Measuring Winds and Currents with Ka-Band Doppler Scatterometry: An Airborne Implementation and Progress towards a Spaceborne Mission

Wineteer, Alexander; Perkovic‐Martin, Dragana; Rodríguez, Ernesto; Rodríguez, Ernesto; Gál, Tamás; Niamsuwan, N.; Nicaise, Fabien; Srinivasan, Karthik K.; Baldi, Chad; Majurec, Ninoslav; Stiles, B.

doi:10.3390/rs12061021

Cited by 13 publications

(15 citation statements)

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“…Previous studies using GPM and AltiKa data focused on much larger water bodies or ocean surfaces, with emphasis on estimating wave heights and shapes for much higher wind speeds (up to 20 m/s) [8,23,25]. Nonetheless, our results agree with a highresolution (600 m × 36 m) scatterometry study over the Gulf of Mexico, which determined that wind speeds of >2-3 m/s are necessary to retrieve meaningful radar returns [12]. Similarly, a bridge-mounted Ka-band system identified very strong backscatter (20 dB) from a river surface at wind speeds >2-3 m/s [29].…”

Section: Discussionsupporting

confidence: 79%

“…Previous scatterometry studies focused on large water bodies and open oceans having background surface wave roughness higher than the roughness seen from smaller water bodies, asserting minimum speeds of 2 m/s [8,12,25]. However, this study points to slightly higher wind speeds (i.e., at least 3 m/s) for successful water surface elevation retrieval over small inland water bodies.…”

Section: Discussioncontrasting

confidence: 54%

“…Such retrievals are traditionally obtained through scatterometry over oceans [7,8]. Classic scatterometry studies were conducted to establish geophysical model functions (GMFs) relating wind parameters to water surface scattering over ocean surfaces via the mechanism of wind-driven water surface roughening [7][8][9][10][11][12]. Due to oblique viewing geometries and poor calibration for SAR-wind GMFs, few have used SAR imagery to assess surface water scattering [13,14].…”

Section: Introductionmentioning

confidence: 99%

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How Does Wind Influence Near-Nadir and Low-Incidence Ka-Band Radar Backscatter and Coherence from Small Inland Water Bodies?

Fayne

Smith

2023

Remote Sensing

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While many studies have been conducted regarding wind-driven Ka-band scattering on the ocean and sea surfaces, few have identified the impacts of Ka-band scattering on small inland water bodies, and fewer have identified the influence of wind on coherence over water. These previous studies have been limited in spatial scale, covering only large water bodies >25 km2. The recently launched Surface Water and Ocean Topography (SWOT) mission is the first Ka-band InSAR satellite designed for mapping water surface elevations and open water areas for rivers as narrow as 100 m and lakes as small as 0.0625 km2. Because measurements of these types are novel, there remains some uncertainty about expected backscatter amplitudes given wind-driven water surface roughness variability. A previous study using the airborne complement to SWOT, AirSWOT, found that low backscatter and low coherence values were indicative of higher errors in the water surface elevation products, recommending minimum thresholds for backscatter and coherence for filtering the data to increase the accuracy of averaged data for lakes and rivers. We determined that the global average wind speed over lakes is 4 m/s, and after comparing AirSWOT backscatter and coherence data with ERA-5 wind speeds, we found that the minimum required speed to retrieve high backscatter and coherence is 3 m/s. We examined 11,072 lakes across Canada and Alaska, with sizes ranging from 350 m2 to 156 km2, significantly smaller than what could be measured with previous Ka-band instruments in orbit. We found that small lakes (0.0625–0.25 km2) have significantly lower backscatter (3–5 dB) and 0.20–0.25 lower coherence than larger lakes (>1 km2). These results suggest that approximately 75% of SWOT observable lake areas around the globe will have consistently high-accuracy water surface elevations, though seasonal wind variability should remain an important consideration. Despite very small lakes presenting lower average backscatter and coherence, this study asserts that SWOT will be able to accurately resolve the water surface elevations and water surface extents for significantly smaller water bodies than have been previously recorded from satellite altimeters. This study additionally lays the foundation for future high-resolution inland water wind speed studies using SWOT data, when the data become available, as the relationships between wind speed and Ka-band backscatter reflect those of traditional scatterometers designed for oceanic studies.

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Section: Discussionsupporting

confidence: 79%

Section: Discussioncontrasting

confidence: 54%

Section: Introductionmentioning

confidence: 99%

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How Does Wind Influence Near-Nadir and Low-Incidence Ka-Band Radar Backscatter and Coherence from Small Inland Water Bodies?

