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This title analyzes distributed Earth observation missions from different perspectives. In particular, the issues arising when the payloads are distributed on different satellites are considered from both the theoretical and practical points of view. Moreover, the problems of designing, measuring, and controlling relative trajectories are thoroughly presented in relation to theory and applicable technologies. Then, the technological challenges to design satellites able to support such missions are tackled. An ample and detailed description of missions and studies complements the book subject.




Chapter 1. Bistatic Synthetic Aperture Radar

Bistatic Synthetic Aperture Radar represents an active research and development area in radar technology. In addition, Bistatic and Multistatic SAR concepts are tightly related to formation flying and distributed space missions that also represent the new space-based remote sensing and surveillance frontiers. This chapter introduces Bistatic SAR, in particular by comparing its peculiarities, operation and performance with respect to conventional monostatic SAR. Some basic concepts of bistatic SAR image formation and the main elements of bistatic SAR geometry are preliminary presented. Performance parameters are then analyzed, including geometric resolution, radiometric resolution and bistatic radar equation. Special emphasis is placed on analytical methods to evaluate the effects of bistatic SAR geometry on image resolution. Further implementation issues, such as footprint, time and phase synchronization are also pointed out. The analysis of past bistatic radar and bistatic SAR experiments and proposed spaceborne bistatic SAR missions supplies essential information to understand how these issues have been faced and can be potentially solved in ongoing and future operational systems. Finally, several scientific applications of bistatic SAR are outlined taking advantages of different techniques and methods.
Antonio Moccia, Alfredo Renga

Chapter 2. Multistatic Radar Systems

This chapter discusses the key elements in the design of a distributed multistatic synthetic aperture radar mission. A number of application domains are discussed, with an emphasis on single- and multi-baseline interferometric techniques, deriving lower and upper bounds to the required spacecraft separation. Several multistatic formation concepts (Cartwheel, Helix, etc.) are discussed within the general framework provided by the Clohessy-Wiltshire equations. Several canonical multistatic acquisition modes (bistatic, alternate bistatic, etc.) are introduced, and the particularities of standard SAR modes in a distributed mission scenario are discussed. Here, the drawbacks of burst-modes such as ScanSAR or TOPS in a multistatic configuration are highlighted. Relevant theory with regard to oscillator phase noise is introduced and followed by a discussion of several phase synchronization approaches. These include TanDEM-X like synchronization links, which are the preferred option for most high frequency mission concepts, and GNSS or data driven approaches, which may be adequate for less demanding or for lower frequency systems. At the end of the chapter, three novel proposed missions are discussed: the high-end Tandem-L mission; SIGNAL, a compact Ka-band mission; and PICOSAR, a C-band low cost passive add-on mission concept.
Paco López-Dekker, Gerhard Krieger, Alberto Moreira


Chapter 3. Relative Trajectory Design

An analysis of orbital relative motion models is presented with emphasis on their application to formation design. Relative motion model evolution from the first Hill’s schematization (circular orbit, close satellites) is described, considering the inclusion of chief’s orbit eccentricity and orbital perturbations. In particular, the inclusion of J2-secular effects is treated in depth considering various approaches in literature. Literature is also reviewed for both small and large eccentricities. Further details are presented to model formations with small chief’s eccentricity (order of 10−3), which are typical of Earth observation missions, for both the case of close formations, i.e. with satellite distance of the order of tens of kilometers, and large formations, i.e. satellite distance up to hundreds of kilometers. Finally, design applications are presented, with derivation of relative trajectories from application requirements. As an example, relative orbits for SAR interferometry are derived from the requested altitude measurement uncertainty and considering different candidate geometries (pendulum, cartwheel, etc.). Relative orbits for SAR tomography and large baseline bistatic SAR applications are also analyzed.
Marco D’Errico, Giancarmine Fasano

