Flight Dynamics Operations of the TanDEM-X Formation

Since end of 2010 the German TerraSAR-X and TanDEM-X satellites are routinely operated as the first configurable single-pass Synthetic Aperture Radar interferometer in space. The two 1340 kg satellites fly in a 514 km sun-synchronous orbit. In order to collect sufficient measurements for the generation of a global digital elevation model and to demonstrate new interferometric SAR techniques and applications, more than three years of formation flying are foreseen with flexible baselines ranging from 150 m to few kilometers. As a prerequisite for the close formation flight an extensive flight dynamics system was established at DLR/GSOC, which comprises of GPS-based absolute and relative navigation and impulsive orbit and formation control. Daily formation maintenance maneuvers are performed by TanDEM-X to counterbalance natural and artificial disturbances. The paper elaborates on the routine flight dynamics operations and its interactions with mission planning and ground-station network. The navigation and formation control concepts and the achieved control accuracy are briefly outlined. Furthermore, the paper addresses non-routine operations experienced during formation acquisition, frequent formation reconfiguration, formation maintenance problems and space debris collision avoidance, which is even more challenging than for single-satellite operations. In particular two close approaches of debris are presented, which were experienced in March 2011 and April 2012. Finally, a formation break-up procedure is discussed which could be executed in case of severe onboard failures

The TerraSAR-X Mission and System Design

This paper describes the TerraSAR-X Mission Concept within the context of a public-private-partnership (PPP) agreement between the German Aerospace Center DLR and industry. It briefly describes the PPP-concept as well as the overall project organization. The paper then gives an overview of the satellite design, the corresponding Ground Segment as well as the main mission parameters. After a short introduction to the scientific and commercial exploitation scheme, the paper finally focuses on the mission accomplishments achieved so far during the ongoing mission

First In-orbit Experience of TerraSAR-X Flight Dynamics Operations

TerraSAR-X is an advanced synthetic aperture radar satellite system for scientific and commercial applications that is realized in a public-private partnership between the German Aerospace Center (DLR) and the Astrium GmbH. TerraSAR-X was launched at June 15, 2007 on top of a Russian DNEPR-1 rocket into a 514 km sun-synchronous dusk-dawn orbit with an 11-day repeat cycle and will be operated for a period of at least 5 years during which it will provide high resolution SAR-data in the X-band. Due to the objectives of the interferometric campaigns the satellite has to comply to tight orbit control requirements, which are formulated in the form of a 250 m toroidal tube around a pre-flight determined reference trajectory. The acquisition of the reference orbit was one of the main and key activities during the Launch and Early Orbit Phase (LEOP) and had to compensate for both injection errors and spacecraft safe mode attitude control thruster activities. The paper summarizes the activities of GSOC flight dynamics team during both LEOP and early Commissioning Phase, where the main tasks have been 1) the first-acquisition support via angle-tracking and GPS-based orbit determination, 2) maneuver planning for target orbit acquisition and maintenance, and 3) precise orbit and attitude determination for SAR processing support. Furthermore, a presentation on the achieved results and encountered problems will be addressed

First In-Orbit Experience of TerraSAR-X Flight Dynamics Operations

TerraSAR-X is an advanced synthetic aperture radar satellite system for scientific and commercial applications that is realized in a public-private partnership between the German Aerospace Center (DLR) and the Astrium GmbH. TerraSAR-X was launched at June 15, 2007 on top of a Russian DNEPR-1 rocket into a 514 km sun-synchronous dusk-dawn orbit with an 11-day repeat cycle and will be operated for a period of at least 5 years during which it will provide high resolution SAR-data in the X-band. Due to the objectives of the interferometric campaigns the satellite has to comply to tight orbit control requirements, which are formulated in the form of a 250 m toroidal tube around a pre-flight determined reference trajectory (see [1] for details). The acquisition of the reference orbit was one of the main and key activities during the Launch and Early Orbit Phase (LEOP) and had to compensate for both injection errors and spacecraft safe mode attitude control thruster activities. The paper summarizes the activities of GSOC flight dynamics team during both LEOP and early Commissioning Phase, where the main tasks have been 1) the first-acquisition support via angle-tracking and GPS-based orbit determination, 2) maneuver planning for target orbit acquisition and maintenance, and 3) precise orbit and attitude determination for SAR processing support. Furthermore, a presentation on the achieved results and encountered problems will be addressed

