Search

The forthcoming Surface Water and Ocean Topography (SWOT) satellite and AirSWOT airborne instrument are the first imaging radar-altimeters designed with near-nadir, 35.75 GHz Ka-band InSAR for mapping terrestrial water storage variability. Remotely sensed surface water extents are crucial for assessing such variability, but are confounded by emergent and inundated vegetation along shorelines. However, because SWOT-like measurements are novel, there remains some uncertainty in the ability to detect certain land and water classes. We study the likelihood of misclassification between 15 land cover types and develop the Ka-band Phenomenology Scattering (KaPS) scattering model to simulate changes to radar backscatter as a result of changing surface water fraction and roughness. Using a separability metric, we find that water is five times more distinct compared with dry land classes, but has the potential to be confused with littoral zone and wet soil cover types. The KaPS scattering model simulates AirSWOT backscatter for incidence angles 1-27°, identifying the conditions under which open water is likely to be confused with littoral zone and wet soil cover types. A comparison of KaPS simulated backscatter with AirSWOT observed backscatter shows good overall agreement across the 15 classes (median r2=0.76). KaPS characterization of the sensitivity of near-nadir, Ka-band SAR to small changes in both wet area fraction and surface roughness enables more nuanced classification of inundation area. These results provide additional confidence in the ability of SWOT to classify water inundation extent, and open the door for novel hydrological and ecological applications of future Ka-band SAR missions.

An approach is proposed for modeling coherent radar echoes from a distributed ground surface in planetary landing simulations. Such simulations pose unique challenges for radar echo modeling given ...

Water surface elevation (WSE), slope and width measurements from the forthcoming Surface Water and Ocean Topography (SWOT) mission will enable spaceborne estimates of global river discharge. WSE will be measured by interferometric synthetic aperture radar (InSAR). InSAR measurements are vulnerable to contamination from layover, a phenomenon wherein radar returns from multiple locations arrive at the sensor simultaneously, rendering them indistinguishable. This study assesses whether layover will significantly impact the precision of SWOT estimates of global river discharge. We present a theoretical river layover uncertainty model at the scale of nodes and reaches, which constitute nominal 200 m and 10 km averages, respectively, along river centerlines. The model is calibrated using high-resolution simulations of SWOT radar interaction with topography covering a total of 41,233 node observations, across a wide range of near-river topographic features. We find that height uncertainty increases to a maximum value at relatively low values of topographic standard deviation and varies strongly with position in the swath. When applied at global scale, the calibrated model shows that layover causes expected height uncertainty to increase by only a modest amount (from 9.4 to 10.4 cm at the 68th percentile). The 68th percentile of the slope uncertainty increases more significantly, from 10 to 17 mm/km. Nonetheless, the 68th percentile discharge uncertainty increases only marginally. We find that the impact of layover on SWOT river discharge is expected to be small in most environments.

The Mars Science Laboratory landing architecture relies on unprecedented onboard ground-relative navigation accuracy, which is provided by the Terminal Descent Sensor. The Terminal Descent Sensor is a Ka-band landing radar that was developed specifically for the Mars Science Laboratory. It employs six narrow-beam antennas to provide high-precision line-of-sight range and velocity measurements over a challenging operational envelope that encompasses the dynamics of terminal descent, the uncertain surface properties at the landing site, and the space environment. Telemetry from landing shows that the radar generally performed very well, with measurement characteristics that match high-fidelity simulations quite closely. Two features in the radar telemetry were not predicted, although they did not adversely affect the landing. These were 1) sporadic velocity errors during the sky-crane phase that were likely caused by the motion of surface material kicked up by the thruster plumes of the descent vehicle and ...

This paper details a specific and unexpected problem faced by the Entry, Descent, and Landing team on the Phoenix Mars Mission that demonstrates two problems in mission architecture: 1) the difficulties inherent in adapting commercial-off-the-shelf technology to a system different from the product’s intended scope; and 2) the design of test environments sufficientlyflightlike as to reveal systemic problems in newarchitectures. Specifically, the interaction of the entry heat shield with the landing radar could not be tested in a flightlike manner; hence, detailed simulation of the landing radarwas conductedwith the expectation that the radar could lock on echoes from the heat shield and report an erroneous altitude, and that phenomenon could be handled with appropriate flight software. Surprisingly, it was discovered that the heat shield spoofed the radar’s internal logic that was designed to prevent locking on ambiguous radar returns. Mitigating the unexpected problem proved to be much more challenging than the anticipated heatshield lock scenario, ultimately requiring last-minute changes to both the radarfirmware and the flight software.

