The spaceborne imaging radar-C, X-band synthetic aperture radar (SIR-C/X-SAR) missions have resulted in important scientific discoveries and provided new insights into Earth system processes. Analyses of SIR-C/X-SAR engineering-mode data have also led to new measurement and mission concepts. The multifrequency, multipolarisation capability provided by SIR-C/X-SAR is unsurpassed from a spaceborne system, making the data set valuable for algorithm development and assessment of optimal imaging parameters more than a decade after the missions were flown.
PAMIR (Phased Array Multifunctional Imaging Radar) is an experimental airborne radar system that has been designed and built by the Research Institute for High Frequency Physics and Radar Techniques (FHR) of Forschungsgesellschaft fur Angewandte Naturwissenschaften (FGAN). The goal is to meet the growing demands for future reconnaissance systems with respect to flexibility and multi-mode operation by the use of an electronically steerable phased array antenna. The X-band system with a bandwidth of 1.8 GHz serves as a platform for different tasks. One of the main objectives is to demonstrate synthetic aperture radar (SAR) imaging at a very high resolution and for a long range. The fine resolution will also be applied for inverse SAR (ISAR) imaging of ground moving targets. Moreover, five parallel receiving channels allow array processing techniques like ground moving target indication (GMTI) via space-time adaptive processing, electronic counter-counter-measures and interferometric SAR with a very high 3D-resolution. A multi-channel scan-MTI mode with a range resolution adapted to the target size allows for a wide area GMTI operation that can be complemented by target tracking. Together with the predecessor system AER-II, operating at a frequency band contained in that of PAMIR, the possibility of experimental investigation of bistatic SAR is given. SAR images of large urban areas and ISAR images of moving objects, both with finest resolution down to the sub-decimetre scale, are presented. Results of GMTI in a wide area scanning mode and broadband bistatic experiments including true bistatic SAR processing are shown as well.
Low-frequency synthetic aperture radar (SAR) systems are of interest primarily because of their ability to see through foliage for a variety of purposes. Unfortunately, the very high frequency/ultra-high frequency bands are used by a large number of services such as television, radio and communications. The authors describe a number of different radio frequency interference suppression techniques that have been investigated and that were tested on wide bandwidth low-frequency SAR data collected on board a helicopter using the Airborne Data Acquisition System (ADAS) low-frequency radar. The least-mean-square (LMS) filter and the Wiener filter are both investigated in detail. The adaptive Wiener filter is found to have superior suppression and sidelobe performance and is also computationally much faster than the LMS filter when applied to the ADAS data.
The authors introduce a new class of modulation, the complex double-binary-offset-carrier (CDBOC) modulation class, which covers most of the modulation types proposed so far for Galileo and GPS signals, namely the binary and quaternary phase shift keying, sine BOC, cosine BOC and alternate BOC modulations. At the same time, the CDBOC class provides a more general framework, with the potential for new applications of wide-band code division multiple access systems and/or future satellite navigation systems. The theoretical derivations of the power spectral density and the auto-correlation function of the CDBOC-modulated signals are introduced, and the theoretical analysis is compared with the results obtained via simulations. The advantage of our method in the context of Galileo and GPS signals is its simplicity and the fact that it provides unified analytical formulas for most of the existent GPS/Galileo signals
An implementation of a fast, factorised, back-projection scheme is presented which is based on interpolation of range-angle sub-aperture data. A method for determining the required angular sampling rate is proposed and the implementation scheme is evaluated for nearest neighbour and Cubic interpolation kernels against image quality and number of operations required. A SAR simulator is used for investigation of the performance and to trade the algorithm parameters. The impulse response function, using interpolation kernels at different sampling rates, is evaluated in terms of peak side-lobe levels and compared with a global back projection implementation. The results show that good processing performance is obtained even when high image quality is required when using cubic interpolation and an adequate sampling rate. This approach is thus suitable for challenging microwave SAR processing problems.
