GigaRad – a multi-purpose high-resolution ground-based radar system

Recently DLR has developed and constructed a new experimental radar instrument for various applications like radar signature collection, SAR/ISAR imaging, motion detection, tracking, etc., where high performance and high flexibility have been the key drivers for system design. Consequently a multi-purpose and multi-channel radar called GigaRad is operated in X band and allows an overall bandwidth of up to 6 GHz, resulting in a theoretical range resolution of up to 2.5 cm. Hence, primary obligation is a detailed analysis of various possible error sources, being of no or less relevance for low-resolution systems. A high degree of digital technology enables advanced signal processing and error correction to be applied. The paper outlines technical main features of the radar, the basic error correction strategy and illustrates some first imaging results

Broadband Polarizer Miter Bend for High Power Radar Applications

Polarizer miter bends are used to alter the polarization in overmoded waveguides. For future space debris observation with broadband high power W-band radar sensors these are important transmission line components. A polarizer miter bend uses a grooved mirror as phase grid for polarization. The present paper addresses a suitable design of such a phase grid for broadband high power radar applications within the frequency range from 90GHz to 100GHz. An appropriate parameter combination is found by a parametric study. For reduction of the required amount of calculation a plane wave approximation and unit cell simulations are used. With a suitable parameter combination a cross polarization of Xpol B −26 dB can be achieved within the considered frequency range. This corresponds to a suitable value for radar applications

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

Inverse Synthetic Aperture Radar Imaging of Space Targets Using Wideband Pseudo-Noise Signals with Low Peak-to-Average Power Ratio

With the number of new satellites increasing dramatically, comprehensive space surveillance is becoming increasingly important. Therefore, high-resolution inverse synthetic aperture radar (ISAR) imaging of satellites can provide an in-situ assessment of the satellites. This paper demonstrates that pseudo-noise signals can also be used for satellite imaging, in addition to classical linear frequency-modulated chirp signals. Pseudo-noise transmission signals offer the advantage of very low cross-correlation values. This, for instance, enables the possibility of a system with multiple channels transmitting instantaneously. Furthermore, it can significantly reduce signal interference with other systems operating in the same frequency spectrum, which is of particular interest for high-bandwidth, high-power systems such as satellite imaging radars. A new routine has been introduced to generate a wideband pseudo-noise signal with a peak-to-average power ratio (PAPR) similar to that of a chirp signal. This is essential for applications where the transmit signal power budget is sharply limited by the high-power amplifier. The paper presents both theoretical descriptions and analysis of the generated pseudo-noise signal as well as the results of an imaging measurement of a real space target using the introduced pseudo-noise signals

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

Orthogonal Waveform Experiments with a Highly Digitized Radar

An innovative highly-digitized radar system called GigaRad is currently under development by the DLR Microwaves and Radar Institute. An objective of the system is having a flexible, high-resolution imaging radar with wide-swath or 3-D capabilities, requiring transmission and reception on multiple channels simultaneously. However, a problem in multi-channel systems, especially in the transmit part, is the interference between simultaneously transmitted signals being operated at the identical frequency range. This paper examines the use of orthogonally coded waveforms as a method to overcome the interference between signals sharing the same bandwidth. First measurement results for a few selected waveforms are presented

First measurement results of a new highly-accurat SAR calibration target

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. An accurate radiometric calibration of the whole SAR system is crucial to cope with the demand on high image quality. Among other factors, the quality of the calibration depends on the utilized reference targets. Also permanent system monitoring, onboard and by reference targets, is required to guaranty the image quality over the whole mission time. In this paper a new highly-accurate active calibration target (transponder) is presented. The device is currently under development in the DLR-project “Kalibri” [1] and features remote control and alignment as well as an improved internal calibration for stable operation. Furthermore, first representative measurement results could be achieved by acquiring TerraSAR X images

Variations of the Transponder’s RCS Due to Environmental Impacts on the Antennas

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

Linearity Measurements of an Accurate Transponder for Calibrating Future Spaceborne SAR Systems

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