Quantum key distribution (QKD) offers exceptionally high levels of data security during transmission by using principles of quantum physics. It is renowned for its provable security features. However, a gap between theoretical models and real-world applications, known as quantum hacking, challenges the reliability of QKD networks. Trojan-horse attacks represent a significant threat to the Bob subsystem in QKD, allowing Eve to infer Alice’s basis choices through back-reflected pulses. This can compromise security without detection in severe cases, especially when quantum bit error rates (QBER) fall below the abort threshold. The proposed method combines a category-based Gaussian Mixture Model (GMM) with the Kolmogorov-Smirnov test to estimate the posterior QBER distribution and assess risks in practical QKD systems. By processing the QBER, the approach also evaluates the dependability of the QKD scenario. Numerical results are presented using a state-of-the-art point-to-point QKD device operating over optical quantum channels of 1 m, 1 km, and 30 km lengths. The results of the experimental analysis of a 30 km optical link suggest that the QKD device provided prior information to the proposed learner. Consequently, our proposed trustworthy monitor offers a defensive mechanism that identifies potential Eve attacks, effectively mitigating the risk of security vulnerabilities.

This paper presents a digital beamforming architecture based on the discrete Fourier transform, designed for medium-Earth orbit satellite payloads to serve multiple ground users. The system leverages a 16×16 16-point two-dimensional fast Fourier transform (2DFFT) to address the growing demand for high-speed data traffic and adaptable satellite communications. The architecture features a routing algorithm for flexible user allocation to any beam position and a cluster-based linear precoding approach to reduce resource and power consumption. Two versions of the 2DFFT module—quantized and non-quantized—are compared in terms of resource usage, power consumption, and performance. Experimental results show that the non-quantized version provides better power efficiency, while the quantized version removes the need for DSP blocks.

This paper proposes a joint optimization framework for energy-efficient linear precoding and feeder-link-beam matching design in a multi-gateway multi-beam bent-pipe satellite communication system. The proposed scheme jointly optimizes the precoding vectors at the gateway antennas and amplifying-and-matching mechanism at the satellite to maximize the system-weighted energy efficiency under the transmit power budget constraint. The technical designs are formulated into a non-convex sparsity problem consisting of a fractional-form objective function and sparsity-related constraints. To address these challenges, two iterative efficient designs are proposed by utilizing the concepts of Dinkelbach's method and the compressed-sensing approach. The simulation results demonstrate the effectiveness of the proposed scheme compared to another benchmark method.

This paper introduces a joint optimization framework for user-centric beam selection and linear precoding (LP) design in a coordinated multiple-satellite (CoMSat) system, employing a Digital-Fourier-Transform-based (DFT) beamforming (BF) technique. Regarding serving users at their target SINRs and minimizing the total transmit power, the scheme aims to efficiently determine satellites for users to associate with and activate the best cluster of beams together with optimizing LP for every satellite-to-user transmission. These technical objectives are first framed as a complex mixed-integer programming (MIP) challenge. To tackle this, we reformulate it into a joint cluster association and LP design problem. Then, by theoretically analyzing the duality relationship between downlink and uplink transmissions, we develop an efficient iterative method to identify the optimal solution. Additionally, a simpler duality approach for rapid beam selection and LP design is presented for comparison purposes. Simulation results underscore the effectiveness of our proposed schemes across various settings.

This paper delves into the dynamics of resource allocation in ultra-dense Low Earth Orbit (LEO) satellite networks within a cell-free massive MIMO framework, focusing on the impact of residual synchronization errors. We conduct various analyses to understand how these errors - encompassing time, phase, and frequency - influence the Signal-to-Interference-plus-Noise Ratio (SINR) and the average number of satellite links connected to each user. Our approach measures the effects of these remaining synchronization errors and uses these values to inform and optimize power and resource allocation decisions. The study reveals that as synchronization errors increase, the number of effective satellite links to users diminishes, consequently reducing the number of satellites actively connected to each user. This research not only highlights the critical impact of synchronization errors on network performance but also demonstrates how advanced knowledge of these error variances can significantly enhance resource allocation strategies and network efficiency in future ultra-dense LEO satellite systems.

