Spaceborne Imaging Radar-C Research Articles

Remote sensing evidence for a possible 10 kilometer in diameter impact structure in north-central Niger

Remote sensing investigations of West Africa reveal a roughly 10 km in diameter, circular feature in north-central Niger. The circular feature is located within Early Devonian fluvio-marine sedimentary strata about 100 km to the north of a suite of igneous ring dikes located in the Aïr Massif. Whether the circular feature is related to the ring dikes, structural doming, or an impact crater can be evaluated from earlier geologic mapping (Black, 1967) and image analyses. Previous mapping indicates that the circular feature is defined by outcrops of Devonian Idekel sandstone and the Late Proterozoic, weakly-metamorphosed molasse of the Proche-Ténéré Formation. Absent in published lithologic descriptions of these sedimentary and metasedimentary rocks are diapiric evaporite deposits that could produce the observed circular morphologies. Lithologies that define the circular feature are confirmed by classification techniques using the ENVI software platform and band ratios/math from Landsat 8 and Sentinel 2A data. Spectral profile comparisons between the igneous intrusive rocks exposed in the Aïr Massif, outcrops of sedimentary rock that are proximal to the circular feature, and desert sand confirm that the sedimentary rocks mapped by Black (1967) define the circular feature. The shallow subsurface structure, revealed through Spaceborne Imaging Radar-C data, and 3-D modeling using Sentinel 1A data, is also not consistent with nearby ring dike structures. These data suggest that the circular feature did not likely form as a magmatic intrusion close to the time of sedimentary rock deposition. We propose that the circular feature formed from either a meteorite impact after deposition of the host sedimentary units or by some other, but less likely, geologic process. Confirmation of an impact explanation for the circular feature would require sample analysis and inspection for shock-metamorphosed materials.

A Modified Three-Stage Inversion Algorithm Based on R-RVoG Model for Pol-InSAR Data

In this paper, a modified two-layer scattering model is applied to a three-stage algorithm for high-precision retrieval of forest parameters from Polarimetric Synthetic Aperture Radar Interferometry (Pol-InSAR) data. Traditional Random-Volume-over-Ground (RVoG) model considers forest target as a two-layer combination of flat ground and volumetric canopy. However, when it comes to sloped terrain, the inversion accuracy of three-stage process deteriorates with the ascending estimation error in volume correlation which is mainly caused by the existence of underlying terrain slope. Aiming at this problem, a Range-sloped RVoG (R-RVoG) model is presented in this paper. By modifying the ground layer as a range-sloped plane, the complex correlation of R-RVoG model can be amended as a function of ground phase, ground-to-volume scattering ratio, forest height, mean extinction and range slope. The introduction of range slope variable makes this modified model better resemble to real scene and thus improves the performance of three-stage algorithm. Both of the simulated data with different terrain slopes and the Space-borne Imaging Radar-C (SIR-C) real data in Tianshan test area are processed to verify the validity of this modification.

Indian Ocean tsunami: The use of MERIS (MTCI) data to infer salt stress in coastal vegetation

The Indian Ocean tsunami of 26 December 2004 removed coastal vegetation and inundated large areas of near‐coastal and low lying land with salt water. There were subsequent reports of early vegetation senescence as salt stress reduced the chlorophyll content of plant canopies. The European Space Agency (ESA) uses data from its Medium Resolution Imaging Spectrometer (MERIS) on Envisat to produce an operational product called the MERIS Terrestrial Chlorophyll Index (MTCI). The MTCI values are related to the relative position of the red edge in the reflectance spectrum of vegetation and so can be used to estimate the chlorophyll content of that vegetation. The difference between pre and post‐tsunami MTCI images was compared with elevation data from the Shuttle Radar Topography Mission's (SRTM) Spaceborne Imaging Radar‐C (SIR‐C) for the Phuket region of Thailand. There was a statistically significant (95% confidence level) decrease in the MTCI after the tsunami in near‐coastal and low lying interior regions. It was hypothesized that this decrease was due to a reduction in chlorophyll content as a result of the salt stress produced by salt water inundation. The recovery of this region is to be monitored using the MTCI.

