Short Aperture Research Articles

Far-field sub-diffraction focusing and controlled focus shaping of circularly polarized light using a dielectric phase plate

In recent years, sub-diffraction focusing has received substantial attention due to its versatility. However, achieving a flexible sub-diffraction focusing in the far field remains stimulating. Existing techniques either require complex fabrication facilities or are limited to the short focal length and high numerical aperture (NA) of the imaging system. Here, we introduce an optimization method for sub-diffraction focusing of a circularly polarized beam in the far field with a lens of large focal length. A cost-effective dielectric phase plate serves the purpose. By employing a phase plate composed of a thin layer of dielectric Si3N4, the phase of the propagating beam is modulated in the beam’s cross-section, which is divided into two regions of the opposite phase by the plate. A sub-diffraction focusing is achieved for a proper tunning between the two regions. In addition to sub-diffraction focusing, the phase plate is also capable of shaping the focus into a doughnut-shaped and a flat-top profile in the far field. This design provides a simple solution for sub-diffraction focusing and focus shaping that will find potential applications in optical imaging, optical trapping, and material processing.

Ultra-short and highly efficient metamaterial Fresnel lens-assisted taper

This paper demonstrates the benefits of leveraging free-space optics concepts in the design of certain integrated photonic components, leading to a footprint reduction without compromising on performance. Specifically, we present ultra-short, highly efficient and fabrication-friendly mode-size converters based on metamaterial Fresnel lens-assisted tapers. This is achieved using a parameterized inverse-design approach, where the metamaterial phase shifters are realized using fabrication-friendly Manhattan geometries, by optimizing the width, length, and position of the phase shifters. This approach overcomes the limitations of the conventional method that uses local periodic approximation, which is not suitable for lenses with a short focal length and high numerical aperture. We also extend the free-space concept of compound lenses and demonstrate a doublet-based taper to further reduce the footprint. The devices are fabricated and experimentally characterized in terms of insertion loss and signal integrity at high data transmission rates, exhibiting high performance. For the singlet, it effectively achieves mode-size conversion from 15 μm to 0.5 μm within a 15 μm distance, leading to ×10 length reduction compared to a linear taper. The insertion loss is under 1 dB over the entire C-band. The doublet achieves the same mode-size reduction within a 10 μm distance, leading to ×15 length reduction compared to a linear taper. The insertion loss is near 1 dB over most of the C-band. In both cases, the signal integrity is maintained for up to 50 Gbit/s.

Dissecting wheat above-ground architecture for enhanced water use efficiency and grain yield in the subtropics

BackgroundGrowing wheat under climate change scenarios challenges, scientists to develop drought and heat-tolerant genotypes. The adaptive traits should therefore be explored and engineered for this purpose. Thus, this study aimed to dissect surface traits and optimizing the leaf architecture to enhance water use efficiency (WUE) and grain yield. Twenty-six wheat genotypes were assessed for five novel leaf traits (NLTs: leaf prickle hairs, groove type, rolling, angle and wettability) under normal, drought and heat conditions following triplicated factorial randomized complete block design (RCBD). The data for NLTs, physiological traits (stomatal conductance, WUE, transpiration, and photosynthesis), and standard morphological and yield traits were recorded. Leaves were sampled at the stem elongation stage (Zadoks 34) to measure the leaf water content (%), contact angle, and to obtain pictures through scanning electron microscopy (SEM). The air moisture harvesting efficiency was evaluated for five selected genotypes. The ideotype concept was applied to evaluate the best-performing genotypes.ResultsThe correlation analysis indicated that long leaf prickle hairs (> 100 μm), short stomatal aperture and density (40–60 mm− 2), inward to spiral leaf rolling, medium leaf indentation, low contact angle hysteresis (< 10°), and cuticular wax were positively associated with WUE. This, in turn, was significantly correlated to grain yield. Thus, the genotypes (E-1) with these traits and alternate leaf wettability had maximum grain yield (502 g m− 2) and WUE supported with high photosynthesis rate, and relative water content (94 and 75% under normal and stress conditions, respectively). However, the genotype (1-hooded) with dense leaf hairs on edges but droopy leaves, spiral leaf rolling, and lighter groove, also performed better in terms of grain yield (450 g m− 2) under heat stress conditions by maintaining high photosynthesis and WUE with low stomatal conductance and transpiration rate.ConclusionThe SEM analysis verified that the density of hairs on the leaf surface and epicuticular wax contributes towards alternate wettability patterns thereby increasing the water-use efficiency and yield of the wheat plant. This study paves a way towards screening and and developing heat and drought-tolerant cultivars that are water-saving and climate-resilient.

