In this study, we present an unsupervised methodology for detecting moving targets using Capella Space’s latest generation SAR sensor. The sensor has the capability to dwell on a target for an extended period of time in its spot-light (SP) mode, which we take advantage of to track moving objects in an acquisition. An approach that combines a signal-processing-based workflow with an image-domain-based one is presented. From one side, the long-dwell SAR image is exploit by doing an interfeometric processing of different azimuth sub-apertures from the long-dwell. From another side, a kernelized cross correlation technique is used. By combining intermediate results from these complementary workflows, a smooth and robust track is obtained on the targets. The algorithm is demonstrated on a long-dwell spotlight obtained over a busy shipping channel with watercraft both small and large successfully tracked.

The ability to automatically detect hard targets is highly beneficial for an imagery analyst. Their initial task in the information gathering process is to identify and interpret these targets in a scene. By automating the detection process, the workflow can be expedited. This paper presents an algorithm for detecting hard targets in long dwell spotlight (SP) Capella Space SAR images. The spectrum of the SP image is divided into non-overlaping sub-bands in order to compute the interferometric coherence between them. The proposed algorithm is tested on real spaceborne Capella SAR data showing the potential to clearly identify hard targets such as buildings, vehicles, and other artificial man-made objects.

In this paper we describe the development and motivation behind the development of a NASA-sponsored airborne instrument, SNOWWI (Snow Water-equivalent Wide Swath Interferometer) that is being developed for exploring the volume scattering and penetration depth characteristics of the snowpack at three different frequencies (5.4 GHz, C-band; 13.64 GHz known as Ku-low; 17.24 GHz known as Ku-high). The system, as it is being constructed is able to receive co- and cross-polarized (VV and VH) returns in an interferometric configuration. By implementing these components of the radar signature on the same platform, we will be able to explore the relationship between snow depth, density and snow water equivalent on the overall radar signature. This work is being done in conjunction with a strong modeling component being led by the University of Michigan, a ground campaign component supported by Boise State University and the US Army Corps of Engineers Cold Regions Research and Engineering Laboratory (CRREL), and a spaceborne concept development being led by Capella Space.

This article presents the first results of the newly conceived airborne Synthetic Aperture Radar system, SNOWWI.SNOWWI is a dual Ku- and C-Band interferometric and dualpolarized (VV and VH) system operating at 13.64 GHz, 17.24 GHz, and 5.39 GHz. The system aims to deliver snowpack observations to quantify Snow Depth (SD) and Snow Water Equivalent (SWE), which have been included as Targeted Observables in the National Academies’ 2017 Decadal Strategy for Earth Observation from Space. This manuscript includes results from the system’s first deployment in Grand Mesa, CO, in January and March 2024.

Taking advantage of Capella’s ability to dwell on a target for an extended period of time (nominally 30s) in its spotlight (SP) mode, an unsupervised methodology for detecting moving targets in this data is presented in this paper. By colourizing short segments (sub-apertures) of the total imaging time, a colourised sub-aperture image (CSI) can be formed. This can be used in conjunction with well-established computer vision techniques to detect moving targets and track them in the SP image. In essence, the moving target detection problem is transformed from temporal image stack identification to colour segmentation in a single image. The presented detection and tracking are wholly unsupervised. Additionally, computer-vision-based tracking algorithms are demonstrated on detected movers and qualitatively assessed for accuracy of tracking.

Capella Space is the first US commercial company to build, launch, and operate a constellation of synthetic aperture radar satellites capable of collecting very high resolution SAR imagery. All satellites in the constellation carry an X-band radar capable of acquiring imagery in spotlight, sliding spotlight, and stripmap modes. In 2023, Capella will launch the first of a new generation of satellites names Acadia. These satellites will provide high quality imagery and lay the platform for advanced SAR data products, such as interferometric SAR and bistatic imagery.

Capella's first commercial Synthetic Aperture Radar (SAR) satellite was launched in August 2020. After more than one year of successful operations, Capella plans a continuous growth of both number of satellites and satellite capabilities. New tasking patterns allow the collection of pairs and time series of very high resolution (VHR) SAR images. A novel repeat tasking request pattern will enable SAR applications such as change detection and the exploitation of interferometric SAR (InSAR) techniques. The combination of on-board GPUs and dedicated on-board processing algorithms will be used for rapid target detection and minimized satellite downlink. On-board processing, applicable in many quasi real-time operational scenarios, can also be used for tipping and cueing other satellites to immediately capture a high resolution imagery. Finally, Capella Space Open Data program started in 2021 with new images added each quarter.

