Successful CDR’s have been held for all LDA components and relevant analyses have been performed: Thermal Analysis, Dynamic Analysis accounting for the AOCS effects on the LDA, Cinematic Analysis for the deployment (including both Arm and Reflector), Normal Mode Analysis for stiffness prediction, Structural Analysis for strength verification, Thermo-elastic Analysis, Meteoroids Analysis, RF Analysis.

The qualification of all LDA components being already achieved, the paper will focus on Reflector assembly qualification (the Reflector assembly deployment will be shown with the support of a video). Reflector surface measurement (connected to mesh tuning) and RF passive intermodulation tests results will be presented.

Comparisons of test results and analysis predictions are made and RF performances given based on accurate mesh surface measurements.

The paper will contain the most recent result achieved within the fast developing realisation and qualification program, including applications for further activities.

This includes a section on a Flight Experiment to demonstrate the capability of the antenna in a real space environmental This paper will conclude therefore with the status on qualification readiness and outlook for the European Large Deployable Antenna system.

Topics/Subtopics:

development scenario basic requirements design analysis performed qualification status main results vs analysis next steps: in orbit demonstration system level applications

Contact Person:

M. MILANO ALCATEL ALENIA SPACE ITALIA Via Saccomuro 24 00131 Roma Italy E-mail: maurizio.milano@aleniaspazio.it

Antenna reflectors usually consist of stiffened or sandwich shells, thus leading to high stiffness and high reflecting surface accuracy. On the other side they are difficult to stow densely and then deploy in orbit if their characteristic in-orbit size exceeds that of the accommodation space available. This is easier for reflecting surfaces consisting of (metallic) meshes to be tensioned in orbit into its reflecting shape by its deployable backside structure. Because of the inherent approximation of such meshes to an e.g. parabolic shape and other possible disturbances like pillow effects, these mesh reflectors are primarily applicable in the lower RF range. So, reflecting surfaces are desirable for increased precision requirements for applications in the higher radar and RF frequency bands. So in order to keep the stowability advantages of meshes but enhancing shape accuracy, reflecting surfaces composed of Carbon fibre reinforced silicones (CFRS) are developed and investigated. Due to their small bending stiffness induced by the silicone matrix they can be manufactured in relatively straightforward processes with double curvature and nearly ideal parabolic shape, and they can also be stowed quite densely. Because of the C-fibers very low thermal expansion is achieved, with no moisture absorption or micro-cracking effects even under strong environmental cycles. Results obtained from different thermo-mechanical and RF characterization tests for such CFRS surfaces will be shown which confirm the expected behaviour. For RF characterization measured properties for insertion loss, cross-polarization and reflectivity will be presented. The deployable reflector concept SMART will be discussed, which is based on such non-tensioned reflecting CFRS surfaces together with a deployable backside structure consisting of thin stowed membranes deployed and tensioned in orbit. This concept leads to a high adaptability in reflector diameter (say from 4m up to 25 m) without changing the principle of the design. Interesting growth potential is also identified e.g. for possible inclusion of future enabling technologies such as active shape control to achieve even higher surface accuracy. This discussion of SMART is based on its design concept together with results from simulation models especially for in-orbit deformations and resulting rms properties. Results from computer models will be complemented by experimental results obtained from scaled laboratory reflector models. These results also include the behaviour under quasi-zero-g conditions as obtained in a parabolic flight campaign with an Airbus aircraft. The surface accuracy achieved by manufacturing and integration is measured via precision photogrammetry under different laboratory conditions and also during aircraft parabolic flight. The presentation is amended with discussion of possible integration concepts for Earth observation and telecom satellites, as well as with an outlook for possible inclusion of active shape control technology.

In the frame of Navigation systems EADS-CASA Espacio has been developing and producing Navigation antennas for different spacecrafts like Inmarsat IV and GIOVE. Now EADS CASA Espacio has been selected to develop and manufacture the Navigation Antennas for GALILEO IOV spacecrafts. This paper presents the description and main performances of the Navigation Antennas developed up to now and the improvements to be included for the new ones. Driving parameters of these antennas are lightweight design and high RF efficiency, including isoflux requirement. Isoflux means that the antenna sends more power to the edge of the coverage than to the nadir direction to compensate for the higher losses due to larger path in the out of nadir directions. Very good correlation has been found between analysis and test has been found that will be presented. Also special emphasis will be put into the very specific test of phase centre and the RF tests performed under temperature at Astrium gmbH.