Chapter 4. Formation Establishment, Maintenance, and Control

This chapter presents continuous and impulsive control methods for formation initialization, maintenance, and reconfiguration. For two-body, circular reference orbits, elementary impulsive control schemes are developed based on the available state transition matrix for relative motion. Formation propagation and control models are presented in the space of differential orbital elements and Cartesian/curvilinear coordinate systems. The J 2-perturbation effects are conveniently modeled with the mean elements and their secular drift rates. Methods for accommodating the disturbance due to the J 2 by modification of the relative orbit initial conditions are discussed. Examples provided include multi-impulse optimal formation initialization maneuvers and a novel inter-satellite fuel balancing concept.
Srinivas R. Vadali, Kyle T. Alfriend

Chapter 5. GPS Based Relative Navigation

The use of Global Positioning System (GPS) measurements provides the primary technique for determining the relative position of cooperative, formation-flying satellites in low Earth orbit. Similar to terrestrial applications, the relative navigation benefits from a high level of common error cancellation. Furthermore, the integer nature of double-difference carrier phase ambiguities can be exploited in carrier phase differential GPS (CDGPS). Both aspects enable a substantially higher relative accuracy than can be achieved in single-spacecraft navigation. Following an overview of spaceborne GPS receivers, the dynamical and measurement models for relative navigation using single- or dual-frequency measurements are presented along with a discussion of estimation schemes for real-time and offline applications. Actual flight results from the TanDEM-X and PRISMA missions are presented to demonstrate the feasibility of mm-level post-facto baseline determination and cm-level real-time navigation using CDGPS.
Oliver Montenbruck, Simone D’Amico

Chapter 6. Radio Frequency-Based Relative Navigation

For distributed systems in space, knowledge of the relative position and velocity is required to maintain the relative geometry of the satellites within certain boundaries. This knowledge can be obtained using an autonomous relative navigation system based on radio frequency (RF) signals. The design aspects of such a system are detailed in this chapter. As all RF-based relative navigation systems are based on GNSS technology, the discussion is limited to this technology only. Navigation is performed by measuring the range (rate) between the satellites, which allows, in combination with a relative dynamics model and the exchange of data between the satellites, an onboard estimation of the relative state of the satellites. Obtaining accurate and unambiguous measurements requires a balanced signal design that minimizes measurement errors, but which also takes into account multiple access and formation safety considerations. Hardware-induced measurement biases should be minimized and hardware (self-) calibration is mandatory to achieve satisfactory performance in space. Details on the design, testing, and performance of one particular system, the FFRF, are provided.
D. Maessen, E. Gill, T. Grelier, M. Delpech

Chapter 7. Vision Based Relative Navigation

Vision is a key technology for the relative navigation of formation flying satellites especially when they operate in close proximity. Indeed, with a vision system the relative position and attitude (usually referred to in the machine vision literature as “pose”) of co-flying satellites can be extracted in real-time. This information can be used either to maintain or change the formation geometry. A crucial aspect affecting vision system design and development is the operation with largely variable lighting conditions and the interference caused by the presence of other celestial bodies in the sensor field of view. This requires the implementation of effective image processing techniques and algorithms by which robust, accurate and reliable pose estimation can be achieved. This chapter provides an overview of sensors, techniques and algorithms enabling the relative navigation based on vision, with specific reference to space missions which have already tested in flight this technology.
Domenico Accardo, Giancarmine Fasano, Michele Grassi


Chapter 8. Autonomy

Missions involving multiple spacecraft, autonomously working together, have become of great interest in the last decade as they offer a number of scientific and engineering advantages. This trend is responsible for an increasing demand on mission planning and scheduling systems able to coordinate the different spacecraft and to allocate tasks amongst them. New approaches are therefore needed to handle this new level of complexity, combining together autonomous solutions for the ground and space segment. The chapter is organized as follows: after an introduction on the motivations of using autonomy solutions for space missions, sect. 8.2 presents a short survey on the applications of autonomy in space Operations. The section identifies the focus of the chapter in Mission Planning and Scheduling, one the most critical aspect for distributed mission. Section 8.3 gives the state of the art regarding mission planning and scheduling applications first on single platforms and then on distributed platforms. Section 8.4 explains the most popular technology for distributed missions, the multi agent paradigm. The section is not meant to cover all the aspects of this technology. It focuses instead on the challenges related to the new trends of self-organizing systems and natural-inspired approaches, techniques that can offer promising advantages for distributed missions.
Claudio Iacopino, Phil Palmer