Fayne

Smith

2023

Remote Sensing

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“…This technology takes advantage of traditional wind scatterometry for the estimation of the surface winds with the benefit of the use of the Doppler phase information for the estimation of the ocean surface currents [17]. The Ka-band frequency, 35.75 GHz, is chosen for Doppler scatterometry in order to improve the spatial resolution for both winds and currents [18] and improve the sensitivity to currents [17]. Given the importance of these measurements, NASA funded an airborne Ka-band Doppler scatterometer, called Doppler-Scatt, under the NASA Instrument Incubator Program (IIP) and Airborne Instrument Technology transition (AITT) program [19].…”

Section: Introductionmentioning

confidence: 99%

A Ka-Band Wind Geophysical Model Function Using Doppler Scatterometer Measurements from the Air-Sea Interaction Tower Experiment

et al. 2022

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Physical understanding and modeling of Ka-band ocean surface backscatter is challenging due to a lack of measurements. In the framework of the NASA Earth Ventures Suborbital-3 Submesoscale Ocean Dynamics Experiment (S-MODE) mission, a Ka-Band Ocean continuous wave Doppler Scatterometer (KaBODS) built by the University of Massachusetts, Amherst (UMass) was installed on the Woods Hole Oceanographic Institution (WHOI) Air-Sea Interaction Tower. Together with ASIT anemometers, a new data set of Ka-band ocean surface backscatter measurements along with surface wind/wave and weather parameters was collected. In this work, we present the KaBODS instrument and an empirical Ka-band wind Geophysical Model Function (GMF), the so-called ASIT GMF, based on the KaBODS data collected over a period of three months, from October 2019 to January 2020, for incidence angles ranging between 40° and 68°. The ASIT GMF results are compared with an existing Ka-band wind GMF developed from data collected during a tower experiment conducted over the Black Sea. The two GMFs show differences in terms of wind speed and wind direction sensitivity. However, they are consistent in the values of the standard deviation of the model residuals. This suggests an intrinsic geophysical variability characterizing the Ka-band surface backscatter. The observed variability does not significantly change when filtering out swell-dominated data, indicating that the long-wave induced backscatter modulation is not the primary source of the KaBODS backscatter variability. We observe evidence of wave breaking events, which increase the skewness of the backscatter distribution in linear space, consistent with previous studies. Interestingly, a better agreement is seen between the GMFs and the actual data at an incidence angle of 60° for both GMFs, and the statistical analysis of the model residuals shows a reduced backscatter variability at this incidence angle. This study shows that the ASIT data set is a valuable reference for studies of Ka-band backscatter. Further investigations are on-going to fully characterize the observed variability and its implication in the wind GMF development.

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“…Unlike the estimates made by altimeters, surface currents estimated by a Doppler scatterometer are sensitive to the full instantaneous surface current field, including the many ageostrophic and divergent processes listed in paragraph two. Doppler scatterometry has been proven from airborne platforms (Rodríguez et al, 2018;Wineteer et al, 2020) and is used prominently in the ongoing NASA Earth Ventures Suborbital-3 S-MODE (Submesoscale Ocean Dynamics Experiment) mission. To scale to a spaceborne mission, Rodríguez (2018) derived the relationships between radar parameters and surface current random error, while Chelton et al (2018) thoroughly analyzed the capabilities of a spaceborne Doppler scatterometer assuming a simple, constant random error, including space and time sampling considerations.…”

Section: Introductionmentioning

confidence: 99%

On the Surface Current Measurement Capabilities of Spaceborne Doppler Scatterometry

Wineteer

Torres

Rodríguez

2020

Geophysical Research Letters

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Wide-swath spaceborne Doppler scatterometry is a promising technique for the simultaneous measurement of global ocean surface winds and currents. The technique has been proven from airborne platforms, and here we use the lessons learned to examine a range of implications for a spaceborne system. We use a Doppler scatterometer simulator and a state-of-the-art global circulation model to generate surface current measurements and their random errors. We find that a feasible instrument could measure 5 km gridded surface currents with typical random errors between 10 and 25 cm/s. For higher wind speeds, the random error in surface current decreases logarithmically. This level of accuracy allows for the computation of surface current relative vorticity and horizontal divergence with typical wavelength resolutions of 15-30 and 25-60 km, respectively. Unlike previous studies, we find that these measurements do not require multiday averaging, opening up new avenues for monitoring global ocean circulation. Plain Language Summary Our understanding of the ocean, the atmosphere, and the Earth system as a whole has been profoundly bettered by the global perspective of space-based sensors. For decades, radar scatterometers have measured global ocean winds from space, contributing significantly to our scientific understanding of the atmosphere and improving forecast models. With a similar instrument, and the addition of "Doppler" capability, a Doppler scatterometer can measure ocean winds and ocean surface currents simultaneously, which opens new avenues to understanding the ocean, the atmosphere, and how they interact. This type of instrument has been successfully built and proven to work on airborne platforms but has yet to be implemented on a satellite. In this work, we have simulated the measurements of a spaceborne Doppler scatterometer to understand the oceanic scales that such an instrument might be able to observe. We find that ocean currents and their derivatives could be measured at scales better than 30 km. This enables the study of many currently unobserved ocean processes, including the vertical circulation of the ocean and the transport of plastics, kinetic energy, heat, and gasses.

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Measuring Winds and Currents with Ka-Band Doppler Scatterometry: An Airborne Implementation and Progress towards a Spaceborne Mission

Cited by 13 publications

References 20 publications

How Does Wind Influence Near-Nadir and Low-Incidence Ka-Band Radar Backscatter and Coherence from Small Inland Water Bodies?

How Does Wind Influence Near-Nadir and Low-Incidence Ka-Band Radar Backscatter and Coherence from Small Inland Water Bodies?

A Ka-Band Wind Geophysical Model Function Using Doppler Scatterometer Measurements from the Air-Sea Interaction Tower Experiment

On the Surface Current Measurement Capabilities of Spaceborne Doppler Scatterometry

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