TDP1 Ground System Design

This paper illustrates the historical development of the TDP-1 ground segment, the system as implemented and operational experience, as well as an outlook to future programs. Aim of the project TDP-1 – Technology Demonstration Payload No.1 - is the demonstration of a data relay service, using an optical High Data Rate Inter-Satellite Link (ISL) between a Laser Communication Terminal (LCT) flown on a low earth orbiting satellite (LEO-LCT) and a second LCT (GEO-LCT) placed on the geostationary communication satellite AlphaSat (of INMARSAT) . The LCT planning system consists of one geostationary satellite (GEO) and up to five low orbiting satellites (LEO) which are also referred to as customers. The main task of GEO within this system is to serve as service provider for the LEOs and one optional optical ground station (OGS). The service consists of an optical data link between the Laser Communication Terminals (LCT) of the satellites (inter-satellite-link,ISL) and a link from a satellite to a ground station (space-to-ground-link, SGL). DLR’s Operations Center (GSOC) role in the TDP-1 program includes design, development and integration of ground infrastructure and operations of the satellites and ground stations. GSOC already gained experience operating Laser Terminals in test scenarios on the TerraSAR spacecraft. This knowledge was be used to develop the TDP-1 operations concept. One major task is the planning of the laser connections and the required coordination between all parties. This paper will illustrate the development from the first activities at GSOC in connection with laser data transfer through the design of the TDP-1 system to an outlook at the EDRS operations concept

Accuracy comparison of Pléiades satellite ortho-images using GPS device based GCPs against TerraSAR-X-based GCPs

Conducting single frame orthorectification on satellite images to create an ortho-image requires four basic components, namely an image, a geometric sensor model, elevation data (for example a digital elevation model (DEM)) and ground control points (GCPs). For this study, orthorectification entailed the use of a single scene Pléiades primary panchromatic image, applying the Pléiades rigorous geometric model, utilising a high-quality 2 m DEM and using GCPs that were acquired from two different collection methods. The application of these different GCPs to the execution of orthorectification encompassed the aim of this paper, which was to investigate and compare the positional accuracies of ortho-images under two scenarios. Firstly, GCPs were manually collected through fieldwork utilising a Trimble GeoExplorer 6000 series handheld GPS device and secondly, by utilising TerraSAR-X based GCPs that were acquired from Airbus Defence and Space. The objective of this study was to determine the geolocation accuracy of a high-resolution satellite ortho-image when different types of ground control are used. This required the execution of two orthorectification tests where only the type of GCPs differed. The results of these tests were interesting since it highlighted the difference in positional accuracy when utilising various sources of ground control to perform orthorectification on satellite imagery. The comparison results showed that utilising the manual GCPs produced a better positional accurate ortho-image as opposed to using the TerraSAR-X based GCPs. Nonetheless, the TerraSAR-X based GCPs still produced a sub 2 m accurate ortho-image, which is more than sufficient for the production of most geospatial products.Keywords: orthorectification, digital elevation model (DEM), ground control point (GCP), high-resolution satellite imagery, TerraSAR-X based GCPs, WorldDEM™, Airbus Defence and Spac

Flight Dynamics Experience on Target Orbit Acquisition and Maintenance Operations for Germany's Hyperspectral Satellite Mission EnMAP

The Environmental Mapping and Analysis Program (EnMAP) is a German hyperspectral satellite mission that aims at monitoring and characterizing Earth's environment on a global scale. The satellite was successfully launched with SpaceX's Falcon 9 Transporter-4 mission on April 1st, 2022. This paper elaborates on the in-flight results obtained during the EnMAP Launch and Early-Operations Phase (LEOP) and the first months of the commissioning phase. Besides the flight dynamics operations, this paper addresses the repeat ground-track orbit control concept and discusses novel flight dynamics functionalities implemented to optimize the scientific return of the EnMAP mission, such as microservices for fast data exchange between the flight dynamics and mission planning systems

Antenna Modeller for Synthetic Aperture Radar Applications. Electromagnetic and Radiometric Considerations

The objective of the present Master Thesis is designing an optimizer of the excitation coefficients of a phased array antenna

Precise autonomous orbit control in low earth orbit: from design to flight validation

