Distributed small satellites systems in Earth observation and telecommunication

K.Schilling

Julius-Maximilians Universität Würzburg, Germany

A paradigm shift from single large, multifunctional satellites to cooperating groups of smaller satellites can be observed in Earth observation as well as in telecommunications. Another trend is to employ modern miniaturization techniques to realize satellites at continuously smaller masses, enabling a cost-efficient realization of systems composed of multiple satellites. Such distributed satellite systems carrying coordinated heterogeneous sensors rise challenges with respect to an efficient implementation of the flow of information and its storage, as well as for optimal control strategies regarding position and attitude. In Earth observation, the innovation potential by employing a distributed network of satellites is obvious in order to provide higher temporal resolution in observation data and to achieve higher availability. Especially in emergency, surveillance and observation tasks, such robust capabilities are important. In case of telecommunications, networks of small satellites in low Earth orbits can offer a cost-efficient approach for robust communication links at low bandwidth.

1. Introduction

Distributed systems of small satellites offer interesting capabilities to complement traditional satellites. Thus in Earth observation multiple satellites can support an increase in temporal and spatial resolution. Observations of surface points from different viewing angles at long baseline distances provide the potential to derive 3-D-images by sensor data fusion approaches. In telecommunications, satellite systems in low Earth orbits offer telecommunication links at a minimum use of resources.

In addition, modern miniaturization technologies enable realization of electro-mechanical components at very small masses. Thus, satellites of few kilograms of mass can already provide interesting functionalities and services. Combination of data from groups of small satellites enables provision of high performance results despite the limitations in resources of each individual small satellite. Technology challenges to implement such innovative distributed spacecraft system concepts relate to robust telecommunication and control capabilities, as will be addressed for formations in this paper.

 

Networks of multiple satellites offer interesting benefits in applications with respect to

-         higher temporal and spatial resolution in observation data,

-         higher availability,

-         graceful degradation in case of failures.

But distributed satellites also raise challenging control and coordination requirements regarding

-         orbits at different altitudes,

-         optimal control strategies for position and attitude of the specific system components,

-         activities of heterogeneous sensors,

-         flow of information and storage in the system.

Multiple coordinated satellites are described as

Constellation, when several satellites flying in similar orbits are organized in time and space to coordinate ground coverage, without on-board control of their relative positions. They are controlled separately from ground control stations.

Formation, if multiple satellites with closed-loop control on-board provide a coordinated motion control on basis of their relative positions to preserve the topology. It is the collective use of several spacecrafts to perform the function of a single, large, virtual instrument.

Swarm or Cluster, if a distributed system of similar spacecraft is cooperating to achieve a joint goal without fixed absolute or relative positions. Each member determines and controls relative positions to the other satellites.

Examples for typical spacecraft constellations are provided in different application fields, such as navigation (GPS, GLONASS, Galileo), telecommunication (TDRSS, Iridium, Globalstar, Orbcomm, Teledesic), remote sensing (Rapid Eye). With respect to formations ESA's planned DARWIN mission points synchronously five free flying telescopes towards one target point in order to achieve enough resolution to detect planets in remote solar systems (for further details see www.esa.int). Formations thus enable higher resolution imagery and interferometry.

 

In order to perform complex tasks in a broad range of applications, groups of vehicles with varying dynamics are to be analyzed, such as groups of aircrafts, UAVs, submarines and land vehicles. In general three different architectural approaches are discussed:

Virtual Structures: the entire formation is treated as one single structure controlled by a centralized planner. The dynamics of the complete structure is translated into a desired motion for each vehicle, which has an individual tracking control.

Behavioral strategies: in this distributed control approach, following inspirations from nature (flock of birds, school of fish), several desired behaviors for each agent are specified. The control action of each agent is the weighted average of the controls for each behavior.

Leader-follower: vehicles are divided into leader(s) and followers, the followers track position and orientation of a designated reference point (leader) with a prescribed offset. It can be implemented as

absolute control architecture, where a central controller sends position and velocity commands to each vehicle regulating its own position, or as

relative control architecture sending absolute position and velocity commands of the leader, while the followers regulate their own position relative to the leader.

While there is a transparent group behavior, the leader is a particularly sensitive position.

 

Note for coordinated observations by swarms of small satellites, challenging technical research problems are to be solved. A necessary requirement is the ability of the satellites to maintain the formation. Thus the position and attitude relative to each other is to be determined with appropriate accuracy, before control actions correct towards the target position in the formation. All satellites of the swarm have to be equipped with suitable sensors and actuators to perform such maneuvers. Especially for pico- and nano-satellites there is still a need for small, low weight sensors and actuators. Within current technology it is by example not possible to integrate a star tracker at pico-satellit level, nevertheless a high accuracy attitude determination is desired. Recent activities in the field of sensor development demonstrate implementation of extremely small components by MEMS technology.

The University Wurzburg's experimental satellite employs a GPS system for position determination and subsequent orbit determination. The companion pico-satellite BEESAT from TU Berlin carries a 3-axis attitude control system by three reaction wheels. The University of Toronto will demonstrate by the CanX-2 satellite at nano-satellite level actuators for formation control by using thrusters and 3-axis-stabilized attitude control. The motivation for this mission is the test of enabling technology for formation flying. In the next step the Can-X4 and Can-X5 satellites are planned for an autonomous formation flight. Thus, future missions will perform complex formation maneuvers with pico- and nano-satellites, but there is still significant research necessary in order to establish appropriate attitude control and formation control systems for satellites in the pico- and nano-satellite class.

 

The paradigm shift from large spacecrafts incorporating multiple payload capabilities to decentralized, distributed small satellite systems raises interesting research topics. Particular advantages in the context of Earth observation and surveillance are higher fault tolerance and robustness of the overall system. Such systems are scalable in a sense that according to application needs additional satellites can be added in order to increase resolution and coverage. The current progress in gun launches (with railguns or light gas guns) to orbit promise interesting quick future reaction capabilities for very small satellites (with a mass of some kg). Nevertheless high resolution data and high bandwidth links can only be provided by traditional large satellites. Thus combinations of coordinated satellite systems composed of few large and many small satellites might complement each other in order to provide the required data quality as well as flexibility and robustness.

Swarms of small satellites offer in particular for Earth observation applications interesting innovative approaches. Satellites in Low Earth Orbit (LEO) enable high spatial resolution on ground and offer interesting potential for applications like disaster monitoring. Due to the low orbit, these satellites exhibit a high relative velocity to reference points on ground, resulting in short observation and communication contact periods in the target areas. One approach to that problem is a higher temporal resolution by satellite constellations with several satellites in the same orbit. The achievable temporal and spatial resolution of such a formation opens new application areas in bio-monitoring and surveillance.