Hierarchical Bayesian Data Analysis in Radiometric SAR System Calibration: A Case Study on Transponder Calibration with RADARSAT-2 Data

Döring, Björn; Schmidt, Kersten; Jirousek, Matthias; Rudolf, Daniel; Reimann, Jens; Raab, Sebastian; Antony, John Mohan Walter; Schwerdt, Marco

doi:10.3390/rs5126667

Cited by 14 publications

(12 citation statements)

References 17 publications

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“…Different methods were analytically compared [8] and conducted in order to increase calibration confidence through cross-comparisons. Thus, a standard uncertainty of 0.2 dB has been achieved [9][10][11]. • DLR's remote controlled trihedral corner reflector is shown in Figure 2b.…”

Section: Introductionmentioning

confidence: 99%

Independent System Calibration of Sentinel-1B

et al. 2017

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Sentinel-1B is the second of two C-Band Synthetic Aperture Radar (SAR) satellites of the Sentinel-1 mission, launched in April 2016-two years after the launch of the first satellite, Sentinel-1A. In addition to the commissioning of Sentinel-1B executed by the European Space Agency (ESA), an independent system calibration was performed by the German Aerospace Center (DLR) on behalf of ESA. Based on an efficient calibration strategy and the different calibration procedures already developed and applied for Sentinel-1A, extensive measurement campaigns were executed by initializing and aligning DLR's reference targets deployed on the ground. This paper describes the different activities performed by DLR during the Sentinel-1B commissioning phase and presents the results derived from the analysis and the evaluation of a multitude of data takes and measurements.

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Section: Introductionmentioning

confidence: 99%

Independent System Calibration of Sentinel-1B

et al. 2017

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“…Although this value does not exclusively explain the difference between the three-transponder method and NWA curves, the sign and the general trend is certainly correct. Despite the encouraging findings for this first demonstration campaign, the derived combined RCS standard uncertainty of 0.38 dB is rather large in comparison to other measurement methods (e.g., 0.2 dB for measurements with corner reflectors (main uncertainty contributors) exploiting satellite overpasses, see [5]). Yet there is no reason in ascribing the uncertainty to the three-transponder method itself.…”

Section: Discussion Of Measurement Resultsmentioning

confidence: 65%

“…During this time, the transponder gain varies slightly despite the thermal control. Previous repeated measurements have shown that the transponder gain is stable within a standard uncertainty of 0.02 dB during 1 h intervals [5].…”

Section: Gain Drift Errorsmentioning

confidence: 91%

“…The most typical ones depend on (possibly compact) indoor or outdoor measurement ranges [14]. Alternatively, one can also use a SAR instrument to directly measure the transponder's equivalent radar cross section (ERCS) [3], an approach which is described in detail in [5]. The advantages of the approach are the end-to-end nature of the measurement setup.…”

Section: Review Of Existing Transponder Rcs Calibration Strategiesmentioning

confidence: 99%

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The Three-Transponder Method: A Novel Method for Accurate Transponder RCS Calibration

Döring¹,

Reimann²,

Raab³

et al. 2014

PIER B

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Abstract-Transponders (also known as polarimetric active radar calibrators or PARCs) are commonly used for radiometric calibration of synthetic aperture radars (SARs). Currently three methods for the determination of a transponder's frequency-dependent radar cross section (RCS) are used in practice. These require either to measure disassembled transponder components, or a separate radiometric measurement standard (like a flat, metallic plate or a corner reflector), leading to additional uncertainty contributions for the calibration result. In this paper, a novel method is introduced which neither requires disassembly nor an additional radiometric reference. Instead, the measurement results can be directly traced back to a realization of the meter, lowering total measurement uncertainties. The method is similar in approach to the well known three-antenna method, but is based on the radar equation instead of Friis transmission formula. The suitability of the method is demonstrated by a measurement campaign for DLR's three new Kalibri C-band transponders, completed by an uncertainty analysis. The method is not universally applicable for all transponder calibrations because (a) three devices are necessary (instead of only one for the known methods), and (b) the transponders must provide certain additional features. Nevertheless, these features have become standard in modern SAR calibration transponder designs. The novel, potentially more accurate three-transponder method is thus a viable alternative for transponder RCS calibration, ultimately contributing to synthetic aperture radars with a reduced radiometric measurement uncertainty.

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“…In Döring et al [10], a new method is applied for the systematic derivation of calibration uncertainties of synthetic aperture radars. This is an important step toward traceable radiometric calibration of these systems.…”

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confidence: 99%

Calibration and Verification of Remote Sensing Instruments and Observations

Müller

2014

Remote Sensing

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Satellite instruments are nowadays a very important source of information. The physical quantities (essential variables) derived from satellites are utilized in a wide field of applications, in particular in atmospheric physics and geoscience. In contrast to ground measurements the physical quantities are not directly measured, but have to be retrieved from satellite observations. Satellites observe hereby the reflection or emission of radiation by the Earth's surface or atmosphere, which enables the retrieval of respective physical quantities (essential variables). The physical basis for the retrieval is the interaction of the radiation with the Earth's atmosphere and surface. This interaction is defined by radiative transfer, which favors the use of radiances and their respective units within retrieval methods.However, the primary measurement quantity (unit) of the sensors consists of voltage or digital counts. Hence, calibration has to be applied in order to relate the digital counts (voltage) given by the sensor to the incoming radiances, consequently, the physical units of interest. The relation between the digital counts and the radiances (calibration coefficients) can be derived by comparison of the sensor signal with an absolute standard reference prior to launch. Nowadays, satellite instruments are usually well designed and calibrated prior to launch. Unfortunately, no matter how sophisticated the instruments are, once in space they degrade with time, e.g., due to thermal, mechanical or electrical effects or exposure to UV radiation. Yet, for the majority of remote sensing retrievals and applications calibrated data are essential. Sensor re-calibration (hereafter referred to as calibration) is therefore the basis of reliable remote sensing and ensuring the quality of the derived variables and products. However, after-launch comparison with reference standards of known accuracy is difficult to manage in terms of metrology. On-board calibration units being interpreted as the reference standard are subject to degradation processes as well, especially in the visible spectra. Moreover, many satellite sensors (especially in the visible spectra) are not adjusted by on-board calibration. Hence, in any case, post launch calibration is quite a challenging task, but is essential for physically defined retrieval of essential variables.

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Hierarchical Bayesian Data Analysis in Radiometric SAR System Calibration: A Case Study on Transponder Calibration with RADARSAT-2 Data

Cited by 14 publications

References 17 publications

Independent System Calibration of Sentinel-1B

Independent System Calibration of Sentinel-1B

The Three-Transponder Method: A Novel Method for Accurate Transponder RCS Calibration

Calibration and Verification of Remote Sensing Instruments and Observations

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