Radar Stereo Research Articles

Improvement of the spaceborne synthetic aperture radar stereo positioning accuracy without ground control points

AbstractCompared with optical remote sensing satellites, the geometric positioning accuracy of synthetic aperture radar (SAR) satellite is not affected by satellite attitude or weather conditions. SAR satellites can achieve relatively high positioning accuracy without ground control points, which is particularly important in global surveying and mapping. However, the stereo positioning accuracy of SAR satellites is mainly affected by the SAR systematic delay and the atmospheric propagation delay of radar signals. An iterative compensation method for the SAR systematic time delay is proposed based on digital elevation model to improve the stereo positioning accuracy of SAR satellites without control points. In addition, to address the non‐real‐time updates of external reference atmospheric param, an iterative compensation method to estimate the atmospheric propagation delay of radar signals is proposed based on standard atmospheric models. In this study, SAR images from the Gaofen‐3 (GF‐3) satellite with 5 m resolutions were used as experimental data to verify the effectiveness of our proposed method. Simultaneously, the 2D positioning accuracy was better than 3 m and increased by 42.9%, and the elevation positioning accuracy was better than 3 m and increased by 90.2%.

Forest biomass change estimated from height change in interferometric SAR height models.

BackgroundThere is a need for new satellite remote sensing methods for monitoring tropical forest carbon stocks. Advanced RADAR instruments on board satellites can contribute with novel methods. RADARs can see through clouds, and furthermore, by applying stereo RADAR imaging we can measure forest height and its changes. Such height changes are related to carbon stock changes in the biomass. We here apply data from the current Tandem-X satellite mission, where two RADAR equipped satellites go in close formation providing stereo imaging. We combine that with similar data acquired with one of the space shuttles in the year 2000, i.e. the so-called SRTM mission. We derive height information from a RADAR image pair using a method called interferometry.ResultsWe demonstrate an approach for REDD based on interferometry data from a boreal forest in Norway. We fitted a model to the data where above-ground biomass in the forest increases with 15 t/ha for every m increase of the height of the RADAR echo. When the RADAR echo is at the ground the estimated biomass is zero, and when it is 20 m above the ground the estimated above-ground biomass is 300 t/ha. Using this model we obtained fairly accurate estimates of biomass changes from 2000 to 2011. For 200 m2 plots we obtained an accuracy of 65 t/ha, which corresponds to 50% of the mean above-ground biomass value. We also demonstrate that this method can be applied without having accurate terrain heights and without having former in-situ biomass data, both of which are generally lacking in tropical countries. The gain in accuracy was marginal when we included such data in the estimation. Finally, we demonstrate that logging and other biomass changes can be accurately mapped. A biomass change map based on interferometry corresponded well to a very accurate map derived from repeated scanning with airborne laser.ConclusionsSatellite based, stereo imaging with advanced RADAR instruments appears to be a promising method for REDD. Interferometric processing of the RADAR data provides maps of forest height changes from which we can estimate temporal changes in biomass and carbon.Electronic supplementary materialThe online version of this article (doi:10.1186/s13021-014-0005-2) contains supplementary material, which is available to authorized users.

A Novel Generation Method of Spaceborne SAR Stereo Image Pair for Geolocation

Stereo SAR geolocation model is an important geolocation model for spaceborne synthetic aperture radar (SAR). Stereo SAR geolocation model need a stereo radar image pair whose generation have much influence on the geolocation accuracy. In this paper,a novel generation method of spaceborne SAR stereo image pair for geolocation is presented which could avoid the geolocation error caused by intersection angle error.Through computer simulation,the results verify the validity of the generation method.

Topographic data generated from Radarsat Images Over West Kalimantan, Indonesia

Indonesia is a tropical country which has dominant cloud coverage, and some of the area has cloud cover almost all year long. Mapping by optical sensors, especially with Photogrammetric method shows a very good result, but the main constraint is cloud cover conditions, and this was the weakness point of this method. This study describe the technique for deriving Topographic data from Radarsat Synthetic Aperture Radar stereo image pair and apply it to an image pair over West Kalimantan, Indonesia. This paper contains also brief discussion of the use of stereo SAR to derive Digital Elevation Model, the site condition and the source of validation data. The result shows that Radarsat data recommended only for 1 : 100.000 or smaller.