In this article, we derive Rytov variance of Adaptive optics (AO) applied to atmospheric turbulence. We chose the modified von-Karman power spectrum, which covers the atmosphere's inner and outer scales. After analytical derivations, we plot Rytov variance against propagation distance and spatial frequency. We organize the plots to see the effect of atmospheric turbulence, scaling factor, and type of correction. Rytov variance is directly proportional to the propagation distance. On the other hand, it has an inverse relation between the ratio of spatial frequencies. Our results show that Rytov variance in adaptive optics corrected spectrum is low for a low turbulent regime. Moreover, we calculate the scintillation index using derived Rytov variance. We believe our results will be used to model adaptive optics corrected random phase screen approach, a model for turbulent channels in wave optics. This way, performance measurements for adaptive optics corrections could be more accurate.

Very High Throughput Satellite (VHTS) systems are typically deployed in Geostationary (GEO) orbit to benefit from the ubiquitous coverage of such orbits. Although the satellites deployed in GEO orbits appear as a static point in the sky from the on-ground user perspective, in practice, the GEO satellite experiences a north-south drift due to the influence of the sun and moon. Such movement may cause a small Doppler effect in the signals sent from geographically distributed cooperative gateways, which may cause significant performance loss when exploiting DVB-S2X SF-Pilot fields and for precoding purposes. This paper presents the first work investigating the effect of GEO Doppler shift in precoded cooperative multi-gateway satellite systems. In addition, to compensate for the frequency variations produced by the GEO movement, we present and test a user-gateway closed-loop compensation procedure. Results using software-defined radio (SDR) in the Lab are provided to validate the proposed method.

Low signal strength, resulting from the distance between satellites and the Earth's surface, remains a primary challenge in integrating them into terrestrial network systems. Nevertheless, the current density of satellite constellations presents an opportunity to design cost-effective receivers capable of utilizing diversity combining techniques to overcome this issue. However, any combining technique relies heavily upon time synchronism between the received signals, which until now has presented a limitation for practical applications in satellite communication systems. This paper proposes a procedure for compensating large symbol time misalignment and estimating the remaining fractional one between two DVB-S2X waveforms (from different satellites) overlapped in time, frequency, and under noise-limited scenarios. We propose a fractional-time-misalignment estimator using the data-aided early late gate (ELG) principle. We evaluate the proposed estimator using the Walsh-Hadamard-based pilot sequences available in the DVB-S2X waveforms. At the same time, we present an estimation of large misalignments based on the correlation properties of the start-of-super-frame (SOSF) field (which uses longer Walsh-Hadamard sequences than the pilots). Simulation results show that the proposed synchronization methods provide an unbiased estimation even when the noise power levels match that of the received signals.

The space communications industry is challenged to develop a technology that can deliver broadband services to user terminals equipped with miniature antennas, such as handheld devices. One potential solution to establish links with ground users is the deployment of massive antennas in one single spacecraft. However, this is not cost-effective. Aligning with recent NewSpace activities directed toward miniaturization, mass production, and a significant reduction in spacecraft launch costs, an alternative could be distributed beamforming from multiple satellites. In this context, we propose a distributed beamforming modeling technique for wideband signals. We also consider the statistical behavior of the relative geometry of the swarm nodes. The paper assesses the proposed technique via computer simulations, providing interesting results on the beamforming gains in terms of power and the security of the communication against potential eavesdroppers at non-intended pointing angles. This approach paves the way for further exploration of wideband distributed beamforming from satellite swarms in several future communication applications.

