. HARDWARE-IN-THE-LOOP TESTING OF CONTINUOUS CONTROL ALGORITHMS FOR A PRECISION FORMATION FLYING DEMONSTRATION MISSION “18 TH INTERNATIONAL SYMPOSIUM ON SPACE FLIGHT DYNAMICS” Bo J. Naasz (1) , Richard D. Burns (2) , David Gaylor (3) , John Higinbotham (4) (1) NASA Goddard Space Flight Center, Code 595, Greenbelt, MD 20771, USA, E-mail: bo.naasz@nasa.gov (2) NASA Goddard Space Flight Center, Code 591, Greenbelt, MD 20771, USA, E-mail: rich.burns@nasa.gov (3) Emergent Space Technologies, Inc., Greenbelt, MD 20770, USA, E-mail: dave.gaylor@emergentspace.com (4) Emergent Space Technologies, Inc., Greenbelt, MD 20770, USA, E-mail: john.higinbotham@emergentspace.com ABSTRACT A sample mission sequence is defined for a low earth orbit demonstration of Precision Formation Flying (PFF). Various guidance navigation and control strategies are discussed for use in the PFF experiment phases. A sample PFF experiment is implemented and tested in a realistic Hardware-in-the-Loop (HWIL) simulation using the Formation Flying Test Bed (FFTB) at NASA’s Goddard Space Flight Center. 1. INTRODUCTION Precision Formation Flying (PFF) refers to the class of distributed spacecraft missions that require precise, continuous control of the relative motion of multiple spacecraft, implemented through inter-satellite crosslinks. PFF technology will enable advanced science missions by using spacecraft Guidance, Navigation, and Control (GNC) systems to place distributed optics and detectors at distances not feasible on traditional spacecraft. Examples of PFF missions include Terrestrial Planet Finder, MicroArcsecond X-ray Imaging Mission, and Stellar Imager. While these missions will most likely occur in orbits near libration points, or in deep space, preliminary on-orbit demonstration of PFF technology is likely to occur in Low Earth Orbit (LEO) (for example in the proposed PFF version of New Millennium Program’s Space Technology 9 mission). In Section 2, we present a plan for demonstration of Precision Formation Flying in low earth orbit, and discuss the guidance and control issues associated with such an experiment. In Section 3, we present the current status of the Formation Flying Test Bed (FFTB) at the NASA Goddard Space Flight Center. In Section 4 we present results from a sample Precision Formation Flying experiment performed using both a non-real time simulation using the FreeFlyer TM software package, and a real-time, hardware-in-the-loop simulation using the FFTB. 2. LOW EARTH ORBIT PRECISION FORMATION FLYING DEMONSTRATION Demonstration of PFF in LEO requires a unique combination of formation flying guidance and control strategies. These strategies must consider the relatively large differential gravitational and atmospheric effects present in LEO, while providing a test environment relevant to more distant orbital regimes. To this end, these strategies must include the use of naturally stable formations for staging and parking, as well as brief experimental periods with formations defined by slight deviations from natural motion so that continuous control is required but not prohibitively expensive. The mission sequence for a LEO PFF demonstration can be broken into 6 distinct phases: 1) launch and early checkout; 2) stack separation; 3) period matching and individual spacecraft checkout; 4) precision formation flying experiments; 5) transfer to, and maintenance of stable parking formations for staging between experiments; 6) safe disposal. In this work we are interested in the precision formation flying experiment phases, and the staging required before and after each experiment. 2.1 Formation Dynamics in Low Earth Orbit Naturally stable formations in LEO, and fuel-efficient means for maintaining these formations in the presence of perturbations and navigation uncertainly have been the subject of a number of recent works. These works present strategies for defining and maintaining relative motion trajectories that will not degrade in the presence of navigation errors and differential perturbations from reference orbit eccentricity, higher order gravitational effects, and drag. A continuing theme in these works is the realization that in order for formation flying in LEO to be feasible, control algorithms must not fight naturally occurring short period relative motion caused by non-