Saab Ericsson Space has recently developed a K/Ka-band antenna family. These antennas are used for satellite applications such as telemetry, command and beacons.

This paper presents the design, bread board work and flight model performance of one of its members, a dual mode K/Ka-band biconical type antenna with toroidal coverage.

The structure basically consists of two radiating elements stacked in height with the receiving Ka-band radiator positioned on top of the transmitting K-band radiator. The radiating elements consist of a circular waveguide with 7 inclined resonant slots around its circumference. The circular polarisation is obtained with an external polariser, consisting of a parallel plate region. In the parallel plate the vertical components phase varies like a free space TEM mode, while the horizontal components phase varies as a TE10 mode. An equal amplitude distribution, obtained by proper inclination of the slots, in combination with 90° phase difference will produce the desired circular polarisation. Outside the polariser corrugations give the desired radiation pattern shape and suppress unwanted lobes.

Each element is fed with a septum polarizer with two waveguide ports. A septum polarizer gives left or right hand circular polarisation depending on which of the two ports is used. The two ports give opposite circular polarisation in the waveguide, but in the radiation pattern only the azimuth phase variation differs. Thus we get two identical power patterns from the two ports without any gain loss.

Two cables from the Ka-band polarizer needed to be routed down to waveguide interfaces at the base plate without affecting the radiation pattern performance of the K-band radiator. Also the K-band radiator had to be robust for structural reasons. A routing of cables on the outside of the K-band radiator caused very high disturbance, even with a routing between the slots. These high disturbances create high omni variation and low cross polar discrimination. We selected a design where the cables are embedded within the K-band radiator wall.

For easier bread board handling and tuning all design work was initially done at X-band, followed by a final scaling to the proper frequency band. The development started with the design of the Ka-band element. Hand calculations followed by simulations and optimization with the method of moment code AKBOR resulted in manufacturing of a bread board with replaceable centre part for radiation pattern tuning possibilities.
Based the Ka-band design work with the K-band radiator started. For the K-band waveguide a thicker radiator wall was needed. This reduces the bandwidth and affects the resonant frequency of the slots. Optimization of the slots was performed using the simulation tool Ansoft HFSS and work with the bread board.

The advent of commercial precision CAD tools for the design of waveguide components for antenna feeds is changing the development cycle and possibilities for these components. Without these CAD tools, engineers use rule of thumb and engineering approximations to create hardware that are often painstakingly tuned on the bench to meet specifications. This method of course has severe limitations when it comes to high performance components. With these precision CAD tools, designs are often limited only by what is practical to manufacture.

In this paper, the author will present a manufacturing technique, known as electroforming, that has much less manufacturing limitations than other techniques such as: conventional machining using a mill or lathe; electro-discharge machining; brazing; or casting.

Some of the more common benefits of electroforming are:

This paper addresses some of the recent developments in the area of multiple beam reflector antennas for communication satellite payloads. These advancements include (a) multiple aperture dual-band reflector antennas, (b) high efficiency horns covering 50% bandwidth, and (c) a novel "stepped-reflector antenna" (SRA) technology. It is shown that by combining the reflector improvements through the use of SRA with the feed improvements through the use of dual-band high efficiency horns (DBHEH), significant improvements in EIRP, G/T, and copolar isolation (C/I) can be achieved.

The SRA system produces "flat-top" radiation patterns for receive beams and highly efficient Gaussian patterns for transmit beams over a geographic coverage as seen by the satellite. The DBHEH designs employ "slope-discontinuities" to generate desired higher order modes over both Tx and Rx bands. The step size required for the SRA is typically odd multiples of quarter wavelength at the Rx band. BY employing a "frequency-dependant" horn design for the DBHEH, it is shown that the step size can be significantly reduced and can be easily blended into the reflector surface for ease of fabrication. Detailed design of a K/Ka band MBA with about 70 spot beams will be presented. The RF performance of this novel MBA has been evaluated and shows about a 1.5 dB EIRP improvement, a 3.0 dB improvement in G/T, and a 4.0 dB improvement in C/I in comparison with a conventional MBA. Detailed comparison results of the advanced MBA with conventional MBA will be presented in the paper.

In order to achieve the cost per bit that will allow the satellite broadband market to grow, next generation systems will have to deliver optimal performance levels while being lighter, smaller and less costly, thus allowing for other revenue generating payloads to be accommodated on the same platform. Current systems often rely on antenna architectures employing four reflectors for the transmit function and four other reflectors for the receive function.