This paper explores how the condition number of the channel matrix affects the performance of different precoding techniques in non-terrestrial network (NTN) communications. Precoding is a technique that can improve the signal-to-interference-plus-noise ratio (SINR) and bit error rate (BER) in massive multi-beam systems. However, the performance of precoding depends on the rank and condition number of the channel matrix, which measures how well-conditioned the matrix is for inversion. We compare three precoding techniques: zero-forcing (ZF), minimum mean square error (MMSE), and semi-linear precoding (SLP), and show that their performance degrades as the condition number increases. To mitigate this problem, we propose a user ordering approach that forms optimally conditioned channel matrices by selecting users with orthogonal channel vectors. We demonstrate that this approach improves the SINR and goodput of all the precoding techniques in full-frequency reuse NTN communications.

Medium Earth orbit (MEO) constellation is an appealing solution between geostationary equatorial orbit (GEO) and lower Earth orbit (LEO) in terms of latency and number of satellites required. On-board processing of digital beam-former in MEO satellites is an efficient solution for achieving wider bandwidth, increased flexibility, and lower latency. Power constraints, however, make it impractical to digitally create thousands of beams at once. In this paper, area-power efficient digital beamformer architectures are proposed considering key metrics of a typical MEO scenario. The proposed efficient digital beamformer is comprised of a sparse-matrix-based user selection, a 2D discrete Fourier transform (DFT)-based digital beam generation, which is implemented by a fast Fourier transform (FFT) algorithm, and a spatial windowing module for selecting the antenna pattern. Furthermore, architectures of digital beam-former using conventional 2D-FFT approach, fully unrolled 2D-FFT, and an area-power efficient twiddle factor (TF) quantized fully unrolled 2D-FFT are proposed. The spatial windowing architecture concerning 10 × 10 radio frequency chains and sparse matrix architecture for user selection is also proposed. The proposed architectures are implemented targeting Virtex ultrascale FPGA and the area-power utilization is reported. It is noticed that more than 50%-reduction in area and power is achieved with the beamformer incorporating the proposed TF quantized fully unrolled 2D-FFT.

Satellite swarms have recently gained attention in the space industry due to their ability to provide extremely narrow beamwidths at a lower cost than single satellite systems. This paper proposes a concept for a satellite swarm using a distributed subarray configuration based on a 2D normal probability distribution. The swarm comprises multiple small satellites acting as subarrays of a big aperture array limited by a radius of 20000λ0working at a central frequency of 19 GHz. The main advantage of this approach is that the distributed subarrays can provide extremely directive beams and beamforming capabilities that are not possible using a conventional antenna and satellite design. The proposed swarm concept is analyzed, and the simulation results show that the radiation pattern achieves a beamwidth as narrow as 0.0015° with a maximum side lobe level of 18.8 dB and a grating lobe level of 14.8 dB. This concept can be used for high data rates applications or emergency systems.

This paper proposes a novel resource allocation scheme for optimizing the downlink of a user-centric cell-free massive multiple-input multiple-output (MIMO) system. The proposed approach aims to optimize the number of users served by each access point based on channel conditions while adapting to variable packet error rate and modulation and coding schemes. To enhance the received signal-to-noise plus interference ratio, the authors use a precoding design approach called the local protective partial zero-forcing that categorizes users based on their channel gain. The problem is formulated as a joint optimization of user assignment, resource allocation, and the precoding design. Closed-form expressions for the data rate are derived, and a new algorithm for resource allocation is introduced that outperforms several different scenarios while keeping the computational complexity reasonable. Compared to fixed parameter schemes, the proposed approach provides an optimal selection of the number of users for each access point and has the potential to significantly improve the system throughput, making it a novel and impactful solution for the practical implementation of user-centric cell-free massive MIMO systems.