Capella Space launched its first commercial Synthetic Aperture Radar (SAR) satellite in August 2020. After commissioning phase, Capella started commercial operations in January 2021, deploying a fully-functional SAR system that demonstrates three engineering milestones: an 8-m2deployable reflector mounted on small and agile satellite; the capability to process and timely deliver 0.5 m resolution spotlight images with 9 looks; and an automated tasking and delivery system that simplifies the ordering of high-quality SAR imagery for expert and novice users. In this paper, we present results from the commissioning and calibration/validation operations. We also compare images collected with the variety of imaging modes that Capella systems can collect.

We summarize the key elements of the operational readiness of Sequoia, the first commercially-operational satellite SAR system built and launched by Capella Space. We provide an overview the radar system, and the space operations and commissioning plan. Sample imagery collected with Capella radar hardware on an airborne platform is used to demonstrate the imaging capability of Sequoia.

Along Track Interferometric Synthetic Aperture Radar (ATI-SAR) is a remote sensing technique intended for measuring surface currents. As presented in [1] and [2] the current vector measurements are biased by the presence of surface winds and waves. It was shown in [3] that these biases are due to the contribution of the orbital velocity of the gravity waves (V o ) to the measured Doppler velocity by the ATI-SAR.

A comprehensive system error source analysis and calibration of an airborne along-track interferometric FMCW SAR for ocean surface currents velocity retrieval is presented. Starting with the observed phase errors from a stationary test site, three major error sources are analyzed and possible calibration approaches are derived. The range-dependent phase offsets are demonstrated to be introduced by the phase imbalance between receive channels and receive antennas of the transceiver. The phase undulations in the along-track direction are likely due to the uncompensated motion errors caused by inaccurate aircraft attitude and velocity measurements. After calibration, most of the system phase errors are removed which greatly improves the accuracy for surface velocity retrieval by ATI-SAR.

The Capella constellation of 36 X-band SAR Satellites will provide global hourly imaging opportunities for the entire earth and InSAR collection intervals of 4 hours. With the successful launch of the first satellite last year, Capella is prepared to launch the first operational this year and the first incrementally launch 36 satelites over two years. In addition to providing short revisit times, Capella will provide game-changing image request and delivery services where users can request imagery on-demand and receive it in a matter of minutes not days or weeks.

Along-Track Interferometric Synthetic Aperture Radar (ATI-SAR) is a remote sensing technique for measuring surface currents. The current vector measurements are biased by the presence of surface winds and waves, so their effect must be accommodated in order to yield accurate currents estimates using ATI-SAR.

The Capella constellation will consist of 36 agile synthetic aperture radars. Each radar will operate at an altitude of around 500 km in an approximately 90-minute polar orbit, providing average imaging revisit times of less than one hour. The radars will be single-pol X-band systems, capable of operating over a 500 MHz bandwidth in stripmap and spotlight imaging modes.

Airborne light detection and ranging (LiDAR) measurements and model simulations are used to investigate the temporal and spatial variability of the water surface elevation at the mouth of the Columbia River. A series of 15-km transects repeated in 2-3-h flights over a two week period resolve processes at a range of scales that are important to the dynamics of the river mouth region, including tides, surface slope, surface gravity waves, and wave setup. Water surface elevations agree well with a nearby tide gauge with an average difference of 0.01 m and an root mean square error of 0.39 m. Significant wave heights derived from the LiDAR measurements agree well with in situ wave drifter measurements and are observed to react to both bathymetry and spatial/temporal variations in currents, increasing up to 228 % on large ebb tides. Surface slopes varied from 2.7 × 10-5to -2.6 × 10-5over the course of a typical tide to as large as 6.3 × 10-5 to -4.8 × 10-5during a larger tide. The magnitude of the setup and set down due to wave height amplification during ebb tide was estimated to be 4×10-6. These observations demonstrate that many of the key dynamical variables at river mouths can be determined from airborne remote sensing measurements of water surface elevation and suggest that forthcoming altimetry products, such as those from the Surface Water Ocean Topography altimeter, may be able to provide new insight on monitoring in these complex regions.