Chapter 9. Relative Navigation

Satellite formations are being considered for a large variety of current and future space missions including in-orbit inspection, SAR interferometry, magnetospheric observation and gravimetry. In the case of cooperative satellite formations, differential GPS, radiofrequency and optical navigation techniques have been demonstrated as viable approaches for relative navigation on a number of recent space missions. Future challenges include accurate relative navigation and positioning in six degrees of freedom, with the limited power and computational resources of small satellites. This article explains the relative navigation requirements and their dependency on the space applications. The software and hardware challenges on relative navigation for future satellite formations are also described.
Nadjim Horri, Phil Palmer

Chapter 10. Communication in Distributed Satellite Systems

The information flow between the different components of a distributed mobile sensor system is crucial in order to enable coordination for an efficient overall performance. This section provides further details for the special situation of network nodes consisting of several satellites and ground stations. Via the communication system partially autonomous functions at each satellite are to be coordinated to enable joint observations. Such self-organized activities of the space segment have to integrate with teleoperations based on supervisory control interaction from ground stations. Related suitable communication design approaches are the central topic of this chapter.
Klaus Schilling, Marco Schmidt

Chapter 11. Ground Station Networks for Distributed Satellite Systems

A space mission is typically divided in space and ground segment, the focus of this chapter lies on ground station networks. Especially highly distributed ground station networks offer new opportunities for the operation of distributed satellite systems. Actual networking concepts are presented and research challenges in the field of ground station networking are discussed.
Marco Schmidt, Klaus Schilling


Chapter 12. Overview of Distributed Missions

Former studies of missions to perform SAR interferometry and multistatic SAR applications are presented and detailed. Missions and studies to perform Earth observations by optical remote sensing are also reported.
Maria Daniela Graziano

Chapter 13. TanDEM-X

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) is a highly innovative Earth observation mission that opens a new era in remote sensing. TanDEM-X comprises two formation flying satellites, each equipped with a synthetic aperture radar (SAR) to map the Earth’s surface with high spatial resolution. Together, the two satellites form a unique single-pass SAR interferometer, offering the opportunity for flexible baseline selection. Primary objective of TanDEM-X is the acquisition of a global digital elevation model (DEM) with unprecedented accuracy and resolution (12 m horizontal and 2 m vertical resolution). Besides the primary mission goal, several secondary objectives based on along-track interferometry and new bistatic SAR techniques have been defined, representing a further important asset of the mission. TanDEM-X was successfully launched in June 2010 and started operational data acquisition in December 2010. This chapter outlines the TanDEM-X mission concept and its implementation, summarizes the main data processing and calibration steps, and provides an overview of the actual performance and mission status. Furthermore, results from several scientific experiments are presented, showing the great potential of future formation flying interferometric SAR missions to serve a wide spectrum of novel applications.
G. Krieger, M. Zink, M. Bachmann, B. Bräutigam, H. Breit, H. Fiedler, T. Fritz, I. Hajnsek, J. Hueso Gonzalez, R. Kahle, R. König, B. Schättler, D. Schulze, D. Ulrich, M. Wermuth, B. Wessel, A. Moreira

Chapter 14. Cartwheel

Radar interferometry gained a lot of interest in very few years in the 1990s. Its main product: the interferogram, is a map of the difference of the phases of two radar images acquired on the same site with a time elapsed between the takes that can range from zero to several years. The phase maps are ambiguous, like contour lines that would not carry any number. They need to be “unwrapped” (i.e. assigned a number) which is generally done by continuity. Several SAR system concepts have been dedicated to interferometry. The Cartwheel concept aims at maximizing the interferometric return of a conventional satellite by adding a cheap constellation of receive-only micro-satellites in a special, and very efficient, orbital configuration, which disturbs the least, or not at all, the Transmitter. Here we describe the system from its design to some specificities of the processing of the products, some of them offering unique new capabilities. Finally, we suggest some more advanced uses of the design.
Didier Massonnet

