Franz J. Meyer

The Potential of Low-Frequency SAR Systems for Mapping Ionospheric TEC Distributions

Ionospheric propagation effects have a significant impact on the signal properties of low-frequency synthetic aperture radar (SAR) systems. Range delay, interferometric phase bias, range defocusing, and Faraday rotation are the most prominent ones. All the effects are a function of the so-called total electron content (TEC). Methods based on two-frequency global positioning system observations allow measuring TEC in the ionosphere with coarse spatial resolution only. In this letter, the potential of broadband L-band SAR systems for ionospheric TEC mapping is studied. As a basis, the dispersive nature of the ionosphere and its effects on broadband microwave radiation are theoretically derived and analyzed. It is shown that phase advance and group delay can be measured by interferometric and correlation techniques, respectively. The achievable accuracy suffices in mapping small-scale ionospheric TEC disturbances. A differential TEC estimator that separates ionospheric from tropospheric contributions is proposed

Prediction, Detection, and Correction of Faraday Rotation in Full-Polarimetric L-Band SAR Data

With the synthetic aperture radar (SAR) sensor PALSAR onboard the Advanced Land Observing Satellite, a new full-polarimetric spaceborne L-band SAR instrument has been launched into orbit. At L-band, Faraday rotation (FR) can reach significant values, degrading the quality of the received SAR data. One-way rotations exceeding 25 deg are likely to happen during the lifetime of PALSAR, which will significantly reduce the accuracy of geophysical parameter recovery if uncorrected. Therefore, the estimation and correction of FR effects is a prerequisite for data quality and continuity. In this paper, methods for estimating FR are presented and analyzed. The first unambiguous detection of FR in SAR data is presented. A set of real data examples indicates the quality and sensitivity of FR estimation from PALSAR data, allowing the measurement of FR with high precision in areas where such measurements were previously inaccessible. In examples, we present the detection of kilometer-scale ionospheric disturbances, a spatial scale that is not detectable by ground-based GPS measurements. An FR prediction method is presented and validated. Approaches to correct for the estimated FR effects are applied, and their effectiveness is tested on real data.

Performance Requirements for Ionospheric Correction of Low-Frequency SAR Data

In recent years, significant progress has been made in developing theory and methods for modeling, detecting, and correcting ionospheric effects in low-frequency synthetic aperture radar (SAR) data. While a large number of correction methods have been developed that differ in sensitivity, data needs, and spatiotemporal accuracy, a lack of performance requirements for ionospheric correction has prevented an evaluation of their suitability for operational implementation. Hence, this paper focuses on the development of performance requirements for the correction of ionospheric effects in low-frequency SAR data. The requirements are derived considering the data quality needs of a set of SAR applications and will ensure the SAR data after ionospheric correction to meet calibration specifications and maintain full performance during all ionospheric conditions. The proposed requirements can serve as a benchmark for a performance assessment of ionospheric correction methods and will help define their suitability for operational implementation. Requirements are determined for SAR polarimetry, SAR imaging, SAR interferometry, and ionospheric research.

Correction and Characterization of Radio Frequency Interference Signatures in L-Band Synthetic Aperture Radar Data

Radio frequency interference (RFI) is a known issue in low-frequency radar remote sensing. In synthetic aperture radar (SAR) image processing, RFI can cause severe degradation of image quality, distortion of polarimetric signatures, and an increase of the SAR phase noise level. To address this issue, a processing system was developed that is capable of reliably detecting, characterizing, and mitigating RFI signatures in SAR observations. In addition to being the basis for image correction, the robust RFI-detection algorithms developed in this paper are used to retrieve a wealth of RFI-related information that allows for mapping, characterizing, and classifying RFI signatures across large spatial scales. The extracted RFI information is expected to be valuable input for SAR-system design, sensor operations, and the development of effective RFI-mitigation strategies. The concepts of RFI detection, analysis, and mapping are outlined. Large-scale RFI mapping results are shown. In case studies, the benefit of detailed RFI information for customized RFI filtering and sensor operations is exemplified.