Synthetic Aperture Radar (SAR) Along-Track Interferometry (ATI) has been used extensively to measure ocean surface currents. Given its ability to measure small velocities (-10 cm/s) of relatively radar-dark water surfaces, there is great potential that this technique can be adapted for ground moving target indication (GMTI) applications, particularly as a method for detecting very slow targets with small radar cross-sections. Herein, we describe preliminary results from an ATI GMTI experiment. The SAR data described were collected by the dual-frequency NASA/JPL airborne radar in its standard dual-baseline ATI mode. The radar system imaged a variety of control targets including a pick-up truck, sport utility vehicles, passenger cars, a bicycle, and pedestrians over multiple flight passes. The control targets had horizontal velocities of less than 5 m/s. The cross-sections of the targets were not purposely enhanced, although the targets' reflectivities may have been affected by the existence of the GPS equipment used to record the targets' positions. Single-look and multiple-look interferograms processed to the full azimuth resolution were analyzed. In the data processed to date, all of the targets were observed by visual inspection in at least one of the four combinations of dual-frequency, dual-baseline interferometric data. This extremely promising result demonstrates the potential of ATI for GMTI applications.

Synthetic aperture radar (SAR) along-track interferometry (ATI) has been used extensively to measure ocean surface currents. Given its ability to measure small velocities (-10 cm/s) of relatively radar-dark water surfaces, there is great potential that this technique can be adapted for ground moving target indication (GMTI) applications, particularly as a method for detecting very slow targets with small radar cross-sections. Herein, we describe preliminary results from an ATI GMTI experiment. The SAR data described were collected by the dual-frequency NASA/JPL airborne radar in its standard dual-baseline ATI mode. The radar system imaged a variety of control targets including a pickup truck, sport utility vehicles, passenger cars, a bicycle, and pedestrians over multiple flight passes. The control targets had horizontal velocities of less than 5 m/s. The cross-sections of the targets were not purposely enhanced, although the targets' reflectivities may have been affected by the existence of the GPS equipment used to record the targets' positions. Single-look and multiple-look interferograms processed to the full azimuth resolution were analyzed. In the data processed to date, all of the targets were observed by visual inspection in at least one of the four combinations of dual-frequency, dual-baseline interferometric data. This extremely promising result demonstrates the potential of ATI for GMTI applications.

Synthetic aperture radar (SAR) along track interferometry (ATI) has been used extensively to measure ocean surface currents. Given its ability to measure small velocities (/spl sim/ 10 cm/s) of relatively radar-dark water surfaces, there is great potential that this technique can be adapted for ground moving target indication (GMTI) applications, particularly as a method for detecting very slow targets with small radar cross-sections. In this paper we describe preliminary results from an ATI GMTI experiment. The SAR data described herein were collected by the dual-frequency NASA/JPL airborne radar in its standard dual-baseline ATI mode. The radar system imaged a variety of control targets including a pickup truck, sport utility vehicles, passenger cars, a bicycle, and pedestrians over multiple flight passes. The control targets had horizontal velocities of less than 5 m/s. The cross-sections of the targets were not purposely enhanced, although the targets' reflectivities may have been affected by the existence of the GPS equipment used to record the targets' positions. Single-look and multiple-look interferograms processed to the full azimuth resolution were analyzed. In the data processed to date, all of the targets were observed by visual inspection in at least one of the four combinations of dual-frequency, dual-baseline interferometric data. This extremely promising result demonstrates the potential of ATI for GMTI applications.

The Soil Moisture Active/Passive (SMAP) mission is scheduled for a late 2014 launch date. The mission will use both active radar and passive radiometer instruments at L-Band in order to achieve the science objectives of measuring soil moisture and land surface freeze-thaw state. To achieve requirements for a wide swath at sufficiently high resolution for both active and passive channels, an instrument architecture that uses a large rotating reflector is employed. In this paper, focus will be placed on the radar design. The radar will employ synthetic-aperture processing to achieve a “moderate” resolution dual-pol product over a 1000 km swath. Because the radar is operating continuously, very frequent temporal coverage will be achieved at high latitudes. This data will be used to produce a surface freeze/thaw state data product.