Analysis of a Microwave Backscattering Mechanism From a Small Urban Area Imaged With SIR-C

We analyzed the Spaceborne Imaging Radar C (SIR-C) imagery of a small urban area to characterize backscattering mechanisms. The results were compared with the building density so that we could relate the polarimetric backscattering characteristics with urban structures. Relative contributions of odd-bounce, double-bounce, and cross-polarized scatterings were evaluated as a function of the building density for small and large incidence angles. Their behaviors were characterized with respect to urban structures at small and large incidence angles.

Spaceborne imaging radar-C/X-band synthetic aperture radar (SIR-C/X-SAR): a look back on the tenth anniversary

The spaceborne imaging radar-C, X-band synthetic aperture radar (SIR-C/X-SAR) missions have resulted in important scientific discoveries and provided new insights into Earth system processes. Analyses of SIR-C/X-SAR engineering-mode data have also led to new measurement and mission concepts. The multifrequency, multipolarisation capability provided by SIR-C/X-SAR is unsurpassed from a spaceborne system, making the data set valuable for algorithm development and assessment of optimal imaging parameters more than a decade after the missions were flown.

Polarization State Conformation and Its Application to Change Detection in Polarimetric SAR Data

For polarimetric SAR (POLSAR) images, it is ideal that scattering geometries of the same target should display resemblance between multidate images, which are used in change detection applications, since the scattering mechanisms may change due to the data acquisition geometry. However, sometimes it is difficult to achieve these conditions. An attempt is made to maximize the resemblance between the scattering geometries in multidate images for a specific target. An algorithm is developed based on the polarimetric basis transformation along with the polarization signatures. As a result, the resemblance between the scattering mechanisms of the same target in both images is maximized. The effects predicted by the theory are confirmed by the change detection analysis of POLSAR data acquired by the Jet Propulsion Laboratory Spaceborne Imaging Radar-C mission.

Radiative transfer modeling of cross-polarized backscatter from a pine forest using the discrete ordinate and eigenvalue method

Radiative transfer models have been widely used to interpret the radar backscatter from forested areas. Most of these models are based on an iterative solution of the radiative transfer equation, usually solved up to first or second order, thus taking into account single and double scattering. Although this method leads to results agreeing well with copolarized backscatter measurements, it produces less accurate estimates for horizontal-vertical (HV) polarization. This paper presents a radiative transfer backscatter model that accounts for multiple scattering by using the discrete ordinate and eigenvalue method applied to a layered medium. Using parameters derived from an architectural tree model, calculations at C- and L-band are compared with HV data acquired for a maritime pine forest in the southwest of France during the Spaceborne Imaging Radar-C missions. Good agreement is found at C-band for all values of forest biomass, and reasonable agreement at L-band for high biomass, when the soil backscatter plays a minor role. For low biomass, the L-band modeling is inadequate because of difficulties in estimating the soil backscatter. Comparison with calculations from a first-order radiative transfer model shows that multiple scattering is significant, especially at C-band.

The Contribution of Multitemporal SAR Data in Assessing Hydrological Parameters

The sensitivity of radar backscattering to the principal hydrological parameters, such as vegetation biomass, soil moisture, and surface roughness, is discussed. Results obtained by using multifrequency synthetic aperture radar (SAR) data measured by the Jet Propulsion Laboratory Airborne Synthetic Aperture Radar, Spaceborne Imaging Radar-C, and European Remote Sensing 1/2 sensors are summarized. The sensitivity of L- and C-bands to spatial variations of plant and soil parameters is masked by the presence of surface roughness, which in turn affects the radar signal. However, from the observation of data collected at different dates and averaged over a relatively wide area that includes several fields, the correlation to soil moisture and vegetation biomass is found to be significant, since the effects of spatial variations are smoothed. On the other hand, the sensitivity to surface roughness becomes appreciable when multitemporal data are averaged in time, thus reducing the effects of temporal moisture variations.

