Stuck Bit Error Identification for the TerraSAR-X and TanDEM-X Onboard Memory

Errors in memory storage devices in the form of erroneous bits induced by radiation are a common issue for every space- craft in orbit. Therefore, well established techniques detect and directly correct these errors in the storage hardware. Sporadically single memory cells can temporarily get “stuck” at a false bit in which case they cannot be corrected. Those stuck bits can persist up to several months generating the same errors during every memory scrubbing cycle. In order to assess the current memory status a method to distinguish between regular and stuck-bit errors is needed. This paper therefore presents a classification approach based on the DBSCAN method where stuck bits are identified as outliers and clustered accordingly. The approach first is verified with simulated data that resembles the error structure of memory errors on TerraSAR-X and TanDEM-X. Subsequently, the method is validated with the memory errors recorded by both satellites throughout their lifetime

MirrorSAR: An HRWS Add-On for Single-Pass Multi-Baseline SAR Interferometry

This paper reports the Phase A study results of the interferometric extension of the High-Resolution Wide-Swath (HRWS) mission with three MirrorSAR satellites. According to the MirrorSAR concept, small, low cost, transponder-like receive-only satellites without radar signal demodulation, digitization, memory storage, downlink, and synchronization are added to the planned German X-band HRWS mission. The MirrorSAR satellites fly a triple helix orbit in close formation around the HRWS orbit and span multiple single-pass interferometric baselines. A comprehensive system engineering and performance analysis is provided that includes orbit formation, MirrorLink, Doppler steering, antenna pattern and swath design, multi-static echo window timing, SAR performance, height performance and coverage analysis. The overall interferometric system design analysis of Phase A is presented. The predicted performance of the global Digital Elevation Model (DEM) is improved by one order of magnitude compared to presently available global DEM products like the TanDEM-X DEM

A Decade of TerraSAR-X and TanDEM-X Operation: A Retrospective on the Performance of the SAR System and an Outlook to the Future

TerraSAR-X and its almost identical twin satellite TanDEM-X have provided high resolution radar images for years and Digital Elevation Models with unprecedented accuracy. The SAR image quality and accuracy has remained constant since launch and is owed to a very stable instrument. A thorough system health monitoring is utilized in order to maintain this stability. On the one hand a short-term verification of the individual acquisitions constantly exploits satellite telemetry from the SAR instrument and the on-board calibration system. Hence performance hanges, e.g. in terms of hardware degradation, can quickly be identified and corrected. For example by monitoring the antenna’s T/R modules via pseudo noise gating, dysfunctional modules could be detected in order to re-adjust the system accordingly. On the other hand a long-term system monitoring approach furthermore was designed to detect long-term system parameter trends and degradations that may affect data quality or imaging capabilities. In particular on-ground measurements are evaluated by continuously imaging globally distributed reference targets such as corner reflectors or test sights with a well-known topography. It was initiated with the launch of TerraSAR-X thus provides a variety of long-term parameter time series. Stimulated by this approach a study has been conducted to analyze the impact of solar events like radiation or the annual eclipse due to the satellites’ orbit geometry. Although the effects are taken into account by calibration the results provide valuable information to interpret monitoring results and gain a deeper understanding of the system. Besides retrospecting the SAR system the goal of this paper is also to give a summary of the TerraSAR-X and TanDEM-X mission with focus on the calibration and the systems’ functionality. The mission status will concentrate mainly on the performance of the global Digital Elevation Model which was completed in September 2016. Having used precise calibration methods and improved processing algorithms, the dataset shows an outstanding quality. In the end up to 20.000 tiles with a size of 1° x 1° were generated yielding an overall absolute height accuracy of 3.5 meters and covering more than 99% of the globe. As the satellites are still in good condition and consumables are sufficient, the mission was extended to generate a change layer as an update of the global DEM. By adjusting acquisition strategies and processing methods, fewer acquisitions are sufficient to achieve the same accuracy as for the global DEM. Furthermore, a brief outline of the proposed future L-band satellite formation is drawn. The ambitious mission shall provide data to help solving pressing climate-related questions. As an example the proposed system will be able to penetrate forest canopy in order to estimate biomass on a global scale. Equipped with a reflector antenna and exploiting the innovative digital beamforming technique, Tandem-L will be capable of illuminating a 350 km wide swath on ground acquiring up to 8 terabyte of data per day. This enables a weekly global coverage, which is a precondition for observing dynamic processes in the bio-, geo-, hydro- and cryosphere

A Simplified Method to Approximate the AOI Coverage Duration in Single Acquisition Direction for SAR Satellites with Repeat Ground-Track Orbits

