Synthetic Aperture Radar Missions Research Articles

Estimation of tree growth in a conifer plantation over 19 years from multi-satellite L-band SAR

The general capability of synthetic aperture radar (SAR) for monitoring forest ecosystems is well documented. However, the majority of SAR studies of forest dynamics use only imagery acquired by one SAR system and are thus limited to the lifecycle of a particular satellite. The synergistic analysis of SAR data from one of the earliest spaceborne SAR missions, the SEASAT mission, with the Japanese JERS-1 satellite-borne SAR is presented. Biophysical parameters frequently retrieved from SAR are tree biomass using backscatter and tree height from the interferometric phase. One potential application that has not been thoroughly examined is mapping of incremental tree growth from SAR backscatter changes. Tree growth measures biomass changes over time, and is correlated to the amount of carbon sequestered by the trees. This paper examines the retrieval of tree growth from multitemporal spaceborne L-band SAR. A SEASAT SAR image from 1978 and a JERS-1 SAR image from 1997 over Thetford forest, UK are used to retrieve tree growth of Corsican Pine stands. Incremental growth was estimated from the changes in backscatter coefficient, and compared to the expected tree growth from general yield class models used by the UK Forestry Commission. The accuracy of the retrieval algorithm depends on the minimum forest stand size included in the analysis. For managed forest plantations, multitemporal L-band SAR has some potential for detecting incremental biomass to support sustainable forest management.

A market-based conflict resolution approach for satellite mission planning

A market-based system has been developed to assist in mission planning for an Earth orbiting synthetic aperture radar mission. This approach was chosen over more traditional systems based on a functional model used to compare the market-based system with a human-expert, thematic and geometric approaches. The market-based system enables participants to signal demands for spacecraft resources needed for data acquisitions by establishing a currency and worth for a particular data acquisition request. We compared a serial draft approach to two market-based approaches; a simple market and a priority market. The market systems utilized a planning period which ended when the value of the time-ordered data acquisition plan did not increase by 10% of the value of the previous round. The market-based approaches were superior to a serial draft approach and that a priority market had a 2% greater value than a simple market.

Investigation of multifrequency/multipolarization radar signatures of rain cells over the ocean using SIR‐C/X‐SAR data

Radar signatures of rain cells are investigated using multifrequency/multipolarization synthetic aperture radar (SAR) images acquired from the space shuttle Endeavour during the spaceborne imaging radar‐C/X‐band SAR (SIR‐C/X‐SAR) missions in April and October 1994. In SIR‐C/X‐SAR images, radar signatures of rain cells over the ocean usually consist of irregularly shaped bright and dark patches that strongly depend on radar frequency and polarization. The radar signatures of rain cells observed in SIR‐C/X‐SAR imagery of the ocean originate from (1) the scattering and attenuation of the microwaves by raindrops and ice particles in the atmosphere and (2) the modification of the sea surface roughness induced by the impact of raindrops and by wind gusts associated with rain cells. Raindrops impinging on the sea surface generate ring waves, which enhance the sea surface roughness, but they also generate turbulence in the upper water layer, which reduces the sea surface roughness. Depending on the radar wavelength, ocean areas struck by rain can have higher or lower normalized radar cross section (NRCS) than the surrounding rain‐free area; in ocean areas where heavy rain is impinging on the sea surface, the X‐ and C‐band NRCS is usually enhanced, and the L‐band NRCS is reduced. From the phase difference between the horizontally and vertically copolarized signals, estimates of the rain rate are obtained. The present analysis shows further that the presently used wind speed retrieval algorithms for the scatterometers aboard the ERS and ADEOS satellites may yield biased wind fields if several rain cells lie within a scatterometer resolution cell.

Image reconstruction from Fourier transform magnitude with applications to synthetic aperture radar imaging

The problem of reconstructing an unknown signal starting from the knowledge of only magnitude information about its Fourier transform is addressed. To this end a phase retrieval (PR) method, based on the inversion of a quadratic operator, is proposed, presented, and discussed. The main feature of the approach is the use of a square amplitude distribution rather than an amplitude distribution. In the lack of any a priori information about the phase to be recovered, the success of the method is related to the availability of a sufficiently large ratio between the dimension of data and the dimension of unknowns; the retrieval procedure could converge to a meaningless trap when this ratio is not large enough. It should be noted, however, that the proposed method has a range of global effectiveness wider than that of previous approaches. Moreover, when available, the use of a priori information, such as the knowledge of the support of the scene to be imaged or the knowledge of a part of the scene, allows one to achieve accurate final images starting from a completely random initial guess of the image values. We apply our PR-based method to a synthetic aperture radar (SAR) case. As is well known, random motion of the platform and/or propagation effects in turbulent random media can affect the correct phase synchronization of the received SAR raw data needed for well-focused SAR images. As a matter of fact, even relatively small phase errors greatly impair the quality of the image. Under common conditions phase errors on the received signal affect just the phase of the Fourier transform of the intensity image without impairing its amplitude. The proposed technique is able to compensate these phase errors by retrieving the phase of the Fourier transform of the image intensity. We also present several experiments performed either on numerically simulated data or on actual data relative to an airborne SAR mission that show the effectiveness of the proposed PR method.

