Analysis Methods of Errors (Motion and Atmospheric) in Synthetic Aperture Radar (SAR) Images

A method to allow the analysis of the effects of motion and atmospheric errors in SAR images is here presented. The objective of the method is to allow the visualization of the effects of motion errors and atmospheric artefacts on the processed (focused) SAR image. The method is intended to allow the analysis of the interaction of motion and atmospheric errors with the adopted SAR processing procedure and motion compensation algorithms. In this article the analysis method has been applied and tested to a C-Band E-SAR (DLR airborne SAR system) data set where we see that the effects of linear and non-linear phase errors observed are in agreement with the theory

On the Gersgorin Theorem applied to Radar Polarimetry

This contribution is concerned with the mathematical formulation and theoretical background of the Ge

Propagation Effects in Satellite mounted Radar Remote Sensing

Abstract—Active microwave SAR imaging of the Earth’s surface is commonly considered to be of all weather capability. However, as the operating frequencies of SAR-systems are increasing, visible image distortions due to heavy precipitation in SAR-images may become present. This holds especially for the case of convective rain events imaged at X–band frequencies and beyond. These rain-cell signatures are thoroughly investigated, and the physical background of the related propagation effects is provided. A review of rain-cell signatures from former missions like SIR-C/X-SAR and the Shuttle Radar Topography Mission are provided. Furthermore, the German spaceborne satellite TerraSAR-X delivered several measurements, which facilitate to study precipitation effects in SAR-images. Based on this SAR-images and simultaneously acquired weather radar measurements, a quantitative estimation of precipitation effects in SAR-images is presented. In a further step, an attempt is made to extrapolate the effects observed in X-band SAR images to images acquired at higher nominal frequency bands such as Ka-band

Investigation of Propagation Effects for SIGNAL (SAR FOR ICE, GLACIER AND GLOBAL DYNAMICS)

SIGNAL is an innovative Earth explorer mission proposal with the main objective to accurately quantify, study, and characterise topographic changes in polar regions (ice masses) and fast flowing glaciers in mountainous areas like the Alps and Himalayan regions. In contrast to Earth observation using sensors in the visible- or infrared regime of the electromagnetic spectrum, the proposed system uses microwaves which are often considered to possess the advantages of both day/night and all weather operational capabilities. Whereas the first argument is true since we are dealing with an active sensor; the second does not hold in cases for which the operating frequencies are above ~3 GHz. Indeed, the SAR performance can be significantly affected by atmospheric effects (losses), especially at unfavourable weather conditions. The principal reason for the restriction on the use of higher frequencies can be found in clear air losses (water vapour and oxygen), cloud attenuation and attenuation due to precipitation, primarily rain. The specific attenuation through rain depends on a number of parameters like the frequency, the polarization, the dropsize distribution (DSD), and rain rate. Attenuation through rain increases with frequency and for the intended frequency of 35 GHz (Ka-band) values of 5 dB/km under rain conditions of 20 mm/hr are prevalent. Depending on the location on Earth, the probability for precipitation events differs to a large extent. At higher latitudes and especially at the polar region the probability is extremely low. Thus, it is valid to conclude that only gaseous attenuation and cloud/fog attenuation will contribute to the atmospheric attenuation budget. For non-polar regions such as Europe, the influence of attenuation due to rain has to be taken into account for a certain amount of data which is far less than 10 %. In order to show how frequently measurements will be effected, pertaining statistical information is provided. The annual path attenuation versus the probability is shown for 3 different climatic regions; the polar regions, Europe and tropical regions, for comparison. As expected, the polar regions are effected the least, whereas European and Tropical regions suffer from higher yearly attenuation percentages. From an interferometric point of view, signals traversing from Earth orbiting satellites to ground are subject to delays caused by the fact that the refractive index in the atmosphere differs from one. The tropospheric delay depends on parameters like the temperature, humidity, air pressure and varies with the height of the surface relative to the platform. The main advantage of a single pass interferometric configuration considered for SIGNAL is that the time between the two acquisitions is far less than the atmospheric decorrelation time. This means that the propagation path does not significantly change between two acquisitions like in dual-pass measurement scenarios. The atmospheric budget of SIGNAL is investigated in detail and conclusions for the proposed mission are drawn