The main purpose of this research is the analysis, development and implementation of a precise autonomous orbit control system for a spacecraft in low Earth orbit. This thesis work represents a step forward in the theoretical formalization and implementation of an on-board orbit maintenance system. Two main approaches are identified for the realization of an on-board orbit control system. The first is the reconsideration and further development of state-of-the-art orbit control methods from the perspective of autonomy. A step forward is then taken in the direction of the definition of a general and rigorous formalization of the autonomous orbit control problem. The problem of the autonomous absolute orbit control is considered as a specific case of two spacecraft in formation in which one, the reference, is virtual and affected only by the Earth's gravitational field. A new parametrization, the relative Earth-Fixed elements, analogous to the relative orbital elements used for formation control, is introduced to describe the relative motion of the real and reference sub-satellite points on the Earth surface. An extensive discussion is dedicated to the reference orbit selection and generation process and the analysis of the free motion of a spacecraft in low Earth orbit. The reference orbit defines the spacecraft's nominal trajectory designed to satisfy the mission requirements. The actual orbit is kept within certain bounds defined with respect to the reference orbit. The generation process of the reference orbit is dealt in detail as it is the fundamental starting point of the orbit control chain. The free motion analysis is essential to understand the orbit perturbation environment which causes the deviation of the actual from the nominal trajectory. The use of the precise orbit determination data of the missions PRISMA and TerraSAR-X guarantee the reliability of the results of this analysis and the understanding of the orbit's perturbation environments at an altitude of 700 and 500 km. This study helps the definition of a proper control strategy. The control algorithms developed in the thesis can be divided into the two broad categories of analytical and numerical. An analytical algorithm for the maintenance of a repeat-track orbit is developed from the state-of-the-art methods and new analytical formulations for the reference orbit acquisition under different constraints and requirements are presented. The virtual formation method for the absolute orbit control is formalized by means of the relative Earth-fixed elements described previously. The state-space representation is used for the mathematical formulation of the problem. A linear and a quadratic optimal regulators, based on this model, are designed for the in-plane and out-of-plane absolute orbit control. Numerical simulations are performed for the validation of the control methods. The test platform includes a very accurate orbit propagator, the flight software and allows the simulation of actuators and navigation errors. The simulation results are evaluated from a performance and operational point of view in order to formulate a first conclusion about the advantages and disadvantages of the different control techniques. The main differences between the considered analytical and numerical control methods are outlined. The practical implementation of a precise autonomous orbit control system for a spacecraft in low Earth orbit is then described in detail. The on-board guidance, navigation and control software development, implementation and testing of the PRISMA mission, to which the author of this thesis contributed, is described. The attention is focused on the technological aspects implied by the realization of the autonomous orbit control system tested in-flight with the autonomous orbit keeping experiment on PRISMA. Among the several innovative aspects of the flight software development, some space is dedicated to the advanced software validation and testing realized on the formation flying test-bed at DLR, the German Aerospace Center, which played a fundamental role in the realization of the PRISMA mission and its experiments. Finally, the flight results of the autonomous orbit keeping experiment on the PRISMA mission, a fundamental milestone of this research work, are presented. This in-flight experiment took place in the summer of 2011 and demonstrated the capability of autonomous precise absolute orbit control using the analytical control method developed in this thesis

PINTA - one Tool to plan them all

In the recent years, the “Program for INteractive Timeline Analysis” PINTA, developed at the German Space Operation Center (GSOC), was continuously improved and experienced several evolution steps. PINTA is a GUI application running on Windows-based computer systems, whose main purpose is to serve as the anchor tool for a mission planning operation’s engineer when generating, modifying or analysing a mission timeline. This is supported by calling automatic planning algorithms of the embedded generic planning library “PLAnningTOol” PLATO, using input of the embedded orbit propagation and event calculation library “SpaceCraft Orbit and GroundTrack Analysis Tool” SCOTA, or its expandability through plugins. PINTA is the generic basis of many semi-automated mission planning systems for past, current and future spacecraft projects operated at GSOC. It is used or has been used for the missions Grace, TET-OOV, FireBird, Grace-FollowOn, Eu:CROPIS and is currently prepared for CubeL. Furthermore, PINTA serves as the timeline analysis tool for validating the TerraSAR-X/TanDEM-X mission planning system. The variety of use cases was further extended to support Launch and Early Orbit Phases (LEOPs) in its special “SoEEditor” configuration as the new generic editing tool for the so-called “Sequence of Events”. It was successfully used for the satellites Biros, HAG-1, PAZ, Grace-FollowOn 1 & Grace-FollowOn 2, Eu:Cropis, EDRS-C and is currently in preparation for EnMAP. In addition to LEOP’s, the SoEEditor was also capable of supporting the constellation maneuvers for the TerraSAR-X/TanDEM-X mission. Besides all these use cases, the paper at hand will especially describe how PINTA was even further extended to not only tackle spacecraft-based but also ground-based scheduling. On the one hand it serves as an “On-Call Tool” to support the on-call shifts by automatically generating conflict-free role-based shift plans for all subsystems by considering various constraints like person outages, working hours, role-conflicts, etc… The plan can then be further adapted manually to cope with user change-requests. On the other hand it is used as a “Multi-Mission-Control-Room-and-pass-Scheduler” (MuMiCoRoS) to coordinate the ground-station booking of all LEO (low-earth orbit) satellites: TerraSAR-X, TanDEM-X, TET, Biros, Grace-FollowOn 1 & 2 and Eu:CROPIS. In order to avoid ground-station and operator conflicts between the missions, an automatic and combined plan for all satellites is generated which can then be further modified manually if necessary. As another use case, PINTA (a.k.a. GPT; Galileo Planning Tool) supports the Galileo Service Operation (GSOp). The planning process involves three timelines: a Short-Term Plan (STP), covering the next ten days, two Mid-Term Plans (MTP) for the Operational (OPE) and the Validation (VAL) chain), covering the next 15 weeks, and a Long-Term Plan (LTP), covering the next 15 months. The activities in these timeframes cover all subsystems of Galileo: Flight Ops, Control segment, Mission segment, remote sites, service operations, hardware, software, hosting, network, etc ... In order to support the GSOp, numerous additional features, like importers, exporters, interfaces and plugins had to be added to PINTA