On-board processing of digital beamforming in satellites is an efficient solution for the higher data rates, more capacity, and lower latency, but the available on-board limited power makes it impractical to digitally create thousands of beams at once. A significant portion of the analog hardware in a satellite communications payload can be replaced with highly integrated digital components, which are often more affordable, lighter, smaller, and reprogrammable by employing digital beamforming. In comparison to matrix-by-vector multiplication beamforming, the discrete Fourier transform (DFT) beamformer enables the finer realization of real-time beamformers with reduced circuit complexity and lower power consumption. Fast Fourier transform (FFT) methods can further reduce the computing cost of the DFT computation. Therefore, in this paper, area-power efficient two-dimensional (2D) FFT digital beamforming techniques are analyzed and implemented. The major implementation challenge is to produce N samples per cycle with lower area-power consumption. Fully unrolled 4-bit twiddle factor (TF) quantized FFT is proposed in this regard. The optimization techniques through quantization, truncation, and complex multipliers are thoroughly discussed for efficient implementation. The behavioral and post-route timing simulations are validated, and implementation results like area and power consumption are estimated and compared among conventional , fully unrolled, and the proposed 4-bit TF quantized 2D-FFT.

The new era of space exploration demands a significant increase in the number of human and robotic missions, thus resulting in novel communication and service requirements. To satisfy such requirements, the fifth generation of mobile communication systems (5G), despite providing connectivity on Earth, has the potential to serve as a communication standard for space resource missions, particularly the ones targeting the Moon. In fact, 5G non-terrestrial networks (NTNs) are already in the standardization process and new techniques are being proposed in order to counteract the peculiarities of the non-terrestrial channel. However, going one step ahead and deploying constellations of satellites around the Earth or the Moon, requires first a detailed analysis and testing of the validity of the proposed techniques. Therefore, in this paper, we introduce the 5G Space Communications Lab, which has been developed with the purpose of simulating space-based 5G communications. The designed testbed proposed here increases the technology readiness level (TRL) of NTN-based 5G systems, demonstrating over a laboratory environment successful 5G communication via space links.

This paper aims to determine linear precoding (LP) vectors, beam hopping (BH), and discrete DVB-S2X transmission rates jointly for the GEO satellite communication systems to minimize the payload power consumption and satisfy ground users' demands within a time window. Regarding constraint on the maximum number of illuminated beams per time slot, the technical requirement is formulated as a sparse optimization problem in which the hardware-related beam illumination energy is modeled in a sparsity form of the LP vectors. To cope with this problem, the compressed sensing method is employed to transform the sparsity parts into the quadratic form of pre-coders. Then, an iterative window-based algorithm is developed to update the LP vectors sequentially to an efficient solution. Additionally, two other two-phase frameworks are also proposed for comparison purposes. In the first phase, these methods aim to determine the MODCOD transmission schemes for users to meet their demands by using a heuristic approach or DNN tool. In the second phase, the LP vectors of each time slot will be optimized separately based on the determined MODCOD schemes.

Satellite communication systems can provide seamless wireless coverage directly or through complementary ground-terrestrial components and are projected to be incorporated into future wireless networks, particularly 5G and beyond networks. Increased capacity and flexibility in telecom satellite payloads based on classic radio frequency technology have traditionally translated into increased power consumption and dissipation. Much of the analog hardware in a satellite communications payload can be replaced with highly integrated digital components that are often smaller, lighter, and less expensive, as well as software reprogrammable. Digital beamforming of thousands of beams simultaneously is not practical due to the limited power available onboard satellite processors. Reduced digital beamforming power consumption would enable the deployment of a full digital payload, resulting in comprehensive user applications. Beamforming can be implemented using matrix multiplication, hybrid methodology, or a discrete Fourier transform (DFT). Implementing DFT via fast Fourier transform (FFT) reduces the power consumption, process time, hardware requirements, and chip area. Therefore, in this paper, area-power efficient FFT architectures for digital beamforming are analyzed. The area in terms of look up tables (LUTs) is estimated and compared among conventional FFT, fully unrolled FFT, and a 4-bit quantized twiddle factor (TF)FFT. Further, for the typical satellite scenarios, area, and power estimation are reported.