Differentiation of Bulinussenegalensis and Bulinusforskalii Snails in West Africa Using Morphometric Analysis

PurposeAccurate identification of medically important intermediate host and vector species is crucial for understanding disease transmission and control. Identifying Bulinus snails which act as intermediate host species for the transmission of schistosomiasis is typically undertaken using conchological and genital morphology as well as molecular methods.MethodsHere, a landmark-based morphometric analysis of shell morphology was undertaken to determine its utility to distinguish the closely related and morphologically similar sister species Bulinus senegalensis and Bulinus forskalii. The method was developed to increase the accuracy of conchological morphology methods to identify Bulinus species in the field. Both species are found in West Africa, but only B. senegalensis is implicated in the transmission of urogenital schistosomiasis.ResultsWe found when scaled down to the same length, 3-whorl and 4-whorl (juvenile) B. senegalensis shells had a longer spire, narrower body whorl and shorter aperture than B. forskalii. In contrast, 5-whorl (adult) B. senegalensis had a shorter spire, but still had a shorter aperture and narrower body whorl than B. forskalii. Canonical Variate Analysis (CVA) showed minimal overlap between B. senegalensis and B. forskalii for 3-whorl and 4-whorl shells, with a clear separation for 5-whorl shells. Overall, B. senegalensis had a consistently shorter aperture size and narrower body whorl than B. forskalii for all development stages. Spire length was variable depending on the stage of development, with 3-whorl and 4-whorl shells having the opposite trends of adult shells.ConclusionsOur study demonstrates the applicability of landmark-based morphometrics in distinguishing the medically important, Bulinus senegalensis from its morphologically similar sister species, Bulinus forskalii. We recommend using measurements based on spire length, penultimate whorl length, body whorl width and aperture size to differentiate B. senegalensis and B. forskalii, when used with the appropriate information for each shell’s development stage.

Investigation of groundwater induced land subsidence in Ludhiana City using InSAR and Sentinel-1 data

Land Subsidence (LS) is a vertically downward motion of land surface due to various reasons such as natural processes and anthropogenic activities especially excessive exploitation of groundwater. LS has adverse effects on substantial infrastructural damage, severe environmental problems, extensive economic losses, and significant societal impacts. In this study, we investigated the LS of Ludhiana city, a densely populated, largest and major industrial city of Punjab state, India. We presented a comprehensive methodology of Short BAseline Subset Interferometric Synthetic Aperture Radar (SBASInSAR) for LS measurement using an open-source computational environment. We generated 197 interferograms from 65 Sentinel-1A images acquired in descending pass between September 2019–July 2022 for deriving radar line of sight (LOS) displacement time-series and mean LOS velocity. Deformation results showed that the southern, and south-eastern parts of the city had been consistently moving downward with a mean subsidence rate of 24.7 mm/yr, while in western, few small patches in eastern and northern regions it ranges from 2 to 21 mm/yr, 3–20 mm/yr and 4–16 mm/yr respectively. The standard deviation of mean LOS velocity was observed between 0.7 and 3.3 mm/yr with majority values < 2 mm/yr. SBASInSAR results were compared with six Groundwater Level (GWL) wells observations, and both measurements generally agreed well at GWL locations (W1–W6) except W1–W2. For assessing the influence of groundwater change on LS, a correlation analysis was performed, and correlation of 0.50, 0.47, 0.81, 0.77, 0.84, and 0.64 was observed at GWL stations (W1–W6) respectively. Overall comparison of InSAR and GWL measurements are found in good agreement and significantly correlated, which can provide sufficiently detailed information about LS.

Detection and Monitoring of Potential Geological Disaster Using SBAS-InSAR Technology

As a global geological environment problem, geological disasters are more likely to induce severe geological disasters and cause loss of personnel and property due to their suddenness and concealment, frequent heavy rainfall and regular extreme weather. In this paper, Sentinel-1A data and DEM data combined with short baseline synthetic aperture interferar (SBAS-InSAR) technology are used to analyze and calculate the spatial-temporal evolution characteristics of surface deformation in Zibo-Weifang area of Shandong Province during 2016–2019. Linear fitting of InSAR monitoring deformation results is performed using level monitoring data to verify the accuracy of the results. The findings indicate that surface deformation in most of the study area is relatively stable, but in some areas, especially near Lucun Town, the maximum negative deformation rate exceeds 80mm/a. Based on the surface deformation results, a total of 377 potential geological disasters are identified using an integrated multi-source stereo observation system of the sky and ground. This identification is done by observing optical remote sensing images and taking the absolute value of surface deformation rate greater than 10 mm/a as the judgment basis, along with ground investigation and review. Several typical geological disaster sites are screened and they are found to be in unstable state. The research results can provide scientific support for geological disaster prevention and control in Shandong Province.

