Robert Metzig

Impact of oscillator noise in bistatic and multistatic SAR

This letter addresses the impact of limited oscillator stability in bistatic and multistatic synthetic aperture radars (SARs). Oscillator noise deserves special attention in distributed SAR systems since there is no cancellation of low-frequency phase errors as in a monostatic SAR, where the same oscillator signal is used for modulation and demodulation. It is shown that the uncompensated phase noise may cause a time-variant shift, spurious sidelobes, and a broadening of the impulse response, as well as a low-frequency phase modulation of the focused SAR signal. Quantitative estimates are derived analytically for each of these errors based on a system-theoretic model taking into account the second-order statistics of the oscillator phase noise

Performance prediction of a phase synchronization link for bistatic SAR

Oscillator phase noise can dictate the performance of bistatic and multistatic synthetic aperture radar imaging. In this letter, the use of a dedicated synchronization link to quantify and compensate oscillator phase noise is investigated. Different synchronization schemes are presented, and their performance is analyzed. The error contribution of the synchronization link itself, which may suffer from receiver noise, aliasing, interpolation, and filter mismatch, is included in the analysis. The synchronization link performance is given in a frequency-domain closed integral form

TerraSAR-X System Performance Characterization and Verification

This paper presents results from the synthetic aperture radar (SAR) system performance characterization, optimization, and verification as carried out during the TerraSAR-X commissioning phase. Starting from the acquisition geometry and instrument performance, fundamental acquisition parameters such as elevation beam definition, range timing, receiving gain, and block adaptive quantization setting are presented. The verification of the key performance parameters-ambiguities, impulse-response function, noise, and radiometric resolution-is discussed. ScanSAR and Spotlight particularities are described.

The joint TerraSAR-X / TanDEM-X ground segment

This paper recalls the essential elements of the joint TerraSAR-X and TanDEM-X ground segment. It elaborates on some topics which are usually not in the primary focus from a pure SAR technical point of view, e.g. the flight formation. Both commissioning and early routine phase results from operating the joint TerraSAR-X and TanDEM-X ground segment are given.

Bistatic spaceborne-airborne experiment TerraSAR-X/F-SAR: data processing and results

Following an original proposal by the authors to the TerraSAR-X (TSX) scientific coordination board, a spaceborne-airborne bistatic experiment was successfully performed early November 2007. TSX was used as transmitter and DLRs new airborne radar system, F-SAR, as receiver; due to the capability of the latter to acquire data quasi-continuously, no echo window synchronisation is needed. Monostatic data were also recorded during the acquisition. This paper includes description and results of the spaceborne-airborne bistatic experiment, with special focus on data processing and image comparison. Given the acquisition scenario, with two-channel sampling and transmitter and receiver clocks operating independently, data processing must necessarily follow a three-step strategy: 1) channel balancing, 2) data synchronisation and 3) bistatic SAR processing. Since neither absolute range nor Doppler references are available in the bistatic data set, synchronisation is done with the help of calibration targets on ground and based on the analysis of the acquired data compared to expected data. Due to the variant nature of the bistatic acquisition and the required precision for the processing, data are processed using a bistatic backprojection approach.

In-orbit SAR performance of TerraSAR-X

TerraSAR-X is the first German Radar satellite for scientific and commercial applications. The project is a public- private partnership between DLR and EADS Astrium GmbH. TerraSAR-X consists of a high resolution Synthetic Aperture Radar at X-Band. The radar antenna is based on active phased array technology that allows the control of many different instrument parameters and operational modes (Stripmap, ScanSAR and Spotlight) with various polarizations. Following the TerraSAR-X launch, it is planned a six month Commissioning Phase covering the characterization and verification of the SAR mission. Within this phase, the Overall SAR System Performance takes care of the correct working and interaction of all SAR system elements essential for obtaining an optimum SAR Performance.

TerraSAR-X Instrument, SAR System Performance & Command Generation

The paper presents selected results from the TerraSAR-X Commissioning Phase from instrument performance, SAR system performance and command generation.

Satellite ground stations with electronic beam steering

In this work, we propose electronic beam steering via antenna arrays as a substitute for large parabolic antennas at satellite ground stations. We concentrate on two array geometries, faceted arrays and hemispherical arrays. A thorough analysis is carried out of the radiation characteristics, the array size, as well as the antenna element distribution and spacing. Moreover, in order to fulfill the requirement of the array design, that is, to achieve a higher gain at low elevation angles where the longer spacecraft to ground station distance leads to a larger range loss, we propose to adjust the number of active antenna elements, i.e., some antenna elements are turned on while others are turned off according to the required level of antenna gain. This also contributes to a concept of an optimized array design for this specific application. In the simulations, the array optimization for both array geometries is further investigated and realized with a realistic ephemeris incorporated. The numerical results support the proposal of replacing large reflector antennas by electronic beam steering via antenna arrays at satellite ground stations.

Performance Prediction and Verification for the Synchronization Link of TanDEM-X

The paper describes the synchronization link of the two satellites of the TanDEM-X mission. The synchronization link is established to track the oscillator phase noise difference which- after appropriate processing - can be used to compensate the effect of oscillator phase noise. A signal model based on the synchronization link hardware is presented which is then used to derive an analytical expression for the compensation phase including effects such as instrument drift, Doppler, antennas, and receiver noise. Specifically the influence of the synchronization link RF hardware on the quality of the derived compensation signal is crucial for the performance. Therefore the synchronization link RF hardware of the TerraSAR-X satellite is characterized to obtain realistic data. These measurements will be used for the calibration of the synchronization link, i.e. to remove systematic errors due to temperature drifts of the instrument.

The tanDEM-X Ground Station Network

This paper describes DLRs Ground Station Network as setup and operated for the TanDEM-X mission payload data reception. It lists the technical layout and operational characteristics of each station. The Station Monitoring and Control System which has been successfully qualified during the TanDEM-X commissioning phase is introduced. Software design aspects are discussed high level. Key features of the Station Monitoring and Control System are provided.