Detection and motion parameters estimation techniques in Forward Scatter Radar

Forward scatter Radar systems designed to take advantage of the greater radar cross section, that is robust to Radar Absorbing Material and other stealth technology, and of the long integration times, due to the little phase and amplitude fluctuations, are attractive for a variety of applications. Many of which fit well with the needs of augmentation of the surveillance capabilities of low-observable targets that may have a small backscatter RCS when observed with the conventional radar systems. This thesis reports on research into this field of radar systems with additional contributions to target detection and motion parameters estimation. Particularly, the first part of the thesis deals with the detection of moving targets that follow a linear trajectory in a single node FSR configuration. The detection scheme based on a square-law detector followed by an appropriate matched filter, here addressed as Crystal Video Detector (CVD) following the traditional terminology (Crystal Video Receiver), has already been put forward in the literature. Performance prediction and FSR system design were key motivator to analytically characterize the detection performance of CVD in terms of both, probability of false alarm and probability of detection. The derived closed-form expressions were validate from Monte Carlo simulations under different geometrical conditions and from experimental data acquired by a passive FSR based on FM signals. Furthermore, new detection schemes based on the CVD ensuring the constant false alarm rate (CFAR) condition were devised and analytically characterized. The performance analysis showed quite small losses of the CFAR-CVD detectors compared to the fixed threshold CVD. The second part of the thesis still handles the problem of target detection through the derivation of innovative detection schemes based on the Generalized Likelihood Ratio Test (GLRT). A comparison with the detection performance of the CVD has proven the better performance of the GLRT-based detectors. In most cases the improvement has an upper bound of 3 dB. However, there are specific circumstances where the standard FSR detector shows significant losses while the GLRT schemes suffer a much smaller degradation. Moreover the possibility to have a set of secondary data assumed target free, drove to the devising of new GLRT schemes. The results demonstrated a non-negligible further improvement over the previous GLRT schemes when the operation conditions get close to the near field transition point. The detection performance of the derived detectors without and with secondary data were analytically characterized. This analytical performance allowed to derive simplified equivalent SNR expressions that relate the GLRT detection performance to the main system and target parameters. These expressions showed to be useful for the design of effective FSR geometries that guarantee desired detection performance for specific targets. In the third part of the thesis the focus is moved to the motion parameters estimation through both, a single baseline and a dual baseline FSR configuration. Accordingly, the Doppler signature extracted from the Crystal Video based scheme is exploited. Following motion parameters estimation approaches already introduced in the literature, a two dimensional filter bank technique was proposed. The main target parameters encoding Doppler rate, main lobe width and crossing time instant were estimated from such technique. The accuracy of the proposed technique was investigated from a theoretical point of view through the derivation of simplified closed-form expression of the Cramer Rao Lower Bound (CRLB). The analysis proved that unbiased estimates of the desired target parameters can be obtained that approach the derived CRLB in the high SNR region. After the dependence of the kinematic parameters on the parameters estimated from the bank was exploited. The cross baseline velocity in a single baseline configuration was estimated under the assumption that the baseline crossing point is known. Meanwhile the dual baseline configuration ensures the possibility to estimate also the baseline crossing point without a priori knowledge on the other target kinematic parameters. Once more, the CRLB of the target motion parameters for both reference scenarios was derived. The analysis proved that unbiased estimates of the target motion parameters can be obtained with high accuracy even for low SNR conditions. The effectiveness of the proposed approach was also shown from experimental data acquired by a passive FSR based on FM signals

Passive forward scatter radar based on satellite TV broadcast for air target detection: preliminary experimental results

The focus of this paper is on the detection of airborne targets and on the estimation of their velocity by means of passive forward scatter radar systems based on the DVB-S as transmitter of opportunity. Results related to an experimental campaign carried out near “Leonardo Da Vinci” airport (Rome, Italy) are shown. Particularly the Doppler signature spectrogram is analyzed for a single node FSR configuration and time delay techniques are analyzed for a multi-static configuration suitable for velocity estimation. Obtained results clearly show the feasibility of the DVB-S based FSR configuration to reliably detect aircrafts and the effectiveness of the proposed velocity estimation techniques even in the near field area

Impact of Ambiguity Statistics on Information Retrieval for Conventional and Novel SAR Modes