We introduce a phase calibration scheme for an interferometric frequency-modulated continuous-wave (FMCW) synthetic aperture radar (SAR) to correct range-dependent phase errors in FMCW SAR interferograms. We demonstrate that the receiver filters operating on the FMCW beat frequency signal account for most of the phase mismatch between the different receiver channels. The scheme presented estimates the phase error in each channel. We present results of the scheme for three estimation approaches (curve fitting, joint least squares, and maximum likelihood) for two different phase models. The results are quantified by computing the reduction in spectral energy associated with the phase mismatch. We find that phase error can be reduced by 14 dB using the approach.

Remote sensing techniques for estimation of high-resolution (sub-mesoscale) ocean surface currents have been of interest to science community for many years. Different radar techniques have been tried, with the most popular being along track interferometry and direct Doppler analysis measurements. Jet Propulsion laboratory (JPL) has recently developed a new airborne Ka-band Doppler scatterometer (DopplerScatt), designed to simultaneously measure ocean surface vector currents and winds. The first validation of DopplerScatt's surface current measurements was performed by the Applied Physics Laboratory (APL) at University of Washington using the C-band airborne ATI SAR collocated measurements during a joint campaign in September 2016. In this paper, we compare the two radial velocity measurements and use a numerical scattering model to aid in understanding the differences.

In this paper, we investigate the effect of measurement error in platform velocity and attitude on the ATI phase measurement through theoretical analysis and numerical simulations. Our ultimate goal is to determine the maximum motion and attitude measurement error that can be tolerated for airborne ATI SAR measurements.

Differences between surface velocity estimates from dual-beam along-track interferometric (ATI) synthetic aperture radar (SAR) and drift rate measurements by surface current drifters are presented for a tidally-driven river mouth. Using hydrodynamic and scattering models, we analyze the contributions from various sources in order to explain the observed differences. We find that the differences are largely explained by bias in the mean Doppler shift due to gravity waves. This work has relevance to improving the accuracy of implementing dual-beam ATI SAR for surface current measurements.

This paper introduces the use of an airborne interferometric synthetic aperture radar (InSAR) to estimate water surface decorrelation times at Ka-Band. Such an assessment is directly relevant to the upcoming Surface Water and Ocean Topography mission, especially for surface water bodies such as lakes and rivers since the surface decorrelation may limit the spatial resolution achievable by the mission to delineate water spatial boundaries. Initial assessments indicate decorrelation times consistent with limited published observations for the ocean and fresh water bodies (several milliseconds). However, there are challenges both in terms of the phenomenology and in the instrument sensitivity to longer decorrelations.

Motion induced by waves on the ocean surface must be removed from measurements of surface velocity made by along-track interferometric synthetic aperture radar (ATI-SAR) in order to estimate surface currents. We focus on the estimation of wave parameters in intermediate to shallow water depths in a challenging bathymetric environment at the mouth of the Columbia River. We compare the ATI-SAR wave parameter measurements with in situ measurements made by a wave buoy offshore of the mouth. For all but two cases, we find that the estimated wave direction and wave period differs by less than 50° and 1.25 s respectively from the buoy measurement.

A radiometric calibration scheme that exploits time-domain backprojection is evaluated using a squinted frequency-modulated continuous wave radar and corner reflectors. We find that the calibration scheme compensates for most of the variation in image intensity due to changes in aircraft attitude, and that for good signal to clutter ratios, the return from the corner reflectors varies by for most cases by around 1 dB, although a 3 dB difference was observed in one case.

The relationship between synthetic aperture radar (SAR) signatures of depth-limited breaking waves and wave height is studied. Wave height is estimated from SAR images using an empirically derived relationship that exploits the azimuthal shift in SAR images associated with moving scatterers. This relationship is derived from in situ measurements rather than from an idealized model of breaking waves as was done in a previous study. We find that the lengths of the SAR signatures are correlated with the observed significant wave height (the correlation coefficient is 0.78) for a range of wave conditions. The relationship between the wave heights and velocity bandwidths from the field data is similar to that between simulated (with a Boussinesq surface wave model) wave heights and velocity ranges (correlation coefficient = 0.82).