Geometric Configuration Design and Fast Imaging for Multistatic Forward-Looking SAR Based on Wavenumber Spectrum Formation Approach

Multistatic forward-looking synthetic aperture radar (Mu-FLSAR) has the potential of high-resolution imaging with short synthetic aperture time, which can improve the transmitter’s survivability, by coherently fusing simultaneously observed measurements of multiple receivers. However, the combined performance of the multiple measurements strictly depends on an appropriate geometric configuration among the transmitter and receivers because the forward-looking application limits the flight directions of receivers. In this paper, to design a geometric configuration for Mu-FLSAR, a wavenumber spectrum formation (WSF) approach is proposed based on the projection relationship between the wavenumber support regions (WSRs) and geometric configuration parameters. On the one hand, the projected pattern of multiple WSRs is deduced, and the relationship between multiple WSRs and the point spread function (PSF) is analyzed. Based on the geometric feature of the kernel WSR, which is formed by the transmitter and the master receiver, and the relationship between the geometric features and the geometric configuration parameters, including synthetic aperture time and azimuthal angle, a WSF method is proposed to visually and quickly deduce the geometric parameter of the salve receivers. On the other hand, based on the designed geometric configuration of Mu-FLSAR, a wavenumber-dependent fast polar format algorithm (WF-PFA) is proposed to efficiently reconstruct the targets relying on the geometric features of WSRs. Simulation results verify the proposed method.

An Aperture Adaptive Scale Transform of Terahertz Radar Imaging Algorithm

Terahertz (THz) radar system can realize high-resolution imaging benefits from its advantages, such as wide bandwidth and short synthetic aperture time. However, due to the short wavelength in the THz frequency range, multi-scattering center effect of targets generally occurs. This paper considers the mismatch between imaging aperture and target characteristics leads to serious resolution degradation, imaging defocus and loss of target details. We propose an aperture adaptive scale transform (AAST) imaging method, which is implemented in three steps. First, the sub-aperture is adaptively divided by matching the azimuth with the scattering characteristics. Then the apertures which contain different details of the target are scaled by weight. Finally, the generalized likelihood ratio test method is used to reconstruct the fine image results of incoherent scale transformation, resulting in more detailed information and higher definition in imaging results. We use a terahertz radar system operating at 0.34 THz and with a bandwidth of 28.8 GHz to scan around a target. The imaging results of measured echo corroborate the validity of the proposed algorithm.

Morphology and morphometric diversity of pollen grains of 26 Indian species of &lt;i&gt;Dioscorea&lt;/i&gt; L. (Dioscoreaceae)

Pollen morphology of 26 species of Dioscorea collected from different regions of India was examined critically under LM and SEM to explore their unique features. Pollen grains were monad, isopolar, small to medium-sized (11.3–31 µm). Peroblate, oblate, suboblate and oblate spheroidal (P/E=0.41–0.90) pollen shape, mono and bi-sulcate aperture type and psilate, reticulate, rugulose, strito-reticulate, reticulate-rugulose, punctate and striate exine ornamentation were believed to be some of the important characters for differentiation of closely related species in this studied genus. The comprehensive qualitative and quantitative characters of the pollen grains served as reliable features in taxonomical studies. The observed data were analyzed using PCA and hierarchical clustering to obtain a steady relationship and grouping of the observed species. Histograms, pie chart and bar graph were performed to enumerate the distribution of the quantitative data of various attributes (long axis, short equatorial axis, pollen shape and aperture length). Correlation scatter plot along with density graph helped to understand the interrelationship amongst the studied taxa.

