This article explores the possibility to exploit global navigation satellite systems (GNSS) signals to obtain radar imagery of ships. This is a new application area for the GNSS remote sensing, which adds to a rich line of research about the alternative utilization of navigation satellites for remote sensing purposes, which currently includes reflectometry, passive radar, and synthetic aperture radar (SAR) systems. In the field of short-range maritime surveillance, GNSS-based passive radar has already proven to detect and localize ship targets of interest. The possibility to obtain meaningful radar images of observed vessels would represent an additional benefit, opening the doors to noncooperative ship classification capability with this technology. To this purpose, a proper processing chain is here conceived and developed, able to achieve well-focused images of ships while maximizing their signal-to-background ratio. Moreover, the scaling factors needed to map the backscatter energy in the range and cross-range domain are also analytically derived, enabling the estimation of the length of the target. The effectiveness of the proposed approach at obtaining radar images of ship targets and extracting relevant features is confirmed via an experimental campaign, comprising multiple Galileo satellites and a commercial ferry undergoing different kinds of motion.

This article investigates passive synthetic aperture radar (SAR) image formation using Terrestrial Digital Video Broadcast (DVB-T) transmitters of opportunity and an airborne receiver. The proposed airborne system does not use separate channels for direct and reflected signal reception. The proposed image formation algorithm suppresses artifacts due to the presence of the direct signal in radar echo data using modified CLEAN method and accounts for unknown irregularities in airborne platform motion using a map-drift autofocus (MDA) technique. The algorithms are theoretically derived and experimentally confirmed via an appropriate airborne campaign, and their experimental performance is measured.

This paper explores radar methods to measure the coordinates (distance and height) of objects on or beside a road. Knowledge of these parameters is essential in the development of autonomous or semi-autonomous cars, as well as to improve driving safety. It is shown that the height is best found by trilateration from sensors at multiple locations on a vehicle. The novelty of the work consists in the formulation of the problem and in its solution using a low-THz radar trilateration approach. The expressions obtained allow calculating the coordinates of targets using a number of different methods of radar trilateration. The influence of the system parameters on the accuracy of measurement using different methods of trilateration has been analyzed. The assumptions made were verified by the experimental system based on a 300-GHz ultra-wideband radar. Experimental results are in good agreement with theoretical calculations; they confirmed the practicality of obtaining high-accuracy height measurements on distributed targets such as pedestrians. The physical basis of the techniques developed would also allow the plan position of a target to be found using the same approach.

Communication satellites in geosynchronous orbit are increasingly broadcasting digital signals with high bandwidth and high power. These signals are in principle well-suited to radar imaging and the study presented here is an initial feasibility study for a passive bistatic synthetic aperture radar using satellites in geosynchronous orbit (GEO). The persistent viewing possible from GEO could enable important new applications. The mission concept is outlined and studies of the available signal formats identify digital TV broadcasts in Ku-band as most suitable for radar imaging. The additional space hardware required is a dedicated receive channel, which could be implemented as a hosted payload at modest cost. Our findings so far suggest that the mission concept is feasible for coarse spatial resolution images and that it could therefore provide a low-cost technology demonstration of geosynchronous radar.

This paper examines the potential in extracting the instantaneous location of maritime moving targets using a passive multistatic radar with Global Navigation Satellite Systems (GNSS) as illuminators of opportunity and a single receiver. This paper presents a theoretical framework for the localization of a moving target from a set of bistatic range measurements. The algorithm and its predicted accuracy are presented. The localization is achieved by what is essentially a multilateration technique, which can be applied while the transmitting platform is also in motion. The algorithms and the accuracy predictions, as a function of the number of transmitters, have been experimentally confirmed via a dedicated experimental campaign, where two different maritime targets were detected by up to 12 GNSS satellites belonging to different satellite constellations (GPS, GLONASS, and Galileo) simultaneously. To the best of the author's knowledge, these are the first results of their kind and on this scale not only for GNSS-based passive radar but also for multistatic radar in general.

