Digital Beamforming and Traffic Monitoring Using the new FSAR System of DLR

In November 2006 the first X-band test flight of DLR’s new FSAR system has been performed successfully and in February 2007 the first flight campaign has been conducted for acquiring experimental multi-channel data of controlled ground moving targets. In the paper the performed experiments and the used setup of the FSAR X-band section are described and preliminary results in the field of ground moving target indication and digital beamforming are presented

Impact of Road Vehicle Accelerations on SAR-GMTI Motion Parameter Estimation

In recent years many powerful techniques and algorithms have been developed to detect moving targets and estimate their motion parameters from single- or multi-channel SAR data. In case of single- and two-channel systems, most of the developed algorithms rely on analysis of the Doppler history. Nowadays it is known, that even small unconsidered across-track accelerations can bias the along-track velocity estimation. Since we want to monitor real and more complex traffic scenarios with a future traffic monitoring system like TRAMRAD, we must know which target accelerations we have to handle in reality. For this reason a common passenger car was equipped with an inertial measurement system and differential GPS to measure accelerations in all three dimensions during rush-hour traffic. In this paper the results of the acceleration measurements are presented and discussed. The standard deviations of the measured accelerations are in the order of 0.5 m/s2 for accelerations in driving direction and 0.6 m/s2 for radial accelerations. A theoretical analysis (which is verified by detailed simulations) of the Doppler slope shows also that at such high across-track accelerations a reliable estimation of the along-track velocity by means of a Doppler slope analysis without further information is unemployable in practice. Also oscillations of the car body along the vertical axis are investigated in this paper. From the field of vehicle dynamics it is known that the eigen frequencies of the car body are in the range from 0.7 to 2.0 Hz. Deflections in the order of one wavelength (X-band) or higher are possible at such frequencies. The simulation results for spaceborne SAR systems with integration times in the order of one second show that the shape and azimuth shift of the impulse response depend beside the oscillation frequency and the deflection also on the initial phase of the oscillation. However, at practical applications the main part of the energy could also be reflected by double bounce from the road surface. Thus, further investigations in the topic of vehicle oscillations by using real radar data are necessary. Finally, some basic ideas are presented which enable a reliable separation between along-track velocity and across-track acceleration. For example, the easiest way to separate both just mentioned motion parameters is the use of a road database, from which the information about the motion direction of the assigned vehicle can be extracted. Hence, the accuracy of along-track velocity estimation is mainly given by the accuracy of the estimated across-track velocity and the angle of the road section in relation to the flight path of the SAR platform

Moving Target Signals in High Resolution Wide Swath SAR

In this paper the impact of two high resolution wide swath synthetic aperture radar imaging techniques, extraction of signals from azimuth subsampled spectra and subpulse azimuth beamsteering on transmit, on ground moving target signals is analyzed. A modelling of the interfering signals in range-Doppler domain is introduced, that is suitable for studying the performance of low PRF sampled GMTI and subpulse beamsteering in azimuth and elevation on transmit and receive for HRWS-SAR imaging and GMTI. The combination of the two techniques yields an operation mode for high resolution wide swath synthetic aperture radar imaging and long observation time wide swath ground moving target indication. The mode is analyzed in terms of signal to interference plus noise ratio and coverage for ground moving target indication

A New Method to Create a Virtual Third Antenna from a Two-Channel SAR-GMTI System

Two-channel SAR-GMTI systems are suboptimal for moving target motion parameter estimation. Indeed, the ATI phase estimate of the across-track velocity component for a moving target is biased to lower values depending on the target signal to clutter ratio and the target across-track velocity. Additional antenna diversity can introduce additional degrees of freedom that can eliminate the bias problem. Aperture switching is an accepted method to virtually increase the number of channels without adding new hardware. One such mode is the RADARSAT-$2$ Toggle mode cite{Chiu_2006}. This paper proposes a new processing method to create a similar effective phase center configuration as the RADARSAT-$2$ Toggle mode from already recorded two-channel SAR data. This is achieved by delaying and combining the recorded two-channel measurements. The combination operation manifests not only a third phase center halfway between the phase centers of the two-channel system, but also a different antenna length of the virtual third antenna which requires a modification of the DPCA-ATI processing algorithm. The DPCA-ATI performance of the new mode is assessed and compared to ATI from the original two-channel mode

