Virginie Kubica

Optimum target detection using illuminators of opportunity

Unlike classical bistatic radars, passive radars make use of illuminators of opportunity to detect targets and to estimate target parameters. One existing radio transmission suitable for passive radar operation is the global system for mobile communication (GSM). For non-cooperative bistatic configurations, one of the major difficulties is the estimation of the reference signal which is required to perform detection. This reference signal, a priori unknown, can be extracted from the signal received at the antenna array provided the direction of arrival of the direct path signal is known. Conventional matched-filter based Doppler filtering offers the possibility of placing the target and interferences in a domain where they can be separated based on Doppler shift. However, slow moving targets residing near mainbeam clutter in the range-Doppler diagram, remain difficult to detect. Internal clutter motion (ICM) exacerbates this issue by spreading the clutter signal power in Doppler frequency. In this paper, we first present a method to estimate autonomously the direction of the illuminating GSM base station from measurements obtained with a two-element antenna array. We passively detect the azimuth of the transmitter without a priori knowledge of the environment. Spatial processing is then employed to attenuate the direct path signal and mitigate its influence on the target detection process. We then propose two methods able to cope with clutter echoes with non zero-Doppler components. We first propose an extension of a CLEAN-like algorithm. We also propose to extend adaptive matched filters to noise-like signals. The adapted matched filter can be used to suppress strictly static clutter but also clutter affected by ICM. These methods are validated by using actual clutter measurements obtained from a passive radar using a GSM base station as illuminator of opportunity.

Feasibility of STAP for passive GSM-based radar

In this paper, we examine the feasibility of applying space-time adaptive processing (STAP) to bistatic passive radars using illuminators of opportunity. The transmitters considered are GSM base stations and are non-cooperative. Although STAP has been extensively applied to signals from pulse-Doppler radars, it was never applied to arbitrary signals arising from illuminators of opportunity. We show that by computing the appropriate mixing product, we essentially convert the signal of opportunity to a pulse-Doppler like signal, hence making the application of STAP to arbitrary signals straightforward. We finally confirm these theoretical results by using real measurements.

Passive Bistatic Radar (PBR) for harbour protection applications

In this paper we investigate the feasibility of using Passive Bistatic Radar, PBR, to provide security for a harbour area. There have been numerous publications on the suitability of passive radar for air surveillance applications. However, this paper focuses predominantly on maritime applications. We investigate the ability of passive radar to detect, track, and eventually manage, marine vessels (boats). The paper includes a case-study of the Livorno Harbour in Italy. Livorno has been chosen as it represents a busy maritime hub. Simulations are provided to determine the theoretical radar performance, which are then compared with experimental results from a DVB-T-based passive radar. It will be shown that passive radars represent a viable solution for harbour protection applications.

A multibeam opportunistic SAR system

This paper discusses the implementation of a passive radar system designed to perform imaging of the ground using radar satellites as emitters of opportunity. The receiver consists in a 4-element microstrip antenna array and is able to receive the signals from most C-band Synthetic Aperture Radar satellites. The array antenna is used to attenuate the direct-path signal by null-steering. The successive processing steps are described with focus on the signal synchronization, signal separation and the image formation algorithm. The current results are presented and discussed.

The use of CLEAN processing for passive SAR image creation

The paper presents the concept of the use of CLEAN processing for strong direct signal removal in passive SAR systems. The CLEAN algorithm proposed has been successfully verified during several measurement campaigns. During the trials the passive SAR receiver was placed at ground level, and different SAR satellites (e.g. EnviSAT, TerraSAR-X) were used as illuminators of opportunity. Almost all SAR radars use the chirp sounding signal, and the presence of a direct signal in the reference channel limits the quality of the image. The removal of the strong direct signal leads to improvement of the image at a short distance from the receiver, which is the case considered in this paper.

Improved cross-range resolution in TOPSAR imaging using Sentinel-1A in bistatic operation

Wide-swath SAR imaging modes such as ScanSAR or TOPSAR offer synoptic observations of large-scale phenomena with a high revisit frequency at the expense of a degraded cross-range resolution. In the bistatic configuration using a SAR satellite as illuminator of opportunity and a ground-based stationary receiver close to the imaged area, this poor cross-range resolution can be improved in some favourable geometries by exploiting the sidelobe emissions of the beams illuminating the adjacent sub-swaths. This has been already demonstrated for ScanSAR mode while this paper analyzes in detail the TOPSAR mode based on measurements of ESAs recently launched satellite Sentinel-1A.

Strategies for the calibration of an array of patch antennas in passive bistatic SAR imaging

One of the main challenges in opportunistic bistatic SAR imaging is the synchronisation of the receiver with the transmitter of opportunity. By adaptive spatial beamforming, we can retrieve the reference signal needed to perform the synchronisation in SAR imaging. This processing also attenuates the direct path interference. In this paper, a four-element patch array antenna is considered. In addition, this phased-array can be used to reject interferences coming from external sources. The calibration of the receiver is of utmost importance considering on the one hand the significant mutual coupling between the antenna elements and on the other hand the differences in phase and gain between the reception chains. This paper discusses two calibration strategies and compares their performance on experimental data.

Along-track resolution enhancement for bistatic imaging in burst-mode operation

Wide-swath synthetic aperture radar (SAR) imaging modes, such as ScanSAR or Terrain Observation by Progressive Scan SAR, share the synthetic aperture length between beam positions. This leads to a degraded along-track resolution compared to the conventional Stripmap mode.We show that this degraded resolution can be enhanced in the case of a bistatic configuration by exploiting the sidelobe emissions of the elevation beams illuminating the adjacent subswaths. If the SNR of the backscattered signals is sufficient, the performance of the Stripmap mode can even be restored. This concept becomes particularly useful when spaceborne illuminators of opportunity are considered. Indeed, the imaging mode of spaceborne SAR instruments is most often a wide-swath mode. Making it possible to exploit those modes to produce images with high azimuthal resolution dramatically increases the number of useful images that can be produced using emitters of opportunity. Signals from any radar satellite in the receiving band of the receiver can be used, thus further decreasing the revisit time of the area of interest. This paper proposes a cross-range resolution-enhancement method that provides an enhanced cross-range resolution compared to the one obtained by the classical burst-mode SAR processing. This method is experimentally validated using measurements acquired in a space-ground bistatic configuration.