The forward scatter radar (FSR) configuration is especially appealing for the detection of low-observable targets since it provides well-known properties for the radar cross section, which includes enhancements with respect to the monostatic and/or moderate bistatic configurations, as well as robustness to the target material and detailed geometrical characteristics. Due to the separation of a transmitter and a receiver, it is customary to use a detection scheme based on a square-law envelope detector followed by an appropriate matched filter, which we address as a crystal video detector (CVD) based on traditional terminology. This paper provides an accurate analytical expression for the detection performance of the FSR target detection using the CVD in terms of both the probability of false alarm (P _fa ) and the probability of detection (P _d ) in order to support performance prediction and FSR system design. The derived expressions are validated by comparison to Monte Carlo simulations under two different geometrical scenarios. Moreover, the comparison of the CVD performance to an ideal optimum detector (upper bound for the FSR achievable detection performance) shows the geometrical conditions, where the CVD suffers significant detection losses so that alternative detectors should be investigated. Finally, to remove the need to operate the CVD with a fixed detection threshold, two fully adaptive detectors are derived, based on the structure of the CVD scheme, which are shown to provide a constant false alarm rate (CFAR). The performance of these CFAR detectors in terms of P _fa and P _d are provided in closed form and validated through Monte Carlo simulations, showing quite small losses with respect to the fixed-threshold CVD. The application to real data acquired by a passive FSR based on frequency-modulated signals is used to show the practical effectiveness and consistency.

The performance tradeoffs of a smallsat synthetic aperture radar (SAR) system for maritime surveillance in the coastal waters of New Zealand are investigated. The lower costs of smallsat platforms allow for a constellation of SAR satellites that can be launched from New Zealand using an existing local launch service provider. The minimum SAR image quality necessary for a smallsat system to achieve a desired detection performance is determined using existing X-band satellite data. The image quality is specified in terms of noise-equivalent sigma zero (NESZ) and resolution. It was found that for a resolution cell of 4 m ² a system NESZ of -1.7 dB is sufficient to detect small fishing vessels with a probability of detection of 0.5, while maintaining the probability of false alarm below 10 ^-10 . These requirements are translated into a preliminary SAR system design.

This paper addresses the design of a low-cost synthetic aperture radar (SAR) for a single dedicated application, namely illegal vessel detection, which can be implemented using a small satellite and is characterized by reduced transmit power and high resolution. Minimum requirements in terms of noise-equivalent sigma zero and resolution that ensure acceptable detection performance are derived based on ship statistics extracted from TerraSAR-X data. One peculiarity of the design is that a pulse repetition frequency much smaller than the nominal Doppler bandwidth is selected to increase the swath width beyond the classical SAR limitation without using digital beamforming, as azimuth ambiguities can be tolerated for this specific application. Several design examples of SAR systems operating in X band demonstrate the potential of this concept for small ship monitoring over swaths of 50-90 km with antennas smaller than 0.6 m2 and very low average transmit powers comprised between 20 and 80 W.

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.

This paper is devoted to the estimation of the target motion parameters with a Forward Scatter Radar (FSR). To provide an upper bound for the estimation performance, a closed form expression of the Cramer-Rao lower bound (CRLB) is provided for the main target signal parameters, namely Doppler rate, baseline crossing instant and main lobe width parameter of the target amplitude modulation pattern. This is performed by operating on the signal at the output of the standard FSR processing scheme, namely on the Crystal Video Detector (CVD) signal: the received signal which is passed through a square law envelope detector followed by a DC removal filter. The estimate of the kinematic parameters of moving targets that follow a linear trajectory can be obtained by inverting the relationship between these parameters and the previously estimated target signal parameters. Especially two FSR configuration are investigated: single node FSR configuration and dual-baseline FSR configuration with a small angular separation between the two baselines. Closed form expression of the corresponding CRLB is derived by using a linearized version of the same inverse transformation. Finally, a practical estimator is presented, based on a 2D filter bank, and its performance is compared with the derived CRLB. The results show that unbiased and highly accurate estimates of the kinematic parameters are obtained. The feasibility and the effectiveness of this estimation scheme is demonstrated by applying this technique to recorded live data acquired with a dual-baseline FSR configuration exploiting FM signals as waveform of opportunity

Maritime surveillance using synthetic aperture radar (SAR) calls for both wide swath and high resolution. This allows frequent monitoring of large areas with high detection probability and low false alarm rate. Conventional SAR modes are, however, limited in that a wide swath can only be imaged at the expense of a reduced azimuth resolution. Ambiguous SAR modes, based on low pulse repetition frequency or continuous variation of short pulse repetition intervals (staggered ambiguous mode), overcome this limitation and allow imaging a wide swath with high resolution for specific ship monitoring applications without the need for digital beamforming or multiple receive apertures. This paper reports on the demonstration of the staggered ambiguous mode via an experimental acquisition with the TerraSAR-X satellite over the North Sea. In spite of technical limitations in the SAR instrument, a ground range swath of 110 km was imaged with an azimuth resolution of 2.2 m, i.e., with a resolution improvement of a factor of eight with respect to TerraSAR-X ScanSAR mode. Despite the higher disturbance level resulting from the presence of range ambiguities of the sea clutter a detection probability higher than 0.8 was achieved for small ships of 21 m × 6 m size. Range ambiguities of the ships were furthermore identified based on their position and signature. The detected ships were validated using maritime positioning data from their automatic identification system. These results motivate the adoption of ambiguous SAR modes in existing and future SAR systems and missions.