An airborne broadband jammer present in the mainbeam of a synthetic aperture radar (SAR) can potentially destroy a large region of the SAR image. In addition to this, multipath reflections from the ground, known as hot-clutter or terrain scattered interference will add a non-stationary interference component to the image. The goal of interference suppression for SAR is to successfully suppress these interferences while not significantly effecting the image quality by blurring, reducing the resolution or raising the sidelobe level. The paper provides an analysis of the degradation from hot-clutter, the limited restoration that multichannel imaging and slow-time space time adaptive processing (STAP) can provide and how fast-time STAP can improve the final image quality.
Quantitative estimation of physical parameters of the Earth's surface is currently an active area of research. Multidimensional synthetic aperture radar (SAR) systems appear in this framework as a tool capable of performing this task on a global scale and independently of the weather conditions. In particular, polarimetric SAR data have been successfully employed, in combination with different electromagnetic inversion algorithms, to extract physical information from measured data. Nevertheless, SAR data are affected by speckle noise. The influence of speckle noise and its filtering in the quantitative estimation of physical parameters is analysed. To perform this study, a novel model for speckle noise in multidimensional SAR data is considered. Finally, the effect on the quantitative estimation of surface parameters is investigated.
On the basis of street vector data and computer-aided design models of buildings, simulated synthetic aperture radar (SAR) images are created using a SAR simulator. By comparing image chips from a SAR image to simulated street data, correspondences are found, which are used as the first step for the geo-referencing. Afterwards, the geo-referencing can be improved using control point information, which is provided from correspondences between the images of buildings created from a SAR simulator and the respective sections of unreferenced SAR images. The hardware-accelerated real-time SAR simulator SARViz is used for the simulation. On the basis of this approach, the initial geo-referencing of airborne SAR images can be improved, using even the results of simplified and fast SAR simulation methods.
Radar entomology has developed such that routine long-term monitoring of insect flight through the atmospheric boundary layer is now practical. Typical entomological radars use X-band (9.4 GHz) marine transceivers with a vertical pencil beam and rotate the plane of polarisation about the beam axis. Ideally, insect species and other parameters (mass etc.) should be estimated from the measured radar cross-section variation with polarisation angle. For this, a library of known insect cross-section polarisation signatures is required. Two models are currently used to parameterise the polarisation signature: the harmonic model and a model using the scattering matrix for symmetric targets (SM3). Data from the literature and a doctoral research project are presented and analysed to obtain parameters for both the harmonic and SM3 models. Knowledge of the measurement errors allows SM3 parameter uncertainties to be quantified in most cases using a maximum likelihood approach. Results for 68 insects representing 24 species are presented. These include several economically significant species (e.g. bees and locusts), with individual insect masses ranging from 9 mg to 3 g.
Rihaczek and Hershkowitz have proposed that radar scattering from man-made targets is dominated by cavity-type reflectors that do not meet the assumptions of point scatterer theory. The effect of such dispersive scattering is to produce spurious bright responses in the radar image that can appear as a number of peaks lying along an arc. If such arc-type responses do exist in radar imagery then their removal would enhance the ability to focus moving target imagery and to perform automatic target recognition. Rihaczek and Hershkowitz have outlined a method for identifying such arc-type responses that use the image phase. Algorithms have been written to automate the outlined procedure and used to analyse a large number of synthetic and inverse synthetic aperture radar images. Arcs are present in most radar images of static and moving ground targets. To investigate whether the arcs are due to a physical mechanism or a chance occurrence, randomised target imagery was generated and also analysed. No significant differences are seen between the distributions of arc numbers obtained from randomised and real imagery. In conclusion, the arcs present in the real imagery are likely to occur by chance, rather than because of an underlying scattering mechanism.