Chapter 15. Sabrina

SABRINA mission was conceived as a dual satellite mission based on COSMO/SkyMed constellation to perform and exploit bistatic Synthetic Aperture Radar in both interferometric and large baseline modes. Analysis of identified application and techniques are presented along with the relative trajectory selection, pointing strategies and safety.
Antonio Moccia, Marco D’Errico, Alfredo Renga, Giancarmine Fasano

Chapter 16. TOPOLEV and C-PARAS

The aim of the European Space Agency funded study on “Concepts for demonstration of advanced techniques and technologies on an EO small mission” was to assess ideas for EO missions compatible with implementation on a small satellite such as PROBA, and which may benefit from Formation Flying. The output of the study was a definition of various small satellite missions and their required developments. The study was led by Astrium Ltd, with support from Astrium SAS, Astrium GmbH, ENVEO, GMV and Verhaert Space. Following initial selection in the first part of the study, three candidates were analysed in detail in Phase 2. Of these three candidates, two of the missions (the Topographic Levelling mission “TOPOLEV”, and C-band PAssive RAdar Satellite(s), “C-PARAS”) require Formation Flying for single pass SAR interferometry, and these are presented in this chapter.
Tony Sephton, Alex Wishart

Chapter 17. The SAR Train

The concept implements the coherent combination of N separate SAR flying along a same orbital arc as seen from ground. A “Signal Cleaning” mode of SAR train keeps unchanged the antenna area requirement of each individual SAR and brings a factor N advantage that applies on SNR and ambiguity protection. The main formation flying constraint is the width of the tube containing the satellite trajectories. The multiplication by N of the total antenna area is the other counterpart to these advantages. A “Antenna Dilution” mode of SAR train enables the distribution of an unchanged total antenna area into N smaller elementary antennas, together with the multiplication by N of the SAR Merit Factor (Swath over Resolution ratio). With respect to the first mode, the tube width constraint is increased and the space-time separation along the track has to be very accurate. Use of appropriate spread spectrum waveforms instead of conventional pulse waveforms removes the major part of the extra orbit constraints introduced by the “antenna dilution” class. A train of N SAR in visibility with a single transmit makes the concept more robust against lost of coherence and eases the metrology of the formation (DGPS). Moreover the global energy efficiency is increased by N since with only a single transmit SAR the same performance is achieved. However, the along track separation constraints for antenna dilution are made more stringent because restrained to the space domain, which reinforces the spread spectrum interest. As part of its applications, the concept can circumvent the matter of huge antenna size for SAR mission in very low frequency (P band) or at high altitude (surveillance).
Jean Paul Aguttes

Chapter 18. P-Band Distributed SAR

This chapter discusses a spaceborne P-band synthetic aperture radar concept based on a distributed architecture and formation flying technologies. This approach can in principle allow overcoming physical constraints that limit the performance of monolithic SARs, leading in the P-band case to huge antennas and hard swath/resolution trade-offs. The proposed SAR is based on a larger transmitting satellite and a set of lightweight receiving-only platforms. This architecture also enables multi-mission capabilities. In particular, forests observation and biomass estimation based on side-looking SAR data can be in theory combined with near nadir interferometric ice sounding. Payload concept is clarified, and a preliminary performance analysis in terms of ambiguity and coverage is proposed. Then, mission analysis, preliminary spacecraft design, and formation control architecture are briefly described.
Giancarmine Fasano, Marco D’Errico, Giovanni Alberti, Stefano Cesare, Gianfranco Sechi

Chapter 19. GRACE

The GRACE (Gravity Recovery And Climate Experiment) satellite mission is in general to a large extent based on hardware and experience of the German CHAMP (CHAllenging Minisatellite Payload) satellite mission with the addition of a second satellite plus an ultra-precise intersatellite K-band link. Since March 2002 the two identical spacecrafts are flying on the same orbit 220 (±50) km apart. The two satellites weigh 485 kg each and were launched to an orbit with an initial altitude of 500 km. After a brief mission overview the main scientific results are presented as well as operational aspects dedicated to the GRACE mission. Here two aspects are addressed more in detail from the flight dynamics point of view, the formation keeping and the swap of the satellite position.
Michael Kirschner, Franz-Heinrich Massmann, Michael Steinhoff