The effect of soil and crop residue characteristics on polarimetric radar response

Although the interaction between linear polarized microwaves and agricultural targets has been studied extensively, far less is understood about the added information provided from polarimetric Synthetic Aperture Radars (SARs). Using 1994 Spaceborne Imaging Radar-C (SIR-C) data, this study examines the sensitivity of linear polarizations and polarimetric parameters to conditions present on agricultural fields during the period of preplanting and postharvest. The polarimetric parameters investigated include circular polarized backscatter, pedestal height, and co-polarized phase differences (PPD). The co-polarizations signature plots are also discussed. Results indicate that the dominant scattering mechanism from these fields varies depending on the type and amount of residue cover, and whether the crop had been harvested. Radar parameters most sensitive to volume and multiple scattering perform best at characterizing these surface conditions. These parameters are the pedestal height, as well as the linear cross-polarization (HV) and the circular co-polarizations (RR). The co-polarizations signature plots and the standard deviation associated with the PPD are also useful in categorizing these cover types. However, the field average PPD provides little information on residue and soil characteristics.

A summary of experimental results to assess the contribution of SAR for mapping vegetation biomass and soil moisture

This paper is an overview of the most recent results obtained by Italian groups involved in the spaceborne imaging radar-C,X-band synthetic aperture radar (SIR-C/X-SAR) hydrological experiment, by using multi-frequency and multi-polarization synthetic aperture radar (SAR) data measured by JPL/AIRSAR, SIR-C, EMISAR, ERS-1, and JERS-1 sensors. The sensitivity of backscattering coefficients to some geophysical parameters that play a significant role in hydrological processes, such as vegetation biomass, soil moisture, and surface roughness, is discussed. Experimental results show that P band appears to be suitable for the monitoring of forest biomass, whereas L band is mainly sensitive to the biomass of wide-leaf crops and C band to narrow-leaf crops. Moreover, the L-band sensor gives the highest contribution in estimating soil moisture and surface roughness. The sensitivity of backscatter to soil moisture and surface roughness for individual agricultural fields is rather low, since both parameters affect the radar signal. However, by observing data collected at different dates and averaged over a relatively wide agricultural area, the correlation with soil moisture becomes considerable, since the effects of spatial roughness variations are smoothed. Retrievals of both soil moisture and surface roughness were performed using a semi-empirical model.

First spaceborne observation of an Earth‐reflected GPS signal

We present the first spaceborne observation of a Global Positioning System (GPS) signal reflected from the Earth's surface, specifically from the Pacific Ocean. This result is scaled to obtain the expected voltage signal‐to‐noise ratio (SNR) and altimetric accuracy for a generic GPS reflections altimetry mission and the current SAC‐C and CHAMP missions. Cross‐correlating a three‐parameter phase model with both a 1‐s and 4‐s segment of spaceborne imaging radar‐C (SIR‐C) calibration data, recorded before and after a Galapagos Islands imaging pass, results in beam‐limited signals having voltage SNRs of 10 and 334, respectively. Evidence for these results being reflected GPS signals includes: (1) The signals' temporal shapes agree closely with that predicted using a detailed scattering model, at two different observation geometries, and differ significantly from the expected direct signal shapes. (2) The signal in the 4‐s data has a measured coherence time of 1.0 ms, which agrees closely with that expected for a reflected signal and is completely inconsistent with the direct signal's coherence properties. (3) The 1‐ and 4‐s signals' voltage SNR is maximized by shifting the model frequency −2740 Hz and 497 Hz, respectively from that expected from their respective specular reflection points, or −2875 Hz and 690 Hz from the expected direct signal frequencies. These values agree with the −2900 Hz and 510 Hz Doppler frequency shifts expected from those points on the surface corresponding to the antenna's pointing direction, thus illustrating beam‐steering effects on the surface. (4) Plausible hypotheses for the detected waveform being a corrupted direct signal, including second‐order mismodeling effects, shuttle multipath effects, or a band‐pass cutoff of the GPS spread spectrum, are shown to be inconsistent with the data. Space‐based observations of reflected GPS signals, like the ones presented here, may enable a new class of ocean topography measurements unavailable from traditional altimeters, such as TOPEX/Poseidon, and perhaps surface wind vector measurements. Making such observations with sufficient SNR will require unusually large, high‐gain antennas. The measurement presented here is scaled to assess the expected SNR for those applications. Because this result lies in a nonlinear scaling regime, the correct scaling equations are presented, and the expected signal strength from a generic GPS reflections altimetry mission is derived to illustrate the most important contributions to the signal SNR. An SNR estimate is also derived for the SAC‐C and CHAMP missions, which are expected to make GPS reflection measurements in the near future. Finally, a qualitative wind speed determination is extracted from the observed signal.

