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The radiometric calibration of synthetic aperture radar (SAR) systems is typically based on the known backscatter of calibration point targets such as transponders. Before a SAR calibration campaign can begin it is therefore necessary to determine the backscatter (radar cross section or RCS) of the transponder itself. Known methods suffer from unnecessarily high uncertainty contributions and therefore also affect the radiometric uncertainty of the calibrated SAR system. In this paper we present a novel calibration approach which neither requires to disassemble the amplifier path nor necessitates a separate RCS calibration target such as a corner reflector or a circular disk. Significant sources of measurement uncertainties can therefore be outright avoided. The method is not applicable to all transponders though, and a certain type of transponder design is assumed. Nevertheless, direct metrological traceability to a realization of the SI base unit "meter" can be achieved for the first time. The reduced measurement uncertainties of the novel approach will subsequently allow to reduce the radiometric measurement uncertainties of air- and spaceborne SAR sensors in the future.

Polarimetric synthetic aperture radar (SAR) systems require verification and calibration of their polarimetric performance after launch. Dual-antenna calibration transponders with freely rotatable, linearly-polarized antennas are one of the most common calibration targets for this task. Because the transponders are used as a measurement standard, any misorientation of the polarization direction of the transponder antennas leads to characterization errors for the SAR instrument. In this paper, we present a novel calibration approach with which the polarization direction of linearly polarized antennas can be accurately determined; no additional reference antenna with a previously known polarization is required. The novel approach will reduce polarimetric transponder calibration errors in the future and will lead to a higher accuracy in calibrated, fully polarimetric SAR systems.

For the radiometric calibration of spaceborne synthetic aperture radar systems, active targets with known backscatter, so called transponders, are used. These serve as an external absolute reference, and the quality of the derived calibration parameters depends on the quality of the deployed transponders. Due to the temperature dependent behavior of the implemented active radio frequency components the transponder radar cross section is sensitive to the temperature of the components. This effect requires the implementation of a reliable and precise temperature management System to stabilize the temperature of the radio frequency components to a constant value under all relevant environmental conditions. In order to fulfill the increasing requirements for future transponders towards radiometric accuracy new temperature control concepts must be developed. For this purpose, existing temperature management systems are analyzed and a promising concept is investigated. In the final version of this paper the results of measurements in a climatic chamber are presented in order to analyze the performance of the new control concept.

Transponders are, besides trihedral corner reflectors, the most commonly used measurement standards in radiometric SAR calibration. They allow signal recording for the reconstruction of the azimuth pattern of the SAR system, adjustments of the backscattering matrix for polarimetric applications, and radar cross sections (RCSs) which are potentially much larger than those of passive point targets. These advantages led DLR to develop, manufacture, and install three new, accurate C-band “Kalibri” transponders in South Germany, which are now being used for the calibration and monitoring of the Copernicus Sentinel-1A satellite. Before the transponders could be used as radiometric measurement standards, they needed to be calibrated themselves. In an effort to find the most accurate RCS calibration approach for the given transponder design, several existing methods were compared [1], and a new, potentially highly accurate method, devised which exploits the specific design of the Kalibri transponders [2]. The new “three-transponder method” is similar in principle to the known “three-antenna method”, but is based on the radar equation instead of the Friis transmission formula. The approach exploits the fact that modern transponders like the “Kalibri” device can also be operated as radars because of the integrated digital sub-system (which is needed to implement a digital delay line and incorporates an AD and DA converter). To conduct a complete measurement, three transponders and three measurements (with one transponder pair each) are required; refined measurement schemas are also possible. In comparison to existing methods, no additional radiometric measurement standard is needed, which so far has been one of the limiting factors in accomplishing lower calibration uncertainties. Measurement traceability is achieved by tracing a comparatively simple length measurement back to a national realization of the meter. Such a length measurements can be performed with high accuracy. The presentation will include the setup and the measurement results of a first demonstration measurement campaign. Despite remaining challenges in the practical implementation, the uncertainty analysis shows that the method is a good candidate for highly accurate transponder RCS calibrations in the future.

