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 frequencydomain 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.Index Terms-Airborne radar, bistatic synthetic aperture radar (SAR), focusing. I. CONTEXTL OW TO MEDIUM (1 m to 30 cm) resolution synthetic aperture radar (SAR) imaging at Office National d'Études et Recherches Aérospatiales (ONERA) in the 80s and 90s (before high-accuracy GPS-hybridized inertial measurement units allowed direct SAR synthesis from measured trajectory) used local map-drift [1] autofocus. This technique, measuring distortion field between two (or more) independent looks (stripmap images), produced estimates in low frequency trajectory errors (typical inertial unit fluctuations were longer in time than the typical integration time needed for these resolutions), and images were synthesized again with corrected trajectory [2].High-resolution (10 cm) SAR imaging initiated in 2000 at ONERA requires autofocus for estimating velocity error fluctuations much shorter than the integration time; thus, an autofocus based on multilateration from phase tracking of pointlike reflectors was designed [3] with automatic detection of the appropriate stable pointlike reflections on the ground. This algorithm uses range-clipped and narrow Doppler-band Manuscript filtered profiles around the pointlike reflection to measure and unwrap the echo phase. From these phase variation measurements in at least three independent directions for any point of the acquisition trajectory, an accurate trajectory update can be derived.With circular SAR imaging with high banking angle and bistatic SAR imaging (and some wild remote areas without enough appropriate man-made opportunistic reflectors), this autofocus method failed. The reason for circular SAR is that high banking angle masks GPS satellites, thus ruining GPS hybridizing, raising the Doppler jitter of the point echoes which implied widening the Doppler filter, and hence possibly mixing the tracked point echo with neighboring echoes. For bistatic SAR, very few (if any) pointlike echoes are usually present due to the fact that 90 • corners (present in many man-made landscape features) scatter the radio waves back to the transmitter and not to the sensor when there is any significant bistatic angle. Furthermore, true bistatic autofocus (not relyin...

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International audienceSynthetic aperture radar (SAR) is one of the favorite tools for earth observation applications, i.e., oceanography, land use mapping, climate change since this device can offer the data at a high spatial resolution and in most meteorological conditions. This is more significant when the data acquired by the Sentinel-1, a new C-band satellite, are exploited. For high-resolution wind field extraction, two different approaches are proposed. In the scatterometry-based approach, wind direction is first extracted by the local gradient method at different scales, i.e., 1–5-km wind resolutions. It is then applied to the empirical geophysical model functions, i.e., CMOD (C-band), for surface wind speed estimation. The advantage of this approach is to deliver accurate wind speed estimates in the range of 2–25 m/s from different SAR data. However, it requires wind direction as an input parameter. This can lead to errors in wind speed estimation due to uncertain wind directions. Therefore, for comparison, in the second approach, we propose the use of the model without wind direction input proposed by Komarov et al. In general, the obtained wind fields based on two proposed approaches are quite similar, and they have good agreement with in situ measurements from the meteorological stations along the Iroise coast

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