Spaceborne imaging radar-C (SIR-C) observations of groundwater discharge and wetlands associated with the Chicxulub impact crater, northwestern Yucatan Peninsula, Mexico

Analyses of spaceborne imaging radar-C (SIR-C) data and field data from the northwestern Yucatan Peninsula, Mexico, demonstrate that spaceborne multifrequency polarimetric radars are excellent tools for characterizing patterns of wetland flooding. Seasonal flooding can be detected in most types of forest and marsh in the radar backscatter magnitude and phase data of both L and C band. Field observations made in the wet and dry seasons concurrent with the space missions and chemical analyses of floodwaters confirm that flooding is the product of discharge from the Yucatan aquifer, which consists of a fresh-water lens floating on seawater. This discharge controls the distribution of wetlands. Therefore, vegetation and flooding patterns, mapped with SIR-C imagery, provide valuable information on the hydrogeology of the region. Radar-image maps of wetlands and flooding indicate that there are three major zones of groundwater discharge that correlate with structures of the buried Chicxulub crater—zone 1 with the peak ring, zone 2 with the crater rim, and zone 3 with the exterior ring. Zone 1 has sulfate-poor discharge, unlike the sulfate-rich discharge in zones 2 and 3. The highest discharge is in zone 3, where the buried crater is closest to the surface. This groundwater-discharge pattern can be explained by tidal pumping of fresh water to the surface through high permeability zones developed in the Tertiary carbonates overlying crater faults and escarpments.

Effects of Digital Elevation Model Accuracy on Hydrologic Predictions

The effect of vertical accuracy of digital elevation models (DEMs) on hydrologic prediction accuracy was evaluated by comparing three DEMs and associated streamflow simulations for the 7.2 km 2 USDA-ARS watershed at Mahantango Creek, PA. The DEMs were the standard 30 m USGS 7.5′ DEM, a 5 m product derived from low altitude aerial photography, and a 30 m product derived from interferometric processing of Spaceborne Imaging Radar-C (SIR-C). Statistical analysis of the DEMs showed that the USGS DEM had the typical stipling errors resulting from processing of the source digital contour maps, and a vertical error structure related to the topographic attributes of the watershed. The SIR-C DEM had a vertical offset of about 50 m from the high resolution and USGS DEMs, as well as error features that were somewhat related to topographic features. Inaccuracies in both the USGS and SIR-C DEMs were apparent in the drainage network, as well as in spatial images of elevation, slope, and contributing area. Comparisons of runoff predicted using a hydrologic model based on the three DEMs showed that mean annual predicted runoff volumes were 0.3% and 7.0% larger for the USGS and SIR-C DEMs, respectively, as compared to the reference DEM. Much larger differences were apparent in individual hydrographs; and the USGS and SIR-C DEMs predicted lower peaks, and higher base flows, than did the reference.