The problem of adaptive constant false alarm rate detection of a pulse-to-pulse partially correlated target with 2K degrees of freedom in pulse-to-pulse partially Rayleigh correlated clutter and multiple-target situations is addressed. Both the target and the clutter covariance matrices are assumed to be known and are modelled as first-order Markov Gaussian processes. An exact expression for the probability of false alarm (Pfa) for the mean level detector is derived. It is shown that it depends on the degree of pulse-to-pulse correlation of the clutter samples. The probability of detection (Pd) is shown to be sensitive to the degree of correlation of the target returns and the degree of correlation of the clutter returns as well. Swerling's well-known cases I, II, III and IV are handled as extreme limits of the proposed model.
The design and analysis of a knowledge-aided detector for airborne space-time adaptive processing (STAP) applications are addressed. The proposed processor is composed of a training data selector, which chooses secondary cells best representing the clutter statistics in the cell under test, and an adaptive processor for detection processing. The data selector is a hybrid algorithm, which pre-screens training data through the use of terrain information from the United States Geological Survey. Then, in the second stage, a data-driven selector attempts to eliminate residual non-homogeneities. The performance of this new approach is analysed using measured airborne radar data, obtained from the multi-channel airborne radar measurements program, and is compared with alternative STAP detectors proposed in the open literature.
Bistatic synthetic aperture radar (SAR) uses a separated transmitter and receiver, flying on different platforms, which enables the exploitation of additional information contained in the bistatic reflectivity of targets. The feasibility of the bistatic concept has already been demonstrated although technical problems such as the synchronisation of the oscillators still persist. The processing of bistatic raw data has still not been resolved sufficiently, either. The omega-k (or range migration) processor has proven popular in the monostatic case because of its relatively simple implementation, its close-to-optimum performance and its numerical efficiency, but has not been generalised for the bistatic case. A method to eliminate the deviation of the bistatic range history from the monostatic hyperbolic shape in the omega-k domain making subsequent omega-k processing possible has been recently proposed, and generalisations of the omega-k algorithm to the bistatic case can also be found. Adding to the literature, an omega-k-type processor for the special case of equal-velocity vectors of transmitter and receiver, the so-called 'spatial invariant' case, is introduced. This processor is not optimum in the sense of the monostatic omega-k processor but degradation for wide ranges of geometrical parameters proved negligible. During flight testing in November 2003, different spatially invariant flight geometries were tested and high resolution bistatic SAR images were generated successfully.
The analysis of jamming signals emitted by electronic countermeasure (ECM) systems after phase quantisation, performed by radio frequency digital memory devices, and after introduction of increasing time delay, is approached. The analysis shows the exact expression of the spectrum of a phase-quantised jamming signal when the quantisation levels are equally spaced. The spectrum of the signal presents a main term that is simply an attenuated version of the signal received by the ECM system, and spurious terms whose amplitude and position depend on the number of quantisation levels. The effect of time delay quantisation introduced by range gate pull-off devices is analytically evaluated, in combination to the phase quantisation. Results show that this second distortion can be generally neglected with respect to the spurious terms because of the phase quantisation, particularly when the number of phase quantisation bits is low
Synthetic aperture radar (SAR) allows all-weather, day-and-night surface surveillance and has the ability to detect, classify and geolocate objects at long stand-off ranges. Bistatic SAR, in which the transmitter and the receiver are on separate platforms, is seen as a potential means of countering the vulnerability of conventional monostatic SAR to electronic countermeasures, particularly directional jamming and avoiding physical attack of the imaging platform. As the receiving platform can be totally passive, it does not advertise its position by RF emissions. The transmitter is not susceptible to jamming and can, for example, operate at long stand-off ranges to reduce its vulnerability to physical attack. This paper presents part of the work undertaken at QinetiQ examining the techniques and additional complications involved in producing high-resolution bistatic SAR imagery. The work presented here focuses on a fully airborne, synchronised bistatic SAR demonstration using QinetiQ's enhanced surveillance radar and the Thales/QinetiQ airborne data acquisition system, which took place in September 2002. Some of the bistatic imagery from the trial are presented here and compared and contrasted with the monostatic imagery collected at the same time.