Satellite-based repeat-pass Interferometric Synthetic Aperture Radar (InSAR) provides a synoptic high spatial resolution perspective of Earth's changing surface, permitting one to view large areas quickly and efficiently. By measuring relative phase change from one observation to the next on a pixel-by-pixel basis, maps of deformation and change can be derived. Variability of the atmosphere and the ionosphere leads to phase/time delays that are present in the data that can mask many of the subtle deformation signatures of interest, so methods for mitigation of these effects are important. Many of these effects have been observed in existing ALOS PALSAR data [1], and studies are underway to characterize and mitigate the ionosphere using these data. Since the ionosphere is a dispersive medium, it is possible in principle distinguish the ionospheric signatures from the non-dispersive effects of deformation and the atmosphere. In this paper, we describe a method for mapping the ionosphere in InSAR data based on a multi-frequency split-spectrum processing technique. We examine a number of PALSAR data sets, including fully polarimetric and single-polarization 28 MHz bandwidth data, where anomalous effects in phase, amplitude and image registration have been observed. We demonstrate the estimation of the ionosphere by means of the split spectrum technique for estimating differential TEC, whereby a radar waveform is transmitted over the full PALSAR spectral band and widely separated portions of the receive spectrum are processed independently and compared for dispersive effects, and quantify its performance.

The soft-touchdown, “sky-crane” concept employed by the 2009 NASA Mars Science Laboratory mission requires an order-of-magnitude improvement from previous missions in the sensing of vehicle velocity and altitude. This paper describes the development of a new radar “Terminal Descent Sensor” that provides decimeterper-second velocity accuracy while also providing better than 2% range accuracy on six unique beams. This sensor design uses a millimeter-wave center frequency (Ka-band) and pencil beam antennas to achieve the required velocity precision and to overcome the problems that angle-of-arrival errors can cause in velocity reconstruction. Included are discussions of the design concept, driving requirements, hardware architecture, and results from a high fidelity performance simulation.

We consider two spacecraft flying in formation to create interferometric synthetic aperture radar (InSAR). Several candidate orbits for such in InSar formation have been previously determined based on radar performance and Keplerian orbital dynamics. However, with out active control, disturbance-induced drift can degrade radar performance and (in the worst case) cause a collision. This study evaluates the feasibility of operating the InSAR spacecraft as a formation, that is, with inner-spacecraft sensing and control. We describe the candidate InSAR orbits, design formation guidance and control architectures and algorithms, and report the (Delta)(nu) and control acceleration requirements for the candidate orbits for several tracking performance levels. As part of determining formation requirements, a formation guidance algorithm called Command Virtual Structure is introduced that can reduce the (Delta)(nu) requirements compared to standard Leader/Follower formation approaches.

We present a flow down error analysis from the radar system to topographic height errors for bi-static single pass SAR interferometry for a satellite tandem pair. Because of orbital dynamics the baseline length and baseline orientation evolve spatially and temporally, the height accuracy of the system is modeled as a function of the spacecraft position and ground location. Vector sensitivity equations of height and the planar error components due to metrology, media effects, and radar system errors are derived and evaluated globally for a baseline mission. Included in the model are terrain effects that contribute to layover and shadow and slope effects on height errors. The analysis also accounts for non-overlapping spectra and the non-overlapping bandwidth due to differences between the two platforms' viewing geometries. The model is applied to a 514 km altitude 97.4 degree inclination tandem satellite mission with a 300 m baseline separation and X-band SAR. Results from our model indicate that global DTED (digital terrain elevation data) level 3 can be achieved.

We have quantified the impact that the ionosphere would have on a L-band interferometric synthetic aperture radar (SAR) mission using a combination of simulation, modeling, Global Positioning System (GPS) data collected during the last solar maximum, and existing spaceborne SAR data. Using the Jet Propulsion Laboratory's Global Ionospheric Maps (GIM) total electron content (TEC) estimates derived from the worldwide array of GPS stations, we determined that the sun synchronous orbit which would minimize TEC at the time of imaging has dawn and dusk equator crossings. Such an orbit also avoids the equatorial post-sunset irregularities. We used the GIM data to examine the day-to-day variability in the background ionosphere and to quantify the impact of the background ionosphere on single pass SAR performance. With the exception of Faraday rotation related effects on single polarization systems, degradation due to the background ionosphere can be avoided if a reasonable model for the ionosphere is used during processing. Our studies reveal that Faraday rotation angles rarely exceeded the 10/spl deg/ threshold that impacts biomass retrieval and that repeat pass interferometric SAR decorrelation due to variations in the background ionosphere causing variable Faraday rotations is a negligible effect. Even a dawn-dusk orbit will not avoid high latitude ionospheric irregularities. We evaluated the strength of the ionospheric irregularities using GPS scintillation data collected at Fairbanks, Alaska and modeled the impact of these irregularities on azimuth resolution, azimuth displacement, peak sidelobe ratio (PSLR), and integrated sidelobe ratio (ISLR). Our examination of ionospheric artifacts in InSAR data has revealed that the artifacts occur primarily in the polar cap data, not auroral zone data as was previously thought.