A new robust phase calibration technique to estimate and correct the range-dependent phase differences between two receivers in an FMCW ATI SAR has been developed. Based on linear regression, this calibration scheme jointly estimates the phases for both SAR receivers and reproduces the range-dependent phase ripple seen in the ATI interfergrams. After applying the calibration results to the post-processing of SAR data, the phase ripple seen in the previous interferograms is significantly reduced (by about 10 dB). This work has relevance to implementing compact low-cost FMCW interferometric SAR systems for geophysical research measurements.

Surface flow is estimated from a dual-beam along-track interferometric synthetic aperture radar at the mouth of a tidally-driven estuary. The measurements show strong variation in the surface flow around jetties and seem to be associated with bathymetry. The surface flow estimates compare well with in situ drifter measurements made around the same time.

Techniques to estimate and correct range-dependent phase differences between receivers in an FMCW ATI SAR are studied. Both techniques reproduce the range-dependent phase ripple seem in the ATI interferograms.

We present a technique to extract geophysical parameters of nearshore ocean breaking waves (e.g., wave height, wave velocity) from ATI SAR data. The technique exploits the azimuthal shift induced by moving scatterers in SAR imagery. We find that length of the azimuthal streaks are correlated with breaking wave height (correlation coefficient is 0.78) for a range of wave conditions. We also outline a technique for highlighting breaking waves in SAR imagery using the ATI SAR images.

A contrast-based phase calibration algorithm for digital beamforming remote sensing radars using three contrast metrics is presented. The algorithm corrects time-varying antenna array phase errors that defocus digital beamforming remote sensing radar imagery. Amplitude errors are treated by equalizing the received powers in all elements. As such, the algorithm does not produce an absolute (or radiometric) calibration vector for the array. The performance of the algorithm is studied using a combination of simulated and real radar data under various conditions and is compared with a clutter-based calibration algorithm. An analytical proof showing that maximizing the expected value of the 4-norm metric is equivalent to phase-calibrating the image, except for a linear phase offset, is provided. We find that the clutter calibration algorithm performs best for statistically homogeneous scenes but that the contrast-calibration algorithms perform better with scenes with larger contrast ratios.

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.

Vertically polarized backscattered power, Doppler velocity, and interferometric height were measured in the nearshore ocean region using a high-resolution imaging microwave radar. The interferometric surface displacement measurements are compared with normalized radar cross sections (NRCS) and Doppler velocities, and with in situ pressure sensor estimates of wave height. The rms uncertainty in the interferometric surface displacement is computed from the data. The interferometric surface displacement measurements are intuitively satisfying in that positive displacements are generally associated with pixels that have larger NRCS values. Interferometric surface displacements compare well with heights derived from in situ pressure sensors, and we provide some possible explanations for the differences.

The Dual-Beam Interferometer is an airborne instrument that combines two along-track interferometric synthetic aperture radars observing the surface below at different squints. This configuration allows retrieving vector velocities of surface flows in a single aircraft pass. The system was designed by the University of Massachusetts and saw several deployments in the early 2000s. An imagery of a boat with a rather pronounced wake system captured during one of these flights is the subject of this paper. The velocity of the vessel is estimated based on the “train off the tracks” displacement in one of the looks. This estimate, which will be affected by uncertainty in target position due to smearing, is then used to remove unknown phase biases in the interferometric channels. Retrieved velocity variations are examined along cuts traversing the wake, with the focus on its narrow “turbulent” part. We find that the reconstructed velocities 150 m behind the boat are intuitively satisfying, but we are unable to fully account for the cross-wake component of velocity at 400 m behind the vessel.

A high-altitude airborne millimeter-wave radar for atmospheric research is being developed by the National Cen-ter for Atmospheric Research. The radar will be mounted on the High-Performance Instrumented Airborne Platform for Environmental Research (HIAPER). We present simulations of the minimum detectable signal, the reflectivity accuracy, the polarimetric purity, and the expected accuracy of cloud liquid water retrievals to describe the expected performance of the radar. We also describe the phased approach to implementing the system which starts with a W-band Doppler radar, and through phases, adds pulse compression, polarimetric capability, and a second wavelength (Ka-band) radar.