Radar stereo- and interferometry-derived digital elevation models: comparison and combination using Radarsat and ERS-2 imagery

In this experiment, we derive and compare radar stereo and interferometric digital elevation models (DEMs) of a study site in Djibouti, East Africa. A Radarsat stereo pair, as well as Radarsat and ERS-2 interferometric data, comprise the test images. The primary objective of the study was to analyse and compare the results obtained by the two techniques and explore possible synergisms between them. We find that in regions of high coherence, the DEMs produced by interferometry are of much better quality than the stereo result. However, the corresponding error histograms also show some pronounced errors due to decorrelation and phase-unwrapping problems on forested mountain slopes. On the other hand, the more robust stereo reconstruction, with an error standard deviation of 45 m, is able to capture the general terrain shape, although finer surface details are lost. In the second part of our experiment, we demonstrate that merging the stereoscopic and interferometric DEMs by applying a user-defined weighting function to a filtered coherence map can significantly improve the accuracy of the computed elevation maps.

Error tracking of radargrammetric DEM from RADARSAT images

In the 1960s, stereoscopic methods were first applied to radar images to derive ground elevation. Unfortunately, research uncovered contradictions between error propagation theory and practical results. These contradictions combined with the lack of stereo radar pairs led to the decline of radargrammetry. The launch in 1995 of RADARSAT, with its various operating modes and specific geometric characteristics, has turned the tide. The error propagation of the radargrammetric DEM generated from different RADARSAT stereo configurations is then quantitatively evaluated along the full processing chain (stereo model set-up with ground control points (GCPs), image matching and three-dimensional (3-D) intersection). Two matching algorithms are used: 1) automatic and 2) computer-assisted visual matching. The GCP collection method using stereoscopic plotting is a requisite to achieve the best results for the stereo model and DEM. The automatic matching gives slightly better results than the computer-assisted visual matching, except when the radiometric disparities in the stereo images are too large. Since the geometric advantage (not involved in the automatic matching) can compensate for the radiometric disadvantage, visual matching, which combines both aspects, is a better method in these conditions. Consequently, these two algorithms can be used in a two-step method to generate the best DEM whatever the stereo configuration. Since the relief is an important parameter in the final accuracy, geometric versus radiometric disparity tradeoffs and general guidelines are suggested for selecting RADARSAT stereo pairs for DEM generation as a function of terrain relief.

Opposite side ERS-1 SAR stereo mapping over rolling topography

Opposite-side radar stereo images have been considered unsuitable for stereo viewing due to illumination differences which limit the ability to identify the same features in the image pair. In some contexts, like a rolling topography (slope less than 10/spl deg/), the shadow, layover, and foreshortening effects, specific to radar images, will not be overwhelming with an opposite-side stereo pair. This paper reports on some issues of stereo viewing and plotting, as well as on quantitative results of mapping and features extraction from ascending and descending orbit ERS-1 SAR stereo images. Planimetric accuracy of 17 m and altimetric accuracy of 23.9 m have been achieved for lake shorelines and DEM extractions, respectively. Impacts of different parameters on the accuracy are also evaluated.

Determining heights and slopes of fault scarps and other surfaces on Venus using Magellan stereo radar

This report presents relationships and techniques for determining the heights and slopes of discretely dipping surfaces, such as normal fault scarps, on the surface of Venus from measurements of their widths in Magellan stereo synthetic aperture radar (SAR) images. These surfaces are clearly recognizable as distinct bands of increased or decreased radar backscatter relative to flat‐lying areas in the 120 to 280‐m resolution Magellan SAR images, but are not generally imaged in the >10‐km resolution Magellan altimetry. Our methods take into account radar distortion effects, and allow one to determine whether a slope is foreshortened, laid over, elongated, or in radar shadow. The techniques make use of graphs constructed for the Magellan incidence angle profiles so that investigators can determine the local height and slope of an individual surface in a straightforward manner, but the techniques are applicable to any stereo radar data set if the radar incidence angles are known. Additionally, the techniques can be used to improve digital elevation models constructed using radar stereoscopy in areas that have steep local terrain where stereoscopic fusion may be impossible due to the effects of layover or radar shadow. Using these techniques we show that there are normal fault scarps on Venus with heights in the 700‐m range that have remarkable topographic slopes of close to 60°, something unheard of on Earth and which suggests a high effective cohesive strength of the Venusian crust.