A more optimal balance between performance and cost can be obtained by combining transmitting and receiving functions, thus reducing the number of reflectors and feed chains required by a factor of two. Several enabling technologies had to be developed in order to alleviate, and nearly eliminate, the RF performance degradations resulting of the dual-frequency operation.

These technologies include smooth-wall dual-band horns with high aperture efficiency that illuminate the reflector such as to obtain high combined transmit and receive performance. This high aperture efficiency feed horn is combined with compact feed components that yield excellent isolation and polarization purity for two-polarizations over the two operating-bands and can be accommodated within the tight element lattice of the multibeam antenna. The element bandwidth is sufficient to support the user and gateway uplink and downlink signals.

An antenna beamwidth equalization method has also been developed to obtain optimal RF performance for the two widely space apart Tx and Rx frequencies over the same coverage cells. This method can also be used to fine-tune the balance between the Tx and Rx EOC gain and C/I performance.

Finally, in order to mitigate the adverse effects of pointing error, the feed components can integrate a high performance multi-channel RF sensing system that simultaneously supports communications and tracking signals. Beacon tracking off-axis is possible using a wide-variety of tracking receivers operating at various rates.

The paper will demonstrate how these enabling technologies address the needs of present and future generations of multimedia satellites.

For more than 5 years CNES, ESA and ALCATEL ALENIA SPACE-F have initiated a large development program on Focal Array Fed Reflector Receive antenna for broadband multimedia application at 30GHz. In the dual linear polarization version, the study is based first of all, on the realization of a feed array Engineering Model funded by CNES in the frame of TCS21 program, and then a feed array EQM associated with an EM Gregorian-type antenna optics developed in the frame of ARTES 3 Domino 2.

The proposed presentation will be concentrated on the final TCS21 results, which have not been published so far and that are fully consistent with CDR predictions. A synthesis of study main achievements will be presented.

The TCS21 Rx active array is presented hereafter :

The feed array concept is the following : several elementary feeds are adjusted in amplitude and phase to shape the desired beams, these feeds may be re-used for several beams to ensure overlap, compact OMTs are implemented in a tight mesh and LNAs placed close to horns to improve G/T and, finally, compact low loss waveguide BFN realize the amplitude and phase laws. Active thermal control has also to be integrated near Low Noise Amplifiers to limit the temperature variation on the array equipment, as well as the temperature gradient between the amplifiers.

This concept associated with a dedicated reflector optics permits to reduce the number of Rx antennas at satellite level (typically one FAFR antenna instead of 4 separate antennas, considering a 20 to 30 adjacent beams coverage). Antenna performances have been derived from focal array results. The good agreement between theoretical (ideal antenna, without manufacturing errors) and achievable antenna performances (simulated from feed measurements) is depicted in following table. From antenna system errors analysis, allowable uncertainty on main performances is –0.7dB for DEOC and -1.8 dB for C/I.

Antenna performances at 29.75 GHz	Directivity max	Directivity EOC	Roll-Off	XPD	C/I
Theoretical (ideal case)	47.4dBi	44.2dBi	3.2dB	26.1dB	16.4dB
Expected ( from array measurements)	47.1dBi	43.5dBi	3.6dB	26.3dB	14.9dB

Recently, many Ka-band satellite communication systems with high data rates have been proposed for future global satellite communication services. The Japan Aerospace Exploration Agency (JAXA) is developing the Wideband InterNetworking engineering test and Demonstration Satellite (WINDS) to demonstrate the technologies necessary to construct future satellite-based communication architectures that enable users to benefit from advanced information services, such as high-speed Internet access, information distribution, and disaster mitigation. WINDS has Ka-band transmitting and receiving Active Phased Array Antennas (APAA) that can flexibly establish communication with any area by electronic beam scanning. These antennas utilize two scanning spot beams. The direction of each beam can be controlled flexibly and rapidly. These beams can be also utilized for the Satellite Switching Time Division Multiple Access (SS-TDMA) communications functions. The scanning spot beam function and SS-TDMA communications systems will be used for broadband communication experiments covering the Asia-Pacific region.

The key features of the antenna are the minimal number of elements, the development of RF components operating over a wide bandwidth, MMIC devices, and high-density packages for miniaturization and light weight. Antenna performance factors such as beam steering and gain control of the two multiple-beam antennas has been confirmed. Also, the test results of the components have satisfied the APAA performance requirements. As a next step, APAA assembly will be integrated on and tested.