Recently, there is an increasing incidence of unauthorized ground transmitters targeting civil communications satellite transponders with the intent to take advantage of fraudulent access or interfere with the wanted signals causing a consequent reduction of the transponder capacity, typically known as Denial-of-Service (DOS). Consequently, there is an increasing interest of satellite operators in feasible solutions to mitigate interfering source. The ESA study “Adaptive Antenna for Telecommunication Links” addresses the request to investigate antenna architectures able to shield a single selectable transponder channel of 36 MHz by the modification of an existing payload for broadcasting coverage application.
The objectives of the ESA activity is to investigate the baseline adaptive architectures applicable to existing satellite antenna systems with Focal Array Fed Reflector Antenna or Shaped Reflector Antenna designs. The adaptive antenna exhibits the capability to adapt the broadcasting radiation pattern to provide a null along the interferer direction, in a particular frequency channel, and minimize the DOS coverage area.
The strategy proposed in the study is able to provide this feature and it foresees to combine two radiation patterns of the same antenna where the first one is the main pattern, corresponding to the nominal shaped coverage and the second is the auxiliary pattern that realizes a pencil beam pointed towards the interferer and correctly weighted to create a null.
The main challenges tackled in this activity are the following:
- Design of the adaptive antenna system to be integrated in the FAFR and Shaped Reflector configurations.
- Trade-off and identification of the most efficient Nulling Algorithms for this application.
- Design of the RF chains to be integrated in the existing antenna system for down and up conversion of the selected 36 MHz channel signals useful in the digital processing.
- Design of the digital system.
- Manufacturing of the adaptive demonstrator antenna that is representative of the FAFR antenna configuration able to implement the feature of the designed architecture.
- Test of the Adaptive Algorithm performance using the Demonstrator antenna and evaluation of pros and cons among the different Nulling Techniques.
The outcomes of the study and the experience gained during the test campaign of the adaptive antenna demonstrator have provided the know-how for design antenna systems as spatial filtering for jamming suppression.
The developed architecture is flexible and it can be applied to different existing antenna systems. In any case an “ex-novo” adaptive antenna system can be designed providing the desired characteristics in terms of performance, complexity and cost.
The adaptive antenna architecture has been thought up to be implemented modifying an existing antenna system design (e.g. FAFR Antenna or Shaped Reflector Antenna).
Both the configurations foresee to use the same reflector of the main antenna system also to generate the auxiliary pencil beam.
In the first case, the configuration foresees to integrate, after the LNAs, the directional couplers that allow to extract the signals used to realize the auxiliary antenna.
The Main Antenna signal is extracted at the I-MUX output after the stage used for the down-conversion in the lower Ku-Band. The selection of the Main Antenna channel to protect is performed by a switch matrix that is able to re-introduce the signal coming from the combination of main and auxiliary antennas.
All the signals are managed by the Adaptive Antenna System adopting a Digital Beam Forming Network that is dedicated to perform the raster scanning and the interferer cancellation. In order to manage the A/D and D/A conversion several blocks for down-conversion and up-conversion are foreseen.
All the channels of the transponder can be protected singly by the proposed architecture: the selection of the 36 MHz channel to protect is made operating on the frequency of the Local Oscillator of the first mixer used in the stage for the down-conversion Ku-L band and the second mixer for the up-conversion L-Ku band. The Digital BFN receives all the signals provided by the elementary beams and also the signal coming from the main antenna that are converted to the base band by the two stages of down conversion.
Since the dynamic range of the signals provided by each elementary beam is different, after the digital conversion a numerical AGC is carried out on each digital input signal, controlling the analogue amplification in front each ADC. The received digital signal is then correspondingly renormalized in order to restore the original feeds signals dynamic: this approach allows to optimize the quantization noise, maximizing the system performance degradation from the numerical precision point of view.
The adaptive beam former has been selected to be implemented in digital technology instead of analogue: in fact, the analogue solution appears too complex to be implemented and it includes in any case a digital signal acquisition block to pick-up input data for the nulling algorithms. In the second case, the configuration is similar to the previous with the only distinction the auxiliary beam are realized adding eight smaller auxiliary horns around the main horn, that feeds the Shaped Reflector, in order to provide the signals used to realize the auxiliary antenna. It is evident that the elementary beams of the auxiliary antenna will suffer from a distortion due to the surface shape of the reflector.
The adaptive antenna architecture adopts different nulling algorithms (i.e. Capon beam forming, Power Minimization and LMS algorithm), that are able to achieve the requested performances exhibiting different advantages and disadvantages.
The nulling performance obtained with the adaptive antenna system integrated to a FAFR antenna configuration is very good and it can be improved by the increasing of the main reflector. Typically the coverage in terms of percentage that suffer of a gain degradation larger than 3 dB is in the order of 10%.
In the case of Shaped Reflector configuration, the coverage in terms of percentage that suffer of a gain degradation larger than 3 dB is in the order of 25%.
In order to check the nulling algorithms performance for the developed architecture in real time, a demonstrator has been manufactured which is applied to a scaled version of FAFR antenna configuration. The test setup shows the figure where two signals (user and interferer) impinge the Adaptive Antenna Demonstrator coming from two different directions of arrival.
The test results have given evidence of the good performance achievable by the adaptive algorithms providing an attenuation of the interferer signal in the order of 40 dB (see figure) when it is operating.
The overall activities of this project have been split into two phases (Phase 1 and Phase 2). Phase 1 and the Phase 2 of the project have been carried out with the following milestones, with the related major achievements:
- KO (Kick-Off),
- BDR (Mission and Antenna Baseline Designs, Threat Scenarios, Vulnerabilities Identification and Mitigation),
- PDR (Adaptive Antenna System Preliminary Design and Analysis).
- FR (Adaptive Antenna System Detailed Design, Demonstrator Manufacturing and Tests, Analysis and Cost Effectiveness Evaluation),
- FP (Final Review and Final Presentation).
All the activities foreseen for Phase 1 and Phase 2 of the contract have been carried out.