Proposal for a Spaceborne Dual-Beam Rain Radar with Doppler Capability

Abstract The potential characteristics and performances of a spaceborne dual-beam radar (or stereo radar) operating at 24 GHz, and devoted primarily to the retrieval of rain-rate structure by using the stereo-radar analysis, were presented in a previous study. This constitutes the starting point of the present paper, which analyzes the feasibility and scientific interest of adding a Doppler capability to the instrument. The constraints imposed by the Doppler mode are elaborated and discussed. Accordingly, the basic design and the expected performances of a low-altitude (≈ 500 km) spaceborne dual-beam Doppler radar are proposed. It is shown that a slight increase in system complexity is needed to perform significant additional Doppler measurements without jeopardizing the primary objective, that is, the quantitative measurement of rain at the global scale. The scientific interest for Doppler data from space is investigated. Two components of the air velocity can he determined from the dual-beam spaceborne ...

Radar Stereo Mapping with Crossing Flight Lines

RESUMEL'utilisation d'orbites polaires a des latitudes elevees combinee a des orbites inclinees peut se traduire, pour des missions de satellites radar, par des intersections de lignes de vol pouvant produire des couples stereoscopiques. La geometrie de cette configuration de visees est plutot unique et peut etre utilisee pour obtenir des mesures d'elevation topographiques. Une serie de donnees produites par la mission SIR de la navette spatiale a ete selectionnee en vue d'une evaluation numerique. Ces donnees radar ainsi que certaines simulations ont servi a mieux evaluer le potentiel de cette technique de mesure. L'auteur conclut que ce mode de production d'images radar sterescopiques est valable pour des applications a des missions de satellites radar a des latitudes elevees et pour des combinaisons d'orbites ascendantes et descendantes dans le cas d'orbites non polaires.

Model for optimal parallax in stereo radar imagery

Simulated stereo radar imagery is used to investigate parameters for a spaceborne Imaging radar. Incidence angles ranging from small to intermediate to large are used with three digital terrain model areas which are representative of relatively flat, moderately rough, and mountainous terrain. The simulated radar imagery was evaluated by interpreters for ease of stereo perception and information content, and rank ordered within each class of terrain. The interpreter's results are analyzed for trends between the height of a feature and either parallax or vertical exaggeration for a stereo pair. A model is developed which predicts the amount of parallax (or vertical exaggeration) an interpreter would desire for best stereo perception of a feature of a specific height. Results indicate the selection of angle of incidence and stereo intersection angle depend upon the relief of the terrain. Examples of the simulated stereo imagery are presented for a candidate spaceborne imaging radar having four selectable angles of incidence.

Simulation of Spaceborne Stereo Radar Imagery: Experimental Results

Computer simulation of radar imagery was used to investigate spaceborne stereo radar. The major conclusions reached are that stereo pairs representing different angles of incidence with the same look-direction appear best and that the angles of incidence and stereo intersection angle be matched to the slope and relief of terrain. A parametric tradeoff analysis was performed for eleven different angles of incidence over three different model areas. The eleven angles spanned the range from small-to large-incidence angles. The three model areas were selected as typical of relatively flat, moderately rough, and mountainous terrain. The stereo pair judged best for each model area are included.

Performance Comparison of Techniques for Obtaining Stereo Radar Images

The performance of three stereo radar techniques is compared with respect to both theoretical performance and performance in the presence of errors. The three techniques are: an improved single flight technique, a previously proposed single flight technique, and a two flight technique. Theoretical data for comparison of image parameters affecting perf ormance have been generated with computer simulations of the imaging geometries for the techniques. The comparison shows that the improved technique will have images with greater similarity resulting in improved stereoviewability and measurability. Computer simulations have also been used to generate sensitivities of computed terrain point coordinates to system errors. These sensitivities are shown and overall error comparisons are made for assumed error values. These comparisons show that the improved single flight technique has the best error performance of the three techniques for the assumed reasonable set of system errors. This technique is followed by the previously proposed single flight technique and then the two flight technique.

An Improved Single Flight Technique for Radar Stereo

An improved technique for obtaining stereo radar image pairs is described. The technique uses a unique combination of two different radar beam pattern characteristics. The resulting stereo radar images are illuminated from very nearly the same aircraft position which results in radar shadow and backscatter characteristics which are very similar and in improved image registration possibilities. The reduction in illumination angle and aircraft position differences is shown to be nearly an order of magnitude or more when compared with a previously proposed single flight technique which used two vertical fan beam patterns at different azimuth angles. As a related sidelight it is shown that this previous technique requlres the two fan beam patterns to generate parallax on the images and thus can not be implemented with synthetic arrays squinted at two different squint angles.