The feasibility of traffic monitoring with TerraSAR-X - analyses and consequences

This paper analyzes the potential of the upcoming German satellite mission TerraSAR-X to monitor traffic from space. As it is well-known, an object moving with a velocity deviating from the assumptions incorporated in the focusing process will generally appear both displaced and blurred in azimuth direction. To study the impact of these (and related) distortions in focused SAR images, the analytic relations between an arbitrary moving point scatterer and its conjugate in the SAR image have been derived and adapted to the TerraSAR-X specifications. To be able to monitor traffic under these boundary conditions in real-life situations, a specific detection strategy is proposed. This strategy makes use of knowledge derived from external sources, as e.g. GIS and semantic models for traffic flow.

The Impact of the Ionosphere on Interferometric SAR Processing

The impact of ionospheric propagation effects on the signal properties of SAR systems is significant and increases with decreasing carrier frequency. Besides polarimetric applications, also interferometric SAR processing can be significantly affected. Relative range shifts, internal image deformations, range and azimuth blurring, and interferometric phase errors are the most significant effects to be considered. In this paper we provide the theoretical background for ionospheric effects on InSAR. We quantify expected magnitudes of the respective effects for various existing SAR sensors and discuss methods for their detection and correction. Real data examples, mainly stemming from the ALOS PALSAR mission, are presented to verify the derived theory.

A review of ionospheric effects in low-frequency SAR — Signals, correction methods, and performance requirements

Ionospheric signal distortions are commonplace in low-frequency space-borne SAR observations and can lead to the degradation of SAR data quality and data consistency if no signal compensation is applied. In this paper we will give an overview of the problem of ionospheric influence in SAR, PolSAR, and InSAR data. We will characterize the spatio-temporal signal properties of ionospheric signals, introduce a selection of currently available correction methods, and present a list of performance requirements to be met by ionospheric correction.

Towards traffic monitoring with TerraSAR-X

This article presents an overview of the traffic monitoring project initiated in the context of the TerraSAR-X mission and the German Aerospace Center (DLR) transportation research program. A short description of the TerraSAR-X instrument is presented, followed by an analysis of the necessary TerraSAR-X system settings for traffic monitoring and an assessment of the expected image quality. Based on a brief revision of the theory of moving object effects in synthetic aperture radar (SAR) images and a quantification of these effects for the TerraSAR-X case, an outline is presented of the processing system that is currently implemented to detect moving vehicles and estimate their velocities. Special emphasis is placed on the integration of a priori information derived from road databases for improving detection rates and velocity estimation accuracy.

The Influence of Equatorial Scintillation on L-Band SAR Image Quality and Phase

It is well known that many of the nighttime acquisitions of the L-band Advanced Land Observing Satellite (ALOS) Phased Array-type L-band Synthetic Aperture Radar (PALSAR) instrument over equatorial regions show significant distortions of the image amplitude information. These distortions have the form of amplitude stripes that are roughly aligned with the local geomagnetic field. While ionospheric scintillation has been identified as the source of these distortions, the exact nature of the induced artifacts on synthetic aperture radar (SAR) image quality and SAR signal phase has not yet been studied in sufficient detail. Hence, this paper provides a quantitative analysis of equatorial scintillation effects on SAR image quality and SAR phase. We have performed a statistical analysis of ALOS PALSAR images over equatorial regions to describe the observed distortions and relate them to ionospheric parameters. An ionospheric simulator was developed and validated that is capable of simulating ionospheric distortions based on ionospheric scintillation parameters. Using this simulator, we found that ionospheric scintillation in the equatorial zone can cause significant distortions of SAR image amplitudes, image focus, and SAR signal phase. We determined threshold ionospheric environmental conditions that lead to the formation of these image distortions. Based on these thresholds, we quantified the likelihood of occurrence of ionospheric distortions for the global equatorial belt and for L-band sensors ALOS PALSAR, ALOS-2 PALSAR-2, and NASA-ISRO SAR (NISAR).

Ionospheric effects in SAR interferometry: An analysis and comparison of methods for their estimation

For spaceborne SAR (Synthetic Aperture Radar) systems, the dispersive effects of the ionosphere on the propagation of the SAR signal can be a significant source of phase error. While at X-band frequencies the effects are small, current and future P-, L- and C-band systems would benefit from ionospheric compensation to avoid errors in topographic retrieval. In this paper the focus is on the effects of the ionosphere on repeat-pass SAR interferometry from P- through X-bands and methods for their estimation which are demonstrated on L-band ALOS-PALSAR acquisitions1.