Review article Synthetic aperture radar (SAR) frequency and polarization requirements for applications in ecology, geology, hydrology, and oceanography: A tabular status quo after SIR-C/X-SAR

The Spaceborne Imaging Radar-C, X-Band Synthetic Aperture Radar (SIR-C/X-SAR) was the first multi-frequency and multi-polarization SAR system to be launched into space. SIR-C/X-SAR imaged over 300 sites around the Earth returning 143 terabits of data. There has been a tremendous advancement of knowledge in the field of radar remote sensing accomplished in the last two years, as well as verification of earlier findings since the two successful SIR-C/X-SAR missions. This review article presents the current status of optimal SAR parameters for various key issues within the disciplines of ecology, geology, hydrology, and oceanography. A polarimetric X- and L-band radar is suggested as a result of our review for future SAR sensors. The design of a single frequency, albeit polarimetric, SAR satellite, limits applications, as can be deduced from the tables.

Monitoring local environmental conditions with SIR-C/X-SAR

This article examines temporal changes in the Spaceborne Imaging Radar-C (SIR-C) C- and L-band radar backscatter over an agricultural test site centred on Altona, Manitoba, Canada in relation to changing environmental conditions during April and October 1994. The results indicate that environmental events such as frost and rain can be monitored. However, care must be taken as some soil targets at high moisture contents may behave as specular reflectors at longer wavelengths, such as L-band at large incidence angles, thus masking soil moisture effects. The strongest correlation between radar backscatter and soil moisture ( r=0.84) was obtained for the 0–2.5 cm soil profile at HH polarizations. The strength of the correlation was approximately the same at C- and L-band. All polarizations displayed a decrease in soil moisture correlation with increasing soil profile depth. It was observed that soil texture effects on the estimation of soil moisture from radar backscatter were relatively small and consequently can be neglected. Although the sensitivity to surface roughness is greater at HV and VV than at HH polarization, results indicate that roughness could be neglected when measuring soil moisture over relatively short periods of time at a given site, without significantly reducing the sensitivity of the relationship between radar backscatter and soil moisture.

SIR-C/X-SAR observations of rain storms

The spaceborne imaging radar-C, X-band synthetic aperture radar observations of rain storms are the first multipolarization and multifrequency observations of precipitation from space. In addition to numerous, often dramatic images of severe weather systems obtained by forming a synthetic aperture in the usual side-looking attitude, several data takes were performed while the radar antennas were parallel to the ground and the radar beams were pointing at nadir. These opportunities coincided with the passage of the Shuttle over Tropical Cyclone Odille in the southern Indian Ocean during the first flight and over Typhoon Seth in the western Pacific during the second flight. The resulting observations, or, more appropriately, the resulting measurements, demonstrate for the first time the capability of a spaceborne multifrequency multipolarization microwave radar system to quantify precipitation rates, to detect hydrometeor phase, and to classify rain type.

Estimation of bare surface soil moisture and surface roughness parameter using L-band SAR image data

An algorithm based on a fit of the single-scattering integral equation method (IEM) was developed to provide estimation of soil moisture and surface roughness parameter (a combination of rms roughness height and surface power spectrum) from quad-polarized synthetic aperture radar (SAR) measurements. This algorithm was applied to a series of measurements acquired at L-band (1.25 GHz) from both AIRSAR (Airborne Synthetic Aperture Radar operated by the Jet Propulsion Laboratory) and SIR-C (Spaceborne Imaging Radar-C) over a well-managed watershed in southwest Oklahoma. Prior to its application for soil moisture inversion, a good agreement was found between the single-scattering IEM simulations and the L-band measurements of SIR-C and AIRSAR over a wide range of soil moisture and surface roughness conditions. The sensitivity of soil moisture variation to the co-polarized signals were then examined under the consideration of the calibration accuracy of various components of SAR measurements. It was found that the two co-polarized backscattering coefficients and their combinations would provide the best input to the algorithm for estimation of soil moisture and roughness parameter. Application of the inversion algorithm to the co-polarized measurements of both AIRSAR and SIR-C resulted in estimated values of soil moisture and roughness parameter for bare and short-vegetated fields that compared favorably with those sampled on the ground. The root-mean-square (rms) errors of the comparison were found to be 3.4% and 1.9 dB for soil moisture and surface roughness parameter, respectively.