This article presents a closed-loop differential phase compensation system for a precoding-enabled multibeam satellite forward link and its validation by live experiments on a GEO satellite scenario. The precoding operation avoids inter-beam interference and maximizes the spectrum efficiency by full frequency reuse as an alternative to the traditional two-color or four-color reuse methods proposed in the DVB-S2 standard. However, the satellite payload introduces differential phase and frequency impairments, which can degrade the precoding performance. This work describes the implementation of the differential phase and frequency tracking and compensation loop in an end-to-end testbed over a multibeam satellite system with independent local oscillators. The developed system performs end-to-end real-time communication over the satellite link, including channel measurements and precompensation. Results are validated by an over-the-air demonstration using two beams of the SES-14 multibeam satellite. Each beam is transmitted by independent transponders, which results in differential frequency and phase offsets due to the transponder undisciplined local oscillators. This phase offset makes it impossible to use precoding without the phase compensation loop. We prove that the implemented system can successfully track and compensate the differential phase and frequency to improve precoding performance.

This paper address the design of a phase tracking block for the DVB-S2X user terminals in a satellite precoding system. The spectral characteristics of the phase noise introduced by the oscillator, the channel, and the thermal noise at the receiver are taken into account. Using the expected phase noise mask, the optimal parameters for a second-order PLL intended to track channel variations from the pilots are calculated. To validate the results a Simulink model was implemented considering the characteristics of the hardware prototype. The performance of the design was evaluated in terms of the accuracy and stability for the frame structure of superframe Format 2, as described in Annex E of DVB-S2X.

Distributed antenna arrays, fractionated payloads and cooperative platforms can provide unprecedented performance in the next generation of spaceborne communications and remote sensing systems. Remote phase synchronization of physically separated oscillators is the first step towards a coherent operation of distributed systems. This work shows the preliminary results of a TDD remote phase synchronization algorithm with a master-follower architecture. Herein, we describe the implementation and validation of the proposed algorithm. The implementation has been conducted in a Cubesat-ready software defined radio and validated at the end-to-end satellite communications testbed available at the University of Luxembourg.

Satellite Communications offer complementary benefits to terrestrial 5G/6G infrastructure, covering a wide range of use cases in need of ubiquitous coverage and reliability. However, to be as competitive as the terrestrial counterpart in terms of supplied throughput, satellite communications require a highly efficient use of the limited available spectrum. Linear precoding has demonstrated the ability to boost the spectral efficiency in the satellite domain, but raising a new issue: the bandwidth requirements of the feeder link. Deployment of several gateways, each of which precoding an independent cluster of beams causes performance degradation. Therefore, in this paper, we investigate the centralized gateway concept, where all digital baseband processes (including precoding) are implemented in a remote server connected via high speed fibers to the distributed remote gateways responsible for the downlink and uplink of the satellite radio frequency signals. In particular, we highlight the main technical challenges and provide a preliminary vision of potential solutions.

Remote Sensing from distributed platforms has become attractive for the community in the last years. Phase, frequency, and time synchronization are a crucial requirement for many such applications as multi-static remote sensing and also for distributed beamforming for communications. The literature on the field is extensive, and in some cases, the requirements an complexity of the proposed synchronization solution may surpass the ones set by the application itself. Moreover, the synchronization solution becomes even more challenging when the nodes are flying or hovering on aerial or space platforms. In this work, we discuss the synchronization considerations for the implementation of distributed remote sensing applications. The general framework considered is based on a distributed collection of autonomous nodes that synchronize their clocks with a common reference using inter-satellite links. For this purpose, we implement a synchronization link between two nodes operating in a full-duplex fashion. The experimental testbed uses commercially available SDR platforms to emulate two satellites, two targets, and the communication channel. The proposal is evaluated considering phase and frequency errors for different system parameters.

