Jamming is the act of intentionally directing powerful electromagnetic waves toward a victim receiver with the ultimate goal of denying its operations. This paper describes the main types of Global Navigation Satellite System (GNSS) jammers and reviews their impact on GNSS receivers. A survey of state-of-the-art methods for jamming detection is also provided. Different detection approaches are investigated with respect to the receiver stage where they can be implemented.

As more and more attention is received by the underlying passive potential of Global Navigation Satellite Systems (GNSS) signals, reflectometry and scatterometry are visited in this paper in order to assess water surface identification and aircraft altimetry performance. This is possible thanks to the high reflectivity of Global Positioning System (GPS) signals in the L-Band frequency range on water as well as ice and snow covered surfaces which compensates for the otherwise low signal intensity. Indeed, GNSS reflectometry belongs to the class of bistatic radar systems where transmitter and receiver reside at different locations and signals pass through a reflecting surface. Code phase and carrier phase based time delay measurements between the direct path and reflected signals, are used to determine the small aircraft's height with respect to the reflecting surface. On the other hand, specular reflection points are computed, and a comparison of the reflected power to the direct received power is shown to yield a good water surface identification performance. Code and carrier phase measurements are implemented in this paper to obtain cm level height estimates.

It is known that satellite radiolocalization was born in the military environment and was originally conceived for defense purposes. Nevertheless, the commercial explosion (dated to 20 years ago) of global positioning system (GPS) in the civil market (automotive, tourism, etc.) significantly changed the original perspectives of this technology. Another big change is expected when other global navigation satellite systems (GNSSs) such as the European Galileo or the Chinese COMPASS become operational and commercial. In fact, modern GNSSs are conceived principally for the civil market (at the opposite of GPS, whose civil employment is given as a sort of “kind gift,” with lower performance than that one granted to military users). The scope of this paper is to provide readers with a clear focus about the potentialities of current and forthcoming GNSSs and associated technologies in a renewed mass-market perspective. The paper also opens a window to the future of radiolocalization technology beyond GPS and GNSS, dealing with the role of digital signal processing and software-defined radio (SDR) in next-generation navigation systems and with the seamless integration of satellite-based navigation with other technologies in order to provide reliable position information also in hostile environments.

This paper considers the possibility to integrate external altitude measurements with a Kalman based Global Navigation Satellite System (GNSS) receiver in a peer to peer scenario. The performance of such a system is investigated for different characteristics of the aiding measurement and for different degree of trust of the receiver upon the aiding measurement. The aiding measurement is obtained starting from the altitude measurements that the other peers in the network send to the aided user. The experiments highlight the need for a parameter that points out the effectiveness and the consistency of the computed aiding measurement. To this purpose, a reliability index is proposed, on the basis of some considerations on the variance and on the number of the received measurements.

In the array processing literature several statistically optimum beamformers have been proposed, to cope with the problem of optimal source separation. Optimum signal to interference plus noise power ratio (SINR) beamforming is probably one of the best techniques because it can circumvent most of the drawbacks of other beamforming techniques, but, in many already proposed approaches, the knowledge of the correlation matrices of the desired and the interference signals, is simply assumed. Unfortunately, the estimation might not be easily evaluated in practical scenarios wherein both kind of signals are simultaneously present at the receiver. In this paper we propose a novel direction of arrival (DOA) driven statistically optimum SINR beamformer whose aim is to exploit the inherent structure of the signal space. The beamformer response is optimized in such a way that power contributions of signals belonging to the desired signal subset (DSS) are maximized at the beamformer output, while power contributions of signals belonging to the complementary subset, named the rejected signal subset (RSS) and to the noise are minimized. In addition, a practical way to estimate the correlation matrices of desired and rejected signals is also provided, along with a closed form of the optimum weight vector characterizing the beamformer. This proposed technique yields to a family of SINR optimum beamformers, parameterized by different partitions of the DOA set. Another important feature of our technique is its inherent modularity, that makes it suitable for a parallel hardware implementation, so that real-time applications can be carried out.

Abstract A novel approach to space communications is represented by the Software Radio (SR) concept, a new technology currently deeply investigated by the scientific community, especially for wireless applications such as mobile communications or wireless network access. The SR approach leads to the definition of a new generation of radio equipment, whose communication functions are almost completely defined by means of software modules. The main outcome of the deployment of this technology is the great flexibility of the radio interface. As a matter of fact the communication standards can be easily reconfigured by simply replacing the corresponding software modules. This paper describes the SDRsat research project that aims at designing a complete software radio reconfigurable regenerative transponder for space applications. Copyright © 2002 John Wiley & Sons, Ltd.

Carrier frequency estimation plays an important role in digital communication receivers. A progressive refinement approach to maximum-likelihood estimation is proposed as a means to improve the performance of a suboptimal estimation algorithm and achieve different tradeoffs among complexity, accuracy and dynamic range. As an example, an application is described for carrier frequency recovery in burst-mode digital transmissions. The Luise and Reggiannini (1995) synchronization algorithm is employed as the master frequency estimator. The performance of the algorithm is demonstrated by means of a simulation study, assuming quadrature phase-shift keying modulation; the results show that the progressive refinement approach can be usefully exploited, for example to improve the estimation range of the LR algorithm, without losing the accuracy and the computational complexity.