The technological advancement of the synthetic aperture radar (SAR) principle leads to an innovative challenge for the calibration as well. In order to provide an active reference target for an accurate absolute radiometric calibration the knowledge of the target’s backscattering characteristics is essential. For the recently developed DLR C-band transponder named “Kalibri” several strategies for an accurate determination of the radar cross section (RCS) have been analyzed. Based on a comparison with respect to accuracy and feasibility, several recommendations for the best transponder calibration strategy were established. The resulting RCS of the transponders retrieved from the most suitable measurement method is presented as well as a cross-validation to prove the plausibility of these results.

Spaceborne synthetic aperture radars (SAR) are systems capable of acquiring high-resolution earth-observation data independent on time of day and weather. These SAR systems have to be calibrated before first use and at regular intervals. For these radiometric calibrations active targets with known backscatter, so called transponders, serve as an external reference. For determining the frequency-dependent radar cross section (RCS) of these transponders, different measurement methods are available. This paper gives an overview of four different strategies with a following uncertainty analysis according to the Guide to the expression of uncertainty in measurement (GUM). After comparison with respect to accuracy and feasibility, a recommendation for the best absolute radiometric transponder calibration method is given.

The progressive development of upcoming satellite missions using synthetic aperture radar (SAR) lead to novel challenges also for the reference targets. In order to provide a reference target for an accurate absolute radiometric calibration the knowledge of the backscattering characteristics is essential. Whereas usually one single measurement is used for the estimation of the radar cross section (RCS), this paper presents two conducted, independent methods to derive the RCS of three C-band transponder leading to cross-validated results based on a compatibility analysis.

Since the conception of synthetic aperture radar (SAR) systems in 1953, radar cross section (RCS) has been used as the measurement quantity for reporting the backscatter of point targets in SAR images. Consequently, radiometric calibration targets like corner reflectors or transponders have also been characterized by their RCS. This presentation will go into detail why using “RCS” as the measurement quantity should be reevaluated, and how better (standardized) radiometric measurements can become possible in the future. Parallels will be drawn to the field of photometry in astronomy. In photometry, the intensity of an astronomical object's radiation is measured. In the first half of the last century, measurement passbands (frequency bands) were defined “by chance” through the physical properties of used photographic films, lenses, and so on. In the second half of the last century, more accurate measurements became possible, and standard passbands were developed to allow comparable, repeatable measurements while fully embracing the frequency dependent characteristics of the measurement equipment and calibration objects. This advancement in methodology lies still ahead of the SAR community. At the moment, radiometric SAR image calibration and radiometric measurements are still influenced by non-standardized factors. As a point in case, the amplitude and phase responses of calibration targets do, with the current state-of-the-art methods, influence the resulting calibration factor. The effects are especially pronounced for current and emerging high-accuracy, high-resolution SAR systems. As a way forward, the authors propose to introduce a new measurement quantity. Instead of reporting the radar cross section (RCS) of a target, the equivalent radar cross section (ERCS) shall be reported. The ERCS shall be equal to the radar cross section of a perfectly conducting sphere which would result in an equivalent pixel intensity if the sphere were to replace the measured target. This definition allows distinguishing between the body property and the quantity that is actually “seen” by the SAR instrument. The RCS of an object might be (and has been since 1953) approximated by a constant for a low-resolution SAR system; for a modern, high-resolution SAR system, the frequency and aspect angle dependent RCS of a target simply cannot be summarized in a single figure anymore. Furthermore, RCS is defined as a ratio of powers alone, whereas the pixel intensity in a SAR image also depends on the target’s phase response due to the processing in the complex domain. The authors believe that a discussion of current radiometric measurement praxis is necessary and a prerequisite for higher quality, comparable measurements with low to very high resolution SAR systems. The introduction of equivalent radar cross section as the new measurement quantity is an important first step in this direction.

