Die Nutzung stationärer Empfänger zur Charakterisierung von Radar-Instrumenten und zur bistatischen SAR-Bildgebung

Under the title “A Golden Age for Spaceborne SAR Systems”, Prof.~Alberto Moreira, Director of the DLR Institute of Radar Technology and Radio Frequency Systems, proclaimed in 2014 that a new era of high-resolution SAR satellite remote sensing began with the launch of TerraSAR-X in 2007. With the launch of the TanDEM-X mission, with Sentinel-1 of the ESA Copernicus programme, the Spanish reconnaissance mission PAZ, as well as ALOS-2 and many other systems, but especially with the entry of commercial spaceflight, the New-Space and the launch of the first small satellites with a SAR instrument by the Finnish start-up ICEYE, a spin-off of Aalto University, synthetic aperture radar is in fact expanding its former areas of application of military reconnaissance and scientific use for the first time to include a wide range of commercial applications in the field of environmental observation, since a large SAR satellite constellation with a high observation frequency provides a consistent and continuous global situation picture.
This environment of highly available transmitters of radar signals in orbit now enables the practical application of bistatic SAR, especially in connection with ground-based interferometry using a stationary radar receiver. Furthermore, the radar receiver can be used simultaneously for the calibration of satellite systems. These two topics are addressed in this thesis.
After investigating the bistatic acquisition geometry for SAR imaging with a stationary receiver, a corresponding signal model is developed to describe the expected measurement. This also includes the definition of the coordinate system in which the focused radar image is to be available later.
A back-projection method is proposed and developed to obtain the image from the measurement data. By numerical analysis of the recording geometry, this processor makes it possible in the general case to focus bistatic measurement data in the previously mentioned radar coordinates. The processor uses a polynomial model to describe the range of a target and can be directly extended by more complex propagation models and by a model of the antennas by applying this model in the spatial domain. Furthermore, for the special bistatic configuration with a linear transmitter trajectory and stationary receiver, two different algorithms to process the data in the frequency domain are developed and presented.
To enable experiments on bistatic SAR imaging, a radar reception system was developed and realised in the HITCHHIKER project. This system has four receive channels with a bandwidth of 500MHz each in the X band. Through a comprehensive characterisation of the system, corresponding physical quantities can be determined from the recorded data.
With this system, bistatic experiments were carried out with the radar satellites TerraSAR-X, TanDEM-X, PAZ and the ICEYE constellation and - in collaboration with Fraunhofer FHR - also with the airborne PAMIR radar system. To process the data from the experiments, the position of the phase centres of the antennas used has to be determined and the transmitting and receiving systems have to be synchronised as a basis of the coherent imaging method. In order to determine the state of the transmission system from these measurements, a system model of the participating radar satellites and the PAMIR radar is being developed and implemented.
In cooperation with the Spanish space agency INTA, the HITCHHIKER receiver is also used in this way to characterise the SAR instrument of the PAZ mission. In addition to the radiation pattern of the instrument's main X band antenna, the data collected during the experiments can be used to measure the frequency of the reference oscillator on board the satellite and evaluate its time synchronisation.