Synthetic aperture radar (SAR) along track interferometry (ATI) has been used extensively to measure ocean surface currents. Given its ability to measure small velocities (/spl sim/10 cm/s) of relatively radar-dark water surfaces, there is great potential that this technique can be adapted for ground moving target indication (GMTI) applications, particularly as a method for detecting very slow targets with small radar cross sections. In this paper we describe preliminary results from an ATI GMTI experiment. The SAR data described here were collected by the dual-frequency NASA/JPL airborne radar in its standard dual-baseline ATI mode. The radar system imaged a variety of control targets including a pickup truck, sport utility vehicles, passenger cars, a bicycle, and pedestrians over multiple flight passes. The control targets had horizontal velocities of less than 5 m/s. The cross sections of the targets were not purposely enhanced, although the targets' reflectivities may have been affected by the existence of the GPS equipment used to record the targets' positions. Single-look and multiple-look interferograms processed to the full azimuth resolution were analyzed. In the data processed to date, all of the targets were observed by visual inspection in at least one of the four combinations of dual-frequency, dual-baseline interferometric data. This extremely promising result demonstrates the potential of ATI for GMTI applications.

We examine the performance of amplitude-based height-estimation techniques for use with airborne synthetic aperture ladar (SAL) sensors in generating three-dimensional reconstructions of ground targets. Such techniques lend themselves to implementation more readily than phase-based techniques and are also more tolerant to phase instabilities that might be associated with SAL systems. For pairwise amplitude-comparison monopulse processing, we present analyses of the expected height sensitivity and bias of SAL systems in terms of the system parameters. We verify this analysis with simulations, and we also provide an overview of other SAL phenomena that affect height-estimation accuracy. We then propose an array-based joint-processing approach that can be applied instead of pairwise monopulse processing. We show that the joint-processing approach represents the maximum-likelihood estimator for obtaining the target height, and we demonstrate that the proposed approach significantly reduces bias-induced errors.

The area accessible from a spaceborne imaging radar, e.g. a synthetic aperture radar (SAR), generally increases with the elevation of the satellite while the map coverage rate is a more complicated function of platform velocity and beam agility. The coverage of a low Earth orbit (LEO) satellite is basically given by the ground velocity times the relatively narrow swath width. The instantaneously accessible area will be limited to some hundreds of kilometers away from the sub-satellite point. In the other extreme, the sub-satellite point of a SAR in geosynchronous orbit will move relatively slowly, while the area which can be accessed at any given time is very large, reaching thousands of kilometers from the sub-satellite point. To effectively use the accessibility provided by a high vantage point, very large antennas with electronically steered beams are required. Interestingly, medium Earth orbits (MEO) will enable powerful observational systems which provide large instantaneous reach and high mapping rates, while pushing technology less than alternative systems at higher altitudes. Using interferometric SAR techniques which can reveal centimeter-level (potentially sub-centimeter) surface displacements, frequent and targeted observations might be key to developing such elusive applications as earthquake forecasting. This paper discusses the basic characteristics of a SAR observational system as a function of the platform altitude and the technologies being developed to make such systems feasible.

This paper discusses large aperture, high orbit radar concepts for measuring sub-centimeter-level surface displacements from space. These measurements will enable applications such as earthquake simulation, modeling and forecasting. We explain the need for large aperture, high orbit arrays and discuss the technologies required to achieve these missions.

This paper describes a modified polar format algorithm for SAR image-formation processing. The algorithm focuses on raw data, assuming a spherical reference surface (ground surface), and, unlike standard polar format algorithms, it assumes spherical rather than planar signal wavefronts. The algorithm, or some variant of it based on the game geometry, is therefore more suitable than the standard polar format algorithm in spaceborne applications for which there is significant curvature of the platform flight track and/or curvature of the Earth surface. The algorithm is described here in the context of spotlight-mode data acquisition.

Along-track interferometry (ATI) is an interferometric synthetic aperture radar technique that can be used to measure Earth-surface velocities. As such, the ATI technique holds promise for the detection of slowly moving ground targets. However, the models often used to characterize ATI performance were developed mainly in the context of mapping ocean currents, and they do not necessarily apply to the case of discrete, moving ground targets amidst clutter. We provide expressions for more accurately modeling the behavior of an ATI system in the context of ground moving target indication. Analysis and design equations are given for topics including target defocus, signal-to-noise and signal-to-clutter ratios, interferometric correlation, interferometric phase bias, target detection, geolocation accuracy, and area coverage rate.

