Impacts of Radar Echoes on Internal Calibration Signals in the TerraSAR-X Instrument

For calibrating and monitoring the required radiometric stability, the radar instrument of TerraSAR-X features an internal calibration facility coupling into an additional port of the TRMs. Calibration pulses are routed through the front-end to characterise critical elements and parameters of the transmit (TX) and receive (RX) path. Changes in the signal path appear due to thermal effects, degradation, or extreme conditions in space. Especially the front-end TRMs controlling the phased array antenna are of crucial significance for the instrument reliability. There are many indications that the interference of the RX-Calibration signals is caused by an echo from a transmitted TerraSAR-X chirp pulse of the same data take. As consequently implemented in the TerraSAR-X system, different approaches solve these effects of signal interference. In orbit, the commanding sequence can be optimised for avoiding interference. At processing level, averaging techniques minimise the noise effects inside the calibration signals. This paper presents the effects of the radar echoes on the whole internal calibration process and how they can be detected and minimised

Equivalent Radar Cross Section: What Is It and Why Is It Important?

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

Accurate Antenna Pattern Modelling for Spaceborne Active Phased Array Antennas

The paper describes the calibration and monitoring of the TerraSAR-X phased array antenna

Bistatic Experiment Using TerraSAR-X and DLR’s new F-SAR System

A bistatic X-band experiment was successfully performed early November 2007. TerraSAR-X was used as transmitter and DLR’s new airborne radar system F-SAR, which was programmed to acquire data in a quasi-continuous mode to avoid echo window synchronization issues, was used as bistatic receiver. Precise phase and time referencing between both systems, which is essential for obtaining high resolution SAR images, was derived during the bistatic processing. Hardware setup and performance analyses of the bistatic configuration are pre-sented together with first processing results that verify the predicted synchronization and imaging performance

Accurate Transponder Calibrations with the Novel Three-Transponder Method

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

Highly Accurate Radar Cross Section and Transfer Function Measurement of a Digital Calibration Transponder without Known Reference - Part I: Measurement & Results

Active Radar Calibrators (ARC), also called calibration transponders, are often used as reference targets for absolute radiometric calibration of radar systems due to their large achievable Radar Cross Section (RCS). But before using a transponder as a reference target, the hardware has to be calibrated itself. A novel method, called three-transponder-method, was proposed some years ago and allows for RCS calibration of digital transponders without using any RCS target as reference. In this paper, this technique is further refined and applied to a setup utilizing only one digital transponder. The accurate measurement design is described and a novel, elaborated data processing scheme is developed to minimize remaining noise and clutter effects in the data. A comprehensive error analysis will be presented in the second part of this paper

Absolute Radiometric Calibration of Broadband X-Band Transponders

Spaceborne synthetic aperture radar (SAR) systems are often used for earth observation capable for acquiring accurate high-resolution data. In order to ensure the quality of these SAR data, the SAR system has to be calibrated first. For this purpose active targets with well-known backscatter properties, called transponders, serve as an external reference. The enhancement of the operational bandwidth up to 1.2 GHz of future civil SAR systems requires the development of appropriate broadband transponders and their accurate calibration. In order to be well prepared for these missions, DLR has been developed a broadband X-Band transponder and an innovative technique for the frequency-dependent determination of the transponder’s radar cross section (RCS) which promises an accurate measurement over the full transponder bandwidth. In this paper the calibration of a broadband transponder according to this new approach is described including the analysis of corresponding measurements. The derived results are verified with a second independent calibration method and finally evaluated

Study of archaeological collection from Arroyo La Glorieta Guaraní site (Buenos Aires province, Lower Paraná River Delta)

El sitio Arroyo La Glorieta (Delta inferior del río Paraná, pcia. de Buenos Aires) fue excavado en 1926 por el técnico del Museo de la Plata Antonio Castro. Debido a las urnas funerarias recuperadas entonces, fue considerado como un cementerio. Los materiales arqueológicos y los restos humanos fueron guardados en las divisiones de Arqueología y Antropología del Museo de La Plata. Pasados casi noventa años de su excavación recién en los últimos tiempos estos materiales empezaron a ser estudiados de manera sistemática. En este trabajo se presentan los primeros resultados de los análisis realizados sobre la alfarería y los restos humanos procedentes del sitio. Con esto se pretende aportar al conocimiento de las ocupaciones indígenas de la región y, en particular, de las guaraníes que arribaron a estas latitudes en momentos anteriores a la conquista.The Arroyo La Glorieta site (Lower Delta of Parana River, Buenos Aires) was excavated in 1926 by technician of the La Plata Museum Antonio Castro. Because of the funerary urns recovered back then, it was regarded as a cemetery. The archaeological materials and human remains were kept in the divisions of Archaeology and Anthropology at the Museum of La Plata. After almost ninety years of the excavation just recently these materials began to be studied systematically. This paper presents the first results of analyzes performed on pottery and human remains from the site. With this, it is intended to contribute to the knowledge of indigenous occupation of the region and, in particular, of the Guarani that arrived at these latitudes in earlier times to conquest.Facultad de Ciencias Naturales y Muse