We propose an on-board selection scheme for aerial and space images, based on linear feature detection in a feature hyperspace. The detection task is performed by means of the Radon transform (RT) and the wavelet transform; a fast algorithm for the RT computation is described, and counteractions against the discretization errors are proposed. A new, wavelet-based algorithm is introduced, which performs a fine analysis of the waveforms of the RT peaks, yielding a possibly error-free detection in images corrupted by a high level of noise. A technique, based on the feature hyperspace, is proposed, able to significantly exploit all the available pieces of information on these peaks. Results of the tests on synthetic and real images are reported, which show that this method achieves satisfactory results, making the detection task highly reliable in the presence of both noise and clutter.

An efficient sampling algorithm for image scanning is proposed, suitable to represent "interesting" objects, defined as a set of spatially close measured values that springs out from a background noise (as in applied geophysics in the process of anomaly detection). This method generates a map of pixels randomly distributed in the plane and able to cover all the image with a reduced number of points with respect to a regular scanning. Simulation results show that a saving factor of about 50% is obtained without information loss. This result can be proved also by using a simplified model of the sampling mechanism. The algorithm is able to detect the presence of an object emerging from a low energy background and to adapt the sampling interval to the shape of the detected object. In this way, all of the interesting objects are well represented and can be adequately reconstructed, while the roughly sampling in the background produces an imperfect reconstruction. Simulation results show that the method is feasible with good performances and moderate complexity.

Turbo estimation algorithms (TEAs) for non-random parameters are able to yield high accuracy estimates by means of an iterative process. At each iteration, a noise reduction is performed by means of an external denoising system (EDS), which exploits the estimation results obtained at the previous step; the enhanced data are then input to the master estimation algorithm (EA) for next iteration. A basic TEA scheme has been previously proposed in the context of modal analysis, which makes use of the Tufts and Kumaresan (1982) algorithm as the master EA, and of a multiband IIR filter as the EDS. In this paper, two improvements of this basic scheme are proposed; the former implies a different design of the EDS, able to achieve better estimation accuracy while reducing the outlier probability; the latter permits the autodetermination of the number of modes making up the signal.

This paper proposes a beamforming design method for interference cancellation using a special thinned array structure, termed minimum redundancy array (MRA). MRA is analyzed with partially adaptive beamforming design techniques and compared with corresponding uniform regular array (URA) over a set of likely interference scenarios. The partially adaptive beamformer design is subject to worst-case performance constraints. Extensive numerical simulations with several sets of interference scenarios show that MRA almost always outperforms URA, in both cases of fully adaptive beamformers and of partially adaptive beamformers. We also introduce a design criterion to take into account both the array geometry and partially adaptive beamforming methods.

Direct digital frequency synthesizers (DDFSss) are generally implemented by successively scanning trough a lookup table stored in a read-only memory (ROM) and converting the recalled sine samples to an analog waveform via a digital to analog converter (DAC). This paper describes a new implementation of a DDFS using a second order IIR resonator, whose impulse response is a sine wave; as this impulse response can be expressed in a second order recursive form, sample generation only requires few multiplications and additions. Therefore, the idea is to use a fast DSP microprocessor that can implement the recursive equation in "real-time". This technique guarantees a larger set of output frequencies, a high spectral purity and a very simple, compact, and reliable hardware design.>

The filter minimizes the mean-square error between the sampled input to the decision device and the corresponding transmitted symbols. It is shown that the structure of this optimal receiving filter corresponds to a bank of matched filters, each followed by a linear transversal filter. Applications of this theory cover nonlinear channels with memory, linearly or nonlinearly modulated signals, and disturbances other than Gaussian noise (e.g. interference from other data sources).>

The bit error rate (BER) of channels including a memoryless bandpass nonlinearity is evaluated by simulation. This would typically be an onboard travelling wave tube (TWT) amplifier in a satellite repeater, when the noise at the input of such a nonlinear element is non-negligible. The usual evaluation technique, error counting, requires a large amount of computer time if small error probabilities are to be estimated. It is shown that faster semianalytic procedures can be used, provided that a proper model for the nonlinear element is adopted. The output process is decomposed into a signal component plus an additional term representing an equivalent noise component, and an equivalent nonlinear transformation, relating the input useful signal to the output signal component, is derived. In addition, several modes for the probability density function (PDF) of the uplink noise component at the output of the transmission chain are discussed and compared. The procedure has been tested on a transparent satellite link using 4-CPSK modulation format. The results compare well with those of the error-counting technique if a composite rectangular PDF with exponential tails as adopted.>

The performance of a TFM scheme applied to a satellite downlink with an on-board regenerative repeater is evaluated by using a time domain simulation software package TOPSIM III. The TFM bit error rate in the presence of nonlinearities and adjacent channel interference is evaluated by means of semianalytic methods. The capabilities of convolutional codes for reducing the overall channel bit error rate are also investigated. Design guidelines are finally outlined for advanced satellite communication systems in which the joint use of nonconventional modulation schemes (like TFM) and of convolutional codes results in improved performance with respect to both bandwidth exploitation and system flexibility.

TOPSIM III is a flexible, easy to use, stand-alone Fortran package for the digital simulation of communication systems in the time domain. TOPSIM III provides a simple high-level language for the description of the system. Programs can be written following the same structure used in drawing the system block diagram. The paper discusses the time domain simulation approach used in the design of the package and describes its capabilities. The contents of the subprogram libraries are presented, describing some of the building blocks that can be used in the simulation as well as the performance evaluation facilities available in the package. Some simple examples illustrate the use of TOPSIM III as a tool for analysis and design of digital communication systems.

A mixed analytic and simulation method is described for the study of a DCPSK up-link used in a regenerative satellite transmission system. A great variety of impairments is considered and their effects on error probability are evaluated, thus emphasizing the performance evaluation in a "real situation." A case study is worked out in detail to stress the design-oriented approach.