Next-generation interferometric synthetic aperture radar (InSAR) systems may provide the basis for establishing an earthquake-forecasting capability within a twenty-year time frame. Such systems would need to provide data with fine temporal resolution, so the system architecture would need to allow for wide-area coverage in order to minimize the effective interferometric repeat time. This paper discusses the coverage advantages associated with medium-Earth orbit (MEO) InSAR systems for observing geophysical phenomena. As MEO architectures dictate the need for large radar antennas, this paper also presents a discussion of advanced antenna technologies—and associated challenges—that might provide revolutionary decreases in the mass densities of large radar aperture antennas.

Interferometric radar techniques often necessitate two-dimensional (2-D) phase unwrapping, defined here as the estimation of unambiguous phase data from a 2-D array known only modulo 2pi rad. We develop a maximum a posteriori probability (MAP) estimation approach for this problem, and we derive an algorithm that approximately maximizes the conditional probability of its phase-unwrapped solution given observable quantities such as wrapped phase, image intensity, and interferogram coherence. Examining topographic and differential interferometry separately, we derive simple, working models for the joint statistics of the estimated and the observed signals. We use generalized, nonlinear cost functions to reflect these probability relationships, and we employ nonlinear network-flow techniques to approximate MAP solutions. We apply our algorithm both to a topographic interferogram exhibiting rough terrain and layover and to a differential interferogram measuring the deformation from a large earthquake. The MAP solutions are complete and are more accurate than those of other tested algorithms.

Two-dimensional (2-D) phase unwrapping, that is, deducing unambiguous phase values from a 2-D array of values known only modulo 2pi, is a key step in interpreting data acquired with synthetic aperture radar interferometry. Noting the recent network formulation of the phase unwrapping problem, we apply here some well-established ideas of network theory to formalize the problem, analyze its complexity, and derive algorithms for its solution. It has been suggested that the objective of phase unwrapping should be to minimize the total number of places where unwrapped and wrapped phase gradients differ. Here we use network constructions to show that this so-called minimum L0-norm problem is NP-hard, or one that complexity theory suggests is impossible for efficient algorithms to solve exactly. Therefore we must instead find approximate solutions; we present two new algorithms for doing so. The first uses the network ideas of shortest paths and spanning trees to improve on the Goldstein et al. residue-cut algorithm [Radio Sci. 23, 713 (1988)]. Our improved algorithm is very fast, provides complete coverage, and allows user-defined weights. With our second algorithm, we extend the ideas of linear network flow problems to the nonlinear L0 case. This algorithm yields excellent approximations to the minimum L0 norm. Using interferometric data, we demonstrate that our algorithms are highly competitive with other existing algorithms in speed and accuracy, outperforming them in the cases presented here.

A principal goal of the Mars Science Laboratory (MSL) rover Curiosity is to identify and characterize past habitable environments on Mars. Determination of the mineralogical and chemical composition of Martian rocks and soils constrains their formation and alteration pathways, providing information on climate and habitability through time. The CheMin X-ray diffraction (XRD) and X-ray fluorescence (XRF) instrument on MSL will return accurate mineralogical identifications and quantitative phase abundances for scooped soil samples and drilled rock powders collected at Gale Crater during Curiosity’s 1-Marsyear nominal mission. The instrument has a Co X-ray source and a cooled charge-coupled device (CCD) detector arranged in transmission geometry with the sample. CheMin’s angular range of 5° to 50° 2θ with < 0.35° 2θ resolution is sufficient to identify and quantify virtually all minerals. CheMin’s XRF requirement was descoped for technical and budgetary reasons. However, X-ray energy discrimination is still required to separate Co Kα from Co Kβ and Fe Kα photons. The X-ray energy-dispersive histograms (EDH) returned along with XRD for instrument evaluation should be useful in identifying elements Z >13 that are contained in the sample. The CheMin XRD is equipped with internal chemical and mineralogical standards and 27 reusable sample cells with either Mylar® or Kapton® windows to accommodate acidic-to-basic environmental conditions. The CheMin flight model (FM) instrument will be calibrated utilizing analyses of common samples against a demonstrationmodel (DM) instrument and CheMin-like laboratory instruments. The samples include phyllosilicate and sulfate minerals that are expected at Gale crater on the basis of remote sensing observations.