Analysis and Prediction of Regional Land Subsidence with InSAR Technology and Machine Learning Algorithm

As a worldwide environmental and geological disaster, land subsidence may cause serious harm to urban development. Therefore, the prediction of land subsidence is a key scientific problem. Decheng county, Shandong Province in China is taken as the research object. Based on BP neural network (BPNN) and random forest (RF) method, the analysis and prediction of regional land subsidence are carried out by applying multi-source monitoring data, Geographic Information System (GIS), and machine learning algorithm. Combined with Short Baseline Synthetic Aperture Interferometric Radar (SBAS-InSAR) and GIS technology, the spatiotemporal evolution characteristics of land subsidence from 2017 to 2020 are analyzed. The impact of different groundwater levels on land subsidence is quantitatively analyzed by BPNN and RF algorithm. The real-time prediction model of regional land subsidence is established. The results show that: 1) The area with the most serious land subsidence is located in Songguantun town, the maximum annual average subsidence rate is −40.71 mm/yr. 2) Land subsidence is mainly affected by deep groundwater and shallow groundwater in the research area. 3) The accuracy of the prediction results of the BPNN model is higher than that of the RF model when groundwater level change is used to predict land subsidence.

A SAR Imaging Method for Walking Human Based on mωka-FrFT-mmGLRT

Synthetic aperture radar (SAR) human-imaging technology has been widely used in the fields of security screening and activity recognition. However, most of the existing methods aim at stationary human targets, resulting in severe restrictions on their potential applications. In this article, an SAR imaging method for walking human is proposed with short aperture terahertz (THz) radar. This method is based on a modified wavenumber domain approximate algorithm ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}{\omega }$ </tex-math></inline-formula> ka), whose interpolation mapping is modified according to the approximation of region of interest (RoI). Because the nonrigid motion phase error caused by the walking human would lead to severe deformation and blur in imaging results, a compensation processing is essential. To figure it out, the fractional Fourier transform combined with the maximum and minimum generalized likelihood ratio test (FrFT-mmGLRT) is devised in this article. Within a short aperture time, the nonrigid motion of walking human can be approximately assumed as a superposition of the rigid movements of different human body parts. Particularly, by taking advantage of the superposition property of FrFT, the motion phase errors of different human body parts are distinguished and estimated by jointly searching for the peaks of FrFT energy spectrum. Furthermore, the mmGLRT-based composite imaging technique is utilized to obtain a whole body result from the compensated results of different body parts. Finally, numerical simulation and real experiments are conducted to verify the feasibility and capability of the proposed method for walking human SAR imaging.

THz Radar Security Screening Method for Walking Human Torso With Multi-Angle Synthetic Aperture

Terahertz (THz) radar imaging has widely adopted for security screening of standoff human body in recent years, owing to its capacities of high resolution, harmless, contactless, and penetrate clothing. However, THz radar security screening for walking human is still a challenge due to the non-rigid motion of walking. To figure it out, a THz radar security screening method for walking human torso is proposed with multi-angle synthetic aperture. In this method, the echo data of walking human torso component is assumed as a rigid motion model within a short time. Particularly, the torso component has the maximum ratio of surface area and echo energy, which means that its motion phase error can be detected after fractional Fourier transform processing. Besides, after the correction and imaging processing, the imaging results under different angles are fused by coherent integration with registration processing to improve the integrity of scattering information, which is incomplete within a short aperture due to self-occlusion and the changes in incident angles. The simulation experiments are conducted to verify the imaging quality and feasibility of the proposed method. Moreover, the real small aperture echo data of moving torso collected by our THz radar (that operates at 330GHz with the bandwidth of 30GHz and the output peak power of 5mW) is processed via the proposed method to indicate the significance of multi-angle imaging.

Separability of Mowing and Ploughing Events on Short Temporal Baseline Sentinel-1 Coherence Time Series

Short temporal baseline regular Synthetic Aperture Radar (SAR) interferometry is a tool well suited for wide area monitoring of agricultural activities, urgently needed in European Union Common Agricultural Policy (CAP) enforcement. In this study, we demonstrate and describe in detail, how mowing and ploughing events can be identified from Sentinel-1 6-day interferometric coherence time series. The study is based on a large dataset of 386 dual polarimetric Sentinel-1 VV/VH SAR and 351 Sentinel-2 optical images, and nearly 2000 documented mowing and ploughing events on more than 1000 parcels (average 10.6 ha, smallest 0.6 ha, largest 108.5 ha). Statistical analysis revealed that mowing and ploughing cause coherence to increase when compared to values before an event. In the case of mowing, the coherence increased from 0.18 to 0.35, while Sentinel-2 NDVI (indicating the amount of green chlorophyll containing biomass) at the same time decreased from 0.75 to 0.5. For mowing, there was virtually no difference between the polarisations. After ploughing, VV-coherence grew up to 0.65 and VH-coherence to 0.45, while NDVI was around 0.2 at the same time. Before ploughing, both coherence and NDVI values were very variable, determined by the agricultural management practices of the parcel. Results presented here can be used for planning further studies and developing mowing and ploughing detection algorithms based on Sentinel-1 data. Besides CAP enforcement, the results are also useful for food security and land use change detection applications.