Echolocation is the ability to use sound-echoes to infer spatial information about the environment. Some blind people have developed extraordinary proficiency in echolocation using mouth-clicks. The first step of human biosonar is the transmission (mouth click) and subsequent reception of the resultant sound through the ear. Existing head-related transfer function (HRTF) data bases provide descriptions of reception of the resultant sound. For the current report, we collected a large database of click emissions with three blind people expertly trained in echolocation, which allowed us to perform unprecedented analyses. Specifically, the current report provides the first ever description of the spatial distribution (i.e. beam pattern) of human expert echolocation transmissions, as well as spectro-temporal descriptions at a level of detail not available before. Our data show that transmission levels are fairly constant within a 60° cone emanating from the mouth, but levels drop gradually at further angles, more than for speech. In terms of spectro-temporal features, our data show that emissions are consistently very brief (~3ms duration) with peak frequencies 2-4kHz, but with energy also at 10kHz. This differs from previous reports of durations 3-15ms and peak frequencies 2-8kHz, which were based on less detailed measurements. Based on our measurements we propose to model transmissions as sum of monotones modulated by a decaying exponential, with angular attenuation by a modified cardioid. We provide model parameters for each echolocator. These results are a step towards developing computational models of human biosonar. For example, in bats, spatial and spectro-temporal features of emissions have been used to derive and test model based hypotheses about behaviour. The data we present here suggest similar research opportunities within the context of human echolocation. Relatedly, the data are a basis to develop synthetic models of human echolocation that could be virtual (i.e. simulated) or real (i.e. loudspeaker, microphones), and which will help understanding the link between physical principles and human behaviour.

In this letter, we report initial experimental results which provide the foundation for low-terahertz (low-THz) radar imagery for outdoor unstructured scenarios as expected in automotive sensing. The requirements and specifications for a low-THz single imaging radar sensor are briefly outlined. The imaging capabilities of frequency-modulated continuous-wave (FMCW) radar operating at 150 GHz are discussed. A comparison of experimental images of on-road and off- road scenarios made by a 150-GHz FMCW radar and a reference 30-GHz stepped-frequency radar is implemented, and their performance is analyzed.

This paper considers the exploitation of navigation satellite systems as opportunity transmitters for bistatic and multistatic synthetic aperture radar (SAR). The simultaneous availability of multiple satellites over a scene of interest at different viewing angles allows multistatic SAR acquisitions using a single receiver on or near the ground. The resulting spatial diversity could be used to drastically improve image resolution or to enhance image information space. To exploit the availability of multiple satellites, two data fusion approaches are here considered. In the former, point features of the single images obtained from different perspectives are extracted and then combined, whereas in the latter, a multistatic image is first obtained by combining the single channel data at the image level and then the point features are extracted. This is achieved by considering ad hoc CLEAN-like techniques. These techniques have been developed on both the analytical and simulation levels and experimentally verified with real GNSS-based SAR imagery. The techniques described here are not limited to GNSS-based SAR but may be applied to any multistatic SAR system.

This letter demonstrates the feasibility of improving range resolution in passive synthetic aperture radar with the Galileo navigation satellites as transmitters of opportunity. This can be done by an appropriate coherent combination of the Galileo E5a and E5b signals. The proposed approach is analytically derived and is confirmed both by simulation and experiments.

This paper analyzes the spatial decorrelation between repeat-pass bistatic synthetic aperture radar (BSAR) images with Global Navigation Satellite Systems as transmitters and a fixed receiver. This study is needed in the development of such a system to monitor temporal changes in a scene. The main challenge is that, in this bistatic configuration, spatial coherence heavily depends on the data acquisition geometry. The appropriate theoretical framework to describe spatial coherence for this case is developed by extending well-established monostatic models and, in principle, can be applied to any fixed-receiver BSAR with a spaceborne transmitter. Theoretical results are initially supported by Monte Carlo simulations. Finally, the validity of the model is confirmed by comparing real images.

