Peter Hoogeboom

Signal Processing for FMCW SAR

The combination of frequency-modulated continuous-wave (FMCW) technology and synthetic aperture radar (SAR) techniques leads to lightweight cost-effective imaging sensors of high resolution. One limiting factor to the use of FMCW sensors is the well-known presence of nonlinearities in the transmitted signal. This results in contrast- and range-resolution degradation, particularly when the system is intended for high-resolution long-range applications, as it is the case for SAR. This paper presents a novel processing solution, which solves the nonlinearity problem for the whole range profile. Additionally, the conventional stop-and-go approximation used in pulse-radar algorithms is not valid in FMCW SAR applications under certain circumstances. Therefore, the motion within the sweep needs to be taken into account. Analytical development of the FMCW SAR signal model, starting from the deramped signal and without using the stop-and-go approximation, is presented in this paper. The model is then applied to stripmap, spotlight, and single-transmitter/multiple-receiver digital-beamforming SAR operational mode. The proposed algorithms are verified by processing real FMCW SAR data collected with the demonstrator system built at the Delft University of Technology.

Modified range-Doppler processing for FM-CW synthetic aperture radar

The combination of compact frequency-modulated continuous-wave (FM-CW) technology and high-resolution synthetic aperture radar (SAR) processing techniques should pave the way for the development of a lightweight, cost-effective, high-resolution, airborne imaging radar. Regarding FM-CW SAR signal processing, the motion during the transmission of a sweep and the reception of the corresponding echo were expected to be one of the major problems. In FM-CW SAR, the so-called stop-and-go approximation is no longer valid due to the relatively long sweeps that FM-CW radars transmit. The main effect of the continuous motion is a Doppler frequency shift throughout the SAR observation time. This Doppler frequency shift can be compensated for by modifying the range migration compensation.

Design and Analysis of Compressed Sensing Radar Detectors

We consider the problem of target detection from a set of Compressed Sensing (CS) radar measurements corrupted by additive white Gaussian noise. We propose two novel architectures and compare their performance by means of Receiver Operating Characteristic (ROC) curves. Using asymptotic arguments and the Complex Approximate Message Passing (CAMP) algorithm, we characterize the statistics of the ℓ1-norm reconstruction error and derive closed form expressions for both the detection and false alarm probabilities of both schemes. Of the two architectures, we demonstrate that the best performing one consists of a reconstruction stage based on CAMP followed by a detector. This architecture, which outperforms the ℓ1-based detector in the ideal case of known background noise, can also be made fully adaptive by combining it with a conventional Constant False Alarm Rate (CFAR) processor. Using the state evolution framework of CAMP, we also derive Signal to Noise Ratio (SNR) maps that, together with the ROC curves, can be used to design a CS-based CFAR radar detector. Our theoretical findings are confirmed by means of both Monte Carlo simulations and experimental results.

Classification of Agricultural Crops in Radar Images

For the past few years an accurate X-band SLAR system with digital recording has been available in The Netherlands. The images of this system are corrected to indicate radar backscatter coefficients (gamma) instead of arbitrary greytones. In 1980 a radar measurement campaign was organized in the Flevopolder area. Ground-based measurements were carried out and at certain prefixed times flights were made. The optimum flight times had been derived from a simulation study, based on previous measurements. The results of this campaign are now being analyzed. The one-dimensional case, consisting of one flight over the test area at a previously derived optimum flight time gives only 35-percent correct classification in 7 classes. Multidimensional cases with flights at different dates improve the classification result to over 85-percent. The small differences in backscatter between the various types of crops clearly demonstrate the need for an accurate radar system which does not show too much speckle. The results of these analyses are reported in the following.

On the use of multi-frequency and polarimetric radar backscatter features for classification of agricultural crops

Abstract The significance of several key multi-frequency, polarimetric back-scatter parameters extracted from calibrated and noise-corrected NASA/JPL DC-8 SAR data are examined. The data were collected during the 1989 MAESTRO-1 campaign over the Flevoland agricultural test site. Calibration uncertainty estimates are used to specify minimum separations between features. Thirteen different backscatter types were identified from the test site data, including eleven different crops, one forest and one water area. Using the parameters with the highest separation for a given class, a hierarchical algorithm was developed to classify the entire image. All three frequencies and all polarizations were used to construct the rules for the classifier. Results indicate that multi-frequency, polarimetric radar backscatter signatures can be useful in classifying several different ground cover types in agricultural areas.