Synthetic aperture radar (SAR) is an all-weather imaging tool with several applications in the field of Earth observation. In recent years, novel SAR modes have been developed, where either the pulse repetition interval (PRI) or the transmitted waveform or phase is continuously varied, leading to smeared ambiguities. Ambiguities with the same total energy but different statistical distributions are likely to have different influences on the retrieval of information from SAR images. This paper analyzes the ambiguity statistics and addresses their impact on selected applications for conventional and novel SAR modes through simple models and simulations based on TerraSAR-X and TanDEM-X data. The reported results show that an understanding of the ambiguity statistics and of the impact of their smearing on product performance is fundamental to select the SAR operation mode within the design of future SAR systems

High-Resolution Wide-Swath Ambiguous Synthetic Aperture Radar Modes for Ship Monitoring

This paper proposes two high-resolution, wide-swath synthetic aperture radar (SAR) acquisition modes for ship monitoring that tolerate ambiguities and do not require digital beamforming. Both modes, referred to as the low pulse repetition frequency (PRF) and the staggered (high PRF) ambiguous modes, make use of a wide elevation beam, which can be obtained by phase tapering. The first mode is a conventional stripmap mode with a PRF much lower than the nominal Doppler bandwidth, allowing for the imaging of a large swath, because the ships’ azimuth ambiguities can be recognized as they appear at known positions. The second mode exploits a continuous variation of the pulse repetition interval, with a mean PRF greater than the nominal Doppler bandwidth as the range ambiguities of the ships are smeared and are unlikely to determine false alarms. Both modes are thought to operate in open sea surveillance, monitoring Exclusive Economic Zones or international waters. Examples of implementation of both modes for TerraSAR-X show that ground swaths of 120 km or 240 km can be mapped with 2 m2 resolution, ensuring outstanding detection performance even for small ships. The importance of resolution over noise and ambiguity level was highlighted by a comparison with ScanSAR modes that image comparable swaths with better noise and ambiguity levels but coarser resolutions

Generalized Likelihood Ratio detection schemes for Forward Scatter Radar

This paper introduces innovative detection schemes for Forward Scatter Radar (FSR) based on the GLRT (Generalized Likelihood Ratio Test) for both cases, where a fixed threshold can be used and where a fully adaptive CFAR scheme is desired. The detection performance of the newly proposed detectors is characterized analytically and compared to the performance of the standard detection scheme. This shows that the new detectors always outperform the standard FSR detector. In most cases the improvement has an upper bound of 3 dB, but there are specific cases where the standard FSR detector shows significant losses, while the new GLRT schemes suffer a much smaller degradation. Finally, simplified equivalent SNR expressions are introduced that relate the GLRT detection performance to the main parameters describing the FSR observation geometry and the target size and motion. These expressions are shown to be useful for the design of effective FSR geometries that guarantee desired detection performance for specific targets

Kinematic parameters extraction from a single node forward scatter radar configuration

This paper focuses on the exploitation of a single node Forward Scatter Radar configuration for the estimation of the kinematic parameters of targets following a linear trajectory. Accordingly, the target signature extracted from a square law detector followed by a DC removal filter is exploited. A four step processing technique is proposed for the estimation of the main target parameters encoding Doppler rate and derivative of the Doppler rate that are exploited for the extraction of the target motion parameters. In particular, the target velocity and the baseline crossing angle are both estimated under the assumption that the baseline crossing point is known. The proposed approach is assessed firstly through synthetic data. Then, experimental data collected by a passive Forward Scatter Radar exploiting FM signals as waveform of opportunity are processed to confirm the effectiveness of the proposed technique

Design of a Low-Cost Synthetic Aperture Radar for Continuous Ship Monitoring

Synthetic aperture radar (SAR) systems have unique image capabilities that are suitable for several Earth observation applications. The focus of this paper is the design of a lowcost SAR for a single dedicated application, namely illegal vessel detection, that can be implemented using a small satellite and is characterized by reduced transmit power and high resolution. Minimum requirements in terms of noiseequivalent sigma zero (NESZ) and resolution that ensure acceptable detection performance are derived, using TerraSAR-X data to retrieve the intensity distribution of typical ships. One peculiarity of the design is that a pulse repetition frequency (PRF) much smaller than the nominal Doppler bandwidth is selected to ensure a wider swath and that azimuth ambiguities are exploited to further improve the detection performance. A design example of a system in Xband imaging a 50-km ground swath and characterized by an average transmit power of only 15 W is presente

Theoretical performance prediction for the detection of moving targets with Forward Scatter Radar systems

The focus of this paper is on the detection of moving targets with a Forward Scatter Radar system. More precisely, a non-coherent detection strategy is considered and theoretical closed-form expressions of the probability of false alarm and of the probability of detection are derived in order to allow performance prediction and support system design. Finally the theoretical contribution is verified through simulations