The dual-beam interferometer is an airborne instrument that combines two vertically polarized C-band along-track interferometric synthetic aperture radars (AT-InSARs) observing the surface below at different squints. The system was designed by the University of Massachusetts and saw several deployments in the early 2000s. An imagery of a small vessel with a rather pronounced wake pattern captured during one of such flights is the subject of this letter. Specifically, the interferometric phase in the turbulent wake exhibits a conspicuous banding structure that is still visible at distances more than 1 km behind the craft. The phase signatures from the fore and aft looks are combined to retrieve both longitudinal and lateral velocity components along cuts traversing the wake 400 and 750 m behind the boat. The results identify appreciable variations in the longitudinal velocity across the turbulent wake which are apparently consistent with the combined effect of the hull drag and the propeller backwash. A persistent pattern for the lateral component is also observed but is harder to interpret without the detailed knowledge of the vessel. The examples demonstrate the utility of AT-InSAR and, particularly, of a dual-beam AT-InSAR, for studies of centerline ship wakes. Readily available velocity signatures of a turbulent wake obtained with such systems can help with vessel classification tasks.

This study takes on the challenge of resolving upper ocean surface currents with a suite of airborne remote sensing methodologies, simultaneously imaging the ocean surface in visible, infrared, and microwave bands. A series of flights were conducted over an air-sea interaction supersite established 63 km offshore by a large multi-platform CASPER-East experiment. The supersite was equipped with a range of in situ instruments resolving air-sea interface and underwater properties, of which a bottom-mounted acoustic Doppler current profiler was used extensively in this paper for the purposes of airborne current retrieval validation and interpretation. A series of water-tracing dye releases took place in coordination with aircraft overpasses, enabling dye plume velocimetry over 100 m to 10 km spatial scales. Similar scales were resolved by a Multichannel Synthetic Aperture Radar, which resolved a swath of instantaneous surface velocities (wave and current) with 10 m resolution and 5 cm/s accuracy. Details of the skin temperature variability imprinted by the upper ocean turbulence were revealed in 1–14,000 m range of spatial scales by a mid-wave infrared camera. Combined, these methodologies provide a unique insight into the complex spatial structure of the upper ocean turbulence on a previously under-resolved range of spatial scales from meters to kilometers. However, much attention in this paper is dedicated to quantifying and understanding uncertainties and ambiguities associated with these remote sensing methodologies, especially regarding the smallest resolvable turbulent scales and reference depths of retrieved currents.

We present novel experimental evidence that demonstrates the effectiveness of exploiting scene motion information for the analysis of scene structure in maritime imaging applications. We analyze data captured by our novel airborne Multi-channel SAR (MSAR) system that is particularly suited to sampling the velocity profile of scatterers in the maritime environment. While previous works have shown the utility MSAR systems for correcting scene motion induced blurring artifacts, our work shows, for the first time, how the information furnished by an MSAR system can systematically render accurate classification of maritime scenes into different perceptual categories. We offer a methodology that is superior to traditional classification techniques that are based purely on the spatial structure of an image. Furthermore, the simplicity of the feature space involved together with the demonstrated classification performance on imagery captured by our airborne MSAR system underscore the strength of the methodology.

We report the initial results of an experimental investigation into the measurement of ocean surface currents with an ultrahigh frequency synthetic aperture radar (UHF SAR) that supports an along-track interferometric (ATI) mode. We compare results from two different approaches to the current measurement: analysis of wave dispersion in a time-sequence of subaperture images and standard ATI-SAR processing. We find that while both methods clearly show a surface velocity front over the north wall of the Gulf Stream, the change in velocity indicated by the dispersion method is significantly less than that indicated by ATI-SAR processing. We offer methodological and geophysical reasons for the observed differences.

We summarize a theoretical analysis of the velocity resolution of an ultra-wideband (UWB), along-track interferometric SAR (ATI-SAR) when used to measure ocean surface currents. In particular, we investigate the question of whether spreading the total system power, P_T, over multiple channels spanning an ultra-wide RF band, B_tot, can improve the surface current velocity resolution. We analyze the resolution achievable when a frequency modulated continuous wave (FMCW) system is channelized to effectively operate as p independent ATI-SARs, each with a bandwidth of B_tot/p, and an average power of P_T/p, but with different center frequencies. Our results indicate that for moderate and higher wind speeds and for the low altitude, drone-based configurations we envision for such a system, spreading the energy across multiple RF sub-bands does indeed produce finer velocity resolution than if the energy is concentrated into a single channel. While a single channel produces a higher signal-to-noise ratio (SNR) than the individual sub-band channels due to higher average power and pulse compression gain, this higher SNR does not always decrease the interferometric phase noise and current speed uncertainty. In contrast, the independent samples in the p sub-bands can be used to reduce the phase noise through a weighted average, resulting in lower overall uncertainty. However, as the altitude and thus the range increases, the phase noise shifts into a regime where it is more sensitive to SNR. The performance of the single-channel system is then superior over a progressively wider span of wind speeds.

In this paper, we report initial results of an experimental investigation into measurement of ocean surface currents with an ultra high frequency synthetic aperture radar (UHF SAR) that supports an along-track interferometric (ATI) mode. We first describe the unique system, the NRL Multiband SAR, used to collect the data. We then present initial results related to two different approaches to current measurement. In the first approach, the surface current is estimated using standard ATI-SAR processing. We show progress in the implementation of this method with an interferometric phase cut across the north wall of the Gulf Stream. In the second, a time-sequence of images of propagating wave-like patterns is first generated using sub-aperture processing of the wide-beam, strip-map data, and this sequence is then transformed into wavenumber-frequency space through a 3D FFT. The surface current is then estimated from the displacement of the wave energy relative to the theoretical surface wave dispersion relation in the absence of any underlying current. We illustrate our progress in this approach with a wavenumber-frequency analysis of a 41-second sub-aperture image sequence of propagating waves.