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

Multistatic SAR Imaging: First Results of a Four Phase Center Experiment with TerraSAR-X and TanDEM-X

Multichannel synthetic aperture radar (SAR) imaging offers the possibility to overcome the pulse repetition frequency (PRF) constraints inherent to single-channel SAR systems. The multichannel approach enables the acquisition of wide swathes with high azimuth resolution. Using a constellation or swarm of small satellites, a cost efficient, faulttolerant system can be envisaged. This paper describes the first results of a multistatic four phase center experiment conducted with TerraSAR-X and TanDEM-X. The experiment is intended to increase the know-how and demonstrate the capabilities for the design of future SAR systems. Key challenges are addressed, the experimental acquisition is described and an evaluation approach is presented. Finally, first results focusing on the azimuth ambiguity performance are shown

Potentials of TanDEM-X Interferometric Data for Global Forest/Non-Forest Classification

This paper presents a method to generate forest/nonforest maps from TanDEM-X interferometric SAR data. Among the several contributions which may affect the quality of interferometric products, the coherence loss caused by volume scattering represents the contribution which is predominantly affected by the presence of vegetation, and is therefore here exploited as main indicator for forest classification. Due to the strong dependency of the considered InSAR quantity on the geometric acquisition configuration, namely the incidence angle and the interferometric baseline, a multi-fuzzy clustering classification approach is used. Some examples are provided which show the potential of the proposed method. Further, additional features such as urban settlements, water, and critical areas affected by geometrical distortions (e.g. shadow and layover) need to be extracted, and possible approaches are presented as well. Very promising results are shown, which demonstrate the potentials of TanDEM-X bistatic data not only for forest identification, but, more in general, for the generation of a global land classification map as a next step

The cis-acting CTTC-P1BS module is indicative for gene function of LjVTI12, a Qb-SNARE protein gene that is required for arbuscule formation in Lotus japonicus

Lota F, Wegmueller S, Buer B, et al. The cis-acting CTTC-P1BS module is indicative for gene function of LjVTI12, a Qb-SNARE protein gene that is required for arbuscule formation in Lotus japonicus. The Plant Journal. 2013;74(2):280-293.The majority of land plants live in symbiosis with arbuscular mycorrhizal fungi from the phylum Glomeromycota. This symbiosis improves acquisition of phosphorus (P) by the host plant in exchange for carbohydrates, especially under low-P availability. The symbiosome, constituted by root cortex cells accommodating arbuscular mycorrhizal fungal hyphae, is the site at which bi-directional exchange of nutrients and metabolites takes place. Uptake of orthophosphate (Pi) in the symbiosome is facilitated by mycorrhiza-specific plant Pi transporters. Modifications of the potato Pi transporter 3 (StPT3) promoter were analysed in transgenic mycorrhizal roots, and it was found that the CTTC cis-regulatory element is necessary and sufficient for a transcriptional response to fungal colonization under low-Pi conditions. Phylogenetic foot-printing also revealed binary combination of the CTTC element with the Pi starvation response-associated PHR1-binding site (P1BS) in the promoters of several mycorrhiza-specific Pi transporter genes. Scanning of the Lotus japonicus genome for gene promoters containing both cis-regulatory elements revealed a strong over-representation of genes involved in transport processes. One of these, LjVTI12, encoding a member of the SNARE family of proteins involved in membrane transport, exhibited enhanced transcript levels in Lotus roots colonized with the arbuscular mycorrhizal fungus Glomus intraradices. Down-regulation of LjVTI12 by RNA interference resulted in a mycorrhiza-specific phenotype characterized by distorted arbuscule morphology. The results highlight cooperative cis-regulation which integrates mycorrhiza and Pi starvation signaling with vesicle trafficking in symbiosome development

In-Flight Performance Monitoring of TerraSAR-X Active Phased Array Antenna

TerraSAR-X is a high resolution synthetic aperture radar (SAR) satellite launched in 2007. Its active phased array X-Band antenna hosts 384 transmit/receive modules (TRMs) controlling the beam shape and steering in azimuth and elevation. A mathematical antenna model approach is used for pattern optimisation and calculation of all possible operation beams. For keeping the excellent radiometric performance of the SAR system the front-end antenna must be monitored throughout mission life time and calibrated if necessary. A calibration network provides monitoring of the instrument behaviour by evaluating internal calibration pulses. Evaluation of these calibration pulses for every image acquisition guarantees high radiometric stability of all SAR products. This paper shows the latest in-orbit results of the antenna performance and TRM stability complementary to the traditional approach of external antenna measurements over calibration targets. For this purpose, the novel PN Gating method is applied in a spaceborne environment for the first time ever

TerraSAR-X Antenna Calibration And Monitoring Based on a Precise Antenna Model

The paper describes the theory behind, the methods and the results of the calibration of the TerraSAR-X antenna model

An Efficient Method for Performance Monitoring of Active Phased Array Antennas

Modern synthetic aperture radars (SARs) are equipped with active phased array antennas to electronically generate various antenna beams. The TerraSAR-X satellite is a high resolution SAR system launched in June 2007. Its active phased array X-band antenna hosts 384 transmit/receive modules (TRMs) for controlling the electronic beam steering in azimuth and elevation direction. The precise modeling of the antenna performance is only possible if the actual characteristics of each individual TRM are monitored. TerraSAR-X has been equipped with an innovative characterization mode based on a coding technique, which is the so-called pseudonoise gating method. The individual and simultaneous characterization of all TRMs is realized under most realistic conditions with power supply loads like in nominal radar operation. For the first time, this novel technique has been applied on a spaceborne SAR system

In-flight Monitoring of TerraSAR-X Radar Instrument Stability

For calibrating and monitoring the radiometric stability, the radar instrument of TerraSAR-X hosts a passive calibration network coupling into an additional port of the T/R modules. Three different types of calibration pulses are applied, whereby sets of these pulses are needed at the start and end of each data acquisition. Additionally, TerraSAR-X has been equipped with an additional calibration mode based on the so-called "PN Gating method". The PN Gating mode allows operating the T/R modules under most realistic conditions with the advantage that all modules can be characterised simultaneously. The individual characteristics are found by assigning special calibration pulse sequences with orthogonal codes to the T/R modules. During on-ground characterisation of the TerraSAR-X instrument this technique has established as a crucial diagnostic tool for functional checks as well as T/R module drift and failure monitoring. For the first time ever, this innovative method of PN Gating measurements will be applied for in-flight monitoring of a satellite SAR instrument. Starting with the TerraSAR-X commissioning phase the instrument behaviour can be accurately characterised to guarantee the radiometric stability of the system throughout the mission lifetime. In this paper, the latest results from the in-orbit calibration of TerraSAR-X are shown and compared to on-ground measurements of pre-launch characterisations. The techniques described above are also applicable for characterisation and calibration of other advanced SAR systems coping with multiple mode radar instruments