In early stages of SAR mission design the coverage capabilities of a system are either tailored to fulfil the mission objectives or drive their development. Traditionally, this involves using complex software that evaluates the intersections of an Area of Interest (AOI) with the footprint of the SAR instrument throughout the orbit using a grid-point approach. Although this methodology yields precise results, it requires cumbersome software setups and scenario configurations in order to achieve a complete representation of the system's coverage performance. To address this challenge, this paper proposes a simplified methodology based on the geometric relations between the instrument's swath width, the ascending orbital ground-tracks, and the potential positions of an AOI. By adopting this approach, streamlined analysis of the minimum and maximum access possibilities for various AOIs, or different system and orbit configurations becomes feasible. Consequently, an approximate representation of the system's coverage capabilities can be obtained straightforwardly

Long-Term System Monitoring Tool User Manual and Functional Description

The document was initially prepared for the PAZ project and is now adapted for the TerraSAR-X/TanDEM-X- project. The User Manual gives an introduction how to use the LTSM Tool. Chapter 3 explains how the tool is used as well as describes the inputs and outputs. The general design of the software is described in Chapter 4 of this document. Chapter 5 summarizes the procedure of creating the LTSM-Report and the configuration file schema is provided in Chapter 6

Observation Strategies for TanDEM-X and Tandem-L

TanDEM-X is a unique mission to derive digital elevation models (DEMs) with two SAR satellites in orbit since 2007 and 2010. Being still in good shape, an update of the global DEM, called "Change DEM", is currently in acquisition. DLR is also pushing for the time after TanDEM-X with Tandem-L, a proposal for a sophisticated satellite-duo operating in L-band, serving a variety of applications. For both missions the acquisition concepts are of crucial importance for combining the scientific with the processing demands

Innovative MirrorSAR Concept for Multi-Static HRWS

DLR funded a phase 0/A study High-Resolution Wide-Swath (HRWS) at Airbus DS DLR/HR provided mission concept, analysis and performance of HRWS extension by 3 Rx satellites This allows for DEM acquisition with unprecedented accuracy DLR owned approach MirrorSAR is the basis for the Extension Presentation is about MirrorSAR in context of HRW

A MirrorSAR Case Study Based on the X-Band High Resolution Wide Swath Satellite (HRWS)

The paper reports results of a concrete MirrorSAR mission analysis study with the planned X-band HRWS satellite as transmitter. The driving goal is a next generation global DEM with a much better height performance compared to TanDEM-X. Three small passive receive satellites are spanning differently sized interferometric baselines by flying in-terleaved Helix orbits. Several system engineering topics associated to the MirrorSAR concept are discussed. The multi-static echo window timing is investigated including the along-track separation between transmitter and receivers. The interaction between helix orbit baseline design, Doppler steering and phase preserving MirrorSAR link geometry is ana-lyzed. Advanced SAR and DEM performance estimations conclude the paper

IQ Bias Channel Deviation on TerraSAR-X and TanDEM-X

The SAR satellites TerraSAR-X and TanDEM-X are experiencing periodically varying solar conditions due to their orbit geometry. As a consequence multiple system parameters are affected by these variations mainly induced by corresponding temperature fluctuation. A long-term analysis has been conducted showing that in particular one parameter, the in-phase and quadrature bias (IQ bias), deviates considerably from expected results. This deviation occurs between the respective channels and also between both spacecraft. In order to identify the driving mechanisms behind this behaviour, the signal path between the antenna front-end and the analogue to digital conversion (ADC) block is analysed, putting a special focus on the IQ demodulation block. Within this block, mixer imbalances are the main contributors to the observed behaviour and provide an explanation for the differing channel outputs. The mixer is also the reason for IQ bias deviations between the satellites since the channel imbalance shows a reversed tendency when comparing both systems. Although the effects are corrected in calibration, the results provide valuable information to interpret monitoring results and to gain a deeper understanding of the system

An Analysis of Battery Preservation Measures on the DEM Product Performance of TanDEM-X

The satellites TerraSAR-X and TanDEM-X are in orbit since 16 and 13 years respectively. This is much longer than the planned life time of 5.5 years. Hence, components like the lithium-ion batteries age and their performance degrade increasingly. In consequence, operational scenarios need to be adapted, e.g., by reducing the maximal duration of an acquisition. This paper assesses an idea to reduce the stress on the battery by lowering the transmit duty cycle of an acquisition. It provides an analysis on how a decreased duty cycle reduces the power consumption and the voltage drops in the batteries. In addition, the deterio-ration of the DEM product performance is evaluated and justified