Conventional radar absorbent materials rely on the absorption and conversion into heat of the electromagnetic energy incident upon them. In an alternative approach, the phase-switched screen (PSS) uses a switchable resistive layer to apply binary phase modulation to the reflected signal so that its energy is redistributed into sidebands that lie outside the receiver passband. Here, the characteristics of an alternative type of PSS, whose efficacy depends on periodic changes in the permittivity or permeability of a dielectric layer, are considered. This leads to the concept of the pseudo-multi-layer dielectric PSS, which mimics the performance of a multi-layer absorber but comprises of only a single layer.
A fast-matched filter algorithm in time domain is presented to correlate signals for optimal detection of signals in noise, to extract ranging measurements, for synchronisation, and so on. The matched filters are discussed in the context of global positioning system receivers, but the algorithms presented here can be directly used for other direct sequence spread spectrum receivers as no constraining assumptions are made in the derivations. The method proposed here outperforms the conventional time-domain method several times in arithmetic complexity. It is also competitive with transform-domain techniques based on fast Fourier transform (FFT). Unlike FFT-based methods, however, this approach is accurate and does not use rounding or scaling operations.
Defence Research and Development Canada, Ottawa, has completed Phase II of a multifunction X-band wideband experimental airborne radar. The system consists of a high average power transmitter, a digital waveform generator, two wideband 8-bit channels for synthetic aperture radar (SAR)/inverse synthetic aperture radar and two narrowband 14-bit channels for ground moving target indication (GMTI). The reflector antenna uses a novel multimode feedhorn to derive two phase centres displaced in azimuth. The radar was designed to support research into SAR imaging of fixed and moving targets (ocean and land), time-frequency analysis of moving targets, clutter suppression for GMTI radar and ocean surveillance for small and large target detection. Highlights of the data collection capabilities, include a swath width 16 K points wide in the single-channel SAR modes, 8 K points wide in the two-channel integrated SAR-GMTI modes and 4 K points wide in the GMTI surveillance modes. The architecture of the radar, its modes of operation with respect to the SAR and GMTI data collection capabilities, the MATLAB-based GMTI processor, and the real-time kernel developed for the SAR processor are discussed; and results from its high resolution stripmap, landspot, seaspot and GMTI modes are presented from trials in July 2003.
In addition to powerful earth imaging and remote sensing, a multistatic spaceborne synthetic aperture radar (SAR) configuration offers the possibility of detecting efficiently the presence of slowly moving targets and surfaces as a result of the large baselines. Such a configuration can be achieved by a constellation of coherent passive satellites receiving the echoes of pulses transmitted by a separate SAR satellite and reflected from the earth. It also enables resolution of the design difficulty of space-based ground moving target indication (GMTI) systems caused by the severe spatial conditions. Owing to the additional degrees of freedom available in a multisatellite constellation compared with a monostatic system, the problem of blindness against certain directions of target motion can be overcome and the sensitivity can be extended to almost any direction of motion, depending on the configuration. For this purpose, non-classical algorithms have to be developed. Because of the high system complexity and the huge amount of data to be processed, only suboptimum methods can be implemented. A suboptimum method is proposed for multistatic spaceborne moving target detection based on the auxiliary beam approach. The main feature of this processing is estimation of both velocity and direction of target motion. According to this method, an analysis of multistatic SAR satellite configurations for their GMTI capability is carried out and the choice of the constellation baselines inferring directly on the expected GMTI sensitivity is discussed.
The author introduces and analyses the performance of different techniques to estimate the parameters of ground moving targets in multi-channel synthetic aperture radar (SAR) data. Candidates are matched filter banks, the along-track interferometric phase and direction-of-arrival estimation methods, which can work in either the raw data or in the compressed SAR image domain. Of particular interest are systems with only two channels because many existing or near-future SAR systems, such as RADARSAT-2 and TerraSAR-X, are restricted to a maximum of two sub-apertures. Desired parameters are the two velocity components (along- and across-track), acceleration if present and the true azimuth location. Theoretical results are evaluated and illustrated with experimental airborne SAR data.