Motion Parameter Estimation of Doppler-Ambiguous Moving Targets in SAR-GMTI

In an along-track interferometric SAR system, the discrete sampling of moving target signals can give rise to two types of ambiguity: Doppler ambiguity, and interferometric angle ambiguity. These ambiguities lead to ambiguities in target velocity estimation. Range cell migration of moving targets is unambiguous in target velocity. Hence, it can be used for resolving the ambiguities in target velocity estimation mentioned above. The wave number domain algorithm as well as the chirp scaling algorithm is adapted to moving target signals. In order to focus moving target signals with arbitrary velocities both approaches are extended to arbitrary Doppler frequency ands. Moving target signals distributed over two neighbouring PRF bands are especially difficult to detect and analyze because the sgnal splits into two parts. It is shown that the two parts appear at different positions in the SAR image and have different ATI phases. They show up as two weaker targets since the energy is split between them. It is demonstrated how the two targets can be identified as possibly the same target, and how they can be properly focussed by adaptation of the SAR focussing algorithms

GMTI Performance Of A High Resolution Wide Swath SAR Operation Mode

In this paper an operation mode for space-based SAR/GMTI is suggested which uses a rectangular antenna array and employs subpulse beamsteering on transmit in order to yield wide swath coverage. A signal modeling in range- Doppler domain is introduced that is suitable for studying the SAR/GMTI performance. Simulation results are given

Airborne Road Traffic Monitoring with Radar

To ensure mobility, future road traffic management needs actual and reliable information about the traffic over wide areas. Nowadays, outside of motorways the actual traffic situation is almost unknown due to the lack of sensor installations. Traffic monitoring with radar from high altitudes delivers a new approach which allows the capture of traffic data in wide areas and at every position in the road net. Radar can look through clouds and can operate independently of weather and sight conditions at day and night and it can detect vehicles independently of their instrumentation onboard. At the German Aerospace Center (DLR) the practical feasibility of airborne radar for road traffic management in general but also in specific situations of big events or occurred disasters is under investigation. This paper gives an overview over the actual status of project works and will present aspects for future applications of airborne radar for road traffic and disaster management

Comparison of System Concepts for Traffic Monitoring with Multi-Channel SAR

This paper addresses the ground moving target indication (GMTI) performance of space-based multi-channel synthetic aperture radar (MSAR) systems as they suffer from the high speed of the radar platform leading to a wide spread clutter spectrum and compares it with the air-based case. Opposite to classical GMTI systems near future space-based SAR systems offer only few simultaneous channels and relatively low pulse repetition fre-quency (PRF). The influences of PRF, antenna size and number of channels on detection performance are stud-ied. The analysis is based on a post-Doppler approach of the optimal processor since this has the advantage that in case of stationary clutter the clutter contributions can be assumed statistically independent for each Doppler cell

Multichannel Systems for Ground Moving Target Indication

The development of a fully operational wide area traffic monitoring system like in the TRAMRAD project (TRAffic Monitoring with spacebased RADar) requires detailed knowledge of the capabilities and limitations of Ground Moving Target Indication (GMTI) techniques. Only with this knowledge it is then possible to end up in an optimized system design. In principle, two processing techniques come into question for radar-based traffic monitoring: Synthetic Aperture Radar (SAR) due to its high resolution imaging capability and Space Time Adaptive Processing (STAP) due to its moving object indication capability. SAR is a well established remote sensing technique which is now routinely used to provide radar images from airborne and spaceborne platforms independent of daylight and weather conditions. On the other hand, several powerful radar techniques have been developed for detecting ground moving targets and estimating their motion parameters mainly for airborne applications in military context. Most of these developments rely in some form on the STAP principle, which evaluates the signals from multiple receive apertures in a spatiotemporal array processing framework. The joint space-time processing is well suited to mitigate the Doppler spreading of ground clutter induced by radar platform motion, thereby enabling a reliable detection of even slow and small vehicles with low radar cross section in heterogeneous environments. However, until today none of these multi channel techniques has been implemented operationally on a spaceborne platform to monitor traffic on the ground. To develop a future fully operational and high resolution traffic monitoring system as envisaged in TRAMRAD it is hence necessary to recapitulate the existing airborne GMTI techniques and algorithms and to adapt them to the traffic scenarios to be observed. This document recapitulates existing GMTI techniques which are based on STAP because from STAP important principles for GMTI using radar can be gained. The influence of system parameters which have to be designed is shown as well as the influence of channel and clutter mismatches to STAP performance. An adaptive system can add robustness to system errors and can handle nonstationary interference. However in real-world environments the adaptation process is critical because clutter statistics vary fast and therefore in general only few samples are available which provide the same clutter statistics as the clutter in the cell under test. A multitude of STAP methods and algorithms has been developed which is aimed on providing fast convergence in the adaptation process. An overview on the methods is given. However the transfer of a typical air-based GMTI system to a space-based GMTI system is not simple: Aspects such as height above ground, platform velocity and platform stability have to be taken into account. A spaceborne GMTI system rather has to combine SAR with STAP principles. The transfer of a STAP processing sheme on a linear multi-channel SAR-system is shown as well as a possibility to transfer STAP on a three-dimensional multistatic array configuration