Track association performance of the best hypotheses search method

Uncontrolled space objects in the geostationary orbit domain are hazardous threats for active satellites. Catalogs need to be build up, in order to protect this precious domain. The Swiss ZimSMART telescope, located in Zimmerwald, regularly scans the geostationary ring in order to provide a homogenous coverage. This surveying technique typically yields short measurement arcs, called tracklets. Each tracklet provides information about the line-of-sight and the rates of change but typically not about the full state of the observed object. Computationally intensive multi-hypothesis filter methods have been developed to associate tracklets with each other. An effective implementation to this approach is presented that uses an optimization algorithm to reduce the number of initial hypotheses. The method is tested with a set of real measurements of the aforementioned telescope

Development of the TanDEM-X Calibration Concept: Analysis of Systematic Errors

The TanDEM-X mission, result of the partnership between the German Aerospace Center (DLR) and Astrium GmbH, opens a new era in spaceborne radar remote sensing. The first bistatic satellite synthetic aperture radar mission is formed by flying the TanDEM-X and TerraSAR-X in a closely controlled helix formation. The primary mission goal is the derivation of a high-precision global digital elevation model (DEM) according to High-Resolution Terrain Information (HRTI) level 3 accuracy. The finite precision of the baseline knowledge and uncompensated radar instrument drifts introduce errors that may compromise the height accuracy requirements. By means of a DEM calibration, which uses absolute height references, and the information provided by adjacent interferogram overlaps, these height errors can be minimized. This paper summarizes the exhaustive studies of the nature of the residual-error sources that have been carried out during the development of the DEM calibration concept. Models for these errors are set up and simulations of the resulting DEM height error for different scenarios provide the basis for the development of a successful DEM calibration strategy for the TanDEM-X mission

System Concepts for Bi- and Multi-Static SAR Missions

The performance and capabilities of bi- and multistatic spaceborne synthetic aperture radar (SAR) are analyzed. Such systems can be optimized for a broad range of applications like frequent monitoring, wide swath imaging, single-pass cross-track interferometry, along-track interferometry, resolution enhancement or radar tomography. Further potentials arises from digital beamforming on receive, which allows to gather additional information about the direction of the scattered radar echoes. This directional information can be used to suppress interferences, to improve geometric and radiometric resolution, or to increase the unambiguous swath width. Furthermore, a coherent combination of multiple receiver signals will allow for a suppression of azimuth ambiguities. For this, a reconstruction algorithm is derived, which enables a recovery of the unambiguous Doppler spectrum also in case of non-optimum receiver aperture displacements leading to a non-uniform sampling of the SAR signal. This algorithm has also a great potential for systems relying on the displaced phase center (DPC) technique, like the high resolution wide swath (HRWS) SAR or the split antenna approach in the TerraSAR-X and Radarsat II satellites

Spaceborne Polarimetric SAR Interferometry: Performance Analysis and Mission Concepts

Spaceborne polarimetric SAR interferometry enables quantitative measurements of important bio- and geophysical parameters of the Earth surface on a global scale. We will first give a short review about actual and planned spaceborne SAR missions that can provide the observation space required for the derivation of Pol-InSAR products. This overview includes both repeat pass mission scenarios like ALOS/PalSAR, TerraSAR-L and Radarsat II, as well as single-pass mission scenarios like a fully-polarimetric Interferometric Cartwheel or TanDEM- X. The Pol-InSAR performance of the suggested mission scenarios will then be analysed by introducing the new concept of a phase tube. This concept enables an optimization of the system parameters and a quantitative comparison between different sensor configurations. The performance analysis for the investigated repeat pass mission scenarios reveals that major limitations have to be expected from temporal decorrelation. Some suggestions will be made to alleviate this performance loss by appropriate orbit refinement. Furthermore, important aspects in the design of future Pol-InSAR sensors will be addressed and we demonstrate the potential benefits arising from the use of bi- and multistatic single pass sensor configurations

