J.J.M. de Wit

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 In FM-CW SAR, the so-called stop-and-go approximation is no longer valid due to the relatively long sweeps that FM-CW radars transmit.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.

Orthogonal waveforms for FMCW MIMO radar

Multiple input/multiple output (MIMO) radar system performance benefits from the capability to simultaneously transmit and receive multiple orthogonal waveforms. For pulse radars fitting orthogonal waveforms have been developed. These waveforms are however not necessarily suitable for frequency-modulated continuous-wave (FMCW) radar. The major benefit of the FMCW radar principle is that the bandwidth of the beat signal is generally much smaller than the signal bandwidth, relaxing sampling requirements. Preferably, this benefit should be preserved when applying orthogonal waveforms in an FMCW MIMO radar configuration. In this paper, orthogonal waveforms compatible with the FMCW radar principle will be discussed and tested by experiment or simulation.

Estimating effective radius and liquid water content from radar and lidar based on the CLARE98 data-set

Retrieval methods to estimate the effective radius (re) and the liquid water content (LWC) of stratocumulus clouds from radar and lidar observations are presented here. The retrieval of re is accomplished by combining radar and lidar observations by means of a Z/α − re relation, which is derived from the measurements of in-situ particle probe instruments. The retrieval of LWC, on the other hand, is done solely on the basis of radar observations. However, when plotting a Z-LWC diagram, there was too much scatter, preventing a direct derivation of a Z-LWC relation. It appears that the deviating points in the Z-LWC scatter diagram are mainly caused by the size distributions that have a rather large re. By filtering out these distributions, the scatter in the Z-LWC diagram is significantly reduced and an empirical Z-LWC relation is derived. The estimates of re and LWC using airborne radar and lidar show reasonably good agreements with the profiles measured in-situ. A comparison of the LWC estimated from ground-based radar data, integrated over the cloud height, with the liquid water path provided by a radiometer also shows good z agreement, with a general offset of about 50 g m−2.

High resolution FM-CW SAR performance analysis

The combination of compact FM-CW technology and high resolution SAR techniques seems of special interest for the implementation of a low cost, airborne imaging radar. A project has been started to investigate the feasibility of FM-CW SAR. A system simulation is in development to estimate the performance of FM-CW SAR. The performance is probably limited by phase noise. Therefore, a study has been made of the effects of phase noise and a time domain description of phase noise has been developed.

Motion compensation for a high resolution Ka-band airborne FM-CW SAR

Airborne synthetic aperture radar is a promising new application for FM-CW radars. At the IRCTR, an operational demonstrator system has been developed in order to investigate the practicability of FM-CW SAR and to prove that an FM-CW SAR system can be operated in an efficient manner from a small platform. The platform used to fly the demonstrator system and the integrated navigation system will be introduced. In addition, the digitalization and the synchronization regarding the navigation system will be addressed. Subsequently, the motion compensation details, especially the squint angle compensation, are explained. The working status of the demonstrator system and the integrated navigation system is proven by the results of an airborne FM-CW SAR campaign. The results obtained during the airborne campaign are presented and the effect of squint angle compensation is shown. The airborne results could not be further improved by applying more sophisticated motion compensation algorithms or autofocusing techniques. Therefore, it was decided to develop an improved FM-CW front end in order to enhance the FM-CW SAR image quality

SAR: A Novel Application for FM-CW Radars

For small-scale earth observation applications, there is a special interest in low-cost, high-resolution imaging radars small enough to be operated from small, possibly unmanned, aircraft. The combination of the compactness of FM-CW technology and the high resolution of SAR systems should result in such a small, cost-effective imaging radar. In a new project the feasibility of FM-CW SAR is studied. SAR algorithms that take the typical characteristics of FM-CW signals into account will be developed. Furthermore, a low-cost demonstrator will be built. This demonstrator should prove the feasibility of FM-CW SAR and provide real test data to validate the processing algorithms and the design models. The demonstrator will be based on commercially available components. The system performance will be limited, but is sufficient to prove the feasibility of FM-CW SAR.

Concept for measuring and compensating array deformation

Phased array antennas are increasingly used in modern radar systems. Such systems are very liable to distortion of the relative phases between the radiating elements. For airborne, spaceborne, and naval phased array systems, vibrations of the array structure are an important source of phase distortions. In this paper, a system concept will be presented to measure array deformation and provide phase compensation information with which the received data can be corrected. Such realtime measurement and compensation of array vibrations may considerably improve the performance of operational systems. Therefore, a study was started to investigate the feasibility of this concept. Shaker experiments proved that the required measurement accuracy is within reach using off-the-shelf sensors and suitable signal processing.

Extraction of Building Features from Stand-Off Measured Through-Wall Radar Data

Automated extraction of building features is a great aid in synthesizing building maps from radar data. In this paper, a model-based method is described to detect and classify canonical scatters, such as corners and planar walls, inside a building. Once corners and walls have been located, a building map can be synthesized. To detect and classify the canonical scatterers, sparse reconstruction with an overcomplete dictionary is used. The dictionary is tuned to phase models that are specific to the different canonical scatterers, i.e., a linear phase change for walls and a quadratic phase change for corners. This is a potentially robust method for detecting canonical scatterers because the polynomial degree of the measured phase is preserved, even after propagation through a wall. Building-feature extraction results obtained with actually stand-off measured through-wall radar data are discussed

Multifunction radar concept for through-wall surveillance

Radar systems suitable for stand-off mapping of building structures and through-wall surveillance allow covert reconnaissance of buildings and surveillance of people inside in support of e.g. police raids or search and rescue operations. In this paper the operational concept will be outlined for a multifunction radar system capable of stand-off building imaging and mapping, through-wall tracking of moving people, and blue-force tracking. These capabilities are illustrated with the aid of representative measurements.

Classification of human gaits using interrupted radar measurements

Classification modes aimed for slow events, such as human gait, using micro-Doppler measurements are long, and therefore not suitable for use during time critical radar operation. When these modes are interruptible by other radar modes they might become more acceptable. In this paper, sparse signal processing, in particular the SL0-algorithm, is investigated for interpolating interrupted radar measurements which subsequently can be used for classification of human gaits. The performance of a k-NN classifier with PCA-based feature extraction was improved significantly when the data were interpolated as compared to using the interrupted data without interpolation.