Internal structure of the San Jacinto fault zone at the Ramona Reservation, north of Anza, California, from dense array seismic data

SUMMARYWe image the internal structure of the San Jacinto fault zone (SJFZ) near Anza, California, with seismic data recorded by two dense arrays (RA and RR) from ∼42 000 local and ∼180 teleseismic events occurring between 2012 and 2017. The RA linear array has short aperture (∼470 m long with 12 strong motion sensors) and recorded for the entire analysed time window, whereas the RR is a large three-component nodal array (97 geophones across a ∼2.4 km × 1.4 km area) that operated for about a month in September–October 2016. The SJFZ at the site contains three near-parallel surface traces F1, F2 and F3 from SW to NE that have accommodated several Mw &amp;gt; 6 earthquakes in the past 15 000 yr. Waveform changes in the fault normal direction indicate structural discontinuities that are consistent with the three fault surface traces. Relative slowness from local events and delay time analysis of teleseismic arrivals in the fault normal direction suggest a slower SW side than the NE with a core damage zone between F1 and F2. This core damage zone causes ∼0.05 s delay at stations RR26–31 in the teleseismic P arrivals compared with the SW-most station, and generates both P- and S-type fault zone trapped waves. Inversion of S trapped waves indicates the core damaged structure is ∼100 m wide, ∼4 km deep with a Q value of ∼20 and 40 per cent S-wave velocity reduction compared with bounding rocks. Fault zone head waves observed at stations SW of F3 indicate a local bimaterial interface that separates the locally faster NE block from the broad damage zone in the SW at shallow depth and merges with a deep interface that separates the regionally faster NE block from rocks to the SW with slower velocities at greater depth. The multiscale structural components observed at the site are related to the geological and earthquake rupture history at the site, and provide important information on the preferred NW propagation of earthquake ruptures on the San Jacinto fault.

Regional-scale monitoring of underwater and dry ground subsidence in high phreatic areas of North China Plain.

Land subsidence monitoring provides information required when developing land use plans and allows for proactive management of subsidence issues. However, it has been challenging to accurately detect land subsidence areas, especially those under waterbodies. This study evaluated the applicability of integrated use of the optical Landsat-8 OLI and microwave Sentinel-1A TOPSAR imagery to delineate subsidence areas and quantify subsidence rates in a typical coal mining area of North China Plain. An Enhanced Modified Normalized Difference Water Index (E-MNDWI) was combined with Short BAseline Subset-Interferometric Synthetic Aperture Radar (SBAS-InSAR) image to monitor underwater and dry ground subsidence. The results demonstrated that the method could delineate underwater and dry ground subsidence and quantify its rates accurately. The proposed method estimated subsidence area corresponded to 34.8% (16.7 km2) of the study area. The size of underwater subsidence areas was substantial and accounted for 43.7% of the subsidence areas. Seasonal underwater subsidence areas were generally distributed in the vicinity of perennial ones. Dry ground subsidence covered 9.4 km2 of the study area and generally occurred in urban and rural residential areas with the maximum subsidence of up to 80.1 mm/year. This study demonstrates the efficiency and capacity of integrating optical and microwave images to monitor the subsidence progresses, which thus can help develop effective rehabilitation policy and strategy to mitigate the impacts of land subsidence.

Two-Dimension Joint Super-Resolution ISAR Imaging With Joint Motion Compensation and Azimuth Scaling

The quality of inverse synthetic aperture radar (ISAR) images suffers seriously from the two-dimension (2-D) resolution and noise. The motion errors arising from translational and rotational motion further aggravate the image defocusing. For the limited bandwidth and short aperture (LB-SA) signal, this letter proposes a novel 2-D joint super-resolution (2D-JSR) ISAR imaging with joint motion compensation and azimuth scaling (JMCAS) algorithm. In this technique, a 2D-JSR signal model is established, enabling the 2-D high-resolution ISAR image to be generated by solving a sparsity-driven optimization problem with a modified quasi-Newton solver. In addition, a new JMCAS algorithm is developed to enhance the focusing performance of image. Not only can this algorithm jointly correct the range shift and phase error caused by translational motion, it can also complete the azimuth scaling and range spatial-variant phase error (RSVPE) compensation simultaneously. Through the iterative processing of 2D-JSR reconstruction and JMCAS, the well-focused and scaled high-resolution ISAR image can be obtained. Both simulated and real data experiments are provided to verify the effectiveness of the proposed algorithm.