Study of Tropospheric Propagation Effects in Sace-borne SAR Remote Sensing: Lates Results from TerraSAR-X

TerraSAR-X, the first civil German space-borne synthetic aperture radar (SAR) satellite, has been successfully launched on 15th of June, 2007. The main purpose of Synthetic aperture radar systems is to map the Earth-surface in high resolution and synthetic aperture radars are often considered as day/night and all-weather imaging systems. Whereas the first argument is true, the second does not hold in every case depending on the operating frequencies for the applied system. Indeed, recent examples of typical rain-induced signature modification have been recorded with the X-band TerraSAR-X system. It is well known that the specific attenuation of the signals may be around 1 dB/km assuming a rain-rate of 40 mm/hr and such an occurrence may frequently take place for tropical areas over rain-forest. Attenuation up to 20 dB and beyond may occur through the precipitation volumes in the cases of heavy precipitation, such as the Brazilian rainforest. However, as will be shown, even the northern latitudes are vulnerable to precipitation induced distortions. During the commissioning phase, a total of 12000 SAR-images (scenes) have been investigated for potential “propagation effects” and some scenes have been selected that revealed visible atmospheric effects. In this contribution, we will present a particularly interesting example acquired over New York, where the SAR image will be compared with weather-radar data acquired nearly simultaneously (within the same minute). The comparison of the images show a good overall agreement and it can be clearly shown that reflectivities in the weather radar image of 50 dBz may cause visible artefacts in the SAR images. In addition, several recent examples of SAR-images with from precipitation induced signatures around the globe will given

An Investigation on Atmospheric Effects in Airborne Interferometric SAR data

The results of an experiment on atmospheric effects in interferometric SAR data are presented. The main purpose of the experiment was to investigate on the influence of the troposphere on airborne synthetic aperture radar measurements, acquired by DLR’s experimental E-SAR system. The analysed data was acquired at L-band. The main idea behind the experiment is to collect data sets at different atmospheric conditions and to compare the measurements by performing a differential interferometric analysis. The main difference between atmospheric conditions on “Day-One” and “Day-Two” of acquisition was a cloud layer between sensor and illuminated surface, reaching from 750 to 1500 m above sea-level (msl). The sensor altitude was about 4000 m (msl). The test site is located at 580 m (msl). The results of the experiment will be highlighted and an interpretation of the observed differential effects will be given. In a first investigation [1] no pronounced indication of atmospheric effects in L-band interferograms was found. In the paper here proposed, two different techniques, multisquint [2] and weighted phase curvature autofocus (WPCA) [3], are applied to accurately mitigate residual motion errors allowing a better interpretation of any possible atmospheric effects. [1] A. Danklmayer and K.A.C. de Macedo. An experiment on atmospheric effects in airborne interferometric SAR data, International Symposion on Antennas and Propagation (ISAP), Toki Messe, Niigata, Japan, 2007. [2] Prats, P. and Reigber, A. and Mallorqui, J. J.. Interpolation-Free Coregistration and Phase-Correction of Airborne SAR Interferograms. IEEE Geoscience and Remote Sensing Letters, vol. 1, no. 3, pp. 188-191, Jul. 2004. [3] K.A.C de Macedo, R. Scheiber and A. Moreira. An autofocus approach for residual motion errors with application to airborne repeat-pass SAR interferometry. IEEE Transactions on Geoscience and Remote Sensing, under review

Signatures Of Extended Meterological Targets Measured With The Space-borne Synthetic Aperature Radar TerraSAR-X And Their Comparison With Simultaneous Weather Radar Measurements

