Engineering Notes Review of Formation Flying and Con- stellation Missions Using Nanosatellites

SMALL satellites are enablingmultisatellite missions that were not otherwise possible because of their small size and modular nature [1].Multiple small satellites can be flown instead of amuch bigger and costlier conventional satellite for distributed sensing applications such as atmospheric sampling, distributed antennas [2], and synthetic apertures [3,4]. Missions with multiple small satellites can deliver a comparable or greater mission capability than a monolithic satellite, but with significantly enhanced flexibility (adaptability, scalability, evolvability, and maintainability) and robustness (reliability, survivability, and fault tolerance) [1,5]. Small satellites that weigh less than 10 kg can be broadly classified into nanosatellites (mass between 1 and 10 kg), picosatellites (mass between 0.1 and 1 kg), and femtosatellites (mass less than 100 g) [1,6]. A class of standardized nanosatellites, called CubeSats [7], range in size from 1U (10 × 10× 10 cm) to 6U (30 × 20 × 10 cm), weigh between 1 and 8 kg, and are usually launched using the standardized CubeSat deployment system called Poly Picosatellite Orbital Deployer (P-POD) [8]. In this Note, we survey 39 multisatellite missions in various stages of development, where each satellite’s mass is less than 10 kg. We categorize them based on their mission type and status, number of satellites (see Fig. 1), lead institution, and funding source (see Fig. 2). The objectives of this Note are to recognize state-of-the-art small satellite formation-flying (FF) missions, inspired by many enabling science applications, and to suggest future research directions. Multisatellite missions can be broadly divided into two categories, namely FF missions and constellation missions. The dynamic states of formation flying satellites [9–11] are coupled through a common control law. In other words, in an FF mission, at least one satellite must track a desired state relative to another satellite, and its tracking control law must, at the minimum, depend upon the states of this satellite. FF missions are further subdivided into two categories, namely FF missions that involve rendezvous and docking and FF missions without docking. Multisatellite missions that do not satisfy the definition of FF missions are called constellation missions. For example, even though specific relative positions are actively maintained, the GPS satellites constitute a constellation because their orbit corrections require only the individual satellite’s position and velocity (states) [9,10]. Furthermore, constellation missions are subdivided into controlled constellation missions, where each satellite actively maintains its position (e.g., GPS), and uncontrolled constellation missions, where satellites have no active control over their position. As per these definitions, satellites in FF missions and controlled constellationmissionsmust have active propulsion systems. Multisatellite missions without position control are categorized under uncontrolled constellation missions. In contrast with constellation missions, the main challenges of FF missions stem from dynamic couplings between satellites and the environment. If FF satellites are launched into LEO, they face environmental disturbances, such as air drag, solar pressure, and J2 perturbations [12]. These disturbances can cause the satellites to rapidly drift away from each other unless they are correctly accounted for. Therefore, the satellites have to counter these disturbances whilemaintaining their orbits and relative distances and attitudes. If the desired positions are not in the same altitude, then the satellites have to expend additional control effort to synchronize their orbital periods and relative distances [13,14].Also, the nonlinear nature of thedynamics of the satellites,with coupled formation flying control laws and environmental disturbances, makes formation flying challenging. This challenge is exacerbated by the limited capabilities of the current sensor and actuator technologies for small satellites [6,15]. Because of these challenges, FF missions is an active area for research and development (for example, see [1,9– 12,16–18] and the references therein). In Fig. 1, we have categorized the 39 small satellite missions into constellation missions (controlled or uncontrolled) and FF missions (with or without docking). The key for these missions is given in Table 1 [19–71]. These missions are also grouped into five mission types, namely Earth science [72] (Sec. II), astronomy and astrophysics [73] (Sec. III), planetary science [74] (Sec. IV), heliophysics [75] (Sec. V), and technology demonstrations (Sec. VI). Based on Fig. 1, we conclude that Earth science missions (14) are the most popular among science-driven missions (22). Moreover, 20 of these science-driven missions only require a constellation. Among the 17 technology demonstration missions, 10 missions aim to demonstrate formation flying capability in space using two to three small satellites. Only two FF missions are currently planning to use four or more small satellites. The categorization of the 39 multisatellite missions, based on their current mission status, leading organizations, and their funding sources, is shown in Fig. 2. The primary funding sources for multisatellite missions are NASA and NASA centers like the Jet Propulsion Laboratory (JPL), the National Science Foundation (NSF), the U.S. Department of Defense (DoD), non-U.S. agencies like the European Space Agency (ESA), Canadian Space Agency (CSA), Chinese Academy of Sciences, and private companies. The various mission status categories are 1) concept, where the mission concept Presented as Paper 2015-1623 at the 53rd AIAA Aerospace Sciences Meeting, AIAA Science and Technology Forum 2015, Kissimee, FL, 5– 9 January 2015; received 5 March 2015; revision received 16 July 2015; accepted for publication 7 September 2015; published online 24March 2016. Copyright © 2015 by the American Institute of Aeronautics andAstronautics, Inc. TheU.S.Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights are reserved by the copyright owner. Copies of this paper may be made for personal and internal use, on condition that the copier pay the per-copy fee to the Copyright Clearance Center (CCC). All requests for copying and permission to reprint should be submitted to CCC at www.copyright.com; employ the ISSN 0022-4650 (print) or 1533-6794 (online) to initiate your request. *Graduate Research Assistant, Department of Aerospace Engineering; [email protected]. Graduate Research Assistant, Department of Aerospace Engineering; [email protected]. Graduate Research Assistant, Department of Aerospace Engineering; [email protected]. Associate Professor, Department of Aerospace Engineering, Coordinated Science Laboratory; [email protected]. Senior Member AIAA. SeniorResearchScientist andTechnical Fellow; [email protected]. gov. Fellow AIAA.