Effects on operations of highly adaptive missions

A nonadaptive operations strategy was used by the Magellan mission to Venus, whereas an adaptive strategy was used for the Spaceborne Imaging Radar-C mission to Earth. The operational strategies employed by these missions are described, along with overviews of both missions and a summary of their systems and capabilities. The effects of adaptivity on the operations systems are discussed, and a summary of advantages of each strategy is given.

Interferometric radar observations of Glaciar San Rafael, Chile

AbstractInterferometric radar observations of Glaciar San Rafael, Chile, were collected in October 1994 by NASA’s Spaceborne Imaging Radar C (SIR-C) at both L- (24 cm) and C-band frequency (5.6 cm), with vertical transmit and receive polarization. The C-band data did not yield good geophysical products, because the temporal coherence of the signal was significantly reduced alter 24 h. The L-band data were, however, successfully employed to map the surface topography of the icefield with a 10 m uncertainty in height, and measure ice velocity with a precision of 4 mm d−1or 1.4 m a−1. The corresponding error in strain rates is 0.05 a−1at a 30 m horizontal spacing. The one-dimensional interferometric velocities were subsequently converted to horizontal displacements by assuming a flow direction and complemented by feature-tracking results near the calving front. The results provide a comprehensive view of the ice-flow dynamics of Glaciar San Rafael. The glacier has a core of rapid flow, 4.5 km in width and 3.5° in average slope, surrounded by slower-moving ice, not by rock. Ice velocity is 2.6 m d−1or 0.95 km a−1near the equilibrium-line altitude (1200 m), increasing rapidly before the glacier enters the narrower terminal valley, to reach 17.5 md−1or 6.4 k ma−1at the calving front. Strain rates are dominated by lateral shearing at the glacier margins (0.4–0.7 a−1), except for the terminal-valley section, where longitudinal strain rates average close to 1 a−1. This spectacular longitudinal increase in ice velocity in the last few kilometers may be a fundamental feature of tidewater glaciers.

Interferometric radar observations of Glaciar San Rafael, Chile

AbstractInterferometric radar observations of Glaciar San Rafael, Chile, were collected in October 1994 by NASA’s Spaceborne Imaging Radar C (SIR-C) at both L- (24 cm) and C-band frequency (5.6 cm), with vertical transmit and receive polarization. The C-band data did not yield good geophysical products, because the temporal coherence of the signal was significantly reduced alter 24 h. The L-band data were, however, successfully employed to map the surface topography of the icefield with a 10 m uncertainty in height, and measure ice velocity with a precision of 4 mm d−1 or 1.4 m a−1. The corresponding error in strain rates is 0.05 a −1 at a 30 m horizontal spacing. The one-dimensional interferometric velocities were subsequently converted to horizontal displacements by assuming a flow direction and complemented by feature-tracking results near the calving front. The results provide a comprehensive view of the ice-flow dynamics of Glaciar San Rafael. The glacier has a core of rapid flow, 4.5 km in width and 3.5° in average slope, surrounded by slower-moving ice, not by rock. Ice velocity is 2.6 m d−1 or 0.95 km a−1 near the equilibrium-line altitude (1200 m), increasing rapidly before the glacier enters the narrower terminal valley, to reach 17.5 md−1 or 6.4 k ma−1 at the calving front. Strain rates are dominated by lateral shearing at the glacier margins (0.4–0.7 a−1), except for the terminal-valley section, where longitudinal strain rates average close to 1 a−1. This spectacular longitudinal increase in ice velocity in the last few kilometers may be a fundamental feature of tidewater glaciers.