Chapter 20. Next Generation Gravity Mission

After the successful experience of the gravity missions GRACE and GOCE, several activities are on going in preparation of a “Next Generation Gravity Mission” (NGGM) aimed at measuring the temporal variations of the Earth’s gravity field over a long time span (up to ~11 years) with high spatial resolution (comparable to that provided by GOCE) and high temporal resolution (weekly or better). Its data will find wide application in geodesy, geophysics, hydrology, ocean circulation and many other disciplines. The most appropriate measurement technique identified for such mission is Low-Low Satellite-Satellite Tracking in which two (or more) satellites flying in “loose” formation in a low Earth orbit act as proof masses immersed in the Earth gravity field. The distance variation between the satellites (measured by a laser interferometer) and the non-gravitational accelerations of each satellite (measured by ultra-sensitive accelerometers) are the fundamental observables from which the gravity field is obtained. Suitable satellite formations for this mission include the “In-line” (the simplest one), the “Cartwheel” and the “Pendulum” (more complex but also scientifically more fruitful), with an inter-satellite distance up to 100 km. Polar, circular orbits with altitudes between ~340 and ~420 km are suitable candidates for the NGGM, providing all-latitude coverage, short repeat cycles/sub-cycles and a still excellent gravity signal compatibly with a long lifetime. Each satellite shall be endowed with a complex control system capable of carrying out several tasks in close coordination: orbit maintenance, formation keeping, provision of a “drag-free” environment to the accelerometers, laser beam pointing and attitude control.
Stefano Cesare, Gianfranco Sechi

Chapter 21. PRISMA

PRISMA is a precursor mission for formation-flying and on-orbit-servicing critical technologies. It consists of two spacecraft launched clamped together in low Earth orbit and separated in space after the commissioning phase in August 2010. The mission represents a unique in-orbit test-bed for guidance, navigation, and control (GNC) algorithms, novel relative navigation sensors (GPS, radio-frequency, vision-based), as well as new propulsion systems (high performance green propellant, micro-electro-mechanical). Originating from an initiative of the Swedish National Space Board, PRISMA is run by OHB Sweden with important contributions by the German Aerospace Center, the French Space Agency, and the Technical University of Denmark. After a brief overview of motivations, partners, and objectives, the chapter starts with a comprehensive description of the mission, including spacecraft platform, formation-flying and rendezvous sensors and actuators, as well as GNC key modes and algorithms. The discussion is followed by a summary of the main project phases, including overall schedule, verification process, and mission operations. Actual flight results from the basic PRISMA mission and its numerous GNC experiments are presented along with the achieved relative navigation and control accuracies over a broad range of autonomous operations between 30 km and nearly zero inter-spacecraft separation.
Simone D’Amico, Per Bodin, Michel Delpech, Ron Noteborn

Chapter 22. Formation for Atmospheric Science and Technology Demonstration

FAST (Formation for Atmospheric Science and Technology demonstration) is a cooperative Dutch-Chinese formation flying mission led by Delft University of Technology (TU Delft) in the Netherlands and Tsinghua University in China. FAST shows the potential of a first international micro-satellite formation flying mission in three distinct fields: technology demonstration, earth science and space education. Here, the FAST mission is presented. The mission scenario consisting of different formation flying stages is described, and the system design of both the space and the ground segments is introduced, with emphasis on Dutch contributions. Some key technical issues related to autonomous formation flying are also addressed.
Jian Guo, Daan Maessen, Eberhard Gill

Chapter 23. Future Trend, Potential, Risks

During the last decades, the concept of distributed space systems has significantly progressed in terms of space applications, including Earth remote sensing. This chapter is devoted to a critical analysis of the achieved improvements and of the areas of major key issues in order to analyze potential and risks of distributed space systems. A discussion of future activities needed to prepare more advanced distributed space missions is also provided. In particular, payloads and applications are first discussed. Then, guidance, navigation, and control as well as other technological challenges, including modularity and architecture, follow.
Marco D’Errico, Eberhard Gill, Antonio Moccia, Rainer Sandau


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