TerraSAR-X, the first civil German space-borne synthetic aperture radar (SAR) satellite, has been successfully launched on 15th of June, 2007. The main purpose of Synthetic aperture radar systems is to map the Earth-surface in high resolution and synthetic aperture radars are often considered as day/night and all-weather imaging systems. Whereas the first argument is true, the second does not hold in every case depending on the operating frequencies as will be shown in more detail. Indeed, recent examples of typical rain-induced signature modification have been recorded with the X-band TerraSAR-X system. It is well known that the specific attenuation of the signals may be around 1 dB/km assuming a rain-rate of 40 mm/hr and such an occurrence may frequently take place for tropical areas over rain-forest. Attenuation up to 20 dB and beyond may occur through the precipitation volumes in the cases of heavy precipitation, such as the Brazilian rainforest. However, as will be shown, even the northern latitudes are vulnerable to precipitation induced distortions. In Effect, the disadvantage of rain features in SAR imagery may turn out to be a useful source for assessing precipitation intensity over SAR surveyed areas. For instance, there is a great potential in quantifying precipitation over oceanic surfaces, a problem hitherto only poorly addressed. During the commissioning phase, a total of 12000 SAR-images (scenes) have been investigated for potential “propagation effects” and some scenes have been selected that revealed visible atmospheric effects. In this contribution, we will present recent example of rain cell signatures in SAR-images and we will focus on a particularly interesting example acquired over New York, where the SAR image will be compared with weather-radar data acquired nearly simultaneously (within the same minute). The comparison of the images show a good overall agreement and it can be clearly shown that reflectivities in the weather radar image of 50 dBz may cause visible artefacts in the SAR images. Due to the polarimetric capabilities of TerraSAR-X it was further possible to visualize the polarimetric behaviour of the signal attenuation through extended meteorological targets. The main advantage of assessing weather volumes, using the SAR-method presented here, is the high resolution obtained indirectly from the analysis of propagation effects in SAR-pixels. The methodology permits us to exploit the high ground-resolution of SAR systems (typically 1 m x 1 m) in order to obtain precipitation mapping bearing similar resolutions. The proposed algorithm suggests further studies exploiting data fusions of space-borne SAR measurements with space-borne weather radar measurements in order to improve the intensity mapping of precipitating clouds over Earth-surface

Precipitation Effects for Ka-band SAR

Space-borne Synthetic Aperture Radar (SAR) imaging is often considered to possess both day/night and all weather operational capabilities. Whereas the first argument is true since we are dealing with an active sensor; the second does not hold in cases for which the operating frequencies are above ~ 3 GHz. Indeed, the SAR performance can be significantly affected by atmospheric effects (losses), especially at unfavourable weather conditions. The principal reasons for the restriction on the use of these higher frequencies can be found in clear air losses (water vapour and oxygen), cloud attenuation and attenuation due to precipitation, primarily rain. The scope of this paper covers the activities performed for an ESA-study (Danklmayer, 2009 [1]) under Task 1 (Survey on Precipitation Effects for SAR) and Task 2, the quantitative assessment of the propagation effects and the associated modelling. For the Ka-band rain rates of 10 mm/h are capable to produce visible artefacts. Depending on the climatic region on Earth, the availability of the investigated Ka-band system will vary. Assuming a 5 dB acceptance of the attenuation due to rain, which corresponds to 2 mm/h at 30° incidence angle for the modelled rain cell, the availability will be better than 98% for the European regions and better than 95% for rain-forest in Brazil. Finally, measures and possibilities are suggested, how to mitigate propagation effects in SAR images, together with an outlook on remaining issues to be addressed in future studies

Precipitation Induced Signatures in SAR Images

Active microwave SAR imaging of the Earth’s surface is commonly considered to be of all weather capability. However, as the operating frequencies of SAR-systems are increasing, visible image distortions due to heavy precipitation in SAR-images are present. This holds especially for the case of convective rain events imaged at X–band frequencies and beyond. The German spaceborne satellite TerraSAR-X delivered several measurements, which facilitate to study precipitation effects in SAR-images. Based on this SAR-images and simultaneously acquired weather radar measurements, a quantitative estimation of precipitation effects in SAR-images is presented. In a further step, a first attempt is made to extrapolate the effects observed in X-band SAR images to images acquired at higher nominal frequency bands

Pricipitation Effects for X- and Ka-band SAR

Space-borne Synthetic Aperture Radar (SAR) imaging is often considered to possess both day/night and all weather operational capabilities. Whereas the first argument is true since we are dealing with an active sensor; the second does not hold in cases for which the operating frequencies are above 3 GHz. Indeed, the SAR performance can be significantly affected by atmospheric effects (losses), especially at unfavourable weather conditions. The principal reasons for the restriction on the use of these higher frequencies can be found in clear air losses (water vapour and oxygen), cloud attenuation and attenuation due to precipitation, primarily rain