The dual-beam interferometer consists of two interferometric synthetic aperture radars (InSARs), one squinted at 20/spl deg/ forward of broadside, and the other 20/spl deg/ aft, to allow measurement of vector surface velocity with only a single aircraft pass. Estimates of surface velocity vectors in the coastal region during high tidal flow are presented. The data were gathered over the barrier islands west of Fort Myers, Florida, as part of a March 2004 deployment. Whereas no detailed bathymetry data were available, high-quality aerial photography appears to be a useful tool in inferring bottom topography and possible current obstructions. The retrieved velocity field clearly follows the expected outflow pattern. While comparisons with tidal current magnitudes predicted by the U.S. National Ocean Service do reveal discrepancies of up to 0.5 m/s, these differences are most likely due to the contribution of ocean surface waves to the overall InSAR velocity measurement. Velocity retrievals for the same area based on the data from different tracks show good consistency. The results constitute the first demonstration of vector retrieval of the surface velocity field with a single-pass InSAR system and confirm the robustness of the dual-beam interferometry principle.

We present observations of microwave radar backscattered power in the nearshore region, and results of a surface current estimation algorithm using microwave radar data. In low wind and wave conditions, patches of increased backscatter are observed in radar images. Animations of the images show these patches advected by nearshore flows such as wave induced cell circulations and longshore currents

The Dual Beam Interferometer (DBI) developed by University of Massachusetts (UMass) and an infrared camera operated by the Naval Research Laboratory (NRL) were jointly deployed during a March 2004 flight campaign off the east coast of Florida. Data collected produced simultaneous observations of features over the Gulf Stream's western boundary by both instruments. Use of instruments with different imaging capabilities enabled extraction of wind direction as well as current velocity measurement.

The Dual Beam Interferometer consists of two interferometric SARs, one squinted at 20 degrees forward of broadside, and the other 20 degrees aft, to allow estimation of vector surface velocity with only a single aircraft pass. It was developed by the University of Massachusetts and is operated in collaboration with the Naval Research Laboratory. The paper presents initial estimates of surface velocity vectors in the coastal region during high tidal flow. The data were gathered over the barrier islands west of Ft. Myers, Florida, as part of March 2004 deployment. While no attempt has been made at this stage to correct for Doppler contributions from surface waves, the retrieved velocity field clearly follows the expected outflow pattern, and velocity magnitudes agree well with in-situ data.

A dual-beam along-track interferometric synthetic aperture radar that is entirely self-contained within an aircraft pod has been developed by the University of Massachusetts to study sea surface processes in coastal regions. The radar operates at 5.3 GHz with a bandwidth of up to 25 MHz. System hardware is described. Initial test flights aboard the National Oceanic and Atmospheric Administration's WP-3D research aircraft were performed to evaluate system performance over land and water surfaces. Imagery were collected for fore and aft squinted beams, though no interferometric data were collected. Notable look-angle dependences are observed in the sea surface normalized radar cross section under very low wind conditions.

A dual-beam along-track interferometric synthetic aperture radar which is self contained within an aircraft pod has been developed to study coastal regions. System hardware is described. Initial test flights aboard the NOAA WP-3D research aircraft were performed to evaluate system performance over land and water surfaces. Notable look-angle dependencies are observed in the sea surface NRCS under very low wind conditions.

Radiometric measurements of the microwave emissivity of foam were conducted during May 2000 at the Naval Research Laboratory's Chesapeake Bay Detachment using radiometers operating at 10.8 and 36.5 GHz. Horizontal and vertical polarization measurements were performed at 36.5 GHz; horizontal, vertical, +45/spl deg/, -45/spl deg/, left-circular, and right-circular polarization measurements were obtained at 10.8 GHz. These measurements were carried out over a range of incidence angles from 30/spl deg/ to 60/spl deg/. Surface foam was generated by blowing compressed air through a matrix of gas-permeable tubing supported by an aluminum frame and floats. Video micrographs of the foam were used to measure bubble size distribution and foam layer thickness. A video camera was boresighted with the radiometers to determine the beam-fill fraction of the foam generator. Results show emissivities that were greater than 0.9 and approximately constant in value over the range of incidence angles for vertically polarized radiation at both 10.8 and 36.5 GHz, while emissivities of horizontally polarized radiation showed a gradual decrease in value as incidence angle increased. Emissivities at +45/spl deg/, -45/spl deg/, left-circular, and right-circular polarizations were all very nearly equal to each other and were in turn approximately equal to the average values of the horizontal and vertical emissivities in each case.