HE TanDEM-X project is implemented by a "Public-Private Partnership" between the German Aerospace Centre (DLR) and Astrium GmbH. The primary goal of the TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) mission is to generate a global digital elevation model. To achieve this, two satellites-TanDEM-X (TDX) and TerraSAR-X (TSX), a satellite of almost identical construction which is in orbit since June 2007will form the first configurable SAR (Synthetic Aperture Radar) interferometer in space with a separation of only a few hundred metres. A powerful ground segment, which is interlaced with that of TSX, completes the TanDEM-X system. The satellites will fly in formation and operate in parallel for three years to cover the entire surface of the Earth. DLR is responsible for the scientific exploitation of the TSX/TDX data, as well as for planning and implementing the mission, for controlling the two satellites and for generating the digital elevation model. Astrium has built the satellites and shares in the costs of development and exploitation. As is the case already with TSX data, the responsibility for marketing the data lies in the hands of Infoterra GmbH, a subsidiary of Astrium GmbH. AOCS operations for two spacecraft flying at low altitude with such unprecedented small separations pose several new challenges and require creative solutions, partly implemented on-board and partly on ground. Due to the extremely short reaction times, emphasis lies with the on-board handling of problems. This is achieved by implementation of a data link (one-way) between the satellites, by the regular sending of a "health" signal (twoway), by the design of a new AOCS safe mode that has no effect on the orbit, by the complete rework of the TSX FDIR concept (on-board Fault Detection, Isolation and Recovery) and by autonomous formation control. This paper addresses some of the more interesting on-board implementations in particular the on-board AOCS surveillance and the newly developed and implemented AOCS safe mode with the magnetic torquers as the sole actuators. It also will touch briefly on some ground aspects such as procedures, planning and execution of orbit correction manoeuvres, personnel and on-call strategy, as well as off-line analysis and tools.
The micro-satellite TET-1 carries several technology experiments. It is the first in a series offering the possibility of in-orbit verification of new equipment made in Germany by the industrial and scientific aerospace community. TET-1 was launched 22nd July 2012 and is operated by the German Space Operations Center. Attitude and attitude control is influenced by several of the experiments. Special attitude control modes are required for a number of experiments in order to point the satellite in a prescribed direction or to a specific location on Earth. These comprise an experiment with three infra-red cameras, a pico thruster and finally a new type S-band transponder. The Li Polymer battery was not expected to have any effect on attitude control. However, it was discovered that charging and discharging the battery disturbs the magnetic field sensors, thus a different approach to attitude control is required when it is in use. Implementation of and special demands on the attitude control system for these experiments will be presented. The mission is experimental with high demands on the attitude control system. The envisaged duration was one year only with a possible prolongation of a further year. Components were therefore not chosen for longevity. However, the actual amount of disruptions due to sensor and/or actuator outages and due to idiosyncrasies in the spacecrafts thermal budget was higher than expected. The ensuing challenges for the attitude control system will be discussed. A software upload in May 2013 mitigated several of the issues addressed above. The improvements in the software and autonomous on-board reactions will be presented in the final Section together with some recommendations for the follow-on missions TET-2 and BIROS.
AOCS operations for two satellites flying at ~514 km altitude with unprecedented small separation of 150 m upwards pose various new challenges and require creative solutions. Several on-ground safety measures were installed, but due to the short reaction times emphasis always lies with the on-board handling of problems. This is achieved by implementation of a data link (one-way) between the satellites, by the design of a new AOCS safe mode that has no effect on the orbit, by the complete re-work of the TerraSAR-X FDIR concept (Fault Detection, Isolation and Recovery) and by autonomous formation control with a dedicated cold gas system on the second satellite TanDEM-X. This paper describes AOCS operations for the TanDEM-X mission in close formation. Most of the tasks are, thanks to extended safety measures and automated control mechanisms, routine. This will be described in the first section, where a short summary of all AOCS related operations will be given. The second section describes some special activities in connection with the cold gas system and the star trackers on TDX. The investigations and the fine tuning of the GPS receivers on both satellites are also described in this section. Although there was never a threat to the formation, close monitoring is required all the time and some mitigation measures were called for. Finally, the influence of the approaching solar maximum in 2013 will also be described in this section. The last part presents the conclusions and contains some suggestions for future missions. Nomenclature Δv= increment in velocity e = eccentricity i = inclination
The IMAPS mission is to establish a global solution for protecting the marine environment through the development and implementation of a modular assessment and protection system. The limitations of current technologies are based on them being individually inadequate to fully achieve the mitigation objectives. All the sources of information related to the detection and classification of marine mammals such as active sonar, passive sonar, radar, visual and distribution databases need to be fused to provide a robust protection system. In order to successfully standardize, transport, interface and fuse the information being supplied and requested by the diverse sensor modalities, two essential tools of the IMAPS global system are being developed: the Integrated Tracker and the Integration Architecture. The Integrated Tracker employs a data fusion algorithm that combines sensor information to refine estimates of target position, velocity, identity to provide integrated tracking and classification. A data fusion architecture has been chosen and a Bayesian approach for fusion and classification is developed. A data fusion and transport approach has been taken to develop a set of IMAPS prototypes that consist of simulated active sonar, passive sonar, radar and visual modalities communicating in real-time via the Integration Architecture, and to supply the sensor data, and other information necessary for the fusion, to the Integrated Tracker. The main functionality of the Integration Architecture is to provide information transport and common services in the form of a 'plug and play' capability, implemented with a software framework developed upon the Test & Training ENabling Architecture (TENA). The TENA software framework provides for the implementation of common software interfaces to disparate sensor modalities, thereby enabling the integration of different data sources using a common data representation and transport medium. Sensor outputs such as target detections and environmental information are being transformed in object-orientated data representations that are easily interpreted, manipulated, and interfaced. In addition to the Global IMAPS initiative, the active sonar modality was designed, built, and tested during the MAST 04 experiment producing detections of whales and preliminary synthesis of whale tracks.
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