Phase, frequency and time synchronization is a crucial requirement for many applications as such as multi-static remote sensing and distributed beamforming for communications. The literature on the field is very wide, and in some cases, the requirements of the proposed synchronization solution may surpass the ones set by the application itself. Moreover, the synchronization solution becomes even more challenging when the nodes are flying or hovering on aerial or space platforms. In this work, we compare and classify the synchronization technologies available in the literature according to a common proposed framework, and we discuss the considerations of an implementation for distributed remote sensing applications. The general framework considered is based on a distributed collection of autonomous nodes that try to synchronize their clocks with a common reference. Moreover, they can be classified in non-overlapping, adjacent and overlapping frequency band scenarios.

We present a convex optimization based Symbol-Level Precoding (SLP) for sum power minimization and propose the low-latency closed-form algorithm to find a heuristic solution to the optimization problem. The technique exploits constructive interference at the multi-user MIMO systems and minimizes the sum power of the transmitted precoded signal per each set of MIMO symbols. As a result, the received signals gain extra Signal-to-Noise Ratio (SNR), which improves data rate and energy efficiency of the system. We benchmark the low-complexity algorithm for solving the optimization technique against the conventional Fast Non-Negative Least Squares algorithm (NNLS). The demonstrated design of the SLP technique combined with the proposed closed-form algorithm has low computational complexity and fast processing time, which is applicable in low-latency high-throughput satellite communication systems.

Cognitive Radio (CR) communication has been considered as one of the promising technologies to enable dynamic spectrum sharing in the next generation of wireless networks. Among several possible enabling techniques, Spectrum Sensing (SS) is one of the key aspects for enabling opportunistic spectrum access in CR Networks (CRN). From practical perspectives, it is important to design low-complexity wideband CR receiver having low resolution Analog to Digital Converter (ADC) working at a reasonable sampling rate. In this context, this paper proposes a novel spatio-temporal wideband SS technique by employing multiple antennas and one-bit quantization at the CR node, which subsequently enables the use of a reasonable sampling rate. In our analysis, we show that for the same sensing performance requirements, the proposed wideband receiver can have lower power consumption than the conventional CR receiver equipped with a single-antenna and a high-resolution ADC. Furthermore, the proposed technique exploits the spatial dimension by estimating the direction of arrival of Primary User (PU) signals, which is not possible by the conventional SS methods and can be of a significant benefit in a CRN. Moreover, we evaluate the performance of the proposed technique and analyze the effects of one-bit quantization with the help of numerical results.

We present a precoded multi-user communication test-bed to demonstrate forward link interference mitigation techniques in a multi-beam satellite system scenario which will enable a full frequency reuse scheme. The developed test-bed provides an end-to-end precoding demonstration, which includes a transmitter, a multi-beam satellite channel emulator and user receivers. Each of these parts can be reconfigured accordingly to the desired test scenario. Precoded communications allow full frequency reuse in multiple-input multiple-output (MIMO) channel environments, where several coordinated antennas simultaneously transmit to a number of independent receivers. The developed real-time transmission test-bed assist in demonstrating, designing and benchmarking of the new Symbol-Level Precoding (SLP) techniques, where the data information is used, along with the channel state information, in order to exploit the multi-user interference and transform it into useful power at the receiver side. The demonstrated SLP techniques are designed in order to be computationally efficient, and can be generalized to others multi-channel interference scenarios.

This paper provides experimental results of the edge detection and spectrum sensing algorithms for wideband cognitive radio networks which are recently proposed in [1] using software defined radio (SDR) platform. The considered algorithms employ ratio based test statistics for detecting the edges of all sub-bands and generalized energy detection (GED) for examining the status of each sub-band. In particular, we validate the theoretical detection and false alarm probabilities of the edge detection and GED algorithms of [1] experimentally for a number of practically relevant parameters such as sensing time and bandwidth. We also compare the performances of these algorithms with and without calibrating the Cognitive Radio Device (CRD). Through extensive experiments, we have found that the theoretical performances claimed in [1] can be achieved reliably just by performing appropriate calibration at the CRD. Moreover, we also verify that the considered detection algorithms are robust against noise variance uncertainty, carrier frequency and timing offsets.