Analysis of sea spikes in radar sea clutter data

In this paper three sets of high-resolution, coherent, and polarimetric radar sea clutter data are analyzed and compared with radar sea clutter models. The nature of the data allows a thorough analysis of the power, polarization and velocity of the sea clutter. It is shown that these quantities, especially the velocity, are good measures of many physical properties of the ocean surface. Furthermore, it is shown that these physical properties match well with the sea clutter models. Sea clutter is found to consist of two components, a diffuse background, characterized by low values of backscattered power, HH/VV polarization ratio and Doppler velocity, and a number of spiking events, which possess higher power, polarization ratio and velocity. The background is reasonably well modeled by tilt-modulated Bragg scattering, whereas the spikes may be associated with the scattering on steepened and/or breaking waves. Moreover, it is shown that the influence of microbreakers has to be taken into account to explain the relatively high polarization ratio. A breaking wave origin for the spikes is supported in two ways. First, by a detailed analysis of the temporal behavior of individual spike backscatter properties, and second, by a statistical analysis of the entire population of spikes.

Range Non-linearities Correction in FMCW SAR

The limiting factor to the use of Frequency Modulated Continuous Wave (FMCW) technology with Synthetic Aperture Radar (SAR) techniques to produce lightweight, cost effective, low power consuming imaging sensors with high resolution, is the well known presence of non-linearities in the transmitted signal. This results in contrast and range resolution degradation, especially when the system use is intended for long range applications, as it is the case for SAR. The paper presents a novel processing solution, which completely solves the non- linearity problem. It corrects the non-linearity effects for the whole range profile at once, differently from the algorithms described in literature so far, which work only for very short range intervals. The proposed method operates directly on the deramped data and it is very computationally efficient.

The Dutch ROVE Program

In the Netherlands the ROVE team (Radar Observation of VEgetation) investigates the possibilities of radar remote sensing in agriculture. It is an interdisciplinary working group in which five institutes collaborate. Each year, from 1975 to 1980, noncoherent backscatter measurements have been made on different types of vegetations, crops, and bare soils using groups of test fields laid out on a test farm of one of the participating institutes. This approach warrants adequate control over botanical, soil, and surface parameters. For the radar backscatter measurements a short-range FM/CW scatterometer is used mounted on a carriage which is moved along the fields. The combination of the FM/CW principle with movement of the system guarantees a sufficient number of independent observations in a measurement where the illuminated patch is large enough to contain an adequate number of scatterers. So the radar return parameter γ (or σ°) is determined with sufficient accuracy as a function of grazing angle and of time through the growing season. An accurate X-band side-looking airborne radar (SLAR) with digital recording is available for airborne verification experiments. The program is a continuation of the experiments described by the authors at the URSI meeting in Berne in 1974.

Signal processing algorithms for FMCW moving target indicator synthetic aperture radar

The combination of frequency modulated continuous wave (FMCW) technology and synthetic aperture radar (SAR) leads to lightweight, cost-effective imaging sensors of high resolution. In FMCW SAR applications the conventional stop-and-go approximation used in pulse radar algorithms cannot be considered valid anymore, so the motion within the sweep needs to be taken into account. Analytical development of an FMCW SAR algorithm starting from the deramped signal and without using the stop-and-go approximation is presented in this paper; it is then validated processing simulated and real data. Furthermore, the effects of a moving target as they appear in an FMCW SAR image are described and the results can be used to assist moving target indicator (MTI) capabilities.

Development of signal processing algorithms for high resolution airborne millimeter wave FMCW SAR

For airborne Earth observation applications, there is a special interest in lightweight, cost effective, imaging sensors of high resolution. The combination of frequency modulated continuous wave (FMCW) technology and synthetic aperture radar (SAR) techniques can lead to such a sensor. In this paper, a developed algorithm for SAR imaging that takes into account the special characteristics of FMCW signals is presented. Constrains for the validity of the stop and go approximation are shown. A demonstrator system has been built at Delft University of Technology and some results from the last airborne campaign are presented.