Hubert Cantalloube

An Omega-K algorithm for SAR bistatic systems

Bistatic SAR uses separate flying transmitter and receiver platforms. There are two principal configurations : spatial invariant case, in which velocity vectors are equal; and spatial variant case, in which velocity vectors are not equal. ONERA and DLR performed experimental flights on the spatial invariant case in february 2003 [1]. The experiment was conducted using the X-band front ends that are compatible in terms of frequency (on a 100 MHz-Wide band). Estimation and compensation of the phase drift between the transmitter and the receiver master oscillator was presented in [1]. Until now, data have been processed with a back projection algorithm. This article investigates an Omega-K algorithm - or Range Migration Algorithm - for bistatic SAR systems in spatial invariant cases. First results of this algorithm on real data are included.

Multiscale Local Map-Drift-Driven Multilateration SAR Autofocus Using Fast Polar Format Image Synthesis

This new autofocus method is based on multilateration by ranging to small target areas at independent directions on the ground. Range-clipped Doppler low-filtered profiles around target points are used to compute local images using frequency-domain polar format algorithm. Images obtained from adjacent subapertures are registered, and the displacements yield elevation, trajectory, or clock drift (bistatic case) errors. To alleviate the insensitivity of map drift to error fluctuation faster than subaperture duration, the algorithm is reiterated with coarse-to-fine resolution, yielding high to low frequency errors. This allowed true bistatic synthetic aperture radar (SAR) autofocus (without monostatic image), autofocus in circular SAR on remote areas, and, as a side product, our first successful air-to-air inverse SAR high-resolution imaging.

LORAMbis A bistatic VHF/UHF SAR experiment for FOPEN

LORAMbis is the experimental part of a joint research program between Sweden and France. The objective is to evaluate the performance of low frequency bistatic SAR for clutter suppression in various applications, e.g. under-foliage target detection or mapping of urban scenes. The airborne bistatic SAR data are acquired using the VHF/UHF component of the ONERA SAR system SETHI and the VHF/UHF LORA system operated by FOI. The first data collection campaign was conducted in December 2009. Focused bistatic SAR images have been formed and indicate that the implemented synchronization method is sufficient. It is based on a GPS disciplined 10 MHz reference signal that is generated in both systems.

High resolution SAR imaging along circular trajectories

After a first series of full circle SAR acquisitions in L and P-bands during a 2004 joint FOI-ONERA campaign in Sweden, ONERA experimented in 2006 high resolution (15 cm) polarimetric, full circle acquisitions in France and Germany using its X-band sensor. In order to cope with narrower antenna pattern and aircraft attitude fluctuations, a steerable antenna was used. Furthermore, an experimental setup for retrieving high accuracy trajectory was installed. This paper describes the processing of this signals.

The ONERA RAMSES SAR: latest significant results and future developments

The last SAR campaigns of the French ONERA RAMSES system have been dedicated to various applications, military or civilian, including very-high resolution, FOPEN (Foliage penetration) in cooperation with the FOI, MTI, bistatic SAR and vegetation characterization. These different projects gave us the opportunity to improve our SAR processing procedure (very high resolution) and to develop new algorithms (bistatic scenario). In parallel, the ONERA team has developed and applied many techniques (polarimetric analysis, PolInSAR, wavelet analysis) which proved to be efficient in these different thematic context. The current aircraft platform, a Transall C160 will be decommissioned at the end of 2008. In order to maintain our activities, we are developing a new system family compatible with smaller aircrafts. The challenge is two fold : migrating the existing radars in a airplane four times smaller and maintaining our innovation capabilities with the development of new radar concepts. These paper presents the most recent results on SAR techniques and describes the status of the development of the RAMSES successors : the first, SETHI, dedicated to military applications and earth observation, the second, BUSARD more focused on UAV applications.

Radargrammetric DEM Extraction Over Urban Area Using Circular SAR Imagery

Digital elevation model (DEM) extraction in urban areas, using synthetic aperture radar (SAR) imagery, has become a subject of interest with the increased availability of high-resolution SAR images. One of the main advantages of SAR imagery is that, except from very peculiar situations, the radiometry of these images does not depend on weather conditions. The main drawback is the slant-range geometry which leads to large shadow and layover areas, and, therefore, to an incomplete DEM. In this paper, a new method of DEM extraction using SAR imagery acquired on a circular trajectory is proposed. Results are presented with data acquired with the SETHI system from ONERA, in X-band, in the city of Nîmes, France.

TropiSAR: Exploring the temporal behavior of P-Band SAR data

The TropiSAR campaign has been conducted in August 2009 in French Guiana with the ONERA airborne system SETHI. The main objective of this campaign was to collect data to support the Phase A of the Earth Explorer candidate mission, Biomass. Several specific questions need to be addressed to answer the recommendations of the ESAC group and the data collection strategy has been constructed accordingly. The first part of the paper lists these specific questions. We then describe the selected test sites, followed by a summary on the radar instrument and the radar configuration (geometry and waveform). The data acquisition plan is provided and the temporal behaviour of the P-Band data is explored.

Airborne SAR-Efficient Signal Processing for Very High Resolution

Frequency-domain synthetic aperture radar (SAR) image formation algorithms are of lower computation cost (both in number of elementary operations and in required memory storage) than direct time-domain integration, and do not make the narrowband (monochromatic) assumption. Both advantages are critical to very-high-resolution imaging because a lower complexity yields a drastic computation time decrease as cross-range resolution increases, and the narrowband assumption is more and more a concern as range resolution (hence bandwidth) increases. Though an exact formulation exists (ω- k algorithm) for a perfect linear uniform acquisition trajectory, in a real-life airborne case, the unavoidable trajectory deviation from a straight line needs to be compensated. This motion compensation (MoComp) operation is much more complicated in the case of frequency-domain processing. An efficient technique for this purpose is presented. This method keeps the parallel processing aspect, and has been programmed both for multithread on multicore/symmetrical multiprocessor central processing units (CPUs) and for graphic processor units (GPUs).

The new ONERA multispectral airborne SAR system in 2009

This paper provides an overview of the new airborne SAR system developed by the French Aerospace Lab ONERA over the last three years. This system, called SETHI, was developed according to the standard FAR25 applied to civil application. The main improvement compared to the previous ONERA airborne radar system RAMSES is that the antennas are located in two pods compatible with small aircrafts like the Falcon 20. This pod-based configuration allows the easy integration of any kind of payloads under the single certification of the pods by authorities.

Elevation-dependent motion compensation for frequency-domain bistatic SAR image synthesis

While numerically more efficient, frequency domain SAR image synthesis is less easily adaptable to the irregular real airborne trajectories than time-domain image synthesis. Trajectory nonlinearities have another consequence: The image focusing depends on the terrain elevation, hence motion compensation for irregular trajectories on mountainous areas must take into account terrain elevation data. Bistatic SAR processing is elevation-dependent even if the trajectory are perfectly linear (with the exception of the case where both aircrafts follow the same flight line). Terrain elevation can only be ignored at distance very large with respect to the elevation fluctuations, which is only the case in airborne bistatic SAR imaging when the area flown over is extremely flat. We describe here how the monostatic elevation-dependent motion compensation for omega-k algorithm is adapted to bistatic omega-k synthesis algorithm.