SAR data focusing using seismic migration techniques

The focusing of synthetic-aperture-radar (SAR) data using migration techniques quite similar to those used in geophysics is treated. The algorithm presented works in the omega -k/sub x/ domain. Because time delays can be easily accommodated with phase shifts that increase linearly with omega , range migration poses no problem. The algorithm is described in plane geometry first, where range migration and phase history can be exactly matched. The effects of the sphericity of the Earth, of the Earth's rotation, and of the satellite trajectory curvature are taken into account, showing that the theoretically achievable spatial resolution is well within the requirements of present day and near future SAR missions. Terrestrial swaths as wide as 100 km can be focused simultaneously with no serious degradation. The algorithm has been tested with synthetic data, with Seasat-A data, and with airplane data (NASA-AIR). The experimental results fully support the theoretical analysis.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Mapping small elevation changes over large areas: Differential radar interferometry

A technique that uses synthetic aperture radar (SAR) images to measure very small (1 cm or less) surface motions with good resolution (10 m) over large swaths (50 km) is presented along with experimental results. The method could be used for accurate measurements of many geophysical phenomena, including swelling and buckling in fault zones, residual displacements from seismic events, and prevolcanic swelling. The method is based on SAR interferometry, where two images are made of a scene by simultaneously flying two physically separated antennas. Then the phases of corresponding pixels are differenced, and altitude formation is deduced from some simple computation and image rectification. It is also possible to use one antenna flown twice over the same scene; then, if the second flight exactly duplicates the track of the first, an interesting possibility occurs. There would be no phase changes between the images at all unless there was a physical change in the scene, such as ground swelling, that would alter the distance from some resolution element to the antenna. Since the phase changes all occur at the short carrier wavelength, the basic limitation on sensitivity is only the phase noise in the system. When the two imaging passes are made from flight tracks that are separated (which is the case with the Seasat images used here), it is no longer possible to distinguish surface changes from the parallax caused by topography. However, with some additional computation, a third image made at some other baseline may be used to remove the topography and leave only the surface changes. This method was applied using Seasat data to an imaging site in Imperial Valley, California, where motion effects were observed that were ascribed to the expansion of water‐absorbing clays. Phase change images of this area are shown, along with associated ground truth about the presence of water. Problems with the technique are explored, along with a discussion of future experimental possibilities on upcoming SAR missions like Earth Observing System (EOS), Earth Resources Satellite (ERS 1), SIR‐C, and the Venus imaging radar, Magellan.

On the Estimation of Wave Slope-and Height-Varnance Spectra from SAR Imagery

A procedure is described for using synthetic aperture radar (SAR) imagery to estimate two-dimensional ocean wave slope-and height-variance spectra. The logic underpinning the procedure is based both on the results of the numerical simulation of SAR wave imagery and analytic descriptions of the SAR imaging process. The procedure, when applied to SAR imagery of waves acquired during the recent Shuttle Imaging Radar Mission (SIR-B), is shown to produce spectra that agree with independent measures of both the two-dimensional slope-and height-variance spectra. The implications of these results for future SAR missions aimed at measuring ocean waves are considered.

Concepts and techniques for real-time optical synthetic aperture radar data processing

The paper discusses the processing of synthetic-aperture-radar (SAR) data from an optical perspective, with emphasis on real-time operations. To appreciate the problem of storing and processing the large quantity of data derived from recent, and projected, satellite-borne SAR missions a brief review of optical and digital techniques is presented. It is evident that there are still severe difficulties to be overcome in achieving real-time processing of satellite-borne SAR data with present digital electronic systems. In the paper several specific optical solutions to the data processing bottleneck are examined in detail, all are based on charge coupled devices (CCDs) configured to perform multichannel processing integrated with a variety of acoustooptic architectures. These systems have the potential to rival the performance of present digital electronic systems and offer substantial reductions in size, weight and power consumption. With future increases in SAR data throughput requirements in a wide range of applications it is argued that both present and future optical systems can offer a cost-effective processing facility which may complement digital electronic computers.