Analysis of the ray-based blind deconvolution algorithm for shipping sources.

The ray-based blind deconvolution (RBD) technique for ocean waveguides estimates both the unknown waveform radiated by some source of opportunity and the channel impulse response (CIR) between the source and the receiving elements of an array of hydrophones using only measured signals, knowledge of the array geometry, and the local sound speed. Previous studies have investigated the applicability of this method for shipping sources in a shallow, nearly range-independent waveguide (∼200 m depth), but using a limited set of shipping vessels (typically only the research vessel itself) and operating within a small domain of RBD processing parameters (e.g., integration time and frequency band). This study systematically investigates the performance of the RBD method for estimating the CIR for a large set of shipping vessels recorded on short aperture, bottom-mounted, vertical arrays deployed in the Santa Barbara channel across different frequency bands and integration times, and also in comparison to CIR measured using active sources. Furthermore, the influence of the source motion on the RBD algorithm is quantified both numerically and experimentally.

A Millimeter-Wave GCW-SAR Based on Deramp-on-Receive and Piecewise Constant Doppler Imaging

A novel generalized continuous-wave synthetic aperture radar (GCW-SAR) based on deramp-on-receive operating in millimeter-wave frequency is proposed in this article. With deramp-on-receive, the receiver sampling rate is drastically reduced, and the downsampled 1-D raw data can be obtained from the received beat signal. Further adopting piecewise constant Doppler (PCD) imaging in the digital domain, a GCW-SAR image can be easily reconstructed by using the existing frequency-modulated continuous-wave (FMCW) radar system. The effects of deramp-on-receive in PCD imaging are analyzed accordingly. The short wavelength of the millimeter-wave carrier used in the proposed GCW-SAR enables high azimuth resolution as well as a short synthetic aperture, which, in turn, significantly reduces the imaging computational complexity. Simulation and experimental results confirm the advantages of the proposed GCW-SAR.

Novel ISAR autofocusing method based on Bayesian inference

To achieve well-focused ISAR imaging with low signal-to-noise ratio (SNR), this study constructs a statistical model and derives an effective method for image autofocusing based on Bayesian inference. Particularly, hierarchical sparse-promoting priors are imposed on the weights, which are conjugate to the likelihood. The method has closed-form solution which facilitates computation. Experimental results have demonstrated the effectiveness of the method in low SNR and short aperture scenarios.

Complementarity of X-, C-, and L-band SAR Backscatter Observations to Retrieve Forest Stem Volume in Boreal Forest

The simultaneous availability of observations from space by remote sensing platforms operating at multiple frequencies in the microwave domain suggests investigating their complementarity in thematic mapping and retrieval of biophysical parameters. In particular, there is an interest to understand whether the wealth of short wavelength Synthetic Aperture Radar (SAR) backscatter observations at X-, C-, and L-band from currently operating spaceborne missions can improve the retrieval of forest stem volume, i.e., above-ground biomass, in the boreal zone with respect to a single frequency band. To this scope, repeated observations from TerraSAR-X, Sentinel-1 and ALOS-2 PALSAR-2 from the test sites of Remningstorp and Krycklan, Sweden, have been analyzed and used to estimate stem volume with a retrieval framework based on the Water Cloud Model. Individual estimates of stem volume were then combined linearly to form single-frequency and multi-frequency estimates. The retrieval was assessed at large 0.5 ha forest inventory plots (Remningstorp) and small 0.03 ha forest inventory plots (Krycklan). The relationship between SAR backscatter and stem volume differed depending on forest structure and environmental conditions, in particular at X- and C-band. The highest retrieval accuracy was obtained at both test sites at L-band. The combination of stem volume estimates from data acquired at two or three frequencies achieved an accuracy that was superior to values obtained at a single frequency. When combining estimates from X-, C-, and L-band data, the relative RMSE for the 0.5 ha inventory plots at Remningstorp was 31.3%. For the 0.03 ha inventory plots at Krycklan, the relative RMSE was above 50%. In a retrieval scenario involving short wavelength SAR backscatter data, these results suggest combining multiple frequencies to ensure the highest possible retrieval accuracy achievable. Retrievals should be undertaken to target spatial scales well above the size of a pixel.