Radar cross section has long been the measurement quantity of choice to describe the radar reflectivity of point targets in calibrated synthetic aperture radar (SAR) images. Derived quantities like the backscatter coefficient are equally in wide-spread use to radiometrically describe, in scientific applications often even more relevant, distributed targets. This paper now raises the question if radar cross section (RCS) really is the best choice to link pixel intensities or digital numbers to a physical unit. In fact, it is shown that RCS actually is not a good choice out of two major reasons, both of which result from the RCS being frequency dependent. The authors propose to distinguish the radar cross section, a body property, from the actual radiometric measurement quantity in SAR images by introducing the dedicated term equivalent RCS. Additionally, the implications for measurements and radiometric calibration are described.

Transponders for synthetic aperture radar (SAR) need to be extremely precise in order to qualify as absolute calibration references for the increasingly demanding new SAR systems. To guarantee highest accuracy and stability even components which normally are considered ideal, have to be be taken into account. This paper shows the environmental influence on the antenna gain and thus on the overall transponder gain, compares two different housing designs and explains why this particular design has been chosen for the new transponder currently being developed at the DLR.

The requirements on new spaceborne synthetic aperture radar (SAR) missions are always pushed towards better image quality with respect to signal-to-noise ratio, radiometric accuracy, and spatial resolution. Therefore an accurate calibration of the SAR system and the final product is essential. The quality of the calibration depends on the utilized reference target. In this paper a new active calibration target (transponder), currently under development at DLR, and measurement results of the linearity of the high frequency section is presented.

The goal of radiometric calibration in synthetic aperture radar (SAR) is to achieve comparability between measurement results acquired with different systems (e.g. RADARSAT-2 and Sentinel-1), at different times (e.g. image stacks over many years), or with different system settings (e.g. center frequency or polarization). At the beginning of the calibration process stands the definition of the measurement quantity. We argue that the currently accepted measurement quantity for point targets, radar cross section (RCS), is not actually the quantity that a SAR system measures, and propose to replace the quantity with equivalent radar cross section (ERCS): The equivalent radar cross section (ERCS) shall be equal to the radar cross section of a perfectly conducting sphere which would result in an equivalent pixel intensity if the sphere were to replace the measured target. The concept “ERCS” has been introduced before [1]. In this presentation, we attempt to communicate the problem from different angles to make it more easily comprehensible and to contribute to a discussion in the CEOS community on a new definition of the radiometric measurement quantity in SAR. These topics are: • Mathematical view: By reviewing the basic SAR convolution integral and the definition of RCS it becomes obvious why RCS cannot be the radiometric measurement quantity in SAR. • Historical view: Considering early, comparably low resolution SAR systems with narrow bandwidths and small angular ranges, it is apparent why RCS has been an acceptable quantity in the past. The advancement towards higher accuracy and higher resolution systems makes a distinction between RCS and ERCS paramount though for today’s and tomorrow’s systems. • Comparison with black bodies: The radiation characteristics of certain surfaces are completely specified if their temperature is known. These black bodies do not exist in nature; they are an idealization. The blackbody radiation at a given wavelength depends only on the temperature. A single number (the brightness temperature) is therefore sufficient to summarize the complex Planck spectrum of a blackbody. This is similar to a large, perfectly conducting sphere in SAR which is used as an idealized object in the ERCS definition. A single number (the sphere’s cross sectional area) summarizes its properties. • Comparison with stellar photometry: In the 18 th century, different astronomers used different optical equipment to measure and compare the brightness of stars. Due to varying passbands (transfer functions) of lenses and photographic film, the results were not comparable. The problem was later solved by introducing standard photometric systems, where the passbands of the used equipment is standardized and calibrated. A comparable interaction exists between a SAR instrument and the measured terrain reflectivity due to the convolution operation in the processor. This we call the SAR passband problem [2]. We propose a similar approach to resolve the SAR passband problem by introducing standardized passbands (weighting/apodization functions at defined bandwidths). By adopting ERCS as the measurement quantity in the future, calibration and measurement results become truly compatible across current and future narrow and particularly wideband, high-resolution, and high- accuracy SAR systems.