Edith Maurer

TerraSAR-X Mission Planning System: Automated Command Generation for Spacecraft Operations

On June 15, 2007, TerraSAR-X was successfully launched from Baikonur, Kazakhstan. On board TerraSAR-X, a high-resolution X-band synthetic aperture radar (SAR) instrument is being operated as the primary payload. The user community requesting SAR products is composed of commercial and scientific partners as documented in a public-private-partnership agreement. The operations of the TerraSAR-X bus as well as payload operations are performed by the Mission Operations Segment (MOS). The Mission Planning System (MPS), which is a part of the MOS, has been designed to handle complex payload and standard bus operations in an automated manner. The purpose of this paper is to describe the concepts and the TerraSAR-X realization of the MPS.

The joint TerraSAR-X / TanDEM-X mission planning system

This paper recalls the essential system requirements and elements for the joint TerraSAR-X / TanDEM-X mission planning system. Its commissioning approach, tests and results are described in detail.

Reversal of TanDEM-X’s Relative Motion from Counter-Clockwise to Clockwise

TerraSAR-X and TanDEM-X were launched in 2007 and 2010 in a polar low Earth orbit for the purpose of Earth Observation with Synthetic Aperture Radar. Both satellites fly in a sun-synchronous dusk-dawn orbit at a mean altitude of 514km. Main incentive of the combined mission is the generation of a digital elevation model with unprecedented accuracy on global scale of the Earth’s land surface. For retrieval of 3D data the payloads on both spacecraft are operated since end of 2010 in a combined, so-called bi-static mode, requiring close spatial proximity of TerraSAR-X and TanDEM-X for payload synchronization reasons. TerraSAR-X is maintained on a stringent reference orbit with an eleven day repeat cycle whereas TanDEM-X describes a close helix around TerraSAR-X. Typical inter-satellite distances range from 150 m to several hundreds of meters. The spacecraft are mainly separated horizontally at the equator crossings and mainly radially over polar region. The relative motion is accomplished by means of slightly differing inclination and eccentricity vectors. During the first three years TanDEM-X had been orbiting in counter clockwise motion (as seen in flight direction) around TerraSAR-X. In August 2013 the relative motion was inverted to a clockwise motion, which is referred to as swap. This campaign opened up the possibility for ground target scanning from the opposite point of view as compared to images acquired in the pre-swap phase. In general, 3D radar imagery requires a differing perspective of TerraSAR-X and TanDEM-X on the ground target. Since TanDEM-X is once orbiting around TerraSAR-X in the course of an Earth orbit, the difference in the perspective, the radar baseline, depends on the argument of latitude. Thus, two of four quarters of an orbit, the ones with most distinct baseline, are well suited for 3D imagery. Until August 2013 bi-static data taking was restricted to northern hemisphere targets in ascending orbit parts whereas southern hemisphere targets have been scanned in descending orbit fractions. Inverting the relative motion opens up the possibility to gain complementary datasets from northern hemisphere targets in now descending orbits and southern hemisphere targets in ascending orbits. Especially in mountainous regions this additional information is mandatory for the generation of the digital elevation model in order to compensate radar shadow and layover effects. This paper focuses on the practical implementation of the swap campaign. The maneuver sequence and the transition from original to final formation are presented in graphical form. The operations concepts with special emphasis on safety aspects that are of paramount importance at such close distance are detailed. Additionally, space craft configuration steps required for post-swap payload operations are described.