This letter conducts a point spread function (PSF) analysis for bistatic synthetic aperture radar (BSAR) systems where the transmitter is in medium Earth orbit, and the receiver is fixed on the ground. To achieve a reasonable azimuth resolution under such a configuration, the trajectory of the satellite can no longer be approximated as a straight line; therefore, current methods for PSF analysis are insufficient. The solution proposed involves extension of the generalized ambiguity function to accommodate satellite trajectory curvature. The theoretical analysis shows effects unlike those observed in monostatic or even the general BSAR and is verified by both simulation and experimental results using navigation satellites as the transmitting platforms.

This paper presents proof-of-concept methods and results on a passive synthetic aperture radar for surface change monitoring. The bistatic topology comprises navigation satellites (such as Global Positioning System, GLObal Navigation Satellite System (GLONASS), Galileo, or Beidou) as transmitters of opportunity and a fixed receiver on the ground. Surface monitoring is to be provided through coherent change detection. An experimental test bed built to confirm the concept of the system is described, and the appropriate experimental program is presented. Preliminary signal processing algorithms for change detection are derived. Experimentally measured changes are compared to the theoretical predictions, and the obtained results are discussed.

This letter demonstrates the feasibility of space-surface bistatic synthetic aperture radar using navigation satellites as transmitters of opportunity and a fixed ground-based receiver. Experiments with real satellite signals are described, and the obtained imagery is presented and discussed.

This paper presents algorithms designed for a subclass of bistatic synthetic aperture radar (BSAR) called space-surface BSAR (SS-BSAR). Two SS-BSAR configurations are considered. The first one assumes a stationary, spaceborne transmitter and a moving airborne receiver. The second case is the generalized SS-BSAR configuration, where the transmitter is nonstationary. The transmitter and receiver have essentially different flight paths and velocities. For each configuration under investigation, the characteristics of the corresponding SS-BSAR received signal are examined first. Then, each proposed algorithm is derived analytically, and verified via simulation.

This paper reports progress in the development of an image formation algorithm suitable for stripmap space-surface bistatic synthetic aperture radar. A description of the proposed algorithm, which is a modification of the standard range-Doppler algorithm, is provided for the case when the transmitter and the receiver have parallel flight paths and unequal velocities. Both simulation and initial experimental results are presented to verify our analysis.

The concept and preliminary analysis of the Interferometric SS-BSAR system is discussed in this paper.

The tool for avalanche prediction based on the Space Surface Interferometric Bistatic Synthetic Aperture Radar (SS-InBSAR) is discussed in this paper. The forthcoming Galileo navigation satellites signals are considered as the electromagnetic source of the novel bistatic SAR.

In this paper we describe the concept of a bistatic SAR system which involves a purpose built stationary ground based receiver but which utilizes existing or proposed low earth orbit satellite (LEOS) communication systems, such as Globalstar or ICO as the transmitting signal sources. In general, such a system has a 3-D configuration (in contrast to the 2-D bistatic SAR systems examined previously in the literature). We analyse this system. With earth observation applications in mind, we characterize quantitatively the 2-D resolving capability within the ground plane. This is demonstrated via a meaningful simulation.

Choice of the operational frequency is one of the most responsible parts of any radar design process. Parameters of radars for buried object detection (BOD) are very sensitive to both carrier frequency and ranging signal bandwidth. Such radars have a specific propagation environment with a strong frequency-dependent attenuation and, as a result, short operational range. This fact dictates some features of the radar's parameters: wideband signal-to provide a high range resolution (fractions of a meter) and a low carrier frequency (tens or hundreds megahertz) for deeper penetration. The requirement to have a wideband ranging signal and low carrier frequency are partly in contradiction. As a result, low-frequency (LF) ultrawide-band (UWB) signals are used. The major goal of this paper is to examine the influence of the frequency band choice on the radar performance and develop relevant methodologies for BOD radar design and optimization. In this article, high-efficient continuous wave (CW) signals with most advanced stepped frequency (SF) modulation are considered; however, the main conclusions can be applied to any kind of ranging signals.