The paper presents the development of a ground based bistatic receiver using TerraSAR-X as a transmitter. The receiver subsystems like antennas, low-noise amplifiers, mixers, filters, synthesizers, etc. have been developed using low-cost monolithic devices in order to allow affordable deployment and at the same time offer final year students a challenging SAR engineering project. First raw data have been acquired on the Barcelona harbor area that has been focused producing geocoded images well matched with existing maps. A preliminary interferogram have been also produced.

This paper presents across-track tomography applied to a bistatic geometry with fixed receivers. This kind of geometry can overcome some of the classical monostatic tomography limitations such as temporal decorrelation and irregular baseline distribution. The Remote Sensing Laboratory (RSLab) of the Universitat Politècnica de Catalunya (UPC) has implemented a SAR Bistatic Receiver for INterferometric Applications, SABRINA, with 4-channels. SABRINA has been used to carry out a bistatic tomographic experiment. The acquired data has been processed with different tomographic methods and their performances compared.

This paper presents an along - track interferometry (ATI) study for a bistatic or multiestatic SAR configuration with fixed ground receivers. This technique can be useful for sea current estimation or for any problem of Ground Motion Target Indicator (GMTI). The proximity of the ground receivers to the scene allows to be very sensitivite to velocities with small baselines. This paper also proposes a multibaseline approach for ATI able to diferenciate among different velocity contributions in the same resolution cell. At the end of this paper, some results over real acquired bistatic data will be presented and discussed. The data have been acquired using the C-band SAR Bistatic Receiver for INterferometric Applications (SABRINA) and ESA's ENVISAT satellite, as a transmitter of opportunity.

The presented work discusses the processing of repeat-pass interferometric data acquired with SABRINA (SAR Bistatic Receiver for INterferometric Applications), where the receiver is fixed and the baseline is due to the two different satellite orbits. The paper addresses the particularities of bistatic interferometry like the coregistration of the images and the decorrelation factors affecting the interferometric phase. Theoretical developments are complemented with the first repeat-pass results. The characteristics of the scene and the 35 days temporal baseline have caused the interferogram to be severely affected by the temporal decorrelation. This first result opens the discussion of what kind of targets are seen in a bistatic geometry and which of them can be coherent along time.

This paper characterizes the interferometric phase for a fixed-receiver bistatic SAR system. Also, the expressions for the interferometric phase and image resolution cell are summarized. It will be distinguished between two particular acquisition geometries : back-scattering and forward-scattering. A bistatic interferometric chain has been implemented whose particularities are discussed. The theoretical developments are complemented with the comparison of the Digital Elevation Models (DEM) generated from the bistatic interferometric data acquired with our fixed receiver, named SABRINA (SAR Bistatic Receiver for INterferometric Applications) with a SRTM DEM and a Digital Terrain Model (DTM) from the Institut Cartografic de Catalunya (ICC).

This paper describes the design and construction of a bistatic SAR receiver suitable for airborne applications, using orbital SAR systems (ENVISAT, ERS-2, RADARSAT, TerraSAR-X among others) as opportunity transmitters. The challenge of this design is to reduce the required data throughput of the recorded data. This is achieved storing data only in the time intervals when scattered signal from the target area appears. The task of detecting this time intervals is performed in real time using a matched filter of the signal received directly from the SAR transmitter.

The Universitat Politecnica de Catalunya is developing a ground based bistatic system using ESA's ENVISAT and ERS-2 as transmitters. The spatial resolution of this configuration is similar to that of their monostatic counterpart, although foreshortening effects have a lesser impact. First single-pass interferometric images corresponding to a local test-site are presented and compared to a synthetic interferogram.