AIR MEETS SPACE: SHAPING THE FUTURE OF COMMERCIAL SPACE TRAFFIC: I. STUDY INTRODUCTION AND INITIAL RESULTS

There are high expectations for a globally growing market of commercial space travel which is likely to turn in the next 10 to 20 years into a multi-billion Euro business. Those growth expectations are also backed up by OneWeb’s order of about 700 small satellites which are likely to be brought into LEO via air launches and by a continuously growing LEO launch rate showing an increase of about 60% in the last decade. Advances in electric propulsion and spacecraft design (CubeSats) helped to significantly reduce launch costs, so that space exploitation becomes affordable for the first time also to the private sector (e.g., for school labs, micro gravity research or in the area of human spaceflight). Several key players in space business, companies like Blue Origin, Virgin Galactic, XCOR, Orbital or SNC get ready to serve the commercial manned and unmanned spaceflight market by developing their own ballistic reusable space vehicles which shall carry humans and cargo payload into suborbital and LEO space. Europe’s single stage to orbit concepts, like REL’s Skylon or Airbus’ Spaceplane, even target for commercial manned point to point mass transportation, similar to today’s travel through airspace, but with much shorter flight times. All these developments will likely stimulate demands for launch sites and spaceports, where commercial aviation and space vehicles will have to be safely managed and controlled in parallel granting easy access to potential customers. Today, management of and access to commercial aerospace is lacking a coordinated European and global approach so that the expected growing number of space vehicles passing through the air-space interface in a rather “uncontrolled” way will likely pose significant threats to human health and airspace safety. This issue is further intensified by the flood of CubeSats and Unmanned Aerial Vehicles which increase collision risks in LEO and lower airspace. Without doubt, space and airspace will move closer together in the next decade, which is why Space Tra ffi c Management is expected to become a global undertaking. Because we think that safety in aerospace should not be jeopardised by those developments, we initiated an evaluation study together with ESA aiming at the generation of a roadmap towards a European Space Traffic Management. This is the first in a series of papers which gives an outline of the study and presents initial results from a European perspective

Multi-Target Tracking for SMARTnet: Multi-Layer Probability Hypothesis Filter for Near-Earth Object Tracking

In this paper, a modified version of the finite set statistics-based Probability Hypothesis Density (PHD) filter is developed specifically for the optical multi-target tracking of objects in the near-Earth realm for Space Situational Awareness (SAA). A two-step PHD filter is proposed in a modified version. One labeled PHD filter is used on the orthogonal image plane, in which linear dynamics in a fourparameter state is employed, forming so-called tracklets. Tracklets are associated sets of a few closelyspaced observations covering a negligible part of the overall orbit. Furthermore, tracklets are fed into a second PHD filter in a modified measurement update version, utilizing the full near-Earth astrodynamics with a six parameter state. In the modification, each tracklet leads to only one update in the PHD, but all observations within the tracklet are processed in the single target Markov transition process within the filter. In this case, the single target filter is an Extended Kalman Filter. In addition, the birth process that has been usually in typical SSA applications shifted to the birth step, forcing a data-driven birth with the disadvantage of a severe model mismatch, back to the propagation step, as in the original PHD filter formulation, avoiding the mismatch. In order to overcome the lack of probabilistic description availability (one of the triggers of the shift to the datadriven update step of previous authors), the data is preprocessed. This has the advantage that birth can employ traditional initial orbit determination methods and does not have to rely on the initialization with an incomplete state using, e.g., an admissible regions approach. The results are generated using the optical data of the DLR SMARTnet telescope network and are compared to the DLR BACARDI data processing

Software and Hardware to Improve a Remote Telescope Station

The telescope network SMARTnet, established and operated by the German Aerospace Center (DLR) in close cooperation with the Astronomical Institute of the University of Bern (AIUB), is conceptualised to collect and distribute data from telescope stations. Contributing stations owned and operated by DLR are running remotely and autonomously. When there is no regular personnel at hand, operational tasks include supervising hardware and software states as well as the environmental conditions. The decision in favour of or against performing observations is made by the software alone and shall be based on well-defined criteria. A decision in favour of observations may be revoked any time in the event of changing conditions. Especially in case of declining weather conditions, observations have to be stopped immediately to prevent hardware damage. Furthermore, the set-up has to be failsafe to shut down observations and station hardware in cases of emergency, e.g. power failures or dome errors. Error handling is thus a key element. If components do not receive or send answers within a reasonable time frame, observations shall be stopped or the error may be solved by the software itself. For DLR’s stations within SMARTnet, the main supervision is based on two computers running in a masterslave configuration. Only the computer in the role of the master is commanding the stations despite of both being individually capable of it. This redundancy is fundamental, and all other hardware components were integrated around this concept. Both computers are checking each other’s reachability via network connection permanently. The computer in the role of the master is checking continuously the status of the station components, the environmental conditions, the observations, and the image processing. The software for controlling the telescope and for processing the acquired images is part of a software package which had already been developed by AIUB. We will show how these software modules are incorporated into the station’s software. Due to DLR’s network security policy, a contact from any stations’ computer to the DLR network is not allowed, disabling possibilities to gain ill-intended access to DLR’s high-security projects. Therefore, a superordinate layer of software monitoring each station was established and is located at DLR. We will primarily focus on the stations’ software and explain the superordinate layer in cases where the communication to the station is involved. In this paper, we will describe adaptations of hardware components and the optimisation of the stations’ software after the start of operation in April 2017. This description will focus on improvements which were made in order to stabilise operations. To put the improvements into context, we will give a condensed overview of the prototype and demonstrator versions, which were already addressed in earlier publications. Subsequently, we will set up scenarios, in which different problems occur after the inauguration in 2017, some hindering observations, some allowing for continuation. We will describe how the system is reacting to ensure maximum safety for the equipment and the steps which are taken to resolve the initially unsolved problems which occured before 2017