Multi-Mission Synergies in Routine Operations of Low Earth Orbiting Satellites

The German Space Operations Center currently operates five low Earth orbit satellites in routine phase. The supported missions are the GRACE-mission (two satellites with an along track separation of 200km), the TerraSAR-X/TanDEM-X mission (two satellites in close formation flight at few hundred meters distance) and the Firebird mission with an infra-red camera on the spacecraft TET-1. The Firebird mission will be extended by a second spacecraft BIROS in near future. Effort has been spent to exploit synergy potentials in operations of these spacecraft. Since 2014 they are controlled in a multi-mission control room to facilitate combined operations for the multi-mission flight support team. The concept of the multi-mission layout of the control room will be described in this paper. Control room activities of low Earth orbiting satellites are driven by ground stations passes. Maximal synergies are possible whenever the ground station passes of the different missions are homogenously distributed over the day. In this case a minimum of multi-mission flight personnel is able to support the different missions sequentially. However, the timing of the ground station passes may not be chosen freely as the visibility times of ground stations are given by the combination of orbit and the geolocation of the available ground stations. In order to avoid conflicts with support times of other satellites a choice between different visibilities in sub sequential orbits is, in some cases, compliant with the mission’s operations concept and requirements. Another option might be the selection of alternative ground stations in different parts of the world. The operational integration of new ground stations in the mission’s network with an appropriate connection line is a precondition. The missions TerraSAR-X/TanDEM-X and GRACE comprise two spacecraft each. In both cases the spacecraft orbit fly in close spatial proximity and both missions use ground stations of the German Aerospace Center (DLR) in Germany, namely Weilheim in southern part, and Neustrelitz in northern part. One ground station supports the first satellite of the mission, the other ground station the second satellite of the same mission. As a consequence the support times in the control room for the two spacecraft are practically identical. In order to open-up the possibility to support parallel passes with a minimum of staff the operational task during passes needs to be reduced and simplified. This is done by assistance of automatic processes taking care of certain pass preparation functions, commanding and post pass activities or by the restriction of active interaction to one satellite only. The concept ideas are described in the paper. A further complication exists by the fact that the satellites of the GRACE mission do not have a sun synchronous orbit. The passes of the Grace satellites move in daytime in contrast to the ones of the TanDEM-X/TerraSAR-X mission and the TET-1 spacecraft. As a consequence the overall support pattern changes from day to day. An extreme accumulation of support times is possible. In order to avoid extreme situations a multi-mission pass planning process has been initiated.

Operational aspects of the TanDEM-X Science Phase

In the years 2010-2014 the satellites TSX and TDX collected all the Synthetic Aperture Radar (SAR) data necessary to fulfill the primary TanDEM-X mission objective: the generation of a global digital elevation model with unprecedented accuracy. In September 2014, when the necessary data set was almost complete, a transition to the so-called science phase took place. Its focus was the implementation of the TanDEM-X secondary mission objectives. TSX and TDX fly in close formation in low Earth orbit in order to form a SAR interferometer in space with adjustable interferometric baselines. Due to the diversity of scientific applications, the science phase was marked by several baseline and hence formation changes and, in addition, by unusual formation geometries. Modifications to the proven operational handling of SAR payloads and the data downlink became necessary as well as the adaptation of existing safety concepts. Furthermore, the transition from one baseline setting to the other had to be managed operationally safe and in such a way that downtimes were minimal.

Ten Years of TerraSAR-X Operations

The satellite of the TerraSAR-X mission, called TSX, was launched on 15 June 2007 and its identically constructed twin satellite TDX, which is required by the mission TanDEM-X, launched on 21 June 2010. Together they supply high-quality radar data in order to serve two mission goals: Scientific observation of Earth and the provisioning of remote sensing data for the commercial market (TerraSAR-X mission) and the generation of a global digital elevation model (DEM) of Earth’s surface (TanDEM-X mission). On the occasion of the 10th anniversary of the mission, the focus will be on the development of the TerraSAR-X system during this period, including the extension of the ground segment, the evolution of the product portfolio, dedicated mission campaigns, radar experiments, refinement of the satellite operations and orbit control, and the results of the performance monitoring. Despite numerous interventions in the overall system, we managed to incorporate new scientific and commercial requirements and to improve and enhance the overall system in order to fulfill the increasing demand for Earth observation data without noticeable interruptions to ongoing operations.

Flying with an Umbrella: Operational Strategies for the Tandem-L Mission

Tandem-L is a highly innovative satellite mission consisting of two identical satellites, which shall fly in a close formation in low earth orbit. The mission’s goal is to monitor a wealth of dynamical parameters in the main realms of our environment: The Biosphere, Geosphere, Cryosphere, and Hydrosphere. Key to the mission is a novel design of the radar instrument, which operates in L-band and includes a large deployable reflector. On the one hand the design incorporating the reflector leads to considerable improvements in terms of sensitivity of the instrument and of spatial resolution. On the other hand it raises the bar for the satellite architecture and mission operations significantly. This paper gives an overview of the planned mission, illustrates the distinctive features of the twin Tandem-L satellites, and focuses on requirements on the attitude and orbit control system. It further presents operational strategies, which are in large parts derived from flight heritage of TerraSAR-X and TanDEM-X at the German Space Operation Center.

Tandem-L: Main results of the phase a feasibility study

Tandem-L is a highly innovative SAR satellite mission for the global observation of dynamic processes on the Earths surface with hitherto unknown quality and resolution. Thanks to its novel imaging techniques and its unprecedented acquisition capacity, Tandem-L will deliver urgently needed information for the solution of pressing scientific questions in the areas of the biosphere, geosphere, cryosphere and hydrosphere. The feasibility of Tandem-L has been analyzed and confirmed in the scope of a phase A study, which has been conducted in close cooperation between the German Aerospace Center (DLR) and the German space industry. This paper provides